town of greenwich drainage manual town of greenwich

Town of Greenwich Drainage ManualLow Impact Development and Stormwater Management

February 2012Revised Feb 2014

Town of GreenwichDrainage Manual

Low Impact Developmentand Stormwater Management

Effective Date: January 1, 2012

46 Hartford RoadManchester, CT 06040

800.286.2469

Larry S. CoffmanLNSB, LLLP

8097 Windward Key DriveChesapeake Beach, MD 20732

301.580.6631

Town of Greenwich Drainage Manual Low Impact Development and Stormwater Management

February 2012

Revised February 2014

Department of Public Works Engineering Division

The Town of Greenwich Drainage Manual is available on-line:

http://www.greenwichct.org/Government/Departments/Public_Works/Engineering_Division/Stormwater_Information/

Acknowledgements

Lead Staff

Scott Marucci, Senior Civil Engineer – DPW Engineering James Michel, P.E., Chief Engineer - DPW Engineering

David P. Thompson, Deputy Commissioner of Public Works – DPW Amy Siebert, Commissioner of Public Works – DPW

Fuss & O’Neill

Erik Mas, PE, Project Manager

Jennifer Cavanaugh, EIT, CPESC, Environmental Engineer Daniel DeLany, PE, Civil Engineer

Philip Forzley, PE, LEED AP, Civil Engineer M. James Riordan, AICP, LEED AP, Environmental Planner

Kenneth Sullenger, EIT, Civil Engineer

Larry Coffman, President, LNSB, LLLP

Stormwater Committee

Katie A. Blankley, Deputy Director P&Z/Assistant Town Planner – P&Z Michael N. Chambers, Director – IWWA

Frank Petise, Senior Civil Engineer – DPW Engineering Denise M. Savageau, Director – Conservation

http://www.greenwichct.org/Government/Departments/Public_Works/Engineering_Division/Stormwater_Information/

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Table of Contents

Town of Greenwich Drainage Manual

Low Impact Development and Stormwater Management

Town of Greenwich Drainage Manual i February 2014

1 Introduction .................................................................................................. 1 1.1 Purpose of the Manual................................................................................. 1 1.2 Organization of the Manual ........................................................................ 1 1.3 Relationship of the Manual to Local Land Use Review Process ................ 2 1.4 How to Use this Manual .............................................................................. 3

2 The Importance of Stormwater Management ............................................ 5 2.1 Impacts of Development ............................................................................. 5 2.2 The Three Components of Stormwater Management ................................ 8

3 Stormwater Management Standards ........................................................ 11 3.1 Introduction ............................................................................................... 11 3.2 The Stormwater Management Standards .................................................. 11 3.3 Applicability and Exemptions ................................................................... 20

4 Low Impact Development .......................................................................... 23 4.1 Introduction ............................................................................................... 23

4.1.1 Advantages of LID ......................................................................................... 24 4.2 Fundamental Concepts ............................................................................. 25 4.3 Incorporating LID Into the Site Planning and Design Process ............... 26

4.3.1 Process Overview ........................................................................................... 26 4.4 Non-structural LID Techniques ............................................................... 31

4.4.1 Minimizing Soil Compaction ........................................................................ 31 4.4.2 Minimizing Site Disturbance ......................................................................... 33 4.4.3 Protecting Sensitive Natural Areas ............................................................... 34 4.4.4 Protecting Riparian Buffers ........................................................................... 37 4.4.5 Avoiding Disturbance of Steep Slopes ........................................................ 39 4.4.6 Siting Relative to Permeable and Erodible Soils ........................................ 40 4.4.7 Protecting Natural Flow Pathways ............................................................... 40 4.4.8 Reducing Impervious Surfaces ..................................................................... 41 4.4.9 Stormwater Disconnection ............................................................................ 43

4.5 Structural LID Techniques ....................................................................... 46 4.6 LID Hydrologic Analysis .......................................................................... 46 4.7 LID Applications ....................................................................................... 47

4.7.1 Residential Development ............................................................................... 47 4.7.2 High Density and Commercial Development ............................................ 52 4.7.3 LID Roadway Design ..................................................................................... 54 4.7.4 Alternative Paving Surfaces ........................................................................... 59

5 Structural Stormwater Management Practices ....................................... 60 5.1 Introduction ............................................................................................... 60 5.2 Categories of Structural Practices ............................................................. 60

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5.2.1 Pretreatment BMPs ........................................................................................ 60 5.2.2 Treatment BMPs ............................................................................................ 61 5.2.3 Conveyance BMPs ......................................................................................... 61 5.2.4 Infiltration BMPs ........................................................................................... 61 5.2.5 Other BMPs and Accessories ....................................................................... 62

5.3 Proprietary Stormwater BMPs ................................................................... 62 5.3.1 Evaluating the Use of Proprietary Systems ................................................ 64

5.4 Treatment Train ........................................................................................ 65 5.5 Operation and Maintenance ...................................................................... 65 5.6 Design Criteria .......................................................................................... 66

5.6.1 Runoff Volume Reduction and Groundwater Recharge (Standard 4) ... 66 5.6.2 Peak Flow Control (Standard 5) .................................................................. 71 5.6.3 Pollutant Reduction (Standard 6) ................................................................ 74

5.7 Selection Criteria ....................................................................................... 79 5.7.1 Land Use Factors ........................................................................................... 79 5.7.2 Physical/Site Feasibility Factors .................................................................. 82 5.7.3 Downstream Resources ................................................................................ 86

5.8 LID Retrofits and Redevelopment ............................................................ 90 5.9 Design Guidance for Stormwater BMPs ................................................... 92

6 Drainage Facilities ...................................................................................... 93 6.1 General Design Requirements .................................................................. 93 6.2 Runoff Determination ............................................................................... 96

6.2.1 Design Storm Frequency .............................................................................. 96 6.2.2 Acceptable Methods ...................................................................................... 97 6.2.2.1 Rational Method…………………………………………………….. 97 6.2.2.2 Other Methods……………………………………………………...101 6.2.3 Hydrologic Analysis Submission Requirements ...................................... 103

6.3 Storm Drainage Systems ......................................................................... 104 6.3.1 General Requirements ................................................................................. 104 6.3.2 Pavement Drainage ...................................................................................... 104 6.3.3 Inlets/Catch Basins...................................................................................... 106 6.3.4 Manholes ....................................................................................................... 108 6.3.5 Storm Drains................................................................................................. 110 6.3.6 Headwalls and Trash Racks ........................................................................ 115 6.3.7 Outlet Protection ......................................................................................... 116

6.4 Culverts ..................................................................................................... 116 6.5 Channels ................................................................................................... 117 6.6 Bridges ...................................................................................................... 119 6.7 Storage Facilities ...................................................................................... 119 6.8 Erosion and Sedimentation Control ......................................................... 121 6.9 Structural Design ...................................................................................... 121

6.9.1 Reinforced Concrete Pipe ........................................................................... 121

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Town of Greenwich Drainage Manual iii February 2014

6.9.2 Plastic Pipe .................................................................................................... 121

7 Submittal Requirements .......................................................................... 124 7.1 Stormwater Management Report ............................................................ 124 7.2 Construction Plans .................................................................................. 126

7.2.1 Plan Set Standards ....................................................................................... 126 7.3 Supporting Documents and Studies ........................................................ 130 7.4 Operation and Maintenance Plan ............................................................ 131 7.5 Erosion and Sediment Control Plan ......................................................... 131 7.6 Plan and Report Revisions ...................................................................... 132

7.6.1 Plans ............................................................................................................... 132 7.6.2 Reports .......................................................................................................... 132

7.7 Certifications ............................................................................................ 132

8 References ................................................................................................ 133

Tables Page 1-1 Town of Greenwich Land Use Jurisdictions, Regulations, and Agencies 2 2-1 Summary of Development Impacts on Water Resources 6 5-1 Percent Annual Runoff Reduction for Various Stormwater BMPs 67 5-2 Recharge Target Depth by Hydrologic Soil Group 68 5-3 Stormwater Pretreatment BMPs 70 5-4 Stormwater BMPs for Peak Flow Control 73 5-5 Site Cover Runoff Coefficients 75 5-6 TSS Removal Efficiencies 77 5-7 Zoning and Land Use for Stormwater BMP Selection 80 5-8 Stormwater BMPs for High Load Areas 83 5-9 Stormwater BMPs for Shellfish Growing Areas and Public Swimming Beaches 88 5-10 Stormwater BMPs for Recharge Areas for Public Water Supplies 89 6-1 Design Elevations of Tidal Waters 95 6-2 Design Storm Frequencies 96 6-3 Runoff Coefficients for Various Surfaces (Rational Method) 98 6-4 Runoff Coefficients for Various Land Uses (Rational Method) 98 6-5 Runoff Coefficients for Greenwich, CT (Rational Method) 99 6-6 Frequency Factors (Rational Method) 100 6-7 Greenwich Catch Basin Grate Parameters 108 6-8 Manhole Sizing 110

Tables -9 Minimum Radii (Feet) for Curved RCP Installation 111 6-10 Manning’s Pipe Roughness Coefficients for Storm Drainage 112 6-11 Minimum Allowable Pipe Slopes to Ensure 2.5 ft/s in Storm Drains Flowing Full 115

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6-12 Storm Drainage Pipe Design and Installation Guidelines 122

Figures Page 2-1 Changes in Stream Hydrology as a Result of Urbanization 7 4-1 Minimally Disturbed Residential Development 33 4-2 Reduced Limits of Disturbance 34 4-3 Site Map with Natural Areas Delineated 35 4-4 Three-Zone Riparian Buffer 37 4-5 Relationship Between Slopes and Development Impacts 39 4-6 Conventional and LID Residential Design Concepts 49 4-7 Medium- to High-Density Lot Using LID Practices 50 4-8 Zero Lot Line Configuration 50 4-9 Conventional and LID Designs for Large Lots 51 4-10 LID Design Concept for Single Family Residential Lot 52 4-11 Example LID Site Plan for Commercial Office Building 53 4-12 Example LID Site Plan for Commercial Shopping Plaza 54 4-13 Example of a Dry Swale for Open Section Roadway Design 55 4-14 Example of Open Section Roadway Design 56 4-15 Narrow Road Section with Sidewalks, Shallow Swale and Porous Pavement Shoulders 56 4-16 Examples of Urban Roadway Bioretention Design 57 4-17 Bioretention Curb Extensions Used for Stormwater Treatment and Traffic Calming 57 4-18 Example of a Bioretention Cell Within the Road Right-of-Way 58 4-19 Example of a Curb Cut Leading to a Bioretention Area 58 4-20 Example of the Use of Alternative Paving Surfaces in Urban Roadway Design 59 5-1 NRCS Hydrologic Soil Groups 85 5-2 Critical Areas 87 6-1 Nomograph for Manning’s Equation for Flow in Storm Drains (Flowing Full) 113 6-2 Nomograph for Flow in Storm Drains (Partial Depth Flow) 114

Appendices A Water Quality – Town of Greenwich B Stormwater Infiltration/Recharge Requirements C Credits for Low Impact Development Best Management Practices D Suggested Sources of Information on the Effectiveness of Proprietary BMPs

Appendices E Recommended Process for Evaluating the Proposed Use of Proprietary BMPs F TSS Removal Efficiency Calculations G Design Guidance for Structural Stormwater BMPs H Stormwater Maintenance Declaration and Easement

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I Checklists J Standard Notes K Certification Forms L 24-Hour Design Storm Rainfall Amounts M Glossary

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Acronyms and Abbreviations AHW allowable headwater AVGWLF Generalized Watershed Loading Function with an ArcView (AV) geographic

information system (GIS) interface BMPs Best Management Practices C runoff coefficient CDM Camp, Dresser, & McKee C.O. Certificate of Occupancy cfs cubic feet per second CGS Connecticut General Statutes CN curve number CTDOT Connecticut Department of Transportation CTDEEP Connecticut Department of Energy and Environmental Protection DBH diameter at breast height DPW Department of Public Works DS downstream EIA effective impervious area ETV Environmental Technology Verification FEMA Federal Emergency Management Agency GIS geographic information system GRV groundwater recharge volume HDPE high density polyethylene HGL hydraulic grade line HSG hydrologic soil groups IDF intensity-duration-frequency LEED Leadership in Energy and Environmental Design LEED-ND LEED for Neighborhood Development LID Low Impact Development LIS Long Island Sound NAVD North American Vertical Datum NGVD National Geodetic Vertical Datum NPDES National Pollutant Discharge Elimination System NRCS U.S. Natural Resources Conservation Service O&M Operation and Maintenance PE Professional Engineer PVC polyvinyl chloride RCP reinforced concrete pipe RCV runoff capture volume RRV runoff reduction volume SARA Superfund Amendments and Reauthorization Act SCS Soil Conservation Service STEPL Spreadsheet Tool for Estimating Pollutant Loads SWPPP Stormwater Pollution Prevention Plan TAPE Technology Assessment Protocol – Ecology TARP Technology Acceptance Reciprocity Partnership TC time of concentration

Town of Greenwich Drainage Manual vii February 2014

Acronyms and Abbreviations TMDL total maximum daily load TSS total suspended solids U.S. EPA United States Environmental Protection Agency US upstream USGBC United States Green Building Council USGS U.S. Geological Survey WB western basin WQF water quality flow WQV water quality volume

Town of Greenwich Drainage Manual 1

February 2014

1 Introduction

1.1 Purpose of the Manual

The Town of Greenwich Drainage Manual provides guidelines for land development activities and stormwater management in the Town of Greenwich. The manual is applicable to new development and redevelopment activities undertaken by private or municipal entities, including public works projects and projects over which the Town Planning and Zoning Commission, Building Division, Inland Wetlands & Watercourses Agency, Health Department, and/or other commissions have responsibility for review and approval. The manual provides guidance for developers, engineers, and local regulatory authorities to design and review projects in a technically sound and consistent manner. This manual is intended to augment other existing design guidance, including the Connecticut Department of Energy and Environmental Protection (CTDEEP) Stormwater Quality Manual (as amended) and the Connecticut Department of Transportation (CTDOT) Drainage Manual (as amended). The Town of Greenwich Drainage Manual is generally consistent with these state-wide manuals to ensure consistency with state stormwater management policies and to eliminate potential redundancy with other existing guidance. This manual references applicable sections of the Connecticut Stormwater Quality Manual and Department of Transportation Drainage Manual, but also includes more detailed design guidance; greater emphasis on Low Impact Development (LID), sustainable site design, and green infrastructure; and specific Stormwater Management Standards tailored to the unique characteristics and issues facing the Town of Greenwich. The design practices described in this manual shall be implemented by professional engineers licensed to practice in the State of Connecticut. The design engineer is responsible for field investigations, data collection and analysis, and design of stormwater management and drainage facilities based upon the guidance contained in this manual. Stormwater management is an evolving field. Existing stormwater management practices are being refined and new practices are being developed on a regular basis. The Town may periodically amend this manual to reflect new or modified technologies, practices, and regulatory requirements.

1.2 Organization of the Manual

This manual emphasizes an integrated approach to stormwater management and drainage design, including stormwater quantity and quality issues. The manual also promotes LID, green infrastructure, and sustainable site planning techniques to maintain the integrity of natural site features during the development process, which can reduce or eliminate structural components of a stormwater management system. The organization of the manual reflects this integrated approach.

Section 1 – describes the purpose and organization of the manual, as well as how to use the manual.

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Section 2 – provides a summary of the stormwater problem and issues unique to the Town of Greenwich, and an overview of the stormwater management approaches contained in the manual.

Section 3 – describes stormwater management standards that address both water quality and quantity and their applicability to various types of projects and settings.

Section 4 – describes environmentally sensitive site design and Low Impact Development approaches for stormwater management.

Section 5 – contains a description of structural stormwater management practices that can be used to satisfy stormwater quality and quantity requirements, including their proper selection and design.

Section 6 – addresses the design of storm drainage facilities, including hydrologic and hydraulic analysis methods and specific design guidance and criteria for various types of drainage systems.

Section 7 – describes submittal requirements, including the Stormwater Management Report, construction plans, operation and maintenance plan, erosion and sedimentation control plan, and certifications.

1.3 Relationship of the Manual to

Local Land Use Review Process

The design guidance contained in this manual is applicable to new development and redevelopment activities on all properties within the Town of Greenwich, regardless of whether the manual is referenced by land use regulations or ordinances administered by the local agencies and jurisdictions listed in Table 1-1. This list will be updated as future land use regulatory programs are developed or modified.

Table 1-1. Town of Greenwich Land Use Jurisdictions, Regulations, and

Agencies

Jurisdiction Regulation Agency

Watercourses Inland Wetlands & Watercourses Regulations

Greenwich Inland Wetlands & Watercourses Agency

Inland Wetlands Inland Wetlands & Watercourses Regulations

Greenwich Inland Wetlands & Watercourses Agency

Coastal Area Coastal Area Management Regulations and Policies

Greenwich Planning and Zoning Commission

Site Plan Approval Building Zone Regulations Greenwich Planning and Zoning Commission

Special Permit Building Zone Regulations Greenwich Planning and Zoning Commission

Building Permit CT State Building Code Building Zone Regulations

Division of Building Inspection, Greenwich Department of Public Works

Land Subdivision Subdivision Regulations Greenwich Planning and Zoning Commission

Septic Systems, Health Department Regulations Greenwich Health Department


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Table 1-1. Town of Greenwich Land Use Jurisdictions, Regulations, and

Agencies

Jurisdiction Regulation Agency

Wells, Swimming Pools, Marine Docks

Town Highways, Sidewalks, and Drainage Facilities

Subdivision Regulations Building Zone Regulations

Highway Division, Greenwich Department of Public Works

State Maintained Highways (Putnam Avenue and Drainage Facilities)

CTDOT Design Standards Connecticut Department of Transportation

1.4 How to Use this Manual

This manual can be used by engineers and designers, local land use boards and commissions, municipal department staff, municipal officials, and property owners. Engineers and designers who are responsible for site design and the design of stormwater management systems for new development and redevelopment projects are the most likely users of this manual. Before beginning a project, engineers and designers should familiarize themselves with the Stormwater Management Standards (Section 3) that their project will have to meet. Next, engineers and designers should review the LID site planning and design process described in Section 4 as well as the structural stormwater management practices and drainage design requirements described in Sections 5 and 6, respectively, to determine approaches that would work best at their site. Finally, engineers and designers should refer to the submittal requirements in Section 7 and any other applicable technical guidance in the manual appendices. The recommended process for using the manual is summarized below:

Step 1 – Review applicable zoning, subdivision, and other local land use planning and regulatory requirements and contact local officials to clarify uncertainties. At a minimum, designers are required to adhere to the Stormwater Management Standards and performance criteria in this manual and the requirements referenced herein.

Step 2 – Collect the necessary information and data to inventory and evaluate the site in order to begin developing site design concepts and a stormwater management approach. The information and data required to properly inventory and evaluate a site are discussed in Section 4 (Low Impact Development site planning and design process) and in Section 7 (submittal requirements). This information will allow engineers and designers to make the decisions necessary to develop effective LID site designs and stormwater management plans.


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Step 3 – Confirm the applicable Stormwater Management Standards (Section 3) and required design criteria (Section 5) depending on the type and location of the proposed development project. The basic criteria for both stormwater quality and quantity controls should be established at this stage.

Step 4 – Establish the basic site design and stormwater management approach utilizing the LID site planning and design techniques described in Section 4. Before project details are finalized, such as building and street layouts, engineers and designers must determine how to avoid, reduce, and manage impacts.

The initial objective of the LID site planning and design process is to avoid disturbance of natural features. This includes identification and preservation of natural areas; minimizing the hydrologic alteration of a site is just as important as stormwater treatment for resource protection. Once sensitive resource areas and site constraints have been avoided, the next objective is to reduce the impact of land alteration by minimizing impervious areas to reduce the volume of stormwater runoff, increase groundwater recharge, and reduce pollutant loadings. Runoff is generated primarily from impervious surfaces, such as rooftops, roadways or any hard surface that prevents water from absorbing into the ground. Impervious surfaces can often be reduced with thoughtful site planning.

After making all reasonable efforts to avoid and reduce potential development impacts, the final step is to determine the basic approach for effectively managing the remaining stormwater runoff to meet the stormwater standards using the approaches described in Sections 5 and 6.

Step 5 – Use the approach determined in Step 4 to develop a conceptual design plan at approximately the 25% design stage that utilizes LID site planning and design techniques to the maximum extent practicable as required by Standard 1; identifies the location and types of BMPs to be utilized, the approximate footprint needed, and construction and maintenance access requirements; and establishes the basic profile to verify physical constraints and the overall feasibility of each BMP (see Stormwater Management Report Part One in Section 7 submittal requirements). At this stage, coordination with the local approval agencies is recommended to address potential issues prior to final design. Additional data may need to be collected at this stage (including field testing of soils if necessary) to revise the concept before moving forward with final design.

Step 6 – Move forward with site design, ensuring that the proposed stormwater management system meets the standards described in Section 3, including preparation of a construction erosion and sediment control plan, a Stormwater Pollution Prevention Plan for high load areas, an Operation and Maintenance Plan, and a Stormwater Management Report (Part Two), construction plans, and completed checklists and certifications (see Section 7 for submittal requirements).


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2 The Importance of Stormwater Management

The Town of Greenwich’s exceptional quality of life depends on the development of successful programs to protect, manage and restore its water resources, aquatic living resources and natural environment. Program goals and objectives include protecting the quality and volume of existing and future water supplies; restoration of sensitive riparian and marine fisheries; protection of sensitive wetlands and critical areas; restoration of impaired waters; ensuring safe recreational water uses; addressing flooding impacts; and meeting regulatory requirements, such as Phase II of the National Pollutant Discharge Elimination System (NPDES) municipal stormwater permit program. Achieving these goals is extremely difficult and complicated by an array of political, social, economic and technological issues.

An effective urban stormwater management program is central to the successful protection and restoration of water resources and related water dependent uses. To guarantee protection and restoration goals are achieved, an urban stormwater program must maintain the natural hydrologic regime and maintain a watershed’s capacity to filter and purify runoff and capture/sequester pollutants. Unfortunately, experience over the last 30 years has clearly demonstrated that conventional strategies of natural resource conservation combined with end-of-pipe runoff treatment technology simply has not and cannot restore or prevent continued degradation of receiving waters from the cumulative impacts of urbanization.

The limitations of conventional urban stormwater management practices to meet receiving water goals was a major driver for development of more effective decentralized source control technologies generally referred to as Low Impact Development or LID. LID provides the necessary tools to plan and engineer sites in a manner that mimics predevelopment hydrology, protects water quality by treating runoff and reducing pollutant loads, and provides a wide array of strategies and tools for urban retrofits to restore impaired waters. Greenwich, like many local communities, historically has placed a strong emphasis on the stormwater basics of providing flood control and adequate drainage. The Town recognizes the limitations of these conventional strategies and the multiple benefits of a holistic approach to stormwater management through the use of more natural systems and LID techniques. The stormwater management design criteria and guidance adopted by this manual reflect the trend in stormwater management toward an integrated approach that combines effective site planning and structural stormwater controls to address the full range of hydrologic and water quality impacts resulting from development.

2.1 Impacts of Development

The hydrologic and water quality impacts from urban runoff can be significant. Streams, lakes, and estuaries in urban areas are often impaired by urban runoff. Impervious surfaces (e.g., rooftops, sidewalks, roads, and parking lots), disturbed soils, and managed turf associated with urban development can have multiple impacts on water quality and aquatic life. These impacts are summarized in Table 2-1.


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Table 2-1. Summary of Development Impacts on Water Resources

Increases in: Decreases in:

Impervious cover, compacted soils, managed turf, and other land covers and uses that contribute pollutants

Health and safety of receiving waters

Stormwater runoff volume Groundwater recharge

Stormwater runoff velocity Stream channel stability

Pollutant loads Health, safety, and integrity of water supplies, reservoirs, streams, biological communities, recreational opportunities

Stream channel erosion Stream habitat

Adapted from CWP, 2008.

Water bodies in the Town of Greenwich that are impaired (i.e., do not meet Connecticut Water Quality Standards) due to urban runoff or related nonpoint sources are summarized in Appendix A. Coastal waters are the primary impaired water bodies in the Town of Greenwich. The impairments are for shellfish harvesting, recreation, and habitats for fish and other aquatic life and wildlife. Non-point sources, including urban stormwater runoff and waterfowl, as well as marina/boating and sanitary on-vessel discharges are the predominant suspected sources of the impairments, which are caused by elevated concentrations of bacteria and nutrient enrichment/low dissolved oxygen. Urban development can also impact the timing and quantity of post-development runoff discharging to urban streams, as evidenced by a comparison between pre-development and post-development runoff hydrographs (Figure 2-1). Compared to the pre-development conditions, post-development stormwater discharges can increase the runoff volume, increase the peak discharge, and decrease the infiltration of stormwater, which thereby decreases baseflow in headwater streams and in wetlands. The changes to stream hydrology can have negative impacts on channel stability and the health of aquatic biological communities. Common problems include bank scour and erosion, increased downstream flooding, and loss of in-stream habitat for macroinvertebrates, fish, and other organisms (CWP, 2008). In addition, these impacts not only affect the aquatic environment, but also affect the ability of people to use these areas for recreation, both active and passive. For example, the discharge of stormwater commonly results in beach closures due to high bacteria and pathogen counts in the water. Increasingly, communities are looking for ways to maximize the opportunities and benefits associated with growth while minimizing and managing the environmental impacts of development. Where and how development occurs can dramatically affect a community’s watersheds, infrastructure, and water supplies.


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Source: CWP, 2008.

Figure 2-1. Changes in Stream Hydrology as a Result of Urbanization Impervious cover, which is one measure of the amount of development within a community and watershed, has been widely cited as an indicator of stream health in urbanizing watersheds. However, the original body of work upon which the impervious threshold relationships to stream health has been disputed on many levels. Although there is a correlation between impervious cover and health of streams in urbanizing watersheds, it is an indirect and complex one. Since this relationship is a general correlation and not a direct cause-and-effect relationship, establishing absolute impervious thresholds for stream health cannot be made reliably. In fact, the cause and effect of stream health degradation is not imperviousness itself but, instead, the way that impervious surfaces have been used to quickly collect and dispose of stormwater runoff. This ―good drainage‖ paradigm results in a change from the natural hydrologic regime to one that increases runoff volume, velocity and flows; loss of recharge; reduced capacity of the landscape to capture and assimilate pollutants; loss of habitat structure; increased water temperatures, etc. The cumulative changes cause physical, chemical and biological alterations of vital ecological functions to the terrestrial and aquatic ecosystems that result in degraded stream health. If stormwater management/drainage systems were engineered to mimic nature by using LID techniques, there would be no cause and effect of urbanization and no correlation between imperviousness and stream health. LID’s philosophy, principles and practices provide the tools to mimic vital natural watershed functions, allowing development and urbanization to occur without stream degradation and independent of the amount of imperviousness. This concept is also known as ―effective‖ impervious cover. Impervious cover that is hydrologically disconnected from the drainage system through the use of LID techniques effectively functions similar to pervious surfaces.


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2.2 The Three Components of

Stormwater Management

Consistent with the Connecticut Stormwater Quality Manual, the Greenwich manual also promotes the following hierarchy of stormwater management methods to initially reduce stormwater impacts through site design and source control methods, followed by structural stormwater management controls to collect, detain, and treat stormwater:

First, reduce runoff and site disturbance through design: Use LID site planning and design techniques to reduce effective impervious cover, disturbed soils, and stormwater runoff volume.

Second, reduce pollutants carried by runoff: Use source control and pollution prevention practices to reduce exposure of pollutants to rainfall and runoff.

Third, capture, detain and treat runoff: Design stormwater BMPs to collect, detain and treat the stormwater that is generated after applying the LID site planning and design and source control methods described above.

Site Planning and Design Effective site planning and design consists of preventive measures that address the root cause of stormwater problems by attempting to maintain pre-development site hydrology. Stormwater programs that rely heavily on conventional end-of-pipe stormwater controls can miss opportunities to reduce stormwater impacts because they collect and treat runoff after it has already been generated. This manual emphasizes the use of site planning and design techniques early in the site development process to achieve greater stormwater quantity and quality benefits. Section 4 of the manual describes LID site planning and design techniques (i.e., non-structural LID BMPs). The following site planning and design techniques are recommended for use in Greenwich:

Preservation of undisturbed natural areas

Preservation or restoration of riparian buffers, floodplains, and shorelines

Minimize grading and clearing

Avoid compaction of porous soils

Avoid disturbance of erodible soils

Preservation of natural topography

Avoidance of sensitive areas

Reduced clearing and grading limits

Protect and preserve open space

Conservation development

Reduced roadway lengths and widths

Shorter or shared driveways

Shared parking

Reduced building footprints


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Reduced parking lot footprints

Reduced setbacks and frontages

Use of fewer or alternative cul-de-sacs

Use of open drainage systems

Lengthen flow paths and maximize sheet flow

Disconnection of roof runoff

Saving and replacing topsoil or the use of compost-amended soils

Source Control Practices and Pollution Prevention Source control practices and pollution prevention are operational practices that reduce or eliminate the exposure of pollutants to rainfall and runoff. Similar to the Connecticut Stormwater Quality Manual, the Greenwich manual emphasizes the use of source control practices and pollution prevention, together with effective site planning and design. The following source control and pollution prevention practices are recommended for use at residential and nonresidential sites in Greenwich: Residential Nonresidential

Natural landscaping

Tree planting

Yard waste composting1

Septic system maintenance

Driveway and street sweeping

Household hazardous waste collection programs

Downspout disconnection

Pet waste pickup

Storm drain marking

Impervious cover disconnection

Covered loading areas

Covered fueling areas

Covered vehicle storage areas

Storm drain disconnection

Downspout disconnection

Covered dumpsters

Covered material storage areas

Secondary containment

Spill response plans

Signage

Employee training

Pollution prevention plans

Natural landscaping

Structural Stormwater Management Practices Structural stormwater management practices (also referred to as stormwater Best Management Practices or BMPs) are designed to collect, detain and treat the stormwater that is generated after applying the LID site planning and design and source control methods described above. Structural practices are typically used to meet multiple objectives such as reducing runoff volume, attenuating peak flows, capturing and treating runoff, and providing groundwater recharge.

1Yard waste composting reduces or eliminates the need for outside inputs of chemical fertilizers, thus resulting in

the reduction of pollutants exposed to rainfall and runoff.


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Stormwater management can be accomplished through small-scale, distributed practices close to the source of runoff (also referred to as structural LID BMPs), such as the use of rain gardens, filter strips, and permeable pavement, in combination with the LID site planning and design techniques that are described in Section 4 of this manual. Traditional end-of-pipe controls such as stormwater basins should only be used, if necessary, after exhausting LID approaches. The following structural stormwater BMPs are recommended for use in Greenwich. Structural LID BMPs Other Structural BMPs

Rainwater harvesting (e.g., rain barrels, cisterns) for property irrigation

Bioretention systems including rain gardens, tree filters, stormwater planters, and curb extensions

Dry wells and subsurface infiltration systems (decentralized, small-scale practices distributed throughout the site)

Green roofs

Permeable pavement

Vegetated filter strips

Vegetated swales/channels

Compost-amended soils

Large-scale, end-of-pipe infiltration and filtration practices

Stormwater basins

Constructed stormwater wetlands

Engineered swales

Proprietary BMPs


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3 Stormwater Management Standards

3.1 Introduction

Stormwater management standards establish minimum stormwater management criteria for new development and redevelopment activities. The Town of Greenwich stormwater management standards promote the use of Low Impact Development techniques to protect water quality, reduce runoff volume, maintain groundwater recharge, and address peak flows and flooding during larger storms. The standards are generally consistent with the stormwater management approaches and design guidance contained in the Connecticut Department of Energy and Environmental Protection Connecticut Stormwater Quality Manual and the Connecticut Department of Transportation Drainage Manual, but also reflect the town’s unique natural resources and development characteristics.

3.2 The Stormwater Management

Standards

Standard 1: Low Impact Development Low Impact Development (LID) site planning and design techniques (see Section 4 of this manual) shall be used to the maximum extent practicable2 to reduce the generation of stormwater runoff and pollutant loads. LID practices, both non-structural and structural, are to be given preference over conventional structural stormwater controls.

Standard 2: Protection of Natural Hydrology

A. Site disturbance shall be minimized. The area outside the project disturbance area shall be maintained at natural grade and retain existing, mature vegetated cover. The project disturbance area shall be depicted on the design, construction, and mitigation plans and shall be delineated in the field prior to commencing land disturbance activities. The project disturbance area shall include only the area necessary to reasonably accommodate construction activities. Low areas on a lot shall not be dewatered and filled in unnecessarily.

B. Soil compaction on site shall be minimized by using the smallest (lightest) equipment possible and minimizing travel over areas that will be revegetated (e.g., lawn areas) or used to infiltrate stormwater (e.g., bioretention areas). In no case shall excavation

2 Project proponents seeking to demonstrate compliance with this standard ―to the maximum extent practicable‖

shall demonstrate that: 1. They have made all reasonable efforts to meet the standard; 2. They have made a complete evaluation of possible site planning and design techniques (non-structural

LID BMPs), source control practices and pollution prevention, and structural LID BMPs as described in Section 4 of this manual; and

3. If full compliance with this standard cannot be achieved, they are implementing the highest practicable level of stormwater management, which must be documented by the proponent.


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equipment be placed in the bottom of an infiltration area during construction. All areas to be used to infiltrate stormwater shall be delineated with protective fencing prior to the start of construction.

C. The post-development time of concentration (TC) should approximate the pre-

development3 values when possible. Flow velocity in areas that are graded to natural drainage patterns should be kept as low as possible to avoid soil erosion.

D. Development shall follow the natural contours of the landscape. A grading plan shall be

submitted as part of the site plan review process showing both existing and finished grades for the proposed development. Retaining walls must comply with the requirements of the Building Zone Regulations. Basements that reach grade should be constructed as walk-outs.

E. Compost-amended soils shall be used for areas of fill on development sites prior to

vegetation establishment. Amending a soil with compost increases the soil’s permeability and water holding capacity, thereby delaying and often reducing the peak stormwater runoff flow rate, and decreasing irrigation water requirements. Soil amendments also enhance a lawn’s long-term aesthetics while reducing fertilizer and pesticide requirements.

F. No ground disturbed as a result of site construction and development shall be left as

exposed bare soil at project completion. All areas exposed by construction, with the exception of finished building, structure, and pavement footprints, shall be de-compacted (aerated) and covered with a minimum thickness of six inches of non-compacted topsoil, and shall be subsequently planted with living vegetation such as grass, groundcovers, trees, and shrubs, and other landscaping materials (mulch, loose rock, gravel, stone).

G. Priority shall be given to maintaining existing surface waters and systems, including, but

not limited to, perennial and intermittent streams, wetlands, vernal pools, natural swales, and low-lying areas.

H. Where roadway or driveway crossings of surface waters cannot be eliminated,

disturbance to the surface water shall be minimized, hydrologic flows shall be maintained, direct discharge of runoff from the roadway to the surface water is strongly discouraged, and the area shall be re-vegetated after construction.

I. Roadway and driveway crossings over streams shall comply with the Connecticut

Department of Energy and Environmental Protection Stream Crossing Guidelines (as amended) to accommodate high flows, minimize erosion, and support aquatic habitat and wildlife passage.

3 Refer to the Glossary at the end of this manual for a definition of ―pre-development‖ conditions.


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Standard 3: Stormwater Best Management Practices Non-structural and structural stormwater Best Management Practices (BMPs) shall be used to meet the conditions below for control of runoff volumes and peak flows, pollutant reduction, and maintenance of groundwater recharge.

A. Stormwater management practices shall be selected to accommodate the unique hydrologic and geologic conditions of the site.

B. Proponents shall demonstrate how the proposed control(s) will comply with these

standards, including runoff reduction, groundwater recharge, peak flow control, and pollutant reduction. The proponent must provide design calculations and other back-up materials necessary.

C. At the discretion of the approving authority, conventional structural stormwater

management systems (non-LID BMPs) shall incorporate designs that allow for shutdown and containment (as feasible) in the event of an emergency spill or other unexpected contamination event.

D. Pumping of stormwater (excluding rainwater harvesting systems such as cisterns),

including, but not limited to, from yards, driveways, and roofs, is strongly discouraged and will be prohibited in most situations as part of a proposed stormwater management system design. This is because of the significant runoff volumes, maintenance requirements, standby power requirements, and overflows associated with large storms. All other feasible approaches must be investigated to avoid the use of pumps in stormwater management system designs. In the event the project proponent determines that pumps for stormwater are necessary the proponent must submit required backup information as described in this manual for review by the approving authority. For the use of a pump for stormwater to be approved by the approving authority, the proponent will be required to provide the following at a minimum:

Include on-site stormwater BMPs that are designed to accommodate and manage the pumped stormwater in accordance with the other standards contained in this manual. The stormwater BMPs shall include a system for re-use of the pumped stormwater for lawn or landscape irrigation.

Maintain a backup generator associated with the pump,

Design the system, at a minimum, for the 25-year, 24-hour design storm (the system may be required to be designed for the 50-year or 100-year, 24-hour design storm as determined by the approving authority),

Provide documentation that the pump and stormwater re-use system are designed and inspected by a Professional Engineer licensed in the State of Connecticut.

E. Pumping of uncontaminated groundwater, including, but not limited to, from

basements, and foundations, is discouraged for new development or in the case of redevelopment involving the upgrade of existing sump pump systems. The replacement of an existing sump pump system is acceptable when a direct replacement of the pump


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is needed. All other feasible approaches (footing drains to daylight, slab on grade, crawl space, etc.) must be investigated to avoid the use of pumps in groundwater management system designs for new development or redevelopment. In the event the project proponent determines that pumps are necessary to manage groundwater for new development or redevelopment applications, the proponent must submit required backup information as described in this manual for review by the approving authority. For the use of a pump to manage groundwater to be approved by the approving authority, the proponent will be required to provide the following at a minimum:

Include on-site BMPs that are designed to accommodate and manage the pumped uncontaminated groundwater in accordance with the other standards contained in this manual. A system for re-use of the pumped groundwater for lawn or landscape irrigation shall be considered. Overflows must be directed to an on-site level spreader or connected to the Town drainage system (Highway Permit Required) with review and approval.

A backup generator associated with the pump is recommended but not required,

Design the on-site BMPs to meet the maximum pumping rate of the proposed pumping system.

Provide documentation that the pump and on-site BMPs are designed and inspected by a Professional Engineer licensed in the State of Connecticut.

Standard 4: Runoff Volume Reduction and Groundwater Recharge

A. Runoff Reduction – Control post-development runoff volumes to the corresponding pre-development runoff volumes for up to the 1-year, 24-hour storm to the maximum extent practicable4 through the use of LID site planning and design techniques and structural stormwater BMPs.

B. Groundwater Recharge – Loss of annual recharge to groundwater shall be eliminated or

minimized to the maximum extent practicable through the use of infiltration measures including LID site planning and design techniques, structural stormwater BMPs, and good operation and maintenance. At a minimum the annual recharge from the post-development site shall approximate the annual recharge from the pre-development site conditions. Compliance with the runoff reduction standard in Item A. above using stormwater infiltration shall be considered adequate to demonstrate compliance with the groundwater recharge standard.

C. Runoff Capture – Runoff must be retained on-site for new stormwater discharges

located within 500 feet of and that ultimately discharge to tidal wetlands, which are not

4 For purposes of this standard, ―to the maximum extent practicable‖ means that:

1. The project proponent has made all reasonable efforts to meet the standard, 2. The project proponent has made a complete evaluation of all possible management measures, including

Low Impact Development site planning and design techniques and structural stormwater BMPs, 3. If the post-development runoff volumes and/groundwater recharge do not at least approximate the

runoff volumes and/or annual recharge under pre-development conditions, the project proponent has demonstrated that the highest practicable methods for runoff reduction and/or infiltration have been implemented.


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fresh tidal wetlands, through the use of LID site planning and design techniques and/or structural stormwater BMPs. Compliance with the runoff reduction standard in Item A. above shall be considered adequate to demonstrate compliance with this standard.

Standard 5: Peak Flow Control

A. Stream Channel Protection – Control the post-development peak flow rate (typically the 2-year storm or smaller) that results in bankfull streamflow conditions and the shape and form of stream channels.

B. Conveyance Protection – Provide adequate passage for flows leading to, from, and

through stormwater management facilities.

C. Peak Runoff Attenuation – Control the post-development peak flow rates to the corresponding pre-development peak flow rates.

D. Emergency Outlet Sizing – Size the emergency outlet to safely pass the post-

development peak runoff from large storms in a controlled manner without eroding the outlet works, downstream drainage systems, and property more than would occur during a similar event under pre-development conditions.

Standard 6: Pollutant Reduction

A. Stormwater management systems shall be designed to remove 80% of the average annual post-construction load of Total Suspend Solids (TSS)5.

Standard 7: High Load Areas Stormwater discharges from land uses with higher potential pollutant loads (referred to as ―High Load Areas‖) require the use of specific source controls, pollution prevention measures, and stormwater BMPs, approved by the approving authority for such use.

A. The uses or activities identified in Section 5.7.1 are considered high-load areas, with the potential to contribute higher stormwater pollutant loads, and shall comply with the requirements in Section 5.7.1.

5 Since removal efficiency may vary with each storm, 80% TSS removal is not required for each storm. It is the

average removal over the year that is required to meet the standard.


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B. In addition to site-specific stormwater BMPs, high-load areas shall include a stormwater pollution prevention plan (SWPPP) describing methods for source reduction and pretreatment.

C. If a high-load area demonstrates, through a SWPPP, the use of BMPs that result in no exposure of regulated substances to precipitation or runoff or release of regulated substances, it shall no longer be considered a high-load area.

Infiltration of stormwater from high-load areas is prohibited within critical areas (see Stormwater Management Standard 8). Infiltration of stormwater from high-load areas outside of critical areas (see Stormwater Management Standard 8) is allowed if adequate treatment is provided.

Standard 8: Critical Areas

A. Stormwater discharges to or near6 critical areas (defined in Section 5 of this manual) require the use of source control and pollution prevention measures and structural stormwater BMPs that are suitable for managing discharges to such areas.

B. Infiltration of stormwater runoff from land uses with higher potential pollutant loads (high load areas) near or within a critical area is prohibited.

Standard 9: Redevelopment

A. Redevelopment is defined as construction, alteration, or improvement that disturbs the ground surface or increases the impervious area on previously developed sites. Redevelopment includes maintenance and improvement of existing roadways including widening less than a single lane, adding shoulders, correcting substandard intersections, and improving existing drainage systems; development, rehabilitation, expansion and phased projects on previously developed sites including residential ―teardowns‖ (i.e., demolition and reconstruction or replacement of an existing residential dwelling with another residence of any size); and remedial projects specifically designed to provide improved stormwater management. A redevelopment project is any project site that undergoes redevelopment. The project site can be entirely under redevelopment or the project site can be a combination of redevelopment and new development.

B. Redevelopment of previously developed sites must meet the standards to the maximum

extent practicable7 for the portion of the site undergoing redevelopment. Projects involving redevelopment or reuse activities shall also improve existing conditions.

6 ―Near‖ a critical area means there is a strong likelihood of a significant impact occurring to a critical area, taking into account site-specific factors. 7 For the purposes of this standard, ―To the maximum extent practicable‖ means that: Proponents of redevelopment projects have made all reasonable efforts to meet the standard, considering the benefits of redevelopment as compared to development of raw land with respect to stormwater;

1. They have made a complete evaluation of possible stormwater management measures including LID site planning and design techniques and stormwater BMPs; and,

2. If not in full compliance with the applicable Standard, they are implementing the highest practicable level of stormwater management.


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C. The portion of a property that is currently undeveloped is not a redevelopment and thus does not fall under Standard 9. Any development on previously undeveloped portions of a site shall fully comply with all of the other Stormwater Management Standards.

D. For all redevelopment projects, stormwater controls (retrofits or new controls) shall be

incorporated into the design and result in a reduction in annual stormwater pollutant loads from the site. Proponents of redevelopment projects shall make full use of opportunities for controlling the sources of pollution and reducing runoff volumes by incorporating LID site planning and design techniques, including filtration (e.g., flow-through bioretention planters or rain gardens) and runoff capture and reuse for irrigation. This is particularly important for constrained redevelopment sites where it may not be possible to install BMPs that treat the entire water quality volume or meet the full runoff reduction standard. Redevelopment projects shall also incorporate measures that address water quantity issues by reducing the runoff volume and peak runoff from the site and by increasing groundwater recharge. Actions to improve existing conditions shall address known water quality and water quantity problems such as documented water quality impairments, low stream flow, or flooding.

E. Redevelopment activities shall not infiltrate stormwater through materials or soils containing regulated or hazardous substances or areas with soil or groundwater contamination. In such instances, the approving authority may waive the requirement to comply with other stormwater management standards that may require infiltration.

Standard 10: Construction Erosion and Sediment Control

A. A plan to control construction related impacts, including erosion, sedimentation, and other pollutant sources during construction and land disturbance activities (construction period erosion, sedimentation, and pollution prevention plan) must be developed and implemented in accordance with the Connecticut Guidelines for Soil Erosion and Sediment Control (as amended) and the requirements of the CTDEEP General Permit for the Discharge of Stormwater and Dewatering Wastewaters from Construction Activities for CTDEEP-regulated activities.

B. All development, regardless of the area of disturbance, must implement erosion and sedimentation controls prior to and during construction. Additionally, temporary controls shall be removed from a site and disposed of properly after the site has been stabilized.

Standard 11: Construction Inspections

A. The approving authority may require the proponent to post a bond, cash or other

acceptable surety. The form of the surety shall be approved by the Town, in an amount deemed sufficient to ensure that the work will be completed in accordance with the approved plans, but not less than the total estimated construction cost of the stormwater management facilities. A portion of the surety may be released as each phase of the project is completed in compliance with the approval, but shall not be fully released until the approving authority has received and approved the final inspection


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report. A portion of the surety may be held after issuance of a Certificate of Occupancy until all site work is completed.

B. The proponent shall notify the approving authority before starting land-disturbing

activity. The proponent shall also notify the approving authority before constructing the key components of the stormwater management system.

C. Periodic inspections of the stormwater management system construction shall be

conducted by the approved professional engineer of record (see ―Engineer of Record‖ certification form in Appendix K). The Town reserves the right to conduct inspections at any time. Written inspection reports shall include: the inspection date and location; evaluation of compliance with the stormwater approval; and any deviations from the approved plans.

D. At a minimum, inspections shall include: an initial site inspection prior to approval of

any plan; inspection of site erosion controls; inspection of the stormwater management system prior to backfilling of any underground drainage or stormwater conveyance structures; and a final inspection before the surety is released. The stormwater system shall be inspected to verify its as-built features, and the inspector shall also evaluate the system during a storm event8. If the inspector finds the system adequate, this shall be reported to the approving authority.

E. If the system is found to be inadequate due to operational failure, even though built

according to the approved plans, the system shall be corrected by the proponent before final approval is granted by the approving authority. If the proponent fails to act, the approving authority may use the surety to complete the work. If the system does not comply with the approved plans, corrective action shall be required and a Stop Work order shall be issued until any violations are corrected and all work previously completed has received approval by the approving authority.

F. Upon completion, the proponent shall certify that the project is in accordance with

approved plans and specifications, and shall provide inspections to adequately document compliance. The approving authority will issue a letter certifying completion upon its receipt and approval of the final inspection and reports, and/or upon otherwise determining that all work was completed in conformance with the approved plans.

Standard 12: Operation and Maintenance

A. A long-term Operation and Maintenance (O&M) Plan shall be developed and implemented to ensure that stormwater management systems function as designed. This plan shall be reviewed and approved as part of the review of the proposed permanent (post-construction) stormwater management system. Execution of the O&M Plan shall be considered a condition of approval of a development plan. The approving authority shall require a project proponent to establish a homeowners association or similar entity

8 A ―storm event‖ shall mean a storm forecasted for 0.5 inches or more of precipitation in a 24-hour period.


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to maintain the stormwater management system. For high-load areas or activities under Stormwater Management Standard 7, the O&M Plan shall include implementation of a SWPPP.

B. The O&M Plan shall identify all items described in Sections 5 and 7.

C. The proponent shall include with the development plan a mechanism for implementing and enforcing the O&M Plan. The proponent shall identify the lots or units that will be serviced by the proposed stormwater BMPs. The proponent shall also provide a copy of the legal instrument (deed, homeowner’s association, utility trust or other legal entity) that establishes the terms of and legal responsibility for the operation and maintenance of stormwater BMPs. In the event that the stormwater BMPs will be operated and maintained by an entity, municipality, state agency or person other than the sole owner of the lot upon which the stormwater management facilities are placed, the proponent shall provide a plan and easement deed that provides a right of access for the legal entity to be able to perform said operation and maintenance functions, including inspections. The owner shall keep the O&M Plan current, including making modifications to the O&M Plan as necessary to ensure that BMPs continue to operate as designed and approved. Proposed modifications of O&M Plans including, but not limited to, changes in inspection frequency, maintenance schedule, or maintenance activity along with appropriate documentation, shall be submitted to the approving authority for review and approval within thirty days of change.

D. Parties responsible for the operation and maintenance of a stormwater management system shall keep records of the installation, maintenance and repairs to the system, and shall retain records for at least five years.

E. Parties responsible for the operation and maintenance of a stormwater management system shall provide records of all maintenance and repairs during inspections and/or upon request.

F. When the responsible party fails to implement the O&M Plan, including, where applicable, the SWPPP, the municipality is authorized to assume responsibility for their implementation and to secure reimbursement for associated expenses from the responsible party, including, if necessary, placing a lien on the subject property.

Standard 13: Stormwater Management Report

A Stormwater Management Report shall be prepared for all development and redevelopment activities that are subject to the Stormwater Management Standards. This report shall document how the proposed project complies with the Stormwater Management Standards and shall be submitted with the stamp and signature of a Professional Engineer (PE) licensed in the State of Connecticut.


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Standard 14: Illicit Discharges All illicit discharges to the stormwater management system are prohibited. The stormwater management system is the system for conveying, treating, and infiltrating stormwater on site including stormwater best management practices and any pipes intended to transport stormwater to the groundwater, surface water, or municipal separate storm sewer system. Illicit discharges to the stormwater management system are discharges that are not entirely comprised of stormwater. Notwithstanding the foregoing, an illicit discharge does not include discharges from the following activities or facilities:

Landscape irrigation,

Uncontaminated groundwater discharges such as pumped groundwater, foundation drains, water from crawl space pumps, and footing drains,

Irrigation water,

Lawn watering runoff,

Residual street wash water,

Discharges of uncontaminated air conditioner condensate,

Discharges of flows from fire fighting activities (except training),

Discharges containing no chemical additives (including chlorine) from the flushing of fire protection systems, and

Naturally occurring discharges such as rising groundwater, uncontaminated groundwater infiltration, springs, and flows from riparian habitats and wetlands.

Redevelopment projects shall demonstrate that no illicit discharges exist on the redevelopment site by use of a dry-weather illicit discharge survey.

3.3 Applicability and Drainage Report

Exemptions

The Greenwich Stormwater Management Standards apply to new development, redevelopment, and other activities that will result in an increased amount of stormwater runoff and/or water pollutants flowing from a parcel of land or any activity that will alter the drainage characteristics of a parcel of land (prior to the application of stormwater Best Management Practices), unless exempt. Two types of exemptions may apply.

Categorical Exemptions The Greenwich Stormwater Management Standards shall not apply to the following categorically exempt activities, although application of the standards is still strongly encouraged:

Normal maintenance and improvement of land in agricultural use (as defined by Connecticut General Statutes), provided such activity conforms to acceptable management practices for pollution control approved by the Connecticut Department of Energy and Environmental Protection and the Greenwich Inland Wetlands and Watercourses Commission. This exemption does not apply to construction activities that are not directly related to the farming or agricultural operation.


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Routine maintenance of existing landscaping, gardens (excluding structural modifications to stormwater BMPs including rain gardens) or lawn areas including those maintained by the Town of Greenwich Parks and Recreation Department and Board of Education.

Resurfacing of an existing impervious area on a non-residential lot such as repaving an existing parking lot or drive with no increase in impervious cover.

Routine maintenance to existing town roads that is performed to maintain the original width, line, grade, hydraulic capacity, or original purpose of the roadway.

Customary cemetery management.

Emergency repairs to any stormwater management facility or practice that poses a threat to public health or safety, or as deemed necessary by the approving authority.

Any emergency activity that is immediately necessary for the protection of life, property, or the environment, as determined by the approving authority.

Repair of an existing septic system.

Construction of utilities (gas, water, electric, telephone, etc.), other than drainage, which will not permanently alter terrain, ground cover, or drainage patterns.

Repair or replacement of an existing roof of a single-family dwelling.

Construction of a second (or higher) floor addition on an existing building.

Construction of a maximum 12 foot x 12 foot shed. The construction must include the installation of a 1 foot wide x 1 foot deep crushed stone trench along the sides of the shed that discharge the roof runoff.

The repair of an existing wood, composite, or plastic deck with no proposed enlargement of the deck surface.

The reconstruction or construction of a wood, composite, or plastic deck with the decking boards spaced at least 3/16 of an inch and a pervious surface below the deck. The pervious area below the deck must have the soil tilled 12 to 16 inches and finished with grass seed, sod, or crushed stone. The minimum depth for the crushed stone is 4 inches. A site plan showing the proposed location of the deck and construction details for the deck must be submitted.

The construction of any fence that will not alter existing terrain or drainage patterns.

Conditional Exemptions Requiring PE Certification

Projects Adding Up to 500 Square Feet of Impervious Surfaces Projects adding up to 500 square feet of impervious surfaces9 are exempt from the Greenwich Stormwater Management Standards, provided that all of the following conditions are met:

The project design, including the proposed drainage design, if any, will not have an adverse effect on offsite properties or offsite drainage infrastructure, as certified by a professional engineer.

At least one of the following measures shall be implemented on the project site to help mitigate the effects of site disturbance and new impervious surfaces within its on site watershed and point of concern:

9 Refer to the glossary in the Town of Greenwich Drainage Manual for a definition of ―impervious surface.‖


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o Disconnection of roof down spouts that meet the Simple Disconnection standards in the Town of Greenwich Drainage Manual February 2012 as amended

o A zero increase in peak flow to all points of concern for the 1, 2, 5, 10, and 25-year design storms

o The runoff volume from the new impervious surfaces shall be infiltrated for the 10-year design storm

o Constructing a bioretention area for the Water Quality Volume of the contributing watershed of the project area. The design standards in the Town of Greenwich Drainage Manual February 2012 as amended must be met

o Creating a buffer with a length greater than or equal to the length of the project area and a minimum width of 10 feet planted as a meadow

o Restoring a riparian buffer (may require IWWA permit)

The project proponent submits an exemption request, including professional engineer certification, in lieu of a Stormwater Management Report (Form SE-100).

This exemption can only be used until the cumulative addition of impervious surfaces on a site exceeds 500 square feet, regardless of ownership changes. For projects adding up to 500 square feet of impervious surfaces, application of the Greenwich Stormwater Management Standards is still strongly encouraged. Residential teardowns are not exempt unless the project meets the Conditional Residential Teardown Exemption Requirements. Commercial teardowns are not exempt.

Projects Adding Between 500 and 1,000 Square Feet of Impervious Surfaces Projects adding between 500 and 1,000 square feet of impervious surfaces are exempt from the Greenwich Stormwater Management Standards, provided that all of the following conditions are met:

The project design, including the proposed drainage design, if any, will not have an adverse effect on offsite properties or offsite drainage infrastructure, as certified by a professional engineer,

At least one of the following measures shall be implemented on the project site to help mitigate the effects of site disturbance and new impervious surfaces within its on site watershed and point of concern:

o Disconnection of roof down spouts that meet the Simple Disconnection standards in the Town of Greenwich Drainage Manual February 2012 as amended



o Constructing a bioretention area for the Water Quality Volume of the contributing watershed of the project area. The design standards in the Town of Greenwich Drainage Manual February 2012 as amended must be met

o Creating a buffer with a length greater than or equal to the length of the project area and a minimum width of 10 feet planted as a meadow

o Restoring a riparian buffer (may require IWWA permit)

Town of Greenwich Drainage Manual 22-A February 2014

At least one of the following measures shall be implemented on the project site using LID or conventional stormwater BMPs to help mitigate the effects of site disturbance and new impervious surfaces:



The project proponent submits an exemption request, including professional engineer certification, in lieu of a Stormwater Management Report (Form SE-100).

This exemption can only be used until the cumulative addition of impervious surfaces on a site exceeds 1,000 square feet, regardless of ownership changes. For projects adding between 500 and 1,000 square feet of impervious surfaces, application of the Greenwich Stormwater Management Standards is still strongly encouraged. Residential teardowns are not exempt unless the project meets the Conditional Residential Teardown Exemption Requirements. Commercial teardowns are not exempt.

Conditional Residential Teardown Exemption Requiring Professional Engineering Certification Projects for residential teardowns that reconstruct where the impervious surfaces10 within each point of concern is less than or equal to pre-development conditions and the peak flow and runoff volume for the 1, 2, 5, 10, 25, 50, and 100-Year Storms has a zero increase to all points of concern are exempt from the Greenwich Stormwater Management Standards, provided that all of the following conditions are met:

The project design, including the proposed drainage design, if any, will not have an adverse effect on offsite properties or offsite drainage infrastructure, as certified by a professional engineer

A Stormwater Management Report must be submitted with the following included: 1. Project Narrative 2. Site Inventory & Evaluation

a. Topography b. Soil Evaluation (Soil Evaluation Test Results (Form SC-101) Shall Be Used)

i. Initial Feasibility Evaluation (NRCS Web Soil Survey and similar sources of information)

ii. Concept Design Testing (test pits/borings and saturated hydraulic conductivity testing, as per Appendix B)

3. Evaluate Pre-Development Site Hydrology to all points of concern (Runoff Volume and Peak Flow Rate – 1, 2, 5, 10, 25, 50 and 100-Year Storms)

a. Watershed Map Pre-Development b. NRCS Runoff Curve Numbers Pre-Development c. Time of Concentration Pre-Development

4. Evaluate Post-Development Site Hydrology to all points of concern (Runoff Volume and Peak Flow Rate – 1, 2, 5, 10, 25, 50 and 100-Year Storms)

a. Watershed Map Post-Development

10 Refer to the glossary in the Town of Greenwich Drainage Manual for a definition of “impervious surface.”

22-B Town of Greenwich Drainage Manual

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b. NRCS Runoff Curve Numbers Post-Development c. Time of Concentration Post-Development

5. Peak Runoff to all points of concern must have a zero increase for the 1, 2, 5, 10, 25, 50, and 100-Year Storms

6. Runoff volume to all points of concern must have a zero increase for the 1, 2, 5, 10, 25, 50, and 100-Year Storms

7. Compare & Summarize Pre-&-Post Development Site Hydrology for peak flow and runoff volume to all points of concern

8. Conveyance Protection: 10, 25, 50 & 100-Year Depending on Peak Flow Rate for Downstream Stormwater Facilities

9. Outlet Protection Calculations – Based on Conveyance Protection 10. Emergency Outlet Sizing: Safely Pass the 100-Year 11. Supporting Documents 12. Sealed and Signed By a Professional Engineer

The application of the Greenwich Stormwater Management Standards is still strongly encouraged. For projects that meet the above criteria, the project proponent needs to submit plans which include all items on the:

1. Checklist for Construction Plans – Form CL-102 2. Checklist for Driveway Profile and Sight Distance Plan – Form CL-103

For projects that meet the above criteria, the project proponent must submit an Operations and Maintenance Plan Report. The Operations and Maintenance Plan must be submitted following the Checklist for Operations & Maintenance Plan Report CL-104. For projects that meet the above criteria, the project proponent needs to submit the items on the Checklist for Certificate of Occupancy – Form CL-105 with the request for Certificate of Occupancy. The Improvement Location Survey must include the items on the Checklist for Improvement Locations Survey Depicting „As-Built” Conditions CL-106. The use of this exemption removes any future additional construction on the property from using the Conditional Exemption regardless of ownership changes.


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4 Low Impact Development

4.1 Introduction

Traditionally, stormwater has been managed using large, structural practices installed at the low end of development sites – essentially as an afterthought – on land segments left over after subdividing property. This approach, sometimes referred to as end-of-pipe stormwater management, yields the apparent advantages of centralizing control and limiting expenditure of land. Unfortunately, it is much less efficient than it appears. In the last decade, alternative approaches have been established that employ environmentally sensitive site design and Low Impact Development (LID) techniques with results that surpass the end-of-pipe approach. LID is the cornerstone of stormwater management. LID is an innovative stormwater management approach that uses the basic principle modeled after nature: manage rainfall where it lands. The goal of LID is to mimic a site’s pre-development hydrology by using design techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source. Techniques are based on the premise that stormwater management should not be seen as stormwater disposal. Instead of conveying and managing/treating stormwater in large, costly end-of-pipe facilities located at the bottom of drainage areas, LID addresses stormwater through small, cost-effective landscape features located at the lot level. LID is a versatile approach that can be applied equally well to new development, urban retrofits, and redevelopment projects. Effective LID includes the use of both non-structural and structural stormwater management measures that are a subset of a larger group of practices and facilities known as Best Management Practices or BMPs. The BMPs utilized in low impact development, known as LID BMPs, focus first on minimizing both the quantitative and qualitative changes to a site’s pre-developed hydrology through non-structural practices and then providing treatment as necessary through a network of structural facilities distributed throughout the site. In doing so, LID places an emphasis on non-structural stormwater management measures, seeking to maximize their use prior to utilizing structural BMPs. Non-structural BMPs used in LID seek to reduce stormwater runoff impacts through environmentally sensitive site planning and design. Non-structural LID BMPs include such practices as minimizing site disturbance, preserving important site features, reducing and disconnecting impervious cover, flattening slopes, utilizing native vegetation, minimizing turf grass lawns, and maintaining natural drainage features and characteristics. Structural BMPs used to control and treat runoff are also considered LID BMPs if they perform these functions close to the runoff’s source. As such, they are typically smaller in size than standard structural BMPs. Structural LID BMPs include various types of basins, filters, surfaces, and devices located on individual lots in a residential development or throughout a commercial, industrial, or institutional development site in areas not typically suited for larger, centralized structural facilities. Standard 1 of the Greenwich Stormwater Management Standards requires the use of LID site planning and design techniques to the maximum extent practicable to reduce the generation of


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stormwater runoff and pollutants. Project proponents shall demonstrate why the use of LID site planning and design techniques is not possible before proposing to use traditional, structural stormwater management measures alone.

4.1.1 Advantages of LID

LID can provide a number of advantages over traditional stormwater management approaches that rely solely on end-of-pipe controls, depending on site-specific factors. Some of these potential advantages include:

Reduced consumption of land for stormwater management. LID practices engage the natural capacity of undisturbed land to absorb precipitation thus reducing the need for structural controls. When structural controls are used, they are small, close to the source of runoff, often installed below grade and made to fit well into the general landscape. Little land is expended for stormwater management.

Reduced construction costs. Traditional stormwater management requires significant sewering and earthwork. LID methods apply controls as close to sources of runoff as possible. Wherever practical, conveyances incorporate natural flow paths and swales instead of pipes. Structures installed tend to be smaller, thus reducing the need for excavation and construction materials.

Ease of maintenance. LID practices require limited maintenance. Much of the maintenance that is required can be accomplished by the average landowner.

Takes advantage of site hydrology. LID management mimics natural site hydrology and exploits the tendency of undisturbed land to retain and absorb runoff from impervious surface. Runoff that is absorbed recharges groundwater and stream baseflow and does not need to be managed or controlled by an end-of-pipe practice. Reduced end-of-pipe discharge is also beneficial for streambank stability and habitat.

Better quality of discharge. Recent research indicates that most constructed technologies are unable reduce pollutant concentrations below certain thresholds, which may exceed water quality standards. Landscapes that utilize LID practices minimize discharge and often retain all runoff from events smaller than the 2-year, 24-hour design storm. Pollution is minimized because discharge is minimized.

More aesthetically pleasing development. Traditional stormwater management tends to incorporate the use of large, unnatural looking practices such as detention ponds. When neglected, these practices may present safety and mosquito concerns. LID practices utilize pre-development land features that are small and fit well into the natural landscape.

Improved marketability and property values. The advantages of LID management translate into the marketplace. The benefits to developers include reduced land clearing and earth disturbance costs, reduced stormwater management costs,


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reduced infrastructure costs (roads, storm water conveyance and treatment systems), and increased quality of building lots and community marketability.

4.2 Fundamental Concepts

Successful application of LID is maximized when it is viewed in the context of the larger design process. Using LID to its fullest potential involves adhering to the following fundamental concepts:

Prevent. Then mitigate. A primary goal of LID is preventing stormwater runoff by incorporating non-structural practices into the site development process. This can include preserving natural features, clustering development, and minimizing impervious surfaces. Once prevention as a design strategy is maximized, then the site design — using structural BMPs – can be prepared.

Minimize disturbance. Limiting the disturbance of a site reduces the amount of stormwater runoff control needed to maintain the natural hydrology.

Manage stormwater as a resource — not a waste. Approaching LID as part of a larger design process enables us to move away from the conventional concept of runoff as a disposal problem (and disposed of as rapidly as possible) to understanding that stormwater is a resource for groundwater recharge, stream base flow, lake and wetland health, water supply, and recreation.

Mimic the natural water cycle. Stormwater management using LID includes mimicking the water cycle through careful control of peak rates as well as the volume of runoff and groundwater recharge, while protecting water quality. LID reflects an appreciation for management of both the largest storms, as well as the much more frequent, smaller storms.

Disconnect. Decentralize. Distribute. An important element of LID is directing runoff to BMPs as close to the generation point as possible in patterns that are decentralized and broadly distributed across the site.

Integrate natural systems. LID includes careful inventorying and protecting of a site’s natural resources that can be integrated into the stormwater management design. The result is a natural or ―green infrastructure‖ that not only provides water quality benefits, but greatly improves appearance by minimizing infrastructure.

Maximize the multiple benefits of LID. LID provides numerous stormwater management benefits, but also contributes to other environmental, social, and economic benefits. In considering the extent of the application of LID, communities need to consider these other benefits.


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Consider the use of LID everywhere. LID can work on redevelopment, as well as new development sites. In fact, LID can be used on sites that might not traditionally consider LID techniques, such as in combined sewer systems, along transportation corridors, and on brownfield sites. Broad application of LID techniques improves the likelihood that the desired outcome of water resource protection and restoration will be achieved.

Make maintenance a priority. The best LID designs lose value without commitment to maintenance. An important component of selecting a LID technique is understanding the maintenance needs and institutionalizing a maintenance program. Selection of optimal LID BMPs should be coordinated with both the nature of the proposed land use/building program and the owners/operators of the proposed use for implementation of future maintenance activities.

4.3 Incorporating LID Into the Site

Planning and Design Process

Using LID successfully requires considering the LID principles from the project’s inception through the final design stages. Specifically, LID development approaches and techniques need to be assimilated into the various phases of the site design process, including:

The initial stages of site analysis to determine features to be preserved and avoided during construction,

The program or concept development process to determine what is constructed, and how much construction the site can support, and

The site design and revision process to address stormwater issues that remain.

4.3.1 Process Overview

The LID site planning and design process builds on the traditional approach to site design. An essential objective of the site design process – and of LID – is to minimize stormwater runoff by preventing it from occurring. This can be accomplished through the use of non-structural BMPs in the site design. Once prevention is maximized, some amount of mitigation may be needed to address stormwater peak rate, volume, and water quality from increased impervious surfaces. These mitigative stormwater management objectives can be met with structural BMPs (Section 5). The LID site planning and design process has been consolidated into the following basic steps:

Step 1 – Identify Applicable Land Use Regulations,

Step 2 – Inventory and Evaluate the Site,

Step 3 – Define Development Envelope,

Step 4 – Develop LID Control Strategies.


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Step 1 – Identify Applicable Land Use Regulations The first step in the LID site planning and design process is to identify applicable zoning, subdivision, and other local land use planning and regulatory requirements, which influence the density and geometry of the development, specifying roadway widths, parking, drainage, and other requirements.

Step 2 – Inventory and Evaluate the Site Incorporating LID into site design requires a thorough assessment of the site and its natural systems. Site assessment includes inventorying and evaluating the various natural resource systems which may pose challenges and/or opportunities for stormwater management and site development. Natural resource systems include:

Topography It is imperative to have an accurate topographic map of a development site. This information is critical for the additional site assessments to be performed below.

A topographic map of the site and at a minimum 100 feet beyond the limits of the subject property shall be provided. The topographic mapping shall show contours at two-foot intervals and meet Class T-2 and A-2 accuracy for the site, extending at least 10 feet beyond the property line, if possible. For sites with slopes less than 2% the applicant must include spot elevations and one-foot contours. The topographic mapping shall be obtained from a field topographic survey by a licensed land surveyor. For topographic mapping greater than 10 feet beyond the limits of the property, the applicant may supplement their mapping with available geographic information system (GIS) data. Elevations for topographic maps shall be based upon Connecticut CGS monumentation.

All slopes greater than 25% (4H:1V slope) as measured over a minimum distance of fifty (50) feet shall be determined and shown on the base map.

Soils In order to fully utilize LID systems, a comprehensive understanding of the site soils is necessary. LID emphasizes the evaporation, storage, and infiltration of stormwater in small, lot-scale systems, which are distributed throughout the site. For sites with mixed soils or those sites with some previously disturbed soils, proposed impervious cover should be located on the mixed or disturbed soils with the more permeable soils being preserved for infiltration. An evaluation must be undertaken to classify the Hydrologic Soil Groups (HSG) soils on site using classification methodologies developed by U.S. Natural Resources Conservation Service (NRCS), as described in Appendix B of this manual. The Hydrologic Soil Groups are used in conjunction with impervious areas on a site to calculate the required recharge volume.

Hydrologic Patterns and Features Since a fundamental objective of LID is to maintain the pre-development hydrologic cycle, it is important to understand the existing hydrologic processes, patterns and physical features (streams, wetlands, native soils, vegetation) that can influence site hydrology. The initial hydrologic assessment shall map prominent hydrologic features such as seeps, springs, drainage swales and isolated depression storage areas. Existing drainage patterns shall


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be determined on the base map and verified in the field. Field observations shall be made regarding existing drainage patterns, the presence of erosion and deposition patterns. All site hydrologic features shall be shown on the base map.

Native Forest and Soil Conservation Areas One of the principal design concepts for LID is the conservation of forest land and the use of native soils for stormwater management. By the protection of forest land and native soils, the following LID objectives can be met: (1) reducing total impervious area; (2) increasing stormwater storage, infiltration and evaporation; and (3) providing potential areas for the dispersion of stormwater. Generalized forest types (old field/shrub; early succession and mature hardwood) shall be determined in the field by a qualified professional. Dominant deciduous and evergreen tree species by class shall be determined and noted on the plans. Notations shall be made as to the density of the trees and canopy coverage. The average type of the forest cover shall be determined.

Wetland and Riparian Management Areas Wetlands and watercourses are generally environmentally sensitive areas. Wetlands, in particular provide a myriad of functions to improve the quality of water which flows through them prior to reaching an open water system. Wetlands are capable of trapping sediments and organic debris, filtration and/or uptake of soluble pollutants via the vegetative matrix.

Wetland soils and watercourses (both intermittent and permanent) shall be determined in the field by a Certified Soil Scientist. Wetland and watercourse boundaries shall be delineated in the field by numbered flagging. Delineated wetland/watercourse flags shall be located by a licensed land surveyor and shown on the base map.

A relative assessment of the quality of each wetland system (functions and values) on the site shall be performed by a Certified Soil Scientist using methodologies established by the U.S. Army Corp of Engineers.

Riparian zones are those areas adjacent to streams, lakes and wetlands that support native vegetation species, which are adapted to saturated or moderately saturated soil conditions.

Where mature vegetation exists on stable land forms, these riparian systems can perform the following functions: o Dissipate stream energy and erosion associated with high flow events; o Filter sediment, capture bedload, and aid in floodplain development; o Improve flood water retention and groundwater recharge; o Develop diverse pond and channel characteristics that provide habitat necessary for

fish and other aquatic life to spawn, feed and find refuge from flood events; o Provide vegetation litter and nutrients to the aquatic food web; o Provide habitat for a high diversity of terrestrial and aquatic biota; o Provide shade and temperature regulation; o Provide adequate soil structure, vegetation, and surface roughness to slow and

infiltrate storm water delivered as precipitation or low velocity sheet flow from adjacent areas.


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It is important to protect these riparian corridors in order to maintain the functions noted above.

Floodplains While development is unlikely to occur within defined floodplains, it is important to acknowledge these areas for the environmental benefits that they provide and ensure that they are incorporated into the design process. Some of the important functions, which floodplains provide are (1) the connection between the stream channel, floodplain, and off channel habitats; (2) mature vegetative cover and soils; and (3) pre-development hydrology that supports the above functions, and flood storage. The location of the 100-year flood boundaries should be taken from flood hazard mapping prepared by the Federal Emergency Management Agency for the Town of Greenwich (http://www.fema.gov/hazard/flood/index.shtm) and added to the base map. If necessary, the limits of 100-year flood boundaries shall be verified in the field by a licensed land surveyor.

Step 3 – Define Development Envelope Determine the development envelope in which buildings, roads and other constructed features may be sited with minimal effect to site hydrology and other ecological, scenic, or historic features. Generally, the development envelope will include upland areas, ridge lines, gently sloping hillsides, and slowly permeable soils outside of wetlands. Setting the development envelope should also consider construction techniques, and make efforts to retain and protect mature trees, minimize clearing and grading for buildings, access and fire prevention, and other construction activities, including stockpiles and storage areas. The envelope should also be confined to areas to be permanently altered. Limiting the development envelope also reduces the amount of site disturbance and impervious cover, thereby generating less runoff and requiring smaller stormwater management systems, as demonstrated by the runoff coefficient and water quality volume equations presented in Section 5.6.2. In general, the following sequence shall be followed to determine the development envelope:

1. Determine those environmentally sensitive areas to be protected from development (see Step 2).

2. Delineate the different vegetative cover types on the site. Highlight those areas of special characteristics or environmental sensitivities. Areas with concentrations of trees with a diameter at breast height (DBH) of 18 inches or greater should be noted on the plan. Trees with a DBH of 42 inches or greater within 50 feet of the limits of disturbance shall also be noted on the plan.

3. Determine and delineate steep slopes (slopes greater than 25% or 4H:1V slope as measured over a minimum distance of 50 feet).

4. Determine and delineate those soil areas which have moderate to high infiltration rates (A and B soils). These areas should be reserved for LID infiltration practices for post-development runoff.

5. Determine and define the pre-development runoff patterns on the site in order to provide a preliminary understanding of the sites’ drainage patterns and the ultimate discharge points.

http://www.fema.gov/hazard/flood/index.shtm


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6. Once the above areas have been clearly delineated on the base plan, the remaining areas would become the available development envelope. This is not to say that development cannot extend beyond the defined development envelope; it is however a starting point to develop environmentally sensitive site plans.

Step 4 – Develop LID Control Strategies Information gathered in the first three steps should be used in developing LID control strategies. This step should include the use of non-structural BMPs such as natural area protection, minimizing impervious surfaces, stormwater disconnection, or other techniques described in this section of the manual. Use hydrology as a design element. In order to minimize the runoff potential of the development, an evaluation of pre-and post-development site hydrology should be an ongoing part of the design process using the hydrologic analysis procedures discussed in this section. The hydrologic evaluation approach is based on the NRCS TR-55 method. The effects of non-structural LID control strategies are reflected in the curve numbers and times of concentration selected for the analysis. The hydrologic analysis will quantify both the level of control that has been provided by the site planning process and the additional level of control required through the use of structural BMPs, including LID and traditional practices.

1. Evaluate pre-development10 conditions. Use the results of modeling to estimate baseline values for runoff volume, peak runoff rate, groundwater recharge, and water quality.

2. Implement non-structural site planning techniques:

a. Minimize total site impervious area.

Use alternative roadway layouts that minimize imperviousness.

Reduce road widths.

Limit sidewalks to one side of roads.

Reduce on-street parking. b. Minimize directly connected impervious areas.

Disconnect roof drains. Direct flows to vegetated areas.

Direct flows from paved areas to stabilized vegetated areas.

Break up flow directions from large paved surfaces.

Encourage sheet flow through vegetated areas.

Locate impervious areas so that they drain to permeable areas. c. Modify drainage flow paths to increase time of concentration (TC).

Maximize overland sheet flow.

Lengthen flow paths and increase the number of flow paths.

Maximize use of open swale systems.

Increase (or augment) the amount of vegetation on the site.

10Refer to the Glossary at the end of this manual for a definition of ―pre-development‖ conditions.


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d. Minimize site disturbance.

Use site fingerprinting. Restrict ground disturbance to the smallest possible area.

Reduce paving.

Reduce compaction of highly permeable soils.

Minimize size of construction easements and material stockpiles.

Place stockpiles within development envelope during construction.

Avoid removal of existing trees.

Disconnect as much impervious area as possible.

Maintain existing topography and associated drainage divides to encourage dispersed flow paths.

Locate new development in areas that have lower hydrologic function, such as barren clay soils.

3. Evaluate site planning benefits and compare with baseline values. The hydrologic

analysis is used to evaluate the cumulative hydrologic benefit of the site planning process in terms of peak flows, groundwater recharge/runoff volume, and water quality.

4.4 Non-structural LID Techniques

A core concept of LID is preventing stormwater runoff by integrating site design and planning techniques that preserve natural systems and hydrologic functions, protect open spaces, as well as conserve wetlands and stream corridors on a site. This section provides information on integrating non-structural LID BMPs early into the site design process. Credits for the use of non-structural LID BMPs are provided in Appendix C to promote the consistent use of these techniques.

4.4.1 Minimizing Soil Compaction

Minimizing soil compaction is the practice of protecting and minimizing damage to existing soil quality caused by the land development process. Minimizing soil compaction is not only important for drainage of a site and the successful use of non-structural and structural LID BMPs, but also for minimizing impacts to established vegetation. Heavy equipment used within the drip line of a tree can cause soil compaction, resulting in the death of tree roots. Damage done to a tree’s root system may take 3 to 4 years after construction to become evident in a tree’s canopy. Minimizing soil compaction relates directly to reducing total site disturbance, site clearing, site earthwork, the need for soil restoration, and the size and extent of costly, engineered stormwater management systems. Ensuring soil quality can significantly reduce the cost of landscaping vegetation (higher survival rate, less replanting) and landscaping maintenance. Fencing off an area can help minimize unnecessary soil compaction.


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Soil is a physical matrix of weathered rock particles and organic matter that supports a complex biological community. This matrix has developed over a long time period and varies greatly within the state. Healthy soils, which have not been compacted, perform numerous valuable stormwater functions, including:

Effectively cycling nutrients,

Minimizing runoff and erosion,

Maximizing water-holding capacity,

Reducing storm runoff surges,

Absorbing and filtering excess nutrients, sediments, and pollutants to protect surface and groundwater,

Providing a healthy root environment,

Creating habitat for microbes, plants, and animals,

Reducing the resources needed to care for turf and landscape plantings. Undisturbed soil consists of pores that have water carrying and holding capacity. When soils are overly compacted, the soil pores are destroyed and permeability is drastically reduced. In fact, the runoff response of vegetated areas with highly compacted soils closely resembles that of impervious areas, especially during large storm events (Schueler, 2000). Minimizing soil compaction can be performed at any land development site during the design phase. It is especially suited for developments where significant ―pervious‖ areas (i.e., post-development lawns and other maintained landscapes) are proposed. Consistent with Stormwater Management Standard 2 (Protection of Natural Hydrology), soil compaction shall be minimized by using the smallest (lightest) equipment possible and minimizing travel over areas that will be revegetated (e.g., lawn areas) or used to infiltrate stormwater (e.g., bioretention areas). In no case shall excavation equipment be placed in the bottom of an infiltration area during construction. Minimizing soil compaction can reduce the volume of runoff by maintaining soil functions related to stormwater infiltration and evapotranspiration. Designers that use this BMP can select a lower runoff coefficient (i.e., curve number) for calculating runoff volume and peak rate from the area of minimized soil compaction. All areas that will be considered forest/open space for stormwater purposes must have documentation that prescribes that the area will remain in a natural, vegetated state. Appropriate documentation includes: subdivision covenants and restrictions, deeded operation and maintenance agreements and plans, parcel of common ownership with maintenance plan, third-party protective easement, within public right-of-way or easement with maintenance plan, or other documentation approved by the local authority While the goal is to have forest/open space areas remain undisturbed, some activities may be prescribed in the appropriate documentation, as approved by the local program authority: forest management, control of invasive species, replanting and revegetation, passive recreation (e.g., trails), limited brush hogging to maintain desired vegetative community, etc. Areas that comply (i.e., ―no disturbance areas‖) can use the forested cover and open space site cover runoff coefficient (R) when calculating the required Water Quality Volume (WQV), provided the following conditions are met:

The no-disturbance areas are protected by having the limits of disturbance and access clearly shown in the Stormwater Management Report, on all construction drawings, and delineated, flagged, and fenced in the field.

No-disturbance areas are not to be stripped of existing topsoil.


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No-disturbance areas are not be stripped of existing vegetation.

Vehicle movement, storage, or equipment/material lay-down is not to be permitted in no-disturbance areas.

Use of soil amendments and additional topsoil is permitted in other areas being disturbed. Grading may be performed using low ground pressure equipment (less than 3 pounds per square inch) to reduce the potential for soil compaction.

Lawn and turf grass are acceptable uses. Planted meadow is an encouraged use.

4.4.2 Minimizing Site Disturbance

Disturbance at a development site can occur through normal construction practices, such as grading, cutting, or filling. Minimizing the total disturbed area of the site requires the consideration of multiple BMPs, such as conservation development and identifying and protecting sensitive areas. These BMPs serve to protect natural resources at the site by reducing site grading and maintenance required for long-term operation of the site. Minimizing the total disturbed area of a site specifically focuses on how to minimize the grading and overall site disturbance, maximizing conservation of existing native plant communities and the existing soil mantle of a site. If invasive plant species are present in the existing vegetation, proper management of these areas may be required in order for the vegetation to achieve its greatest hydrological potential. Minimizing the total disturbed area of a site is best applied in lower density single-family developments (Figures 4-1 and 4-2), but can also be applied in residential developments of all types including commercial, office park, retail center, and institutional developments. However, as site size decreases and density and intensity of development increases, this BMP is uniformly more difficult to apply successfully. Consistent with Stormwater Management Standard 2 (Protection of Natural Hydrology), site disturbance shall be minimized. The area outside the project disturbance area shall be maintained at natural grade and retaining existing, mature vegetated cover. The project disturbance area shall be depicted on the design, construction, and mitigation plans and shall be delineated in the field prior to commencing land disturbance activities. The project disturbance area shall include only the area necessary to reasonably accommodate construction activities.

Source: Metropolitan Washington Council of Governments

Figure 4-1. Minimally Disturbed Residential Development


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The disturbance limits should reflect reasonable construction techniques and equipment needs together with the physical constraints of the development site such as slopes or soils. Limits of disturbance may vary by type of development, size of lot or site, and by the specific development feature involved. Site disturbance including earthwork and clearing of vegetation should be limited to 40 feet beyond the building perimeter, 10 feet beyond the primary roadway curbs, walkways, and main utility branch trenches, and 25 feet beyond areas of proposed infiltration in order to limit compaction in the proposed infiltration area. This guidance is not intended to limit lawn areas.

Source: Adapted from Atlanta Regional Commission, 2001

Figure 4-2. Reduced Limits of Disturbance

4.4.3 Protecting Sensitive Natural Areas

Natural areas include woodlands, riparian corridors, areas contiguous to wetlands and other hydrologically sensitive and naturally vegetated areas. To the extent practicable these areas should be preserved. Natural areas can be one of the most important components within a development scheme, not only from a stormwater management perspective, but in reducing noise pollution and providing valuable wildlife habitat and scenic values. New development tends to fragment large tracts of undisturbed areas and displace plant and animal species; therefore it is essential to maintain these buffers in order to minimize impacts. Sensitive natural areas should be conserved at development sites, thereby preserving pre-development hydrologic and water quality characteristics. If an area is permanently protected under a conservation easement, the project proponent can subtract the conservation area from the total site area when computing the water quality volume.


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Figure 4-3. Site Map with Natural Areas Delineated

Priority shall be given to maintaining existing surface waters and systems, including, but not limited to, perennial and intermittent streams, wetlands, vernal pools, and natural swales. Where roadway or driveway crossings of surface waters cannot be eliminated, disturbance to the surface water shall be minimized, hydrologic flows shall be maintained, there shall be no direct discharge of runoff from the roadway to the surface water, and the area shall be revegetated post-construction. Roadway and driveway crossings over streams shall also comply with the Connecticut Department of Energy and Environmental Protection Stream Crossing Guidelines (as amended) to accommodate high flows, minimize erosion, and support aquatic habitat and wildlife passage.

Construction Guidelines Although protecting sensitive areas happens early in the site plan process, it is equally important that the developer and builder protect these areas during construction. The following guidelines describe good planning practices that will help ensure protection of a few common environmentally sensitive resources during construction.

Water Resources

If vegetation needs to be reestablished, plant native species or use hydroseed and mulch blankets immediately after site disturbance.

Use bioengineering techniques, where possible, to stabilize stream banks.

Block or protect storm drains in areas where construction debris, sediment, or runoff could pollute waterways.

During and after construction activities, sweep the streets to reduce sediment from entering the storm drain system.

Avoid hosing down construction equipment at the site unless the water is contained and does not get into the stormwater conveyance system.

Implement spill control and clean-up practices for leaks and spills from fueling, oil, or use of hazardous materials. Use dry clean-up methods (e.g., absorbents) if possible. Never allow a spill to enter the stormwater conveyance system.


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Avoid mobile fueling of equipment. If mobile fueling is necessary, keep a spill kit on the fueling truck.

Properly dispose of solid waste and trash to prevent it from ending up in lakes and streams.

Wetlands Activity that is subject to the Greenwich Inland Wetlands and Watercourses Regulations shall comply with the regulations and conform to the requirements of the Greenwich Inland Wetlands and Watercourses Agency. Project proponents should consult with the Greenwich Inland Wetlands and Watercourses Agency for specific requirements. The information provided in this section is provided in addition to applicable local, state, and federal wetland regulatory requirements.

Determine if local, state, and/or federal wetland permits are required and comply with all applicable regulations and permit requirements.

Avoid impacts to wetlands whenever possible.

Excavate only what is absolutely necessary to meet engineering requirements. Do not put excavated material in the wetland. (Excavated material could be used in other areas of the site to improve seeding success).

If construction activities need to occur within a wetland, activities should be timed, whenever possible, when the ground is firm and dry. Avoid early spring and fish-spawning periods.

Avoid altering and damming flowing systems.

Install flagging or fencing around wetlands to prevent encroachment.

Travel in wetlands should be avoided. Access roads should avoid wetlands whenever possible. Crossing a wetland should be at a single location and at the edge of the wetland, if possible.

Never allow a spill to enter area wetlands.

Avoid trenching utilities through the tree’s critical root zone.

Avoid piling excavated soil around trees.

Replace trees removed during construction with native trees.

Conduct post-construction monitoring to ensure trees impacted by construction receive appropriate care.

Avoid dewatering stormwater ponds and basins (i.e., for maintenance or related purposes) during seasonally-high saturation periods to avoid exfiltration of groundwater through the stormwater pond or basin, which could adversely impact the subsurface hydrology of adjacent wetlands.

Floodplains

Design the project to maintain natural drainage patterns and runoff rates if possible.

Maintain as much riparian vegetation as possible. If riparian vegetation is damaged or removed during construction, replace with native species.

Use bioengineering techniques to stabilize stream banks.

Keep construction activity away from wildlife crossings and corridors.

Stockpile materials outside of the floodplain and use erosion control techniques.


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Woodlands

Protect trees on sites with severe design limitations, such as steep slopes and highly erodible soils.

Preserve trees along watercourses to prevent bank erosion, decreased stream temperatures, and to protect aquatic life.

Protect the critical root zone of trees during construction. This is the area directly beneath a tree’s entire canopy. For every inch of diameter of the trunk, protect 1.5 feet of area away from the trunk.

In general, areas with concentrations of trees having DBH of 18 inches or greater should be avoided.

4.4.4 Protecting Riparian Buffers

Riparian buffers are areas of planted or preserved vegetation between developed land and surface water. Riparian buffer areas protect water quality by cooling water, stabilizing banks, mitigating flow rates, and providing for pollution and sediment removal by filtering overland sheet runoff before it enters the water. An example of a three-zone riparian buffer is shown in Figure 4-4.

The inner zone, also termed the ―streamside zone,‖ begins at the edge of the stream bank of the active channel and extends a minimum distance of 30 feet, measured horizontally on a line perpendicular to the water body. Undisturbed vegetated area aims to protect the physical and ecological integrity of the stream ecosystem. The vegetative target for the streamside zone is undisturbed native woody species with native plants forming canopy, understory, and duff layer. Where such forest does not grow naturally, then native vegetative cover appropriate for the area (such as grasses, forbs, or shrubs) is the vegetative target. Generally, speaking structural BMPs are not allowed in the inner zone.


Figure 4-4. Three-Zone Riparian Buffer


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The middle zone extends immediately from the outer edge of the inner zone for a minimum distance of 50 feet. This managed area of native vegetation protects key components of the stream ecosystem and provides distance between upland development and the streamside zone. The vegetative target for the middle zone is either undisturbed or managed native woody species or, in its absence, native vegetative cover of shrubs, grasses, or forbs. Undisturbed forest is encouraged strongly to protect future water quality and the stream ecosystem. Forested riparian buffers should be maintained and reforestation should be encouraged where no wooded buffer exists. Proper restoration should include all layers of the forest plant community, including understory, shrubs and groundcover, not just trees.

The outer zone extends a minimum of 20 feet immediately from the outer edge of the middle zone. This zone prevents encroachment into the riparian buffer area, filters runoff from adjacent land, and encourages sheet flow of runoff into the buffer. The vegetative target for the outer zone is native woody and herbaceous vegetation to increase the total width of the buffer; native grasses and forbs are acceptable.

Generally, all three zones of the riparian buffer should remain in their natural state. However, some maintenance is periodically necessary, such as planting to minimize concentrated flow, the removal of exotic plant species when these species are detrimental to the vegetated buffer and the removal of diseased or damaged trees. Effective treatment of stormwater runoff is achieved when pervious and impervious area runoff is discharged to a grass or forested buffer via overland flow. The use of a filter strip is recommended to treat overland flow in the green space of a development site. The area draining by sheet flow to a buffer can be subtracted from the total area in the Water Quality Volume calculation, and the impervious area draining to the buffer by sheet flow can be subtracted from the impervious area in the Groundwater Recharge Volume calculation and the post-development impervious area in the Runoff Reduction Volume calculation, provided the following conditions are met:

The minimum stream buffer width (i.e., perpendicular to the stream flow path) shall be 50 feet as measured from the top bank elevation of a stream or the boundary of a wetland.

The maximum contributing path shall be 150 feet for pervious surfaces and 75 feet for impervious surfaces.

The average contributing overland slope to and across the buffer shall be less than or equal to 5%.

Runoff shall enter the buffer as sheet flow. A level spreader shall be utilized where local site conditions prevent sheet flow from being maintained.

The stream buffer remains unmanaged other than routine debris removal.

The buffer is protected by an acceptable conservation easement or other enforceable instrument that provides perpetual protection of the area. The easement must clearly specify how the natural area vegetation shall be managed and boundaries will be marked. [Note: managed turf (e.g., playgrounds, regularly maintained open areas) is not an acceptable form of vegetation management.]


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4.4.5 Avoiding Disturbance of Steep Slopes

Disturbance of long, steep slopes tends to cause soil erosion. The potential for soil erosion is significantly increased on slopes of 25% (4H:1V slope) or greater. Development on steep slopes also results in a larger disturbance footprint than development on flatter slopes, as depicted in Figure 4-5.


Figure 4-5. Relationship Between Slopes and Development Impacts Development on steep slope areas shall be avoided. Unnecessary grading should be avoided on all slopes, as should the flattening of hills and ridges. Development shall follow the natural contours of the landscape. A grading plan shall be submitted as part of the site plan review process showing both existing and finished grades for the proposed development. Retaining walls must comply with the requirements of the Building Zone Regulations. Basements that reach grade should be constructed as walk-outs. Compost-amended soils shall be used for areas of fill on development sites prior to vegetation establishment. Amending a soil with compost increases the soil’s permeability and water holding capacity, thereby delaying and often reducing the peak stormwater runoff flow rate and decreasing irrigation water requirements. Soil amendments also enhance a lawn’s long-term aesthetics while reducing fertilizer and pesticide requirements. Guidelines for the use of compost-amended soils are included in the design references in Appendix G. No ground disturbed as a result of site construction and development shall be left as exposed bare soil at project completion. All areas exposed by construction, with the exception of finished building, structure, and pavement footprints, shall be decompacted (aerated) and covered with a minimum thickness of six inches of non-compacted topsoil, and shall be subsequently planted with living vegetation such as grass, groundcovers, trees, and shrubs, and other landscaping materials (mulch, loose rock, gravel, stone).


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4.4.6 Siting Relative to Permeable and Erodible Soils

Areas with highly permeable soils can be used as non-structural stormwater infiltration zones. Avoiding highly erodible or unstable soils can prevent erosion and sedimentation and water quality impacts. Infiltration of stormwater into the soil also reduces both the volume and peak discharge of runoff as well as groundwater recharge. Soils at development sites should be evaluated using the methods described in Appendix B to identify areas of erodible and permeable soils and to ultimately guide site layout and the placement of impervious surfaces and infiltration practices.

Whenever possible, highly erodible soils should be left undisturbed and protected from disturbance during site construction. Gravel soils tend to be the least erodible. Also as clay and organic matter increase, erodibility tends to decrease. Infiltration practices should be located on those portions of the site with the most permeable soils.

4.4.7 Protecting Natural Flow Pathways

A main component of LID is to identify, protect, and use natural drainage features, such as swales, depressions, and watercourses to help protect water quality. Instead of ignoring or replacing natural drainage features with engineered systems that rapidly convey runoff downstream, designers can use these features to reduce or eliminate the need for structural drainage systems. Naturally vegetated drainage features tend to slow runoff and thereby reduce peak discharges, improve water quality through filtration, and allow some infiltration and evapotranspiration to occur. Protecting natural drainage features can provide for significant open space and wildlife habitat, improve site aesthetics and property values, and reduce the generation of stormwater runoff itself. If protected and used properly, natural drainage features generally require very little maintenance and can function effectively for many years. Site designs should use and/or improve natural drainage pathways whenever possible to reduce or eliminate the need for stormwater pipe networks. This can reduce costs, maintenance burdens, and site disturbance related to pipe installation. Natural drainage pathways should be protected from significantly increased runoff volumes and rates due to development. The design should prevent the erosion and degradation of natural drainage pathways through the use of upstream volume and rate control BMPs, if necessary. Level spreaders, erosion control matting, revegetation, outlet stabilization, and check dams can also be used to protect natural drainage features. This technique is best used in residential development, particularly lower density single-family residential development. Other land uses such as commercial and industrial developments tend to be associated with higher density development. This results in higher impervious cover and maximum site disturbance allowances, making protecting and conserving natural flow pathways/drainage areas more difficult.


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Recommended design considerations for protecting natural flow pathways include:

Identify natural drainage features. Identifying and mapping natural drainage features should be performed as part of the site analysis. Subtle site features such as swales, drainage pathways, and natural depressions should be delineated in addition to commonly mapped hydrologic elements such as wetlands, perennial and intermittent streams, and water bodies.

Use natural drainage features to guide site design. Instead of imposing a two-dimensional paper design on a particular site, designers can use natural drainage features to steer the site layout. Drainage features define contiguous open space and other undisturbed areas as well as road alignment and building placement. The design should minimize disturbance to natural drainage features. Drainage features that are to be protected should be clearly shown on all construction plans. Methods for protection, such as signage and fencing, should also be noted on applicable plans.

Use native vegetation. Natural drainage pathways should be planted with native vegetative buffers and the features themselves should include native vegetation where applicable. If drainage features have been previously disturbed, they can be restored with native vegetation and buffers.

4.4.8 Reducing Impervious Surfaces

Reducing impervious surfaces includes minimizing areas such as streets, parking lots, and driveways. By reducing the amount of paved surfaces, stormwater runoff is decreased while infiltration and evapotranspiration opportunities are increased. More space is also provided for conservation of natural features and design of larger, more functional LID BMPs such as bioretention systems and swales. The following recommendations are good practices to be considered. However, the current Department of Public Works Engineering Division design standards and specifications, Building Zone Regulations, and Subdivision Regulations shall apply.

Streets Streets usually account for the majority of impervious surfaces associated with urban development. To maximize potential water quality benefits, various LID techniques can be incorporated into overall road design. The chosen techniques will depend on the soils, development density, zoning, and use of the receiving water. However, there are several ways to include LID in road design. When implementing LID in road designs, the goal is not just to reduce impervious surface, but to avoid using the roadway surfaces to collect, concentrate, and convey the runoff. Specific focus should be on disconnecting runoff from the drainage system by directing it to LID BMPs such as swales, bioretention, buffers, and infiltration systems. Roadway lengths and widths should be minimized on a development site where possible to reduce overall imperviousness. If it is possible to reduce road width, there is an opportunity to increase the available green space to be used for a wider open swale section to help achieve


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greater filtration, infiltration, or storage. Where allowed, open-section roadways can reduce the need for costly curb and gutter sections and encourage the filtering and infiltration of stormwater. Higher density developments require wider streets, but alternative layouts can minimize street widths. For example, in instances where on-street parking is desired, conventional pavement can be used for the travel lanes, with permeable pavers or porous pavement used in the parking lanes. This design approach reduces impervious area while also providing an area for infiltration of stormwater from the roadway. Porous pavement can also be used in the travel lanes of roads and parking lots. Numerous factors influence street length, including conservation development and similar clustering techniques. As with street width, street length greatly impacts the overall imperviousness of a developed site. While no one prescriptive technique exists for reducing street length, alternative street layouts (open space or hybrid street plans) should be investigated for options to minimize impervious cover. The use of cul-de-sacs introduces large areas of imperviousness into residential developments. When cul-de-sacs are necessary, three primary alternatives can reduce their imperviousness; reduce the required radius, incorporate a landscaped island into the center of the cul-de-sac, or create a T-shaped (or hammerhead) turnaround. A landscaped island in the center of a cul-de-sac can provide the necessary turning radius, minimizing impervious cover. This island can be designed as a depression to accept stormwater runoff from the surrounding pavement, thus furthering infiltration. A flat apron curb will stabilize roadway pavement and allow for runoff to flow into the cul-de-sac’s open center.

Parking Lots Parking lots and rooftops are the largest contributors of impervious area on commercial sites. Parking lot size is dictated by lot layout, stall geometry, and parking ratios. Modifying any or all of these three aspects can serve to minimize the total impervious areas associated with parking lots. Parking ratio requirements and accommodating peak parking demand often provide parking capacity substantially in excess of average parking needs. This results in vast quantities of unused impervious surface. A design alternative to this scenario is to provide designated overflow parking areas. The primary parking area, sized to meet average demand, might still be constructed on impervious pavement to meet local construction codes and Americans with Disabilities Act requirements. However, the overflow parking area, designed to accommodate increased parking requirements associated with peak demand, could be constructed on pervious materials. This design approach, focused on average parking demand, will still meet peak parking demand requirements while reducing impervious pavement.


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To reduce impervious cover, storm flows, and pollutant loads from commercial parking areas, several LID strategies can be employed including:

Establish minimum and maximum or median parking demand ratios and allow additional spaces above the maximum ratio only if parking studies indicate a need for added capacity.

Use a diagonal parking stall configuration.

Integrate bioretention into parking lot islands or planter strips distributed throughout the parking area to infiltrate, store, and/or slowly convey storm flows to additional facilities.

Shared parking.

Driveways A significant percentage of the impervious cover in a residential subdivision can be attributed to driveways. Several techniques can be used to reduce impervious cover associated with driveways including:

Shared driveways provide access to several homes and may not have to be designed as wide as local residential roads.

Minimize front yard setbacks to reduce driveway length.

Reduce minimum driveway widths. Driveways can be reduced further with a bulb-out at the garage.

Use permeable paving materials and aggregate storage under wearing surface.

Direct surface flow from driveways to compost-amended soils, bioretention areas or other dispersion and infiltration areas.

Sidewalks Impervious surface coverage generated by sidewalks can be reduced using the following strategies:

Reduce sidewalks to the minimum widths allowed by local zoning.

Design a bioretention swale or bioretention cell between the sidewalk and the street to provide a visual break and increase the distance of the sidewalk from the road for safety.

Install sidewalks at a two percent slope to direct storm flow to bioretention swales or bioretention cells—do not direct sidewalk water to curb and gutter or other hardened roadside conveyance structures.

Use permeable paving material to infiltrate or increase time of concentration of storm flows.

Plant trees between the sidewalk and streets to capture and infiltrate runoff.

4.4.9 Stormwater Disconnection

Stormwater disconnection reduces stormwater volume by disconnecting roof leaders, impervious roads, and driveways and directing runoff to other BMPs including vegetated areas that infiltrate at the site. Disconnection is ideal for most single-family developments, but can also be applied to many development sites, including larger office parks and retails centers.


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Disconnecting roof leaders and routing road and driveway runoff from conventional stormwater conveyance systems allows runoff to be collected and managed onsite. Runoff can be directed to vegetated areas designed for onsite storage, treatment, and volume control. This is a distributed, low-cost method for reducing runoff volume and improving stormwater quality through increasing infiltration and evapotranspiration, decreasing stormwater runoff volume, and increasing stormwater time of concentration. Stormwater can flow from rooftop areas or from impervious areas such as driveways, walkways, small parking areas, minor roadways, and ancillary outdoor areas such as patios. Bioretention/rain gardens, water quality swales, or other structural filtration/infiltration BMPs are typically required for runoff from roads and parking lots because of their greater potential for runoff volume and pollutant loads. An ideal combination of LID techniques is to minimize the total disturbed area of a site in combination with stormwater disconnection. This not only reduces runoff volumes, peak rates, and pollutant loadings, but also provides multiple decentralized opportunities to receive disconnected flows. Careful consideration should be given to the design of vegetated collection areas. Concerns pertaining to basement seepage and water-soaked yards are warranted, with the potential arising for saturated depressed areas and eroded water channels. Proper design and use of bioretention areas, infiltration trenches, and/or dry wells reduces or eliminates the potential for surface ponding and facilitates functioning during cold weather months. Where basements exist, consider the direction of groundwater flow and proximity. Two types of stormwater disconnection are recommended – (1) Simple Disconnection in which stormwater runoff from impervious areas is directed to a pervious lawn or forested area, and (2) Disconnection to LID BMPs in which stormwater runoff from impervious areas is directed to an LID BMP such as a bioretention system, drywell, or decentralized subsurface infiltration system. Design criteria for these two types of stormwater disconnection techniques are described below.

Simple Disconnection

1. Flows from impervious areas are directed into stabilized vegetated areas, including on-lot swales, forested areas, and onsite depression storage areas.

2. The shape, slope, and vegetated cover in the downstream pervious area is sufficient to maintain sheet flow throughout its length. The entire vegetated ―disconnection‖ area has a maximum slope of 5 percent.

3. The minimum length of the downstream pervious area is 40 feet, and the minimum width is 10 feet. It is recommended that the length of the downstream pervious area equal the length of the contributing impervious flow path.

4. Flow from the impervious surface enters the downstream pervious disconnection area as sheet flow or, in the case of roofs, from downspouts equipped with splash pads, level spreaders, or dispersion trenches that reduce flow velocity and induce sheet flow in the downstream pervious area.


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5. In all cases, flows do not contribute to basement seepage. 6. All discharges onto the downstream pervious disconnection area are non-erosive. 7. Runoff is not directed to vegetated areas if there is reason to believe that pollutant

loadings will be elevated. 8. Roof downspouts or curb cuts are at least 10 feet away from the nearest connected

impervious surface (i.e., driveway) to discourage ―re-connections.‖ a. Limit the contributing impervious area to a maximum of 1,000 sq. ft. per

discharge point. b. Limit the contributing rooftop area to a maximum of 1,000 sq. ft. per

downspout, where the pervious area receiving runoff must be at least twice this size.

c. For contributing areas greater than 1,000 sq. ft., level spreaders are recommended.

9. A maximum contributing impervious flow path length of 75 feet. 10. Simple disconnection is not suggested on lots less than 7,500 square feet due to

minimum sizing and area requirements.

Disconnection to LID BMPs

1. Flows from impervious areas are directed into properly designed and constructed LID BMPs. For the purpose of this manual, an infiltration practice with an impervious contributing drainage area equal to or less than 1,000 square feet (i.e., allowing up to one roof leader, with a maximum contributing drainage area of 1,000 square feet each, to be piped to a single infiltration practice) and no other BMP’s within 25 feet (measured in all directions from the outer edge of the storage layer of the BMP) is considered a LID BMP. Infiltration practices designed with a larger impervious contributing drainage area are considered conventional or non-LID BMPs.

2. The downstream LID BMP must be designed in accordance with the design criteria listed in this manual, and/or the references sited in Appendix G.

3. In all cases, flows do not contribute to basement seepage. 4. In all cases, placement of LID BMPs must meet the setback requirements listed in this

manual. In situations where the above conditions and/or criteria cannot be achieved due to physical site constraints, the use of amended soils or similar methods to enhance infiltration and stormwater disconnection may be allowed by the approving authority. Methods to compute the resultant runoff volumes and peak runoff rates from disconnected impervious areas are discussed in the design references cited in this section of the manual. Once a stormwater disconnection is approved, no modifications will be allowed without an additional Town approval. Furthermore, a detailed survey will be required for this approval such that the Town can evaluate such modification.


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4.5 Structural LID Techniques

The selection and design of the following structural LID BMPs are addressed in Section 5 of this manual:

Rainwater harvesting (e.g., rain barrels, cisterns) for property irrigation,

Bioretention systems, including rain gardens, tree filters, stormwater planters, and curb extensions,

Dry wells and subsurface infiltration systems (decentralized, small-scale practices distributed throughout the site – Maximum contributing area is 1,000 square feet - and no other BMP’s within 25 feet (measured in all directions from the outer edge of the storage layer of the BMP)),

Green roofs,

Permeable pavement,

Vegetated filter strips,

Vegetated swales/channels,

Compost-amended soils. These are generally small-scale structural practices distributed throughout the site, close to the source of runoff.

4.6 LID Hydrologic Analysis

The structural and non-structural LID BMPs described in this manual shall be designed using the NRCS TR-55 hydrologic analysis methods to meet the requirements of the Stormwater Management Standards in terms of peak flow, runoff volume, groundwater recharge, and water quality. The LID analysis and design approach focuses on the following hydrologic analysis and design components:

Runoff Curve Number (CN) – Minimizing change in post-development hydrology by reducing impervious areas and preserving natural areas to reduce the storage requirements to maintain the pre-development runoff volume.

Time of Concentration (TC) – Maintaining the pre-development TC in order to minimize the increase of the peak runoff rate after development by lengthening flow paths and reducing the length of the runoff conveyance systems.

Runoff Reduction – Maintaining predevelopment runoff volumes through canopy interception, soil infiltration, evaporation and evapotranspiration, rainwater harvesting, engineered infiltration, and extended filtration (bioretention or dry swales with under drains that delay the delivery of stormwater).

Retention – Providing retention storage for volume and peak control, as well as water quality control, to maintain the same storage volume as the pre-development condition.

Detention – Providing additional detention storage, if required, to maintain the same peak runoff rate and/or prevent flooding.

The recommended LID hydrologic analysis methods for calculating pre-and post-development Runoff Curve Numbers, Times of Concentration, runoff volumes, and peak runoff rates are described in Chapter 5 of the New Jersey Stormwater Best Management Practices Manual


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(http://www.njstormwater.org/bmp_manual2.htm). The New Jersey Stormwater Best Management Practices Manual provides guidance for modeling various site conditions that may be encountered in the analysis and/or design of structural and non-structural BMPs, including LID BMPs. Runoff reduction resulting from disconnected impervious surfaces should be calculated using the Two-Step Technique described in the New Jersey manual. Other hydrologic analysis and design methods may be used for LID site design and design of LID BMPs, at the discretion of the approval authority, including more detailed continuous simulation models (e.g., HSPF, SWMM, and WINSLAMM) and simplified spreadsheet models.

4.7 LID Applications

This section illustrates the application of LID techniques for residential development, high density/commercial development, and roadway design. The following recommendations are good practices to be considered. However, the current Department of Public Works Engineering Division design standards and specifications, Building Zone Regulations, and Subdivision Regulations shall apply.

4.7.1 Residential Development

Typical residential development determines lot size by dividing the total acreage, minus the roads and regulated sensitive areas, by the number of lots allowed under the applicable zoning. Most, if not all, of the site is cleared and graded. In contrast, LID projects employ conservation or open space design (clustering and other planning strategies) to minimize site disturbance, maximize protection of native soil and vegetation, and permanently set aside the open tracts for multiple objectives including stormwater management. Several general objectives shall guide the placement and orientation of residential lots:

Minimize site disturbance.

Strategically locate lots for dispersing stormwater to open space areas and preserving open space.

Orient lots and buildings to maximize opportunities for on-lot infiltration or open conveyance through bioretention swales or other LID BMPs.

Locate lots adjacent to preserved open space to improve aesthetics and privacy.

Avoid impacts to existing wetlands or watercourses.

Medium and High Density Residential Conservation development is a type of development where buildings are organized together into compact groupings that allow for portions of the development site to remain in open space. The primary focus of conservation development has been to identify important natural features, develop areas less sensitive to disturbance, and create a stormwater management system similar to the site’s original hydrology. LID site design may include the traditional objectives of conservation development (to preserve natural and cultural features, provide recreation, preserve rural character, and produce more affordable housing); however, the primary purpose of the low impact development design is to

http://www.njstormwater.org/bmp_manual2.htm


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minimize the development envelope, reduce impervious cover, and maximize native soil and forest protection or restoration areas. Natural resource protection areas (the preferred strategy) are undisturbed conservation areas. Restoration areas (appropriate where land is or will be disturbed) can be enhanced through soil amendments and native planting to improve the hydrologic function of the site. Both can provide dispersion for overland flows generated in developed areas. LID site design strategies for medium to high density residential development are listed below and shown schematically in Figures 4-6 through 4-8.

Amend disturbed soils to regain stormwater storage capacity.

Drain rooftops to rain barrels or cisterns for non-potable reuse within the house or garden.

Utilize vegetated roof systems to evaporate and transpire stormwater.

Lay out roads and lots to minimize grading to the greatest extent possible.

Stormwater from lots not adjacent to forested/open space infiltration areas can be conveyed in swales or dispersed as low velocity sheet flow to the infiltration areas.

Orient lots to use shared driveways to access houses along common lot lines.

To maximize privacy and livability within cluster developments, locate as many lots as possible adjacent to open space, orient lots to capture views of open space, and design bioretention swales and rain gardens as visual buffers.

Set natural resource protection areas aside as a permanent tract or tracts of open space with clear management guidelines.


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Source: LID Technical Guidance Manual for Puget Sound

Figure 4-6. Conventional and LID Residential Design Concepts


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Figure 4-7. Medium- to High-Density Lot Using LID Practices


Figure 4-8. Zero Lot Line Configuration


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Large Lot Residential Substantial reduction of impervious surfaces can be realized through LID design for large lot development. Additional road network and driveway lengths are the primary reasons for increased imperviousness associated with large lot development. The increased storm flows from the additional road network required to serve rural and large lot designs should be dispersed to bioretention swales, adjacent open space, and/or lawn areas. LID site design strategies for rural and large residential lots are listed below and shown schematically in Figures 4-9 and 4-10.

Reduce the development envelope in order to retain a minimum of 65 percent of the site in native soil and vegetation.

Reduce effective impervious cover to zero (i.e., fully disperse stormwater such that none of the impervious areas on-site are directly connected to the storm drainage system or contribute to direct discharge off-site).

Integrate bioretention and open bioretention swale systems into the landscaping to store, infiltrate, slowly convey, and/or disperse stormwater on the lot.

Disperse road and driveway stormwater to adjacent open space and lawn areas.

Maintain pre-development flow path lengths in natural drainage patterns.

Preserve or enhance native vegetation and soil to disperse, store, and infiltrate stormwater.

Disperse roof water across the yard and to open space areas or infiltrate roof water in infiltration trenches.

Lots may be organized into cluster units separated by open space buffers as long as road networks and driveways are not increased significantly, and the open space tract is not fragmented.

Place clusters on the site and use native vegetation to screen or buffer higher density clusters from adjacent rural land uses.


Figure 4-9. Conventional and LID Designs for Large Lots


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Figure 4-10. LID Design Concept for Single Family Residential Lot

4.7.2 High Density and Commercial Development

High density and commercial land uses inherently have large, continuous impervious areas due to building roofs and parking. There are several major stormwater issues associated with high density and commercial areas. First, the volume of runoff from the site increases significantly for post-development conditions versus pre-development due to the high degree of impervious cover. The peak runoff rate also increases significantly due to shorter times of concentration. Lastly, pollutant loads are significantly higher due to heavy automobile traffic in parking areas. Consequently, the concentrations of sediment, metals, hydrocarbons, and other pollutants are much higher from a high density and commercial areas compared to other land uses. LID site design strategies for high density and commercial sites are listed below and shown schematically in Figures 4-11 and 4-12.

If the land area permits, utilize diagonal parking spaces with a one-way aisle. This change can reduce impervious area by 5 to 10% as the aisle width.

Encourage shared parking agreements with adjacent or nearby properties (within walking distance) that serve land uses with non-competing hours of operation. An example of


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this is use of a parking area for an office building after 5 pm for an adjacent retail use which stays open to 9 or 10 pm.

Where feasible and not cost prohibitive, place parking under a building. This will help reduce the total impervious area on a site.

Use permeable pavement, grass pavers or other non-impervious systems for overflow parking or parking that is only needed for certain times of the year.

Integrate bioretention into parking lot islands or planter strips distributed throughout the parking area to infiltrate, store and slowly convey storm water flows to other structural BMPs, if needed.

The typical LID techniques used for high-density developments include perimeter buffers, swales and bioretention systems; parking lot bioretention/detention islands, planter boxes, green roofs, permeable pavement, infiltration devices, and subsurface detention.

Source: Low Impact Development Center

Figure 4-11. Example LID Site Plan for Commercial Office Building The commercial LID site designs in Figures 4-11 and 4-12 are in contrast to traditional commercial site designs where runoff is concentrated and directed to inlets, pipes and a detention pond.


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Source: Low Impact Development Center

Figure 4-12. Example LID Site Plan for Commercial Shopping Plaza

4.7.3 LID Roadway Design

Roadways generate a major portion of runoff in urban areas and present significant engineering challenges in developing effective LID controls. Despite the challenges, there are effective LID design principles and engineering practices available for most types of roadway systems to meet stormwater management objectives. The use of some techniques may require modification of roadway design standards. Additionally, in highly urbanized areas, site constraints (limited space, poor soils, and utility conflicts) often require more extensive engineering and a greater reliance on structural LID practices.

A LID roadway design does not necessarily require reduction of impervious surfaces but rather optimizing the integration of LID practices by engineering the roadway itself or the surrounding landscape/streetscape to provide storage, detention, infiltration, and filtration as applicable. Reduction of the roadway surfaces is most useful in creating additional space for the use of LID practices; impervious surface reduction alone has a relatively small influence on reducing runoff volume or improving water quality. It is more important to hydraulically disconnect roadway surfaces by directing runoff to LID practices.


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Open Section Roadways One of the simplest ways to disconnect a roadway is to use an open section grass swale roadway design, rather than curb and gutter, when designing a road in rural or suburban areas. Open section roadways consist of a variable-width gravel or grass shoulder, usually wide enough to accommodate a parked car, and an adjoining grassed swale that conveys and treats runoff. When feasible, reducing road width provides greater opportunities to increase the width of grass shoulders and swales for treatment.

Generally, shallow and broad swales are the best design for open roads as they provide more surface area to treat and absorb runoff. The performance of the swales can be enhanced where you have soils that do not filter well. Figure 4-13 shows an example of a swale design to enhance its ability to treat runoff. In this case, several features have been incorporated into the design, including a culvert as a weir for detention control; check dams to increase retention time and decrease velocities; and a trench drain along the bottom of the swale to encourage infiltration and increase runoff storage in the engineered soil. Swales should be designed so that they are shallow with under drains to encourage good drainage and discourage standing water.

Source: Maryland Department of Environment

Figure 4-13. Example of a Dry Swale for Open Section Roadway Design

Street pavements width should be adjusted accordingly depending on off-street parking availability and shoulder requirements. Where feasible, existing vegetation and drainage features adjacent to the shoulder or swale should be preserved. Also where feasible, consider placing utilities under street pavements to eliminate conflicts with tree roots, grass swales, and bioretention areas.


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Figure 4-14 shows a 60-foot roadway design with sidewalks on both sides. The important LID feature is the use of wider swales for treatment and control. The swales are located between the road surface and sidewalks providing greater protection to pedestrians.

Figure 4-14. Example of Open Section Roadway Design

Figure 4-15 shows a narrow road section with sidewalks, shallow swale, and porous pavement shoulders. The paver blocks provide a rough surface to alert drivers if their tires leave the road surface. The pavers also protect the edge of the asphalt surface from breaking off. Generally, very shallow and broad swales are preferred as they provide more surface area to treat and absorb runoff. Swale performance can be greatly enhanced by incorporating infiltration into their design.

Figure 4-15. Narrow Road Section with Sidewalks, Shallow Swale and

Porous Pavement Shoulders

Urban Roadways It is possible to incorporate LID techniques into urban roadway design. For example, Figure 4-16 shows two different types of roadway bioretention filtration systems for high-density urban development. Neither system provides infiltration due to poor soils and high groundwater. Where flow, volume and water quality controls are required or desired and the


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soils permit, both systems could be designed as filtration and infiltration systems. The system on the left is a bioretention cell that requires a larger surface area to treat the water quality volume. The system on the right is a higher-flow media tree box filter with a smaller footprint, which can save space and reduce overall construction and maintenance costs. Both systems provide similar levels of water quality treatment.

Figure 4-16. Examples of Urban Roadway Bioretention Design

Figure 4-17 shows how bioretention curb extensions can be integrated into an urban setting for treatment of roadway runoff and to provide traffic calming. Infiltration was not possible in this urban retrofit; therefore the bioretention systems were constructed only as filtration devices that discharge through an under drain into the nearby storm drain system.

Figure 4-17. Bioretention Curb Extensions Used for

Stormwater Treatment and Traffic Calming


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Figure 4-18 also shows an example of a filtration bioretention cell. In this application, attention must be given to the inlet structures. Generally it is desirable to avoid high velocities of water flowing through the bioretention cell, so in this case, a mini detention flow restriction device was used to reduce velocities (highlighted).

Figure 4-18. Example of a Bioretention Cell Within the Road Right-of-Way

When curb and gutter drainage is desired or required, it is still possible to incorporate LID. Often there is space between the curb and sidewalk that can be used to treat road runoff. Figure 4-19 shows an example of a curb cut that allows runoff to drain into the adjacent green space. If this approach is used it is important that the curb cut is made wide enough to prevent clogging by trash and debris. Generally curb cuts become blocked with sediment over time so they need to be cleaned periodically and if possible designed with sufficient slope to help create enough velocity to flush sediment into the grass area.

Figure 4-19. Example of a Curb Cut Leading to a Bioretention Area


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4.7.4 Alternative Paving Surfaces

Porous pavers, asphalt and concrete are other design options to provide a hard surface suitable for roadways that allow runoff to percolate into underground gravel beds or other storage devices for detention or infiltration. An example is provided below as Figure 4-20. To reduce costs, these surfaces can be strategically placed and sized to allow sufficient runoff volume to enter the underlying storage device rather than placed over the entire roadway surface.

Figure 4-20. Example of the Use of Alternative Paving Surfaces

in Urban Roadway Design


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5 Structural Stormwater Management Practices

5.1 Introduction

Structural stormwater management practices (also referred to as stormwater Best Management Practices or BMPs) are designed to collect, detain and treat stormwater that is generated after applying Low Impact Development (LID) site planning and design and source control (i.e., non-structural) methods. Structural practices are typically used together with non-structural practices to meet multiple objectives such as reducing runoff volume and peak flows, capturing and treating runoff, and recharging groundwater. Stormwater management can be accomplished through small-scale, distributed practices close to the source of runoff (also referred to as structural LID BMPs), such as the use of rain gardens, filter strips, and permeable pavement, in combination with the LID site planning and design techniques that are described in Section 4 of this manual. Traditional end-of-pipe controls such as stormwater basins should only be used, if necessary, after exhausting LID approaches.

5.2 Categories of Structural Practices

Structural stormwater BMPs can be divided into several basic classes according to the function(s) that each BMP serves including pretreatment, treatment, conveyance, infiltration, and other functions. Structural practices that are considered LID BMPs in this manual (i.e., small-scale structural practices distributed throughout the site, close to the source of runoff) are identified in the following sections.

5.2.1 Pretreatment BMPs

The first BMPs in a treatment train, these measures typically remove the coarse sediments that can clog other BMPs. Maintenance removal of sediment is especially critical for pretreatment BMPs, because they receive stormwater containing the highest concentrations of suspended solids. The most common pretreatment BMPs include:

Deep sump catch basins,

Oil grit separators,

Proprietary devices,

Sediment forebays,

Vegetated filter strips. Pretreatment BMPs can be configured as on-line or off-line devices. On-line systems are designed to treat the entire water quality volume. Off-line practices are typically designed to receive a specified discharge rate or volume. A flow diversion structure or flow splitter is used to divert the design flow to the off-line practice. To receive TSS removal credit, oil grit separators and deep sump catch basins must be configured as off-line devices.


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5.2.2 Treatment BMPs

There major types of treatment BMPs include:

Filtration BMPs: Filtration systems use media to remove particulates from runoff. They are typically used when circumstances limit the use of other types of BMPs, such as where space is limited–particularly in a highly urbanized setting–or when it is necessary to capture particular industrial or commercial pollutants (e.g., hydrocarbons). In these circumstances, other BMPs might be cost-prohibitive or not as effective. Filtered runoff may be collected and returned to the conveyance system, or allowed to partially infiltrate into the soil. Filtration BMPs include:

o Filtering bioretention systems, including, rain gardens, tree filters, stormwater planters, and curb extensions (LID BMPs),

o Proprietary media filters, o Sand filters/organic filters.

Stormwater Treatment Basins: These BMPs provide peak rate attenuation by detaining stormwater and settling out suspended solids. The basins that are most effective at removing pollutants have either a permanent pool of water or a combination of a permanent pool and extended detention, and some elements of a shallow marsh. Common stormwater basins include:

o Extended dry detention basins, o Wet basins.

Constructed Wetlands: Constructed stormwater wetlands are designed to maximize the removal of pollutants from stormwater runoff through wetland vegetation uptake, retention and settling. Gravel wetlands remove pollutants by filtering stormwater through a gravel substrate. Common stormwater wetlands include:

o Constructed stormwater wetland, o Gravel wetland.

5.2.3 Conveyance BMPs

These BMPs collect and transport stormwater to BMPs for treatment and/or infiltration. These practices may also treat runoff through infiltration, filtration, or temporary storage. A water quality swale usually functions as a runoff conveyance channel and a filtration practice. The vegetation or turf also prevents erosion, filters sediment, and provides some nutrient uptake. Conveyance BMPs include:

Grass channels (LID BMPs),

Wet and dry water quality swales (LID BMPs).

5.2.4 Infiltration BMPs

Infiltration systems are designed primarily to reduce the quantity of stormwater runoff from a site and provide groundwater recharge. Infiltration techniques reduce runoff volume and direct the water back into the ground. Infiltration BMPs include:


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Bioretention systems, including, rain gardens, tree filters, stormwater planters, and curb extensions specifically designed for infiltration (LID BMPs),

Infiltration systems when designed as decentralized, small-scale practices distributed throughout the site, which typically include dry wells (Maximum contributing area is 1,000 square feet), leaching catch basins, and smaller subsurface infiltration units (LID BMPs),

Infiltration systems when designed as larger centralized or end-of-pipe systems, which typically include infiltration basins, infiltration trenches, and larger subsurface infiltration units.

For the purpose of this manual, an infiltration practice with an impervious contributing drainage area equal to or less than 1,000 square feet (i.e., allowing up to one roof leader, with a maximum contributing drainage area of 1,000 square feet, to be piped to a single infiltration practice) is considered a LID BMP. Infiltration practices designed with a larger impervious contributing drainage area are considered conventional or non-LID BMPs.

5.2.5 Other BMPs and Accessories

Structural BMPs that do not fit into any of the categories above include:

Compost-amended soils (LID BMPs),

Green roofs (LID BMPs),

Permeable pavement (LID BMPs),

Rain barrels and cisterns (LID BMPs),

Dry detention basins. BMP accessories are devices that enable BMPs to operate as designed. Common BMP accessories include the following:

Check dams,

Level spreaders,

Outlet structures.

5.3 Proprietary Stormwater BMPs

Proprietary stormwater BMPs are manufactured systems that use proprietary settling, filtration, absorption/adsorption, vortex principles, vegetation, and other processes to remove pollutants from stormwater runoff. The most common types of proprietary BMPs include hydrodynamic separators, filtration systems, wet vaults, and catch basin inserts. Underground infiltration systems are not considered proprietary BMPs since treatment typically occurs in the soil below the structure, not in the structure itself. This section augments the information contained in the Connecticut Stormwater Quality Manual on proprietary stormwater BMPs.


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1. The appropriate uses and benefits of proprietary BMPs include:

Pretreatment,

Urban/constrained sites – proprietary BMPs may be the best choice: o Not enough land or room for other BMPs, o Need to tie into existing drainage infrastructure, o Site elevations limit the head for certain BMPs,

Redevelopment projects and retrofits,

Target specific pollutants and sources (TMDLs),

Benefits: o Reduced space requirements, o Reduced engineering and design, o Spill containment and control capabilities.

2. Disadvantages and limitations of proprietary BMPs include:

Limited performance data: o Generally not capable of achieving sufficient pollutant removal alone, o Many have not been through evaluation needed to demonstrate treatment capability, o Alone, they provide limited benefits because they generally do not address peak

flows or runoff volume, o Considered ―secondary practices‖ in the Connecticut Stormwater Quality Manual,

Require regular maintenance to achieve design removal efficiencies,

Efficiency affected by sediment particle size, flow rate, and sediment loading rate. Proponents may consider proprietary BMPs, particularly where site constraints limit the use of LID techniques or conventional structural BMPs. If sized properly, proprietary BMPs can serve a useful role in meeting the Stormwater Management Standards, particularly on smaller sites where adequate space for other BMPs is not available. If a proponent proposes to include a proprietary BMP as a component of the stormwater management system, the proponent shall demonstrate that the proprietary BMP can meet the applicable Stormwater Management Standards; if proposed to meet the TSS removal requirements of Standard 6, and provide sufficient information for the approving authority to assess the TSS removal efficiency of the proposed proprietary BMP and assign a TSS removal credit. Proprietary stormwater BMPs that are intended to provide pollutant reduction shall be designed to remove a mean particle size of 100-microns (i.e., very fine to fine sand) with a particle density of 5.14 lb sec2/ft4 (based on a specific gravity of 2.65 – assumed mostly quartz in composition). Proponents shall provide sufficient information to the approving authority so that it can evaluate the proprietary BMP. The approving authority may reasonably deny the use of a proposed technology, if it finds that: (1) there is not sufficient information to assess the effectiveness of the technology; or (2) based on the available information, the proposed use of the technology does not meet the requirements of the Stormwater Management Standards.


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5.3.1 Evaluating the Use of Proprietary Systems

There are two general scenarios for evaluating the proposed use of a proprietary BMPs:

1. Another state has reviewed the performance of a technology as determined by an accepted monitoring protocol and assigned a TSS removal efficiency.

Other states, such as New York, New Jersey, and Massachusetts, have verified proprietary stormwater BMPs based upon accepted stormwater BMP performance monitoring protocols including the Technology Acceptance Reciprocity Partnership (TARP), the New Jersey Corporation for Advanced Technology protocol, the Massachusetts Stormwater Technologies Clearinghouse, EPA’s Environmental Technology Verification (ETV) program, and the State of Washington Department of Ecology Technology Assessment Protocol – Ecology (TAPE). Sources of information on accepted monitoring protocols and stormwater BMP performance verifications are provided in Appendix D of this manual.

In this scenario, the approving authority shall presume that the proprietary BMP achieves the assigned TSS removal efficiency, provided the conditions under which it is proposed to be used are similar to those in the performance testing. Key considerations in making this evaluation include:

Design flow rate or runoff volume,

Particle size distribution,

Pollutant loading,

On-line versus off-line configuration,

Tailwater effects,

Maintenance.

2. The local approving authority makes a case-by-case assessment of a specified proposed use of a proprietary technology at a particular site.

In this more common scenario, the performance claim of a proprietary technology has not been fully verified by an accepted monitoring protocol and/or an efficiency rating has not been assigned by another state or approving authority. It is the responsibility of the project proponent to submit information to the approving authority demonstrating the effectiveness of the proprietary BMP. The approving authority has the responsibility to review the information and determine whether it is sufficient to assess the performance of the technology and if the proposed application meets the Stormwater Management Standards and design requirements of this manual. Appendix E contains a detailed description of the recommended process for evaluating a proposed use of a proprietary stormwater BMP.


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5.4 Treatment Train

Stormwater BMPs can be combined in series to enhance pollutant removal or achieve multiple stormwater objectives. Referred to as stormwater ―treatment trains,‖ BMPs used in series consist of a combination of source control measures, natural features, and structural BMPs to maximize pollutant removal, as well as runoff reduction and groundwater recharge. Combining non-structural and structural measures in series rather than using a single method of treatment improves the levels and reliability of pollutant removal. The effective life of a BMP can be extended by combining it with pretreatment BMPs, such as a vegetated filter strip or sediment forebay, to remove sediment prior to treatment in the downstream ―units.‖ Sequencing BMPs can also reduce the potential for re-suspension of settled sediments by reducing flow energy or providing longer runoff flow paths. In selecting the order or arrangement of individual structural BMPs in a treatment train, BMPs should be arranged from upstream to downstream by their relative ease of maintenance. The BMP from which it is easiest to remove collected sediment and debris should be located at the upstream end of the treatment train. In downstream BMPs, it should be progressively more difficult to remove sediment and debris. The optimal arrangement should also include consideration of site conditions and the abilities and equipment of the party responsible for maintenance of the BMPs. Hydrodynamic separators and wet vaults (i.e., manufactured devices that achieve solids removal primarily through swirling and/or baffles) should generally be placed at the upstream end of a treatment train.

5.5 Operation and Maintenance

Structural stormwater BMPs require regular maintenance to perform successfully. In general, required maintenance needs are often more easily identified for aboveground BMPs than for underground BMPs. Further, BMPs that incorporate natural vegetation as part of the pollutant removal process, such as bioretention areas, generally require less maintenance than engineered and pre-fabricated systems.

Stormwater Management Standard 12 requires that a long-term Operation and Maintenance (O&M) Plan be developed and implemented to ensure that stormwater management systems function as designed. The O&M Plan should address the following BMP maintenance issues:

How and when maintenance is to be performed,

How and when inspections will be performed, and

How these tasks will be financed.

At a minimum, the O&M Plan shall identify:

Stormwater management system(s) owners;

The party or parties responsible for operation and maintenance including how future property owners will be notified of the presence of the stormwater management system and the requirement for proper operation and maintenance;


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The routine and non-routine maintenance tasks to be undertaken after construction is complete and a schedule for implementing those tasks;

Plan that is drawn to scale and shows the location of all stormwater BMPs along with the discharge point;

Description and delineation of public safety features; and

Estimated operations and maintenance budget. For most BMPs, the maintenance requirements include visual inspections (e.g., inspection of sediment forebays) and physical upkeep (e.g., removing and disposing of sediment and mowing water quality swales). The Connecticut Stormwater Quality Manual and the design references in Appendix G of this manual contain maintenance requirements and maintenance inspection checklists for specific types of stormwater BMPs.

5.6 Design Criteria

This section describes the criteria and calculations for the design of structural stormwater management practices to satisfy Stormwater Management Standards 4, 5, and 6 after LID site planning and design techniques have been exhausted. The criteria are similar to the sizing criteria contained in the Connecticut Stormwater Quality Manual but also include explicit runoff volume reduction and TSS removal criteria.

5.6.1 Runoff Volume Reduction and Groundwater Recharge (Standard 4)

Runoff Reduction Volume Surface runoff volume reduction is an important component of the effectiveness of a site’s overall design and stormwater management system. LID site planning and design practices and stormwater BMPs can eliminate runoff volume that would otherwise discharge directly to downstream drainage systems, reducing demand on system capacity and mitigating the hydraulic/sediment entrainment and transport impacts of increased runoff volume. In addition, reduction of runoff volume plays an important role in reducing pollutant loads to surface waters. Runoff reduction seeks to maintain the same pre-development runoff volume from a site after a site is developed. In its simplest terms, this means achieving the same pre-development runoff coefficient for every storm, up to a designated storm event. Runoff reduction is the total runoff volume that is reduced through canopy interception, soil infiltration, evaporation, rainfall harvesting, engineered infiltration, extended filtration or evapotranspiration. Extended filtration includes bioretention or dry swales with under drains that generally delay the delivery of stormwater from small sites to the stream system by six hours or more (Center for Watershed Protection and Chesapeake Stormwater Network, 2008). Runoff reduction performance varies by BMP type and the underlying soils. Table 5-1 summarizes research findings on runoff reduction rates for various types of stormwater BMPs.


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The values in Table 5-1 reflect the range of percent annual runoff reduction (based on the median and 75th percentile values) reported in the literature for common stormwater BMPs. Several BMPs have moderate to high capabilities for reducing annual runoff volume, while others have a negligible effect on runoff volumes and were not assigned runoff reduction rates.

Table 5-1. Percent Annual Runoff Reduction for Various BMPs

BMP Type Annual Runoff Reduction (%)

Infiltration 50 to 90

Bioretention 40 to 80

Soil Amendments 50 to 75

Permeable Pavement 45 to 75

Green Roofs 45 to 60

Dry Swales 40 to 60

Rain Tanks and Cisterns 40

Rooftop Disconnection 25 to 50

Grass Channels 10 to 20

Dry Extended Detention Pond 0 to 15

Wet Stormwater Ponds 0

Wet Swales 0

Constructed Stormwater Wetlands 0

Stormwater Sand Filters 0

For the purpose of meeting the Runoff Volume Reduction criterion in Stormwater Management Standard 5, the project proponent shall calculate pre- and post-development total runoff volumes. The post-development total runoff volume shall not exceed the pre-development total runoff volume for the 1-year, 24-hour design storm11. For new development and redevelopment, the required Runoff Reduction Volume (RRV) is the difference between the pre- and post-development total runoff volumes for the 1-year design storm. Runoff volume calculations shall include runoff onto the project site from the off-site contributing drainage area based on the current condition of the off-site drainage area. If the off-site area enters the proposed Runoff Reduction BMP the BMP must include the off-site area to size the BMP. If the off-site area bypasses the Runoff Reduction BMP the off-site area shall not be used to size the Runoff Reduction BMP. Modifications to the site to bypass a Runoff Reduction BMP are acceptable but these modifications must maintain the existing condition flow segment type (Sheet Flow, Shallow Concentrated Flow, Open Channel Flow). Runoff reduction shall be accomplished using the LID site planning and design techniques (non-structural LID BMPs) described in Section 4 and structural BMPs described in this section of the manual, if necessary.

11 Runoff reduction standards for various jurisdictions typically range from the 0.5-inch storm up to the 2-year

storm. The 1-year storm was selected for use in Greenwich since this amount of runoff reduction is suitable for maintaining stream geomorphology, reducing pollutant loads, and maintaining stream quality in suburban watersheds (Center for Watershed Protection and Chesapeake Stormwater Network, 2008).


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Runoff volume control for larger design storms may be required by the approving authority for large developments and special or sensitive situations. The approving authority may also require the project proponent to evaluate pre- and post-development runoff volumes associated with more intense, shorter-duration storm events or less intense, longer-duration storm events to reflect potential changes in rainfall due to climate change or other factors. The project proponent shall submit runoff volume calculations for pre- and post-development conditions for the 1, 2, 5, 10, 25, 50 and 100-year design storm frequencies unless directed otherwise by the Town. The 24-hour design storm rainfall amounts shall be determined as described in Section 6.2 of this manual.

Groundwater Recharge Volume If a project meets the Runoff Reduction Volume criterion described in the previous section using stormwater infiltration, then the groundwater recharge criterion of Stormwater Management Standard 4 is satisfied, and no groundwater recharge calculations are required. If the project does not meet the Runoff Reduction Volume criterion through the use of stormwater infiltration, the proponent must demonstrate compliance with the Groundwater Recharge criterion of Stormwater Management Standard 4, as described below. The Groundwater Recharge Volume (GRV) is the post-development recharge volume (on a storm event basis) required to minimize the loss of annual pre-development groundwater recharge. The required GRV equals a depth of runoff corresponding to the soil type times the impervious areas covering that soil type at the post-development site.

IFGRV (5.3)

where: GRV = required Groundwater Recharge Volume (cubic feet or acre-feet)

F = target depth factor associated with each Hydrologic Soil Group12 (inches) (see Table 5-2)

I = impervious area on the post-development site for new development projects or the net increase in impervious area for redevelopment projects

Attention must be given to ensure consistency in units. In particular, the target depth factors must be converted to feet.

Table 5-2. Recharge Target Depth by Hydrologic Soil Group

NRCS Hydrologic

Soil Group

Approximate

Soil Texture

Target Depth

Factor (F)

A Gravels, sand, loamy sand or sandy loam 0.60 inch

B Silty loam 0.35 inch

12 The NRCS classifies soils into four hydrologic groups, A through D, indicative of the minimum infiltration

obtained for a soil after prolonged wetting. Group A soils have the lowest runoff potential and the highest infiltration rates, while Group D soils have the highest runoff potential and the lowest infiltration rates. The prescribed stormwater volume that is required to be infiltrated must be determined using existing site conditions and the target depth factors listed in Table 5-2.


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C Sandy clay loam 0.25 inch

D Clay, silty clay loam, sandy clay, silty clay 0.10 inch

Infiltration of the required GRV must be accomplished using appropriate non-structural (i.e., site design) and structural BMPs. Stormwater infiltration BMPs include dry wells, infiltration basins, infiltration chambers and galleys, infiltration trenches, leaching catch basins, and bioretention systems (including rain gardens, tree filters, stormwater planters, and curb extensions) when specifically designed for infiltration. Roof runoff may be infiltrated without any pre-treatment, and the infiltrated volume may be used to meet the groundwater recharge, runoff reduction, and water quality standards.

Runoff Capture Volume The Runoff Capture Volume (RCV) is equivalent to the Water Quality Volume and intended to minimize the discharge of fresh water to sensitive coastal receiving waters and wetlands. As described in the Connecticut Stormwater Quality Manual, the RCV applies to new stormwater discharges located within 500 feet of and that ultimately discharge to tidal wetlands, which are not fresh water wetlands. The RCV must be retained on-site for such discharges through the use of LID site planning and design techniques and/or structural stormwater BMPs such as infiltration systems or stormwater basins.

Designing Infiltration BMPs To size infiltration BMPs, one of three methods may be used – (1) the ―Static‖ Method, (2) the ―Simple Dynamic‖ Method, or (3) the ―Dynamic Field‖ Method (see Appendix B for a more detailed description of these methods). The ―Static‖ Method assumes that the entire volume is discharged to storage instantaneously, is easier to calculate, and generally results in a larger infiltration volume than the ―Dynamic‖ methods. The ―Dynamic‖ methods assume that the infiltration BMP is infiltrating as it fills and requires certain technical calculations that account for this when sizing the infiltration system. When designing infiltration BMPs, adequate subsurface information needs to be obtained to verify that the anticipated field infiltration rate meets or exceeds the minimum required infiltration rate of 0.5 inches per hour (Hydrologic Soil Group A and B soils) at the actual location and soil layer where infiltration is proposed. Stormwater infiltration may be proposed at locations having field infiltration rates of between 0.2 and 0.5 inches per hour (Hydrologic Soil Group C soils), provided that field infiltration rates are field-verified by saturated hydraulic conductivity testing. To determine the field infiltration rate, a soil evaluation must be performed using the methodologies described in Appendix B. Infiltration systems must be installed in soils capable of absorbing the design volume. Infiltration structures must be able to drain fully within 72 hours. In addition, there must be at least a 2-foot separation distance from the bottom of the infiltration structure to seasonal high groundwater or bedrock/ledge (this separation requirement may be waived or reduced by the approving authority on a case-by-case basis). A 3-foot separation distance is required from the bottom of the infiltration structure to seasonal high groundwater for land uses with higher potential pollutant loads (high load areas). Soils under BMPs shall be scarified or tilled to improve infiltration. Non-infiltration BMPs (e.g.,


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filters, dry water quality swales, filtration bioretention systems) may be constructed one foot above seasonal high groundwater or, in some cases, below the water table (e.g., wet water quality swale, gravel wetland, wet basin). Runoff should be pretreated prior to its entrance into most infiltration BMPs to remove materials that would clog the soils receiving the recharge water. Pretreatment is also recommended for most stormwater BMPs to facilitate sediment removal and prolong the lifespan of the treatment mechanism. Pretreatment shall be designed to accommodate a minimum of one-year’s worth of sediment, capture anticipated pollutants, and be easily accessible to facilitate inspection and maintenance. Pretreatment is not required for bioretention systems that are designed with an equivalent sediment volume within the surface storage area. Suitable pretreatment BMPs and BMPs that require pretreatment are listed in Table 5-3. The pretreatment guidance contained in Table 5-3 applies to infiltration as well as other types of stormwater BMPs.

Table 5-3. Stormwater Pretreatment BMPs

Best Management Practice

(BMP)

Pretreatment

BMP? BMP that Requires Pretreatment?

Pretreatment BMPs

Deep Sump Catch Basin Yes No

Oil Grit Separators Yes No

Proprietary Separators Yes No

Sediment Forebays Yes No

Vegetated Filter Strips Yes No

Treatment BMPs

Bioretention - Rain Gardens, Tree Filters, Stormwater Planters, Curb Extensions

Yes No pretreatment required if system is designed with equivalent sediment storage volume

Constructed Stormwater Wetlands No Yes

Extended Dry Detention Basins No Yes

Gravel Wetlands No Yes

Proprietary Media Filters No Yes

Sand/Organic Filters No Yes

Wet Basins No Yes

Conveyance BMPs

Grass Channels No Yes

Water Quality Swales – Dry No Yes

Water Quality Swales – wet No Yes

Infiltration BMPs

Bioretention - Rain Gardens, Tree Filters, Stormwater Planters, Curb Extensions (infiltration design)

Yes No pretreatment required if system is designed with equivalent sediment storage volume

Dry Wells No No pretreatment required for runoff from non-metal roofs

Infiltration Basins No Yes

Infiltration Trenches No Yes

Leaching Catch Basins No Yes

Subsurface Structures No Yes


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(BMP)

Pretreatment

BMP? BMP that Requires Pretreatment?

Other BMPs

Compost-Amended Soils No No

Green Roofs No No

Permeable Pavement No No

Rain Barrels & Cisterns No No

Dry Detention Basins No No

For some LID practices in highly urbanized settings, pretreatment may be economically or physically impractical due to insufficient space, insufficient grades, or utility conflicts, thereby preventing the use of otherwise effective treatment techniques. In these instances, a larger LID BMP system or a more intensive maintenance schedule may be used in lieu of pretreatment, if allowed by the approving authority. This flexibility also applies to pretreatment requirements in other sections of this manual for LID practices at constrained sites. Infiltration of stormwater may be prohibited or subject to additional pre-treatment requirements, at the discretion of the approving authority, for (1) high load areas [see Stormwater Management Standard 7], (2) areas with soil or groundwater contamination such as brownfield sites, and (3) public drinking water aquifer recharge areas, wellhead protection areas, or water supply intake protection areas.

5.6.2 Peak Flow Control (Standard 5)

Stream Channel Protection The 2-year, 24-hour design storm post-development peak flow rate shall be (1) less than or equal to 50 percent of the 2-year, 24-hour design storm pre-development 13peak flow rate or (2) less than or equal to the 1-year, 24-hour design storm pre-development peak flow rate. This stormwater management standard is required for all stormwater flows which discharge directly or indirectly into a water body or watercourse including those discharges which enter a storm sewer system prior to discharging to the water body or watercourse. This standard may be waived under certain conditions, at the discretion of the approving authority, as described in the Connecticut Stormwater Quality Manual.

Conveyance Protection Conveyance systems must be designed to provide adequate passage for flows leading to, from, and through stormwater management facilities based on either the 10- or 25-year, 24-hour design storm post-development peak flow rate for which the facilities are designed for (see Peak Runoff Attenuation requirements). If the stormwater management system is designed to control the 50-year or 100-year post-development peak flow rates, then the conveyance system should also be designed to provide adequate passage for flows leading to, from, and through the stormwater management system based on the same peak flow rate to prevent bypass of the system.

13Refer to the Glossary at the end of this manual for a definition of ―pre-development‖ conditions.


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Peak Runoff Attenuation For projects that rely solely on the use of structural and non-structural LID BMPs for stormwater management, the post-development peak flow rates shall not exceed the pre-development peak flow rates for all flows off-site for the 2-year, 5-year, and 10-year, 24-hour design storms. For projects that do not rely solely on structural and non-structural LID BMPs (i.e., those that use conventional stormwater BMPs), the post-development peak flow rates shall not exceed the pre-development peak flow rates for all flows off-site for the 2-year, 5-year, 10-year, and 25-year, 24-hour design storms. Section 5.2 contains a list of LID BMPs and conventional or non-LID BMPs. For the purpose of this manual, infiltration practices are categorized as LID BMPs or non-LID BMPs based on their contributing drainage area. An infiltration practice with an impervious contributing drainage area equal to or less than 1,000 square feet (i.e., allowing up to one roof leader, with a maximum contributing drainage area of 1,000 square feet, to be piped to a single infiltration practice) is considered a LID BMP. Infiltration practices designed with a larger impervious contributing drainage area are considered conventional or non-LID BMPs. Peak runoff attenuation for larger design storms (i.e., 50-year and 100-year storms) may be required, at the discretion of the approving authority, for large developments and special or sensitive situations. The approving authority may also require the project proponent to evaluate pre- and post-development peak runoff rates associated with more intense, shorter-duration storm events or less intense, longer-duration storm events to reflect potential changes in rainfall characteristics due to climate change or other factors. This stormwater management standard may be waived, at the discretion of the approving authority, for sites that discharge to a large river, lake, estuary, tidal waters, or land subject to coastal storm flows, as described in the Connecticut Stormwater Quality Manual. The project proponent shall submit peak flow rate calculations for pre- and post-development conditions for the 1, 2, 5, 10, 25, 50 and 100-year design storm frequencies unless directed otherwise by the Town. The 24-hour design storm rainfall amounts shall be determined as described in Section 6.2 of this manual. Calculations for pre- and post-development conditions shall utilize Natural Resource Conservation Service TR-55 Tabular Hydrograph or TR-20 methodology. Peak discharge rates shall be calculated using the point of discharge at the downgradient property boundary, prior to discharge to the receiving water body. The topography of the site may require evaluation at more than one location if flow leaves the property in more than one direction. Calculations shall include runoff from adjacent upgradient properties. A proponent may demonstrate that a feature beyond the property boundary is more appropriate as a design point. Hydrograph routing calculations shall be used, including stage-discharge manual analysis or computer modeling methods. When detention is proposed, a downstream hydrologic analysis may be required by the approving authority to determine whether peak flows, velocities, and hydraulic effects are exacerbated downstream of the site for the 2-year, 5-year, 10-year, and 25-year, 24-hour design storms. Analysis of larger design storms may be required by the approving authority for large developments and special or sensitive situations. This analysis must be performed at the


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outlet(s) of the site and at critical downstream locations (stream confluences, culverts, other channel constrictions, and flood-prone areas) to a confluence point where the site drainage area represents 10% of the total drainage area above that point. When detention is not proposed, a downstream hydrologic analysis must be performed, as described above. Table 5-4 indicates whether specific structural stormwater BMPs are generally effective for peak flow control.

Emergency Outlet Sizing Size the emergency outlet to safely pass the post-development peak runoff from the 100-year storm in a controlled manner without eroding the outlet works and downstream drainages and property.

Table 5-4. Stormwater BMPs for Peak Flow Control


(BMP) Stream Channel Protection Peak Runoff Attenuation

Pretreatment BMPs

Deep Sump Catch Basins No No

Oil Grit Separators No No

Proprietary Separators No No

Sediment Forebays No No

Vegetated Filter Strips Partial No

Treatment BMPs


Yes Yes

Constructed Stormwater Wetlands Yes Yes

Extended Dry Detention Basins Yes Yes

Gravel Wetlands Yes Yes

Proprietary Media Filters No No

Sand/Organic Filters No No

Tree Filters and Stormwater Planters Partial Partial

Wet Basins Yes Yes

Conveyance BMPs

Drainage Channels No No

Grass Channels Partial Partial

Water Quality Swales Partial Partial

Infiltration BMPs


Yes Yes

Dry Wells Yes (only for clean runoff) Yes (only for clean runoff)

Infiltration Basins Yes Yes (small sites)

Infiltration Trenches Full exfiltration trench systems Full exfiltration trench systems

Leaching Catch Basins Only if sufficient number of leaching catch basins

Only if sufficient number of leaching catch basins

Subsurface Structures Yes (with sufficient volume or infiltration)

Yes (with sufficient volume or infiltration)


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(BMP) Stream Channel Protection Peak Runoff Attenuation

Other BMPs

Compost-Amended Soils Partial Partial

Green Roofs Partial Partial

Permeable Pavement Partial Partial

Rain Barrels & Cisterns Partial Partial

Dry Detention Basins Yes Yes

“Yes” – BMP can typically be used alone to meet the Stream Channel Protection and Peak Runoff Attenuation criteria “No” – BMP does not provide sufficient runoff attenuation to meet the Stream Channel Protection and Peak Runoff Attenuation criteria “Partial” – BMP typically provides some level of runoff attenuation but may not be suitable for fully meeting the Stream Channel Protection and Peak Runoff Attenuation criteria alone

5.6.3 Pollutant Reduction (Standard 6)

Water Quality Volume The Water Quality Volume (WQV) is the volume of stormwater runoff from a given storm that must be collected and treated to remove 80% of the average annual post-construction load of Total Suspend Solids (TSS). The WQV is calculated using the following equation:

12

"1 ARWQV (5.1)

where: WQV = water quality volume (acre-feet) R = site cover runoff coefficient = RvI * %I + RvT * %T + RvF * %F

RvI = runoff coefficient for impervious cover (see Table 5-5) RvT = runoff coefficient for lawn or managed turf (see Table 5-5) RvF = runoff coefficient for forested cover and open space (see Table 5-5) % I = percent of site in impervious cover (fraction) %T = percent of site in lawn or managed turf (fraction) %F = percent of site in forested cover and open space (fraction) A = site area (acres)

The runoff coefficient in the above equation differs from the runoff coefficient calculation specified in the Connecticut Stormwater Quality Manual. The runoff coefficient in the above equation accounts for impervious cover as well as other land cover types, including the effect of grading, site disturbance, and soil compaction, which can greatly increase the runoff coefficient compared to forested areas. A minimum of 60% of the WQV must be treated by using non-structural and structural LID BMP’s.


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Table 5-5. Site Cover Runoff Coefficients

Land Cover Type Runoff

Coefficient (R)

Forested Cover and Open Space

Land that will remain undisturbed OR that will be restored to a hydrologically functional state:

Portions of residential yards that will NOT be disturbed during construction

Portions of roadway rights-of-way that, following construction, will be used as filter strips, grass channels, or stormwater treatment areas; MUST include compost-amended soils or placement of engineered soil mix as per the design specifications

Community open space areas that will not be mowed routinely, but left in a natural vegetated state (can include areas that will be bush hogged no more than four times per year)

Utility rights-of-way that will be left in a natural vegetated state

Surface area of stormwater BMPs that are NOT wet ponds, have some type of vegetative cover, and that do not replace an otherwise impervious surface. BMPs in this category include bioretention, dry swale, grass channel, ED pond that is not mowed routinely, stormwater wetlands, soil amended areas that are vegetated, and infiltration practices that have a vegetated cover.

Other areas of existing forest and/or open space that will be protected during construction and that will remain undisturbed

Operational & Management Conditions for Land Cover in Forested Cover & Open Space Category Undisturbed portions of yards, community open space, and other areas that will be considered as forest/open space must be shown outside the development envelope on approved plans AND demarcated in the field (e.g., fencing) prior to commencement of construction. Portions of roadway rights-of-way that will count as forest/open space are assumed to be disturbed during construction, and must follow the most recent design specifications for soil restoration and, if applicable, site reforestation, as well as other relevant specifications if the area will be used as a filter strip, grass channel, bioretention, or other BMP All areas that will be considered forest/open space for stormwater purposes must have documentation that prescribes that the area will remain in a natural, vegetated state. Appropriate documentation includes: subdivision covenants and restrictions, deeded operation and maintenance agreements and plans, parcel of common ownership with maintenance plan, third-party protective easement, within public right-of-way or easement with maintenance plan, or other documentation approved by the local authority While the goal is to have forest/open space areas remain undisturbed, some activities may be prescribed in the appropriate documentation, as approved by the local program authority: forest management, control of invasive species, replanting and revegetation, passive recreation (e.g., trails), limited brush hogging to maintain desired vegetative community, etc.

0.02 to 0.05*

Lawn or Managed Turf

Land disturbed and/or graded for eventual use as lawn or managed turf:

0.15 to 0.25*


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Table 5-5. Site Cover Runoff Coefficients

Land Cover Type Runoff

Coefficient (R)

Portions of residential yards that are graded or disturbed, including yard areas, septic fields, residential utility connections.

Roadway rights-of-way that will be mowed and maintained as turf.

Turf areas intended to be mowed and maintained as turf within residential, commercial, industrial, and institutional settings.

Impervious Cover Roadways, driveways, rooftops, parking lots, sidewalks, and other impervious cover. This category also includes the surface area of pools, ponds, and stormwater wet ponds.

0.95

*Range dependent on original Hydrologic Soil Group (HSG) Forested Cover and Open Space - A: 0.02 B: 0.03 C: 0.04 D: 0.05 Disturbed Soils/Lawn - A: 0.15 B: 0.20 C: 0.22 D: 0.25

For sites where off-site areas also contribute to the site’s drainage, the off-site area shall be included in the calculation of WQV based on its current condition. If the off-site area enters the proposed WQV BMP the BMP must include the off-site area to size the BMP. If the off-site area bypasses the WQV BMP the off-site area shall not be used to size the WQV BMP. Modifications to the site to bypass a WQV BMP are acceptable but these modifications must maintain the existing condition flow segment type (Sheet Flow, Shallow Concentrated Flow, Open Channel Flow).

Water Quality Flow The Water Quality Flow (WQF) is the peak flow rate associated with the water quality design storm or WQV. Structural BMPs that are designed based on flow rate, rather than volume, such as grass channels and proprietary BMPs, should be designed to treat the WQF. The WQF should be calculated using the NRCS, TR-55 Graphical Peak Discharge Method or other methods recommended by the manufacturers of proprietary BMPs, as described in the Connecticut Stormwater Quality Manual.

TSS Removal Stormwater BMPs shall be designed to remove 80% of the average annual post-construction load of Total Suspended Solids (TSS). The use of a treatment train approach (multiple BMPs in series) is recommended. If a treatment train is used, the entire treatment train (not each individual BMP) shall remove at least 80% of the annual average TSS load. Where there is more than one outfall or treatment train, each outfall or treatment train shall achieve 80% TSS removal prior to discharge. The 80% TSS removal requirement of Stormwater Management Standard 6 is met when:

Suitable practices for source control and pollution prevention are identified in a long-term pollution prevention plan, and thereafter are implemented and maintained according to an approved O&M Plan.


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Structural stormwater management practices are sized to treat the Water Quality Volume as defined in this manual or the Water Quality Flow, which is defined in the Connecticut Stormwater Quality Manual.

Proprietary stormwater BMPs that are intended to provide pollutant reduction must meet the requirements in Section 5.3.

Appropriate pretreatment is provided, as described in this manual. Table 5-6 contains removal efficiencies for various types of BMPs, which can be used to demonstrate compliance with the 80% TSS removal requirement. Different removal rates and BMPs may be utilized if supporting information is provided and accepted by the approving authority. It is important to note that the TSS removal rates shown in Table 5-6 are based upon multiple sources of BMP research and monitoring data. Actual TSS removal rates of specific BMPs during individual storm events will depend upon a number of site-specific factors and can be highly variable. As such, the TSS removal rates presented in Table 5-6 are based upon recognition of this variability, but accurately represent the relative TSS removal efficiencies of the various BMPs listed in the table. In addition, Table 5-6 also indicates that the TSS removal rates for proprietary stormwater BMPs should be determined on a case-by-case basis, as described in Section 5.3 of this manual.

Table 5-6. TSS Removal Efficiencies


(BMP) TSS Removal Efficiency

Pretreatment BMPs

Deep Sump Catch Basins 25% only if used for pretreatment and only if off-line

Oil Grit Separator 25% only if used for pretreatment and only if off-line

Proprietary Separators Varies – see Section 5.3

Sediment Forebays 25% if used for pretreatment

Vegetated filter strips 10% if at least 25 feet wide, 45% if at least 50 feet wide

Treatment BMPs


90% provided it is combined with adequate pretreatment or equivalent sediment storage volume, 80% without pretreatment

Constructed Stormwater Wetlands 80% provided it is combined with a sediment forebay

Extended Dry Detention Basins 50% provided it is combined with a sediment forebay

Gravel Wetlands 85% provided it is combined with a sediment forebay

Proprietary Media Filters Varies – see Section 5.3

Sand/Organic Filters 85% provided it is combined with sediment forebay

Wet Basins 80% provided it is combined with sediment forebay

Conveyance BMPs

Drainage Channels For conveyance only. No TSS Removal credit.

Grass Channels 70% if combined with sediment forebay or equivalent

Water Quality Swale – wet & dry 80% provided it is combined with sediment forebay or equivalent


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Table 5-6. TSS Removal Efficiencies


(BMP) TSS Removal Efficiency

Infiltration BMPs


90% provided it is combined with adequate pretreatment or equivalent sediment storage volume, 80% without pretreatment

Dry Wells 90% for runoff from non-metal roofs.

Infiltration Basins & Infiltration Trenches

90% provided it is combined with adequate pretreatment (sediment forebay or vegetated filter strip, grass channel, water quality swale) prior to infiltration

Leaching Catch Basins 80% provided a deep sump catch basin is used for pretreatment

Subsurface Structure 90% provided they are combined with one or more pretreatment BMPs prior to infiltration.

Other BMPs

Compost-Amended Soils May reduce required runoff reduction volume and water quality volume. No TSS Removal Credit.

Green Roofs May reduce required runoff reduction volume and water quality volume. No TSS Removal Credit.

Permeable Pavement 90% if designed to prevent run-on and with adequate storage capacity.

Rain Barrels and Cisterns May reduce required runoff reduction volume and water quality volume. No TSS Removal Credit.

Dry Detention Basins For peak rate attenuation only. No TSS Removal credit.

As discussed in Section 5.4, the use of multiple BMPs in series (i.e., a treatment train) is recommended and often necessary to achieve the required 80% TSS removal requirement. In such cases, the total removal rate of the BMP treatment train is based on the removal rate of the second BMP applied to the fraction of the TSS load remaining after the runoff has passed through the first BMP. The removal rates are not additive. In reality, the first BMP in a treatment train will remove a larger percentage of coarse solids, with progressively finer material passing through to subsequent BMPs in the treatment train, thereby reducing the effective TSS removal rates of subsequent BMPs. However, for the purpose of demonstrating compliance with the TSS removal requirement, each BMP in series is assumed to achieve the removal efficiency listed in Table 5-6. The percentage of TSS removed by the entire treatment train shall be calculated by applying the TSS removal rates in Table 5-6 for each BMP in the order in which it is used in the stormwater management system. A simplified equation for the total TSS removal rate (R) for two BMPs in series is:

100/BABAR (5.2)

where: R = total TSS removal rate (%) A = TSS removal rate of the first or upstream BMP (%)

B = TSS removal rate of the second or downstream BMP (%) This approach can be extended to more than two BMPs in series. A worksheet for calculating TSS removal efficiencies for multiple BMPs in series and example TSS removal calculations are provided in Appendix F.


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5.7 Selection Criteria

The Connecticut Stormwater Quality Manual provides information on the selection of structural stormwater management practices for a particular site. In addition to whether or not a stormwater BMP system will achieve the basic objectives of the Stormwater Management Standards (i.e., runoff reduction, groundwater recharge, peak flow control, pollutant reduction), the primary factors to consider in the selection of an appropriate system of stormwater BMPs include:

Land use factors,

Physical/site feasibility,

Downstream resources,

Maintenance factors,

Winter operation. The Connecticut Stormwater Quality Manual describes how each of these factors affects the selection of stormwater BMPs, including a series of tables that summarize the selection criteria for each type of stormwater BMP. This section of the Town of Greenwich Drainage Manual provides additional guidance on the selection of stormwater BMPs based upon land use characteristics and natural resources specific to the Town of Greenwich.

5.7.1 Land Use Factors

Zoning Categories Certain stormwater BMPs are more suitable for some land uses than others. Rural and low density residential areas typically have larger amounts of available land area and larger buffers between lots. A wider range of stormwater BMPs are suitable in rural and low-density residential areas. Conversely, downtown settings and other urban areas are more restrictive in terms of BMP selection, since these sites typically have less available land area, higher population density, significantly more underground utilities, and a wider range of pollutants. The Connecticut Stormwater Quality Manual summarizes the compatibility of stormwater BMPs with various types of developed land uses ranging from rural to ultra-urban land uses. Table 5-7 identifies the predominant land use types (from the Connecticut Stormwater Quality Manual) associated with each Zoning category identified in the Greenwich Building Zone Regulations. While most of the stormwater BMPs described in this manual are potentially applicable to rural, residential, and commercial settings, there are fewer stormwater BMP options for heavily urbanized areas (ultra-urban areas) because of the constraints inherent in building in urbanized areas. Limited space eliminates many space-intensive options (e.g., stormwater basins) and makes BMPs that can be used on a micro-scale and that have smaller ―footprints‖ more attractive.


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The most suitable BMPs for highly urbanized areas include:

Bioretention systems, including rain gardens, tree filters, stormwater planters, and curb extensions,

Grass channels,

Green roofs,

Subsurface infiltration structures,

Leaching catch basins,

Permeable pavement,

Sand/organic filters,

Water quality swales (dry),

Deep sump catch basins,

Dry wells,

Proprietary BMPs,

Infiltration trenches and chambers,

Rain barrels and cisterns,

Vegetated filter strips.

Table 5-7. Zoning and Land Use for Stormwater BMP Selection

Greenwich

Zoning Category1

Connecticut Stormwater Quality Manual

Land Use for Selection Criteria

Rural Residential Commercial/

Industrial Ultra Urban

RA-4 RA-2 RA-1 R-20 R-12 R-7 R-6

R-MF R-C

R-CC R-PHD-E R-PHD-N

R-PHD-TH R-PHD-SU

R-PR LBR1 LBR2

LB CGBR CGB


February 2014

Table 5-7. Zoning and Land Use for Stormwater BMP Selection

Greenwich

Zoning Category1

Connecticut Stormwater Quality Manual

Land Use for Selection Criteria

Rural Residential Commercial/

Industrial Ultra Urban

GB GBO WB

BEX-50 P

H-1 H-2

1Zoning categories from the Greenwich Building Zone Regulations. Excludes overlay zones.

High Load Areas Stormwater discharges from land uses with higher potential pollutant loads require the use of specific source control and pollution prevention measures and specific stormwater management practices, approved by the approving authority for such use. The following uses or activities are considered ―high-load areas,‖ with the potential to contribute higher pollutant loads to stormwater, and shall comply with the requirements set forth in this section.

Areas within an industrial site that are the location of activities subject to the CTDEP Industrial Stormwater General Permit (except where a No Exposure Certification for Exclusion from the General Permit has been executed).

Vehicle salvage yards and recycling facilities.

Auto fueling facilities (gas stations and other facilities with on-site vehicle fueling).

Exterior fleet storage areas (cars, buses, trucks, public works equipment).

Exterior vehicle service, maintenance and equipment cleaning areas.

Commercial parking lots with high intensity use (1,000 vehicle trips per day or more). Such areas typically include fast food restaurants, convenience stores, high turnover (chain) restaurants, shopping centers and supermarkets.

Road salt storage facilities (if exposed to rainfall).

Commercial nurseries.

Non-residential facilities having uncoated metal roofs with a slope flatter than 20 percent.

Outdoor storage and loading/unloading of hazardous substances or materials.

Facilities subject to chemical inventory reporting under Section 312 of the Superfund Amendments and Reauthorization Act of 1986 (SARA), if materials or containers are exposed to rainfall).

Marinas (service, painting and hull maintenance areas).

Confined disposal facilities, disposal sites, landfills or wastewater residuals landfills if stormwater that may come into contact with the confined disposal area, disposal site, landfill or wastewater residuals landfill may cause or contribute to the discharge of


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pollutants to wetlands, surface waters or groundwater or otherwise result in a release or threat of release.

Commercial car wash facilities within 150 feet of a water body including coastal waters.

Other land uses and activities as designated by the approving authority. Infiltration of stormwater from high-load areas is prohibited within critical areas (see Stormwater Management Standard 8). Infiltration of stormwater from high-load areas outside of critical areas (see Stormwater Management Standard 7) is allowed. For such discharges, designs should incorporate one pretreatment BMP, one treatment BMP, and one infiltration BMP (Table 5-8). Alternatively, rather than providing at least one pretreatment BMP, one treatment BMP, and one infiltration BMP, for infiltration of stormwater from high-load areas outside of critical areas, the project proponent may submit a pollutant loading analysis showing the difference between pre- and post-development pollutant loadings. Post-development loads cannot exceed pre-development loads. Stormwater pollutant loads shall be calculated for total suspended solids and any other pollutants of concern (e.g., bacteria, nutrients, metals, hydrocarbons). Several well-documented and relatively simple pollutant loading calculation methods are available for this purpose, including:

Simple Method,

STEPL,

AVGWLF,

WINSLAMM,

P8 Urban Catchment Model.

5.7.2 Physical/Site Feasibility Factors

Physical site constraints such as the infiltration capacity of the soil, depth to bedrock or water table, size of the drainage area, slope, and site topography can limit the selection of stormwater BMPs. Depending on the physical site constraints, certain BMPs may be too costly to install or may be ineffective. Physical feasibility factors are described in the Connecticut Stormwater Quality Manual. In Greenwich, relatively poor soils, high groundwater, and shallow bedrock present challenges for the selection and successful application of infiltration BMPs. The following sections address subsurface conditions in the Town of Greenwich relative to the selection and use of stormwater BMPs.


February 2014

Soil Infiltration Capacity Figure 5-1 shows the distribution of NRCS Hydrologic Soil Groups (A, B, C, and D) throughout the Town of Greenwich. A Hydrologic Soil Group is a group of soils having similar runoff potential (and infiltration potential) under similar storm and cover conditions. As shown in Figure 5-1, approximately 31% of the soils in the Town of Greenwich are classified as type D soils, which have very low infiltration rates. Approximately 17% of the soils are classified as type C (low infiltration rates), while approximately 46% of the soils have moderate to high infiltration rates (type A and B soils). As described in Appendix B (Stormwater Infiltration/Recharge Requirements), when designing infiltration BMPs, a soil evaluation must be performed to verify that the anticipated field infiltration rate meets or exceeds the minimum required infiltration rate and to classify the Hydrologic Soil Group soils on-site for calculating the required groundwater recharge volume. A soil evaluation is conducted in two phases: (1) initial feasibility evaluation, and (2) concept design testing. An initial feasibility evaluation is conducted to determine whether infiltration is feasible and potential locations on the site for infiltration facilities. An initial feasibility evaluation is meant to screen unsuitable sites, reducing testing costs. If the results of the initial feasibility evaluation show that infiltration is feasible, then concept design testing must be performed to support the infiltration system design. The soil evaluation must be conducted by a qualified professional as defined in Appendix B.

Table 5-8. BMPs for High Load Areas

1. Discharges from certain land uses with higher potential pollutant loads may be subject to additional requirements, including the need to obtain a CTDEP individual or general discharge permit or a discharge permit pursuant to the Federal Clean Water Act.

2. All proponents must implement source control and pollution prevention. 3. All BMPs shall be designed in accordance with the Town of Greenwich Drainage Manual.

Pretreatment BMPs Deep Sump Catch Basin

Oil Grit Separator

Proprietary Separators

Sediment Forebays

Vegetated Filter Strip (must be lined)

Treatment BMPs

Bioretention - Rain Gardens, Tree Filters, Stormwater Planters, Curb Extensions (filtration design)

Constructed Stormwater Wetlands (must be lined)

Dry Water Quality Swales

Extended Dry Detention Basins (must be lined)

Gravel Wetlands (must be lined)

Proprietary Media Filter (Does not include catch basin inserts)

Sand /Organic Filters (must be lined)

Wet Basins (must be lined)

Infiltration BMPs Bioretention - Rain Gardens, Tree Filters, Stormwater Planters, Curb Extensions (infiltration design)

Infiltration Basins

Infiltration Trenches

Leaching Catch Basins

Subsurface Infiltration Structures


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Infiltration BMPs should not be installed in Hydrologic Soil Group D soils, as confirmed through the soil evaluation methods contained in Appendix B. When fill materials are present or are added prior to construction of the system, a soil textural analysis must be conducted in both the fill material and the underlying native material below the fill layer, and the Hydrologic Soil Group of the more restrictive layer shall be used. Stormwater infiltration is not permitted through fill materials composed of asphalt, brick, concrete, construction debris, and materials classified as solid or hazardous waste. Alternatively, the debris or waste may be removed in accordance with applicable State solid waste regulations and replaced with clean material suitable for infiltration.

Water Table and Bedrock The depth to the seasonal high water table and depth to bedrock (or other impermeable layers) will also influence the selection and performance of stormwater BMPs. High groundwater may be appropriate for some BMPs where a permanent pool is required, since the interception of groundwater will aid in maintaining such a pool. Other BMPs, such as infiltration systems, may not be appropriate if the separation between the bottom of the infiltration system and groundwater table is not sufficient to allow for water to drain from the device and to adequately remove pollutants from stormwater runoff. Bedrock impedes the downward exfiltration of stormwater and prevents infiltration BMPs from draining properly. Shallow bedrock may preclude the use of some BMPs or limit infiltration rates and result in excessive groundwater mounding below stormwater BMPs. An area is generally not suitable for infiltration BMPs if bedrock is within two feet of the bottom of the BMP.


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Figure 5-1. NRCS Hydrologic Soil Groups


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In Greenwich, a high water table is common in coastal areas and areas with poor soils. Shallow bedrock and rock outcrops are also common in many areas of Greenwich. Similar to soil infiltration capacity, water table and bedrock conditions vary considerably from site to site and therefore must be considered and evaluated for every site. The soil evaluation requirements in Appendix B include field evaluation of depth to seasonal high groundwater and bedrock. In areas with a high groundwater table, incorporating LID may be more challenging and may require additional or more creative site engineering to take advantage of swales, bioretention, and sand filters for filtration of pollutants. In these situations, it may be more feasible to rely on conservation of natural features and vegetation to the greatest extent possible. This approach will also reduce both quantity of runoff and the amount of pollutants generated. While infiltration may not be practical in these areas, bioretention systems designed for water filtration are viable options. Soils and groundwater challenges may make it more attractive to rely on conservation of natural vegetation and use of conservation areas to filter runoff prior to discharging to sensitive waters. There must be at least a 2-foot separation distance from the bottom of the infiltration structure to seasonal high groundwater or bedrock/ledge (this separation requirement may be waived or reduced by the approving authority on a case-by-case basis). The top two feet is the biologically active zone of a plant and soil complex and is where most of the physical, chemical, and biological pollutant removal occurs. A 3-foot separation distance is required from the bottom of the infiltration structure to seasonal high groundwater for land uses with higher potential pollutant loads (high load areas).

5.7.3 Downstream Resources

It is important to consider not only the impacts the development will have at a site, but also how downstream resources may be impacted by development activities. The Connecticut Stormwater Quality Manual provides guidance on the downstream resources that should be considered when selecting stormwater BMPs. In addition to the general guidance provided in the Connecticut Stormwater Quality Manual, the following sections discuss specific types of downstream resources within the Town of Greenwich, referred to as ―Critical Areas‖ in this manual, and recommended and restricted BMPs for stormwater discharges to or near14 these resources. Critical areas (see Figure 5-2) are defined as:

All parcels within the Coastal Area Management Zone that have a property boundary along the water are considered within the ―Critical Area‖.

Shellfish growing areas and public swimming beaches (entire Greenwich coastline),

Recharge areas for public water supplies (groundwater and surface water supplies),

Other sensitive receiving water bodies or wetlands as designated by the Town of Greenwich or the Connecticut Department of Energy and Environmental Protection.

14 A discharge is ―near‖ a critical area if there is a strong likelihood of a significant impact occurring to the area,

taking into account site-specific factors.


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Figure 5-2. Critical Areas


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The stormwater BMPs approved for discharges to or near critical areas shall be designed to treat the Water Quality Volume (WQV) for the post-development site. These stormwater discharges require the use of a treatment train that provides 80% TSS removal, including at least one pretreatment BMP, one treatment BMP, and one infiltration BMP. Alternatively, rather than providing at least one pretreatment BMP, one treatment BMP, and one infiltration BMP, for discharges to or near critical areas, the project proponent may submit a pollutant loading analysis showing the difference between pre- and post-development pollutant loadings. Post-development loads cannot exceed pre-development loads. Stormwater pollutant loads shall be calculated for total suspended solids and any other pollutants of concern (e.g., bacteria, nutrients, metals, hydrocarbons). Several well-documented and relatively simple pollutant loading calculation methods are available for this purpose, including:

Simple Method,

STEPL,

WINSLAMM,

P8 Urban Catchment Model.

Shellfish Growing Areas and Beaches Coastal areas are more sensitive to nitrogen loading than freshwater systems as nitrogen is typically the limiting nutrient in salt water systems. The other major pollutant of concern in coastal waters is bacteria. Public swimming beaches and shellfish beds are extremely sensitive to high bacteria levels, which can result in closures of swimming beaches and shellfish beds. Table 5-9 list stormwater BMPs approved for discharges to or near shellfish growing areas and public swimming beaches in Greenwich, including but not limited to such resources in the following areas:

Byram Harbor,

Captain Harbor,

Cos Cob Harbor,

Greenwich Cove,

Greenwich Harbor,

Indian Harbor,

Smith Cove.

Table 5-9. Stormwater BMPs for Shellfish Growing Areas

and Public Swimming Beaches

1. If applicable, proponent must comply with Coastal Area Management and CT Tidal Wetlands Act requirements

2. Dry detention basins are prohibited. 3. All BMPs must be designed in accordance with the Town of Greenwich Drainage Manual.

Pretreatment BMPs Water Quality Inlets

Deep Sump Catch Basins with Hoods

Sediment Forebays

Vegetated Filter Strips

Drainage Channels

Hydrodynamic Separators or other Proprietary BMPs


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Table 5-9. Stormwater BMPs for Shellfish Growing Areas

and Public Swimming Beaches

Treatment BMPs Constructed Wetlands

Sand Filter/Organic Filters


Water Quality Swales

Extended Detention Basins

Wet Retention Basins

Infiltration BMPs Bioretention, Rain Gardens, Tree Filters, Stormwater Planters, Curb Extensions (infiltration design)

Infiltration Trenches

Infiltration Basins

Dry Wells (uncontaminated roof runoff only)

Infiltration Chambers or Galleys

Public Water Supplies There are two public drinking water supply watersheds in Greenwich. The Greenwich system, which is managed by Aquarion Water Company, includes much of the Mianus River watershed, parts of the East Branch of the Byram River watershed, and the watersheds for the Putnam and Rockwood Reservoir. These areas generally include the northern and eastern portions of Greenwich. The Kensico Reservoir is also a drinking water supply managed by the New York City Department of Environmental Protection. Surface water supplies are particularly susceptible to contamination by bacteria and other pollutants. Table 5-10 list stormwater BMPs approved for discharges to recharge areas for public water supplies in Greenwich.

Table 5-10. Stormwater BMPs for Recharge Areas for Public Water Supplies

1. Drinking Water Supplies:

Unless necessary to manage stormwater from essential drinking water facilities, no stormwater BMPs should be located within the following areas: i. Class I Land. Lands owned by a water company that are within 250 feet of a reservoir used for

a drinking water supply, within 100 feet of its tributary, or within 200 feet of a public water supply well.

ii. Class II Land. Lands within the public drinking water supply watershed but not included in Class I, or completely off the watershed but within 150 feet of a storage reservoir and the tributaries that directly enter it.

Leaching Catch Basins are prohibited within Class I and Class II Land. They can be located within a mapped Aquifer Protection Area (Level A or B) provided a hooded deep sump catch basin is used for pretreatment.

Drainage channels may be used for conveyance only and are not eligible for TSS removal.

Proponents must comply with local source water protection requirements. 2. All BMPs must be designed in accordance with the Town of Greenwich Drainage Manual.

Pretreatment BMPs Water Quality Inlets

Deep Sump Catch Basins with Hoods

Sediment Forebays

Vegetated Filter Strips

Hydrodynamic Separator or other Proprietary BMPs

Treatment BMPs Constructed Wetlands

Water Quality Swales

Sand Filter/Organic Filters

Bioretention - Rain Gardens, Tree Filters, Stormwater


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Table 5-10. Stormwater BMPs for Recharge Areas for Public Water Supplies

Planters, Curb Extensions

Extended Detention Basins

Wet Retention Basins

Infiltration BMPs Bioretention, Rain Gardens, Tree Filters, Stormwater Planters, Curb Extensions (filtration design)

Infiltration Trenches and Basins

Infiltration Chambers or Galleys

Dry Wells (uncontaminated roof runoff only)

Sensitive Receiving Waters and Wetlands Impaired waters and wetlands that have highly rated functions and values are a few examples of receiving waters that may be more sensitive to development activities and could require additional measures to protect or restore their unique characteristics. Toxic pollutants such as metals, soluble organic compounds, and bacteria are of particular concern for waters that could serve as future water supply sources. Rivers that support cold water fisheries are sensitive to increases in water temperature, which are often caused by stormwater running over heated impervious surfaces that lack of sufficient buffers to provide shade. It should be noted that thermal pollution from urban runoff in marine waters is less of a concern, as compared to discharges from power plants and other significant industrial facilities with high-temperature effluent. Downstream flooding and channel erosion are also important considerations. Coastal waters are the primary impaired water bodies in the Town of Greenwich (Appendix A). The impairments are for shellfish harvesting, recreation, and habitats for fish and other aquatic life and wildlife. Non-point sources, including urban stormwater runoff and waterfowl, as well as marina/boating and sanitary on-vessel discharges are the predominant suspected sources of the impairments, which are caused by elevated concentrations of bacteria and nutrient enrichment and low dissolved oxygen. The following guidelines are recommended for stormwater discharges to or near sensitive receiving waters and wetlands:

If cold water fisheries are present, select BMPs that will reduce thermal impacts.

For inland lakes and ponds, select BMPs with high phosphorus and sediment removal to reduce the rate of eutrophication. Select BMPs with high bacteria removal when waters are used for recreation.

For coastal areas, select BMPs with high nitrogen and bacteria removal to reduce closure of swimming beaches and shellfish beds.

5.8 LID Retrofits and Redevelopment

LID retrofitting can be an effective approach to control stormwater pollution in existing urbanized communities and commercial developments. With LID retrofit projects, micro-scale management techniques are introduced into the existing urban landscape (roads, sidewalks, parking areas, buildings, landscaped areas, etc.) to reduce pollution from existing sources. The most economical way to retrofit existing development is to ensure that infill development, redevelopment, and reconstruction projects include the use of LID practices. Over time as urban areas are redeveloped and rebuilt with LID practices, runoff is reduced and more of the


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previously untreated runoff can be treated thereby reducing water quality impacts. Retrofitting over time through the redevelopment process combined with targeted capital improvement projects can have a significant long-term benefit. Retrofits also play an important role in meeting the Stormwater Management Standards for redevelopment projects (Standard 9). Several LID techniques may be used for retrofit and redevelopment. Selection should be made on the level of desired pollutant removal as well as the unique constraints of the site. When selecting the most appropriate LID techniques it is important to match the optimum LID technique to meet the goals of the receiving waters. The following are examples of the types of LID retrofits that may be appropriate for redevelopment sites in Greenwich:

Install filtering practices/bioretention to remove pollutants in high load areas.

Insert stormwater treatment within or on the margins of existing parking lots.

Provide stormwater treatment in open spaces near the downgradient outfall of parking lots.

Look for opportunities with the street, its right of way, cul-de-sacs and traffic calming devices to treat stormwater runoff before it gets into the street storm drain network.

Disconnect, store and treat stormwater runoff generated from residential and commercial rooftops close to the source.

Convert or disconnect isolated areas of impervious cover and treat runoff in an adjacent pervious area using low tech approaches such as a filter strip.

Reconfigure the storm drainage of high visibility urban landscapes, plazas and public spaces to treat stormwater runoff with landscaping and other urban design features.

Provide stormwater treatment in an underground location when no surface land is available for surface treatment. Use this as a last resort at ultra-urban sites.

Add water quality treatment storage to an existing pond that lacks it by excavating new storage on the pond bottom, raising the height of the embankment, modifying riser elevations/dimensions, converting unneeded quantity control storage into water quality treatment storage and/or installing internal design features to improve performance.

Retrofit existing lawns using compost-amended soils. Lawns established by this process are termed Tilled Compost-Amended Turf, which are different than traditional lawns because it results in an eight to ten-inch soil base having an organic content between 8 and 13 percent, by weight.

Other larger-scale stormwater retrofits may be appropriate at the sub-watershed scale and generally should be implemented in the context of a watershed-based management plan. A discussion of stormwater retrofits is provided in the Connecticut Stormwater Quality Manual. Additional information on stormwater retrofits can be found in Urban Subwatershed Restoration Manual No. 3, Urban Stormwater Retrofit Practices, Version 1.0, prepared by the Center for Watershed Protection (August 2007) and the stormwater BMP design references in Appendix G of this manual.


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5.9 Design Guidance for Stormwater

BMPs

This manual does not attempt to duplicate the extensive guidance that is available in other existing design manuals including the Connecticut Stormwater Quality Manual and stormwater manuals of other jurisdictions. Because stormwater management is an evolving field, existing stormwater management practices are being refined and new practices are being developed on a regular basis. Appendix G of this manual contains a summary of recommended references for the selection, design, construction, and maintenance of structural stormwater BMPs (including Low Impact Development or LID practices) that may be used in conjunction with LID site planning and design techniques. The list of recommended design references in Appendix G may be updated by the Town as necessary to reflect new developments and trends in stormwater management. Additionally, the design references in Appendix G may contain information, guidance, or requirements that conflict with information, guidance, or requirements specified in the Town of Greenwich Drainage Manual. In these instances, the information, guidance, or requirements specified in the Town of Greenwich Drainage Manual shall apply. Bioretention systems are one of the most versatile and effective LID BMPs and can be implemented in most areas where landscaping is to be incorporated into new development or redevelopment projects. Bioretention systems include bioretention cells, landscape detention, rain gardens, bio-filters, tree box filters, and stormwater planters. Due to the popularity of bioretention and its central role in LID design, there is a large body of design guidance available from other stormwater manuals and related literature sources. Many of these sources contain regional design information that may not be applicable to Greenwich, as well as outdated or conflicting information since the design of bioretention systems continues to evolve. Therefore, recommended guidance for the design, construction, and maintenance of bioretention systems in the Town of Greenwich is included in Appendix G of this manual. Appendix G may be updated by the Town as necessary to provide similarly detailed guidance for other types of stormwater BMPs.


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6 Drainage Facilities

This section addresses procedures for the design of drainage facilities, including pavement drainage, storm drainage systems, culverts, channels, brides, and storage facilities. Hydrologic and hydraulic analysis methods and drainage report submission requirement are also discussed. The information presented in this section is based on guidance contained in the Connecticut Department of Transportation Drainage Manual, modified to reflect local conditions and drainage design policies.

6.1 General Design Requirements

Site Hydrology: Preserve pre-development site hydrology (i.e., runoff, infiltration, evapotranspiration, and groundwater recharge) to the extent possible through the use of environmentally sensitive design, low impact development design techniques, and structural stormwater management practices. Low areas on a lot shall not be dewatered and filled in unnecessarily.

Runoff Rates: The post-development peak discharge rate shall not exceed the pre-development peak discharge rate, unless it can be demonstrated to the satisfaction of the approving authority that the increased discharge will not exacerbate downstream flooding conditions or contribute to downstream erosion.

Minimum Setback from Neighboring Properties: A minimum setback of 10 feet from the adjacent property line shall be required for surface stormwater discharges or infiltration systems. Flow from surface stormwater discharges and overflows from infiltration systems shall be dispersed on-site in a controlled manner that does not result in erosion or off-site drainage problems.

Minimum Setback from Building Foundations and Retaining Walls: A minimum setback of 10 feet from building foundations and retaining walls shall be required for infiltration systems. Surface stormwater discharges shall also meet this setback, except roof leader discharges (drywell overflows at roof leader discharge locations are also acceptable). Stormwater discharges or infiltration systems should not contribute to basement seepage.

Downstream Problems: Possible adverse effects of increased runoff upon downstream properties and facilities due to increased runoff must be investigated and appropriate preventative measures taken, as if necessary, to avoid possible damage.

Channels and Non-structural Improvements: Except within the street right-of-way, open channels, ditches or swales shall be used in lieu of pipes, whenever possible. In general, a non-structural approach to drainage improvements shall be given preference, primarily through environmentally sensitive site design, LID techniques, and green infrastructure practices.


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Diversion of Drainage: The collection and discharge of stormwater runoff from a watershed area of 100 acres or greater or the relocation, retention, detention, bypass, channelization, piping, culverting, ditching, or damming of waters where the drainage area tributary to such waters is 100 acres or greater may be permitted only after study of the hydraulic and hydrologic impacts of the proposed diversion and approval by the approving authority and other state or federal agencies having jurisdictional control. Additionally, proposed diversions involving drainage areas of less than 100 acres shall also be reviewed by the local approving authority.

Pumped Discharges: Pumping of stormwater (excluding rainwater harvesting systems such as cisterns), including, but not limited to, from yards, driveways, and roofs, is strongly discouraged and will be prohibited in most situations as part of a proposed stormwater management system design. This is because of the significant runoff volumes, maintenance requirements, standby power requirements, and overflows associated with large storms. All other feasible approaches must be investigated to avoid the use of pumps in stormwater management system designs. In the event the project proponent determines that pumps for stormwater are necessary, the proponent must submit required backup information as described in this manual for review by the approving authority (see Stormwater Management Standard 3).

Pumping of uncontaminated groundwater, including, but not limited to, from basements, and foundations, is discouraged for new development or in the case of redevelopment involving the upgrade of existing sump pump systems. The replacement of an existing sump pump system is acceptable when a direct replacement of the pump is needed and in the case of redevelopment when the improvement doesn’t require a sump pump. All other feasible approaches (footing drains to daylight, slab on grade, crawl space, etc.) must be investigated to avoid the use of pumps in groundwater management system designs for new development or redevelopment. In the event the project proponent determines that pumps are necessary to manage groundwater for new development or redevelopment applications, the proponent must submit required backup information as described in this manual for review by the approving authority (see Stormwater Management Standard 3).

Connections to Town Drainage System: Proposed connections to the Town drainage system will only be allowed with the approval of the approving authority based on an investigation by the design engineer demonstrating the Town drainage system to be adequate to accept the proposed discharge or that the peak flow into the system is not increasing.

Roof Leader Discharge: The discharge of roof leaders onto sidewalks and roadways is prohibited. Rooftop runoff may be infiltrated into the ground without pretreatment, discharge to pervious landscaped areas provided that it will not exacerbate flooding or contribute to erosion, or be reused through the use of rain barrels, cisterns, or gray water system.


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Tidal Considerations: In areas affected by tides, consideration must be given to the effects of tidal flooding. For design purposes, and unless otherwise approved by the approving authority, design elevations of tidal waters for drainage systems shall be equal to an elevation midway between mean high water level, and the elevation of tidal flooding for the design storm frequency of the facility under consideration. Design tidal elevations in Greenwich for common design storm frequencies are listed below (see Table 6-1). The Town of Greenwich Building Zone Regulations, as amended, stipulate minimum building design requirements in flood hazard overlay zones.

Easements: Where drainage facilities are located within private property, approved perpetual unobstructed easements shall be indicated on a plan and filed with the Town Clerk. Easements shall generally be at least 20 feet in width for open ditches, channels, etc. 20 feet is also recommended for closed conduits, but a smaller easement can be accepted on a case-by-case basis. A swale, channel or ditch that passes drainage runoff from adjacent upstream public or private property shall be considered a drainage facility requiring an easement.

Table 6-1. Design Elevations of Tidal Waters

Design Storm

Frequency (years)

Design Elevation of Tidal Waters (feet)*

Above mean sea level

datum (NGVD 29)

Above mean sea level

datum (NAVD 88)

10 6.3 5.2

25 7.1 6.0

50 7.7 6.6

100

12.2 (easterly town line to Cos Cob Harbor)

11.1 (easterly town line to Cos Cob Harbor)

11.5 (Greenwich Cove to Flat Neck Point)

10.4 (Greenwich Cove to Flat Neck Point)

In subdivisions, where concentrated roadway drainage discharges into private property, a note designating the private property owner as being responsible for properly passing drainage through his property shall be shown on the record plan. Where open ditches and channels are enclosed by private property owners, the approval of the Inland Wetlands & Watercourses Agency is required and the required size of the conduit shall meet the Stormwater Management Standards of this manual. A design from a registered professional engineer is required unless otherwise approved by the DPW Engineering Division. The Town of Greenwich shall have the right to inspect the drainage facility from time to time and to direct appropriate maintenance measures which shall be performed in a timely manner. Standard easement language is included in Appendix H as well as in the Town’s drainage relocation requirements.


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6.2 Runoff Determination

6.2.1 Design Storm Frequency

The storm frequency chosen for design will significantly impact the sizing of the storm drainage system. Storm drainage systems must be designed to adequately accommodate peak runoff for the chosen design storm to protect the property, safety and convenience of the public. Drainage systems shall be designed for Type III, 24-hour design storm events. The design storm frequencies to be used for design are presented in Table 6-2. These design storm frequencies are based on roadway classification, design speed limits, economic analysis, and policy decisions made by the Town. In areas of potentially high property damage, the design storm frequencies used for design shall be increased as directed by the Town. Reduction of the design storm frequency used for design will be subject to Town approval. The 24-hour design storm rainfall amounts are presented in Appendix L of this manual. These values, which are specific to Greenwich, are derived from an on-line web tool for extreme precipitation analysis developed as a joint collaboration between the Northeast Regional Climate Center (NRCC) and the USDA Natural Resources Conservation Services (NRCS), http://precip.eas.cornell.edu, for New York and New England. The design storm rainfall amounts provided by this web tool offer significant advantages over previous products (e.g., Rainfall Frequency Atlas of the United States‖, Technical Paper No. 40, U.S. Department of Commerce, Weather Bureau and NOAA Technical Memorandum ―NWS Hydro-35‖, June 1977, U.S. Department of Commerce, National Weather Service) since the design storm rainfall amounts are based on a much longer period of record, including ongoing updates as new rainfall data is collected. The Town may periodically update the 24-hour design rainfall amounts listed in Appendix L to reflect future rainfall data. Designers should refer to the web tool for additional information such as design rainfall amounts for shorter design storms (less than 24 hours) and intensity-duration-frequency data.

Table 6-2. Design Storm Frequencies

Type of Facility Design Frequency (Years)

Storm Drains and Inlets

Local Streets and Parking Lots 10

Local Streets and Parking Lots at Sags* 25

Secondary and Major Roads 10

Secondary and Major Roads at Sags* 25

Local Drainage Channels and Ditches 25

Watercourse Channels 50

Culverts**

Watershed Area <1 square mile 50

Watershed Area ≥1 square mile 100

Bridges 100

Storage (Detention/Retention) Facilities See Section 6.7

* Sags include any low point without a reliable means of overland flood relief, where ponding will occur if the drainage system cannot convey storm flows.

http://precip.eas.cornell.edu/


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** Culverts for driveway crossings of watercourses will require design for the 100-year storm, regardless of the size of the upstream watershed area. For those critical activities as defined in CGS Section 25-68b-25-68h, the design storm frequency shall be 500 years.

The project proponent should submit hydrologic and hydraulic design calculations for pre- and post-development conditions for the 1, 2, 5, 10, 25, 50 and 100-year design storm frequencies unless directed otherwise by the Town.

6.2.2 Acceptable Methods

Conveyance systems such as storm drains and open channels are designed based on peak runoff rates. The design of stormwater management and drainage facilities that rely on detention or retention requires the use of runoff hydrographs and hydrograph routing (typically more complex than estimating peak runoff rates) using computer modeling techniques. All methods used for analysis shall be documented and subject to approval by the Town.

6.2.2.1 Rational Method

The Rational Method is one of the most widely used techniques for estimating peak runoff for small watersheds. It assumes that the rainfall intensity is uniformly distributed over the watershed area at a uniform rate throughout the duration of the storm. The use of the Rational Method shall be limited to watersheds smaller than 200 acres and the design of storm drainage networks (not to be used for site design). In the Rational Method, runoff is related to rainfall intensity by the following equation:

CIAQ (6.1)

where: Q = maximum rate of runoff (cubic feet per second) C = runoff coefficient representing a ratio of runoff to rainfall

I = average rainfall intensity for a duration equal to the time of concentration, for a selected return period, (inches/hour)

A = drainage area tributary to the design location (acres) The above equation applies to 1-year to 10-year storm frequencies. The formula for infrequent storms (i.e. larger storm frequencies) is identified as Equation 6.2.

Runoff Coefficient The Runoff Coefficient (C) is a function of the type of cover on the site. As site imperviousness increases, the value of C also increases. The value of C can be determined based upon land use or soil type and slope. Tables 6-3 and 6-4 provide runoff coefficients based on different land use characteristics. Where various land cover conditions exist within a drainage area, a composite runoff coefficient based on the percentage of different types of surfaces within the drainage area may be developed utilizing the coefficients listed in these tables.


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Table 6-3. Runoff Coefficients for Various Surfaces

(Rational Method)

Type of Surface Runoff Coefficient (C)

Forest Cover 0.02 to 0.05

Streets

Asphalt 0.95

Concrete 0.95

Brick 0.85

Gravel 0.70

Permeable Paving

Porous asphalt 0.01 to 0.55

Pervious concrete 0.01 to 0.65

Paving stones (e.g., unit pavers) 0.10 to 0.70

Grass pavers (e.g., turf blocks) 0.15 to 0.60

Drives and Walks 0.85

Roofs 0.95

Artificial Turf 0.90

Disturbed Soils/Lawn, sandy soil

Flat (< 2%) 0.05 to 0.10

Average (2% to 7%) 0.10 to 0.15

Steep (> 7%) 0.15 to 0.20

Disturbed Soils/Lawn, heavy soil

Flat (< 2%) 0.13 to 0.17

Average (2% to 7%) 0.18 to 0.22

Steep (> 7%) 0.25 to 0.35

Table 6-4. Runoff Coefficients for Various Land Uses

(Rational Method)

Type of Area Runoff Coefficient (C)

Downtown Areas (Commercial) 0.70 to 0.95

Neighborhood Areas 0.50 to 0.70

Residential Areas

Single Family Areas 0.30 to 0.50

Multi units, detached 0.40 to 0.60

Multi units, attached 0.60 to 0.75

Suburban 0.25 to 0.40

Residential Areas (1.2 acre lots or larger) 0.30 to 0.45

Apartment Dwelling Areas 0.50 to 0.70

Industrial

Light Areas 0.50 to 0.80

Heavy Areas 0.60 to 0.90

Parks, Cemeteries 0.10 to 0.25

Playgrounds 0.20 to 0.40


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Table 6-4. Runoff Coefficients for Various Land Uses

(Rational Method)

Type of Area Runoff Coefficient (C)

Railroad Yard Areas 0.20 to 0.40

Unimproved Areas 0.10 to 0.30

Table 6-5 lists Runoff Coefficient values for selected zones in Greenwich based on the Greenwich Building Zone Regulation zoning classifications and map. Copies of the Town zoning regulations and map may be obtained at the office of the Greenwich Town Planner. The C values listed in Table 6-5 may be used to estimate runoff under proposed development conditions for currently undeveloped areas within the listed zones. For other zones not included in the table, C values shall be determined at the discretion of the Town.

Table 6-5. Runoff Coefficients for Greenwich, CT

(Rational Method)

Zone Steep Slope Average Slope Flat Slope

RA-4 0.36 0.27 0.22

RA-2 0.37 0.28 0.23

RA-1 0.39 0.30 0.25

R-20 0.44 0.36 0.31

R-12 0.49 0.40 0.36

R-7 0.54 0.49 0.44

Business 0.7 to 0.9 0.6 to 0.8 0.5 to 0.7

For these values, steep slope is assumed to be > 7%, average slope is 2% to 7%, and flat slope is < 2%.

The Runoff Coefficients listed in Table 6-3 through 6-5 are applicable for storms with 1-year to 10-year frequencies. Less frequent, higher intensity storms require modification of the Rational Method formula because infiltration and other losses have a proportionally smaller effect on runoff. For storm frequencies larger than 10-year storms, a frequency factor (Cf) is multiplied by the right side of the Rational Method equation to estimate peak runoff. The rational formula then becomes:

IACCQ f (6.2)

Values for Cf are listed in Table 6-6. The product of C times Cf shall not exceed 1.0.


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Table 6-6. Frequency Factors

(Rational Method)

Recurrence Interval

(Years) Cf Value

25 1.1

50 1.2

100 1.25

Rainfall Intensity The rainfall intensity (I) is the average rainfall rate (inches per hour) for a duration equal to the time of concentration for a selected return period. The rainfall intensity for the design storm is dependent upon the time of concentration, TC. Once a particular return period has been selected for design and a time of concentration calculated for the drainage area, the rainfall intensity can be determined from the rainfall Intensity-Duration-Frequency (IDF) curves. IDF values shall be obtained from the on-line web tool for extreme precipitation analysis developed as a joint collaboration between the Northeast Regional Climate Center (NRCC) and the USDA Natural Resources Conservation Services (NRCS), http://precip.eas.cornell.edu.

Time of Concentration The time of concentration, TC, is the time required for water to flow from the hydraulically most remote point of the drainage area to the point under investigation. Factors that affect the time of concentration are the length of flow, the slope of the flow path, and the roughness of the flow path. For flow at the upper reaches of a watershed, rainfall characteristics, most notably the intensity, may also influence the velocity of the runoff. The time of concentration equals the sum of the travel times on each segment of the principal flow path. Sheet flow occurs in the upper reaches of a watershed. Such flow occurs over short distances and at shallow depths prior to the point where topography and surface characteristics cause the flow to concentrate in rills and swales. The depth of such flow is usually 0.75 inch to 1 inch or less. Concentrated flow is runoff that occurs in rills and swales and has depths on the order of 1.5 inches to 4 inches. Part of the principal flow path may include pipes or small streams. The travel time through these segments would be computed separately. Velocities in open channels are usually determined assuming bank-full depths. The following equation represents the time of concentration (TC) which is the sum of the travel time for each flow regime along the principal flow path.

CHSCOC TTTT (6.3)

where: TC = time of concentration (hours) TO = travel time for overland flow (hours)

TSC = travel time for shallow concentrated flow (hours) TCH = travel time for channel flow (hours)

http://precip.eas.cornell.edu/


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There are numerous methods used to calculate the travel time for each of the flow regimes, including various equations and nomographs. The designer should refer to the Connecticut Department of Transportation Drainage Manual for recommended methods for calculating travel times for overland (sheet) flow, shallow concentrated flow, and channel flow. For design purposes, the minimum time of concentration is 5 minutes for paved areas and 10 minutes for all other analyses.

Drainage Area The limits and area of the drainage area (A) may be established by field surveys, USGS topographic maps or Town topographic maps. To assist in delineating watershed boundaries, Town topographic information is available from the GIS Division. Historic topographic maps at a scale of one inch equals 200 feet are available at the DPW Engineering Division. If topographic information other than a direct field survey is used as the basis for delineating drainage area boundaries (e.g., USGS or Town topographic maps), the topographic map reference and latest map revision date must be identified in the Stormwater Management Report.

6.2.2.2 Other Methods

A variety of computer modeling methods, including numerous proprietary software packages, are available for hydrologic/hydraulic design. These methods should be used in situations where the underlying assumptions of the Rational Method are not valid, thereby rendering the Rational Method inappropriate for use in these applications. Other situations in which the use of computer modeling methods is recommended include:

1. The watershed under study is ungaged and significant reservoir or valley storage is present, thereby preventing the use of regional regression equations.

2. A stormwater detention design or study is to be undertaken and reservoir routing computations are required.

3. The designer must construct a model to simulate an actual storm event and sufficient data is available to facilitate calibration of the results. (May be desirable or necessary in circumstances which involve litigation).

4. Pre/post development peak discharge rates must be determined in order to document the effects of a proposed activity.

5. A dam breach analysis is required. 6. May be used on any watershed, as is limited by individual computer program, and is

recommended when watershed exceeds the limits established by other methods. 7. The Town may require the use of these or similar programs for various site specific

reasons.

The following sections summarize suggested computer modeling methods for hydrologic/hydraulic design.

U.S. Army Corps of Engineers Methods

HEC-1: This computer program simulates a) rainfall and/or snowmelt runoff from


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subbasins within a watershed, b) flow through stream reaches, c) flows from urban areas, d) flow diversions, and e) flow through reservoirs including a breached dam condition with a varying (degrading) failure section. Effectively, there is no limitation related to basin size as the program is suitable for use in the analysis of watersheds which vary in size from small urban catchments to large, multibasin river systems. In order to transform rainfall values to runoff, a unit hydrograph or kinematic wave approach is employed. The unit hydrograph, which is most commonly used, is the recommended method unless it can be demonstrated by the designer that basin characteristics require the use of the kinematic wave method. The designer shall use the 24-hour, Type III storm event as recommended by the National Resource Conservation Service (NRCS), formerly the Soil Conservation Service (SCS). Times of concentration and travel times for use in the HEC-1 program shall be developed using the methodology described in this manual and the Connecticut Department of Transportation Drainage Manual.

HEC-HMS: The Hydrologic Modeling System HEC-HMS is designed to simulate the precipitation-runoff processes of larger watershed systems. It supersedes HEC-1 and provides a similar variety of options but represents a significant advancement in terms of both computer science and hydrologic engineering. In addition to unit hydrograph and hydrologic routing options, capabilities include a linear quasi-distributed runoff transform for use with gridded precipitation, continuous simulation with either a one-layer or more complex five-layer soil moisture method, and a parameter estimation option.

NRCS Methods

TR-55 Urban Hydrology for Small Watersheds, Version 2.0: This rainfall runoff model was developed by the U.S. Soil Conservation Service, now known as the Natural Resources Conservation Service (NRCS). TR-55 presents simplified procedures to calculate storm runoff volume, peak rate of discharge, hydrographs, and storage volumes required for flood water reservoirs. These procedures are applicable to small watersheds, especially urbanized watersheds. The primary functions of the program are for peak runoff computations using the Graphical Peak Discharge Method, the Tabular Hydrograph Method, and Temporary Storage. Support functions include the computation of the runoff curve number, the time of concentration, and travel time. Limitations of the model include NRCS type distributions, 24-hour duration rainfall, 10 subwatersheds, and minimum of 0.1 hour and maximum 10-hour times of concentration. The Tabular Hydrograph Method, or the use of TR-20 as described below, is the preferred NRCS method for hydrologic design in Greenwich.

TR-20: Technical Release No.20 is a computer program which computes direct runoff from a rainstorm, generates flood hydrographs from surface runoff, and routes flow through channel reaches or reservoirs. Output data includes peak discharges with related times of occurrence and water surface elevations at designated cross sections or structures. With the exception of the channel routing routine (Modified Att-Kin


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Method), TR-20 is based upon the methodologies contained in the NRCS (formerly SCS) National Engineering Handbook, Section 4, Hydrology, commonly referred to as NEH 4. Based on the assumption that rainfall depth is uniformly distributed throughout the watershed, the area limitations for subcatchment or overall basin size range from approximately 0.1 square miles to 400 square miles. As required with the HEC-1 program, the designer shall utilize the 24 hour Type III storm as recommended by the NRCS, and methodologies described in this manual and the Connecticut Department of Transportation Drainage Manual shall be used for time of concentration and travel time computations. All hydrologic studies performed utilizing TR-20 for watersheds greater than one square mile shall include multiple flood analyses to include the 1, 2, 5, 10, 25, 50, 100 and 500 year events, except where dam studies require the analysis of probable maximum floods. The computer model shall be calibrated to a gage station if possible.

Other hydrologic/hydraulic analysis methods may be used with the approval of the Town, provided that the procedures are properly documented and submission requirements identified in Section 7 are met. Numerous proprietary software packages are available, utilizing one or moreof the above analysis methods, that may be used for hydrologic/hydraulic analysis and design including:

Applied Microcomputer Systems – HydroCAD,

Bentley Systems – PondPack,

Bentley Systems – StormCAD,

Bentley Systems – FlowMaster,

Bentley Systems – CulvertMaster,

Intellisolve – Hydraflow Hydrographs,

Boss International/U.S. Army Corps of Engineers – HEC-HMS and HEC-RAS.

Other proprietary software may be used with the approval of the Town.

6.2.3 Hydrologic Analysis Submission Requirements

The following hydrologic analysis information shall be submitted to the Town for review as part of the required Stormwater Management Summary Report (see Section 7). Other design calculations and supporting documentation are also required as described elsewhere in this manual.

Mapping that clearly delineates the entire watershed and sub-basin limits used for the analysis.

Watershed schematic representing the physical sub-basin elements and their interrelationships for all cases in which a computer program is used to develop a hydrologic model.

Mapping that identifies the flow paths used for computation of time of concentration and/or travel time values.

Mapping identifying soil types, land cover and/or zoning information as appropriate.


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All input parameters used in hydrologic computer models shall be fully documented for inclusion in a hydrologic design report, including information related to determination of runoff coefficients, times of concentration, and curve numbers.

Stage-storage-discharge relationships for reservoir routing, structure routing and channel routing.

Any additional data or documentation requested by the Town.

6.3 Storm Drainage Systems

6.3.1 General Requirements

The following general requirements apply to the design of storm drainage systems in the Town of Greenwich:

Storm drainage systems shall be designed for the appropriate design storm frequency as indicated in Table 6-2.

Storm drainage systems shall be designed for complete development of the tributary watershed. Offsite areas that drain through the developed property shall be included. o The latest edition of the Planning and Zoning Commission’s ―Building Zone Regulation Map‖ shall be used to estimate the type of future development to be expected in offsite areas. Storm drainage systems shall be designed to collect and convey the anticipated peak discharge that will result from the development of the entire watershed of the proposed system for the appropriate design storm frequency unless otherwise directed by the approving authority.

Where stormwater cannot be managed entirely on-site through infiltration and/or retention, storm drainage systems shall convey stormwater to a suitable outlet, preferably an existing river, stream, coastal waters, or an existing drainage system within the Town, if possible, and flow shall be dispersed on-site in a manner that does not result in off-site drainage problems. Where a suitable outlet is not available within the project site or adjacent Town right-of-way, project proponents will be required to obtain appropriate drainage easements and to construct new and/or improve existing drainage facilities. These requirements may be modified if roadways or other properties will not be impacted by increased runoff. Drainage systems shall not be discharged into any portion of the Town sanitary sewerage system.

6.3.2 Pavement Drainage

Roadway storm drainage facilities collect and convey stormwater runoff in a manner which adequately drains the roadway and minimizes the potential for flooding and erosion to properties adjacent to the right-of-way. The design of roadway storm drainage facilities should account for damage to adjacent property and risk of traffic interruption by flooding. Traditional roadway storm drainage systems consist of curbs, gutters, storm drains, channels and culverts. Roadway features considered during gutter, inlet and pavement drainage


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calculations include longitudinal and cross slopes, curb and gutter sections, and bridge decks. The pavement width, cross slope and profile control the time it takes for stormwater to drain to the gutter section. The gutter cross section and longitudinal slope control the quantity of flow that can be carried in the gutter section. This section addresses requirements for the design of traditional roadway storm drainage systems. Non-traditional approaches are encouraged for the design of roadways and associated storm drainage systems, as discussed in Section 4 of this manual. Non-traditional approaches, also referred to as LID or ―Green Streets,‖ include curbless roads and sheet flow to roadside swales, narrower roads, and the use of permeable pavement. If non-traditional approaches are used in place of traditional curb-and-gutter roadway drainage systems, the requirements of this section do not apply.

Longitudinal Slope Longitudinal slope is an important consideration when designing curbed and uncurbed roadways, since slope impacts gutter flow and ultimately gutter spread. The minimum longitudinal roadway design slope shall be 1%, except in transition areas between grades. Minimum grades can be maintained in flat terrain by using a sawtooth profile. Maximum longitudinal slopes shall be 9% on public roads, 15% on single family driveways, 12% on multi-family drives, and 8% on commercial drives.

Cross Slope The Connecticut Department of Transportation Highway Design Manual has established standard cross slopes for different types of roadways. Median areas should not be drained across travel lanes, and roadside areas should generally be sloped to drain away from the pavement.

Bridge Decks Drainage design of bridge decks is similar to that of other paved roadway surfaces. Bridge parapets tend to collect large amounts of debris. This results in the higher potential for bridge deck inlets to clog. Gutter flow from adjacent roadways should be intercepted prior to reaching the bridge. If at all possible, zero gradients and sag vertical curves should be avoided on bridges.

Under Drains In certain areas groundwater can be a significant problem as it attacks foundations, substructures, subgrades and other aspects of highway components. In most soils where groundwater is a problem, a system of under drains, installed for the removal of excess moisture, can be used in the overall roadway design. Under drains may consist of networks of perforated (or otherwise permeable) pipe, French drains, or collector fields. Adequate provisions shall be made for subsurface drainage systems. The location, details and required size (or capacity) of subsurface drainage facilities shall be reviewed for approval by the Town. In subdivision roadway cuts, under drains shall be required if seepage occurs during construction or if directed by the Town, even though plans may have been approved without under drains.


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Under drains shall consist of a trench located below the road subbase, containing a suitable perforated, slotted or open joint conduit and backfilled with an appropriate pervious material. The conduit shall be not less than twelve inches (12‖) in nominal diameter. Where the conduit carries surface drainage in addition to subsurface drainage, the size shall be not less than fifteen inches (15‖) in diameter and shall be one size larger than required to carry surface stormwater runoff alone. Outlets for under drain conduits shall be connected directly to drainage structures, where practical. If no drainage structure is available, the outlet conduit shall be terminated with an appropriate under drain outlet endwall. Upstream ends of under drains carrying only subsurface drainage shall be extended to a drainage structure (if one is available within 25 feet) and plugged. Lateral foundation under drains and slope under drains crossing the road shall be provided at locations approved by the Town, and responsibility for maintenance of such under drains shall remain that of the Owner.

6.3.3 Inlets/Catch Basins

Inlets (also commonly referred to as ―catch basins‖) are drainage structures that collect surface water through grate or curb openings and convey it to storm drains or directly outlet to culverts. Proper inlet analysis and design are required to define the hydraulic design capacity of the drainage system.

Standard Greenwich Inlets Inlets shall be constructed in accordance with the Town of Greenwich standard construction details, unless otherwise approved by the Town. Inlets shall be Greenwich Type C-1 along all roadways and within Town-maintained parking lots. All catch basins must have a trap on the outlet and sump as shown on the standard details. Catch basins and inlets located within roadways of private condominium developments, private parking lots or at driveway entrances shall have curb openings and may be either Greenwich Type C-l or an approved pre-cast concrete structure. Greenwich Type C-4 catch basins shall only be permitted under extenuating circumstances.

Inlets on Grade The capacity of an inlet depends on a number of factors: inlet geometry, roadway cross slope, roadway longitudinal slope, total gutter flow, depth of flow, and pavement or surface roughness. Depth of flow adjacent to the curb is the major factor in determining the capacity of gutter inlets. The frontal flow, or the water flowing in the area occupied by the grate, is fully intercepted by the inlet at low velocities. The remainder of flow outside the width of the grate will bypass the inlet and travel to the next inlet. Designs shall consider interception of the entire frontal flow and disregard side flow.

Inlets at Low Points In sag vertical curve locations where significant ponding could occur, additional inlet capacity may be necessary to control excess gutter spread. Double grate inlets can be used in lieu of


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standard single grate inlets. Flanking inlets could be used adjacent to the primary inlet located in the low point, to limit gutter spread. Flanking inlets will also work in relief of the primary inlet if it becomes clogged.

Curbing Non-traditional approaches such as curbless roads and sheetflow to roadside swales, narrower roads, and the use of permeable pavement are encouraged for the design of roadways and associated storm drainage systems, as discussed in Section 4 of this manual. Curbing shall be installed on roadways when traditional curb-and-gutter storm drainage systems are proposed. The DPW Engineering Division may recommend that the requirement for curbing be reduced or eliminated for other unique or special conditions.

Inlet Locations Along roadways with curbs or shoulders raised above the edge of pavement, catch basins shall be located to prevent gutter and catch basin capacities from being exceeded and to prevent ice from building up in the gutter. Gutter flow width shall not exceed one-half of the road travel lane during the design frequency storm. The maximum distance of gutter flow to catch basins shall be 300 feet. On roadways which do not have shoulders raised above the edge of pavement, no catch basins will be required, except at low points in gutter profiles which shall be drained by catch basins located outside the edge of pavement. In such cases, the curb inlet shall be set back two and one-half feet (2.5 feet) from the normal edge of pavement. In residential areas where the design flow across a roadway intersection is 2 cubic feet per second (cfs) or more, a catch basin will be required upstream of the intersection to intercept flow. In business districts, a catch basin will be required at all such locations regardless of design flow. At the downstream end of all curbed or cut sections, appropriate catch basins shall be provided as directed by the Town to prevent erosion of the adjacent shoulder or slope and to reduce the discharge of solids into receiving water bodies and wetlands. Paved asphalt leakoffs and inlets installed on top of culverts will not be permitted unless otherwise directed by the Town. Within parking lots, catch basins shall be located at low points and as required to prevent gutter and/or catch basin capacities from being exceeded, to permit the crowning of circulatory drives, and to permit the development of gutters along the high side of parking lots to prevent hazardous ice formation. Leakoffs or curb cuts in lieu of catch basins, or elimination of parking lot curbing altogether, may be used with the approval of the Town provided that the system is designed to prevent erosion and discharge to a bioretention system, infiltration system, water quality swale, or other similar structural stormwater BMP in accordance with requirements of this manual. In business districts, a catch basin is required at driveway entrances whenever a concentrated flow would significantly inconvenience pedestrians or where existing gutter capacity along the road is already or would become exceeded. Catch basins at driveway entrances shall be located outside the road right-of-way.


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Dry wells will be permitted only when approved by the Town to collect snow melt which may freeze and become hazardous. Their use as part of the public stormwater drainage system will not be permitted.

Gutter Flow Calculations Gutter flow calculations are required in order to relate the runoff flow rate (Q) in a curbed channel to the spread of water on the roadway shoulder, parking lot or travel lane. Gutter Flow calculations shall be prepared based on Section 11.9 of the Connecticut Department of Transportation Drainage Manual. The allowable gutter flow width (i.e., gutter spread) shall not exceed one-half of the road travel lane, leaving the other half of the lane dry, during the design frequency storm. Inlet capacities of catch basins shall be determined based on the methods contained in Section 11.9 of the Connecticut Department of Transportation Drainage Manual. A grate inlet in a sag or low point operates first as a weir having a crest length approximately equal to the outside perimeter along which the flow enters. Bars are disregarded and the side against the curb is not included in computing the perimeter. Weir operation continues to a depth of approximately 0.4 foot above the top of the grate. When the depth at the grate exceeds approximately 1.4 feet, the grate begins to operate as an orifice. When the depth over the grate is between 0.4 and 1.4 feet, the operation of the grate inlet is indefinite due to vortices and other disturbances. The capacity of the grate is between that given by the weir and orifice equations. The larger depth is used for design purposes. Equations for weir and orifice flow are provided in the Connecticut Department of Transportation Drainage Manual. Table 6-7 lists catch basin grate parameters for Greenwich Type C-1 and C-4 catch basins.

Table 6-7. Greenwich Catch Basin Grate Parameters

Parameter Greenwich Type

C-1 Catch Basin

Greenwich Type C-4

Catch Basin

Perimeter (feet) 11.63 6.33

Area (square feet) 7.25 2.50

Factor of Safety for Clogging:

Zero (0%) clogging potential 1.0 1.0

Moderate (0 to 50%) clogging potential 1.0 1.0 to 2.0

High (50%) clogging potential 1.0 2.0

6.3.4 Manholes

Manholes provide entry to continuous underground storm drains for inspection and cleanout. Typical locations where manholes should be used are:

Where two or more storm drains converge,

At intermediate points along tangent sections,

Where pipe size changes,

Where an abrupt change in alignment occurs,

Where an abrupt change of the grade occurs.


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Location Storm drains shall typically convey flows from catch basin to manhole rather than between multiple catch basin structures. However, within a building site and under conditions of limited flow (less than 2.0 cfs), storm sewers may be installed between catch basins with the approval of the Town. All new catch basins must be equipped with a trap and sump to remove coarse sediment, floatables, and debris. Manholes must be used to connect a proposed drainage system to the existing Town system. However, a single catch basin within a building site may discharge into an existing Town catch basin. Direct connection to existing drainage pipes without an access-junction structure shall not be permitted unless approved by the Town. In no case will such connection be permitted where the diameter of the connecting pipe is equal to or greater than half the diameter of the existing pipe. Manholes shall be located outside existing or future sidewalks and driveway entrances/ramps unless approved by the Town.

Spacing Maximum spacing for manholes shall not exceed 300 feet. Manholes shall be placed where a change in alignment of pipes is required, including horizontal changes in direction and vertical changes in pipe slope.

Standard Greenwich Manholes All manholes shall be precast concrete structures unless otherwise approved by the DPW Engineering Division. Manholes shall be provided in accordance with the Town of Greenwich standard details for pre-cast concrete manholes, cast iron frames and covers.

Sizing Manholes are fabricated in various sizes to accommodate different pipes sizes and pipe system geometries. When a manhole is used as an angle point, the separation angle between the two pipes inside the manhole is an important factor to ensure the structural integrity of the manhole is not compromised. The following guidelines should be used when designing and specifying manholes:

A 48-inch diameter manhole should be provided when storm drainage pipes are 30 inches or less in diameter.

A 60-inch diameter manhole should be provided when storm drainage pipes are greater then 30 inches, but less then or equal to 42 inches.

A 72-inch diameter manhole should be provided when storm drainage pipes are greater then 42 inches, but less then or equal to 54 inches.

When storm drainage pipe diameter is greater than 54 inches, a specially designed junction box should be provided.

Manholes must be large enough to accommodate all pipes while maintaining adequate separation between pipes. Manhole size should be verified by calculating the manhole coefficient, K, for two pipes connecting at a manhole.


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inchesTRTRK

142211 (6.4)

where: R1 = interior radius of pipe 1 (inches) T1 = wall thickness of pipe 1 (inches)

R2 = interior radius of pipe 2 (inches) T2 = wall thickness of pipe 2 (inches) ∆ = angle between centerlines of pipes 1 and 2 (degrees) K = relative manhole coefficient (in/degree)

Based on the calculated K value, Table 6-8 should be used to determine manhole sizing.

Table 6-8. Manhole Sizing

Manhole Diameter

(inches)

K1

(in/degree)

Maximum Pipe Size

(inches)

48 0.42 30

60 0.52 42

72 0.63 54

1When the computed K value exceeds 0.63, a junction chamber or double

catch basin is required.

6.3.5 Storm Drains

Under ordinary conditions, storm drains should be sized on the assumption that they will flow full or practically full under the design discharge but will not flow under pressure head. The Manning’s equation is recommended for capacity calculations. A minimum of one foot of freeboard shall be provided between the hydraulic grade line and the storm drain grates in all drainage systems. In locations such as depressed roadway sections and underpasses where ponded water can be removed only through the storm drain system, a higher design frequency should be analyzed. The main storm drain downstream of the depressed section should be designed by computing the hydraulic grade line and keeping the water surface elevations one foot below the grates and/or established critical elevations. Less than one foot of freeboard or even surcharge conditions may be allowed, with approval by the Town, in cases where one foot of freeboard is not achievable, such as instances where tailwater is unavoidable or where unnecessarily disruptive and expensive upgrades of the existing storm drainage system would be needed to meet the 1-foot freeboard requirement.

General Guidelines Circular storm sewers for drainage systems shall not be less than 12 inches in diameter, except for systems serving smaller areas on private property or those used to connect roof drains, as substantiated with flow/capacity calculations. Elliptical pipe of equal hydraulic characteristics to the above noted circular conduits may only be substituted when approved by the Town.


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Class IV reinforced concrete pipe (RCP) shall be used on public property and within Town roadways when sufficient cover is maintained on the pipe. Class V RCP will be required when minimal cover exists and as directed by the Town. Plastic pipe may be used on private property, but RCP must be used on public property unless otherwise approved by the Town. Pipe backfill and support conditions shall be in accordance with the Town of Greenwich standard construction details. Where storm sewers cross sanitary sewers, water mains, gas mains or other utilities, minimum clearance shall be 12 inches. If 12 inches of clearance cannot be provided, special provisions shall be taken to protect pipes in accordance with the requirements of the utility company. Storm sewers shall be laid on straight alignments, both in plan and in profile, with structures providing access at all angle points or at the junction of two or more lines. In special cases, when approved by the Town, conduits may be placed on curved alignments. Such curvature must not exceed the manufacturer’s recommendations. Minimum radii (in feet) for RCP laid on curves shall comply with Table 6-9.

Table 6-9. Minimum Radii (Feet) for Curved RCP Installation

Pipe Sections Pipe Diameter

15” 18” 24” 36” 48” 60” 72”

4-ft lengths 108 128 168 250 326 405 442

8-ft lengths 216 256 336 500 652 810 884

Outlets All storm drainage outlets shall be designed with appropriate outlet structures and outlet protection. The flowline elevation of the outfall should be equal to, or higher than the recipient. If this is not the case, excavation may be required to ensure positive gravity flow, or in severe cases pump stations may be required. Where practical, the outlet should be positioned in the outfall channel so that it is pointed in a downstream direction. This will reduce turbulence and the potential for erosion. When the outlet is located in a manner to allow the discharge to impinge on the opposite bank of a channel, that bank should be evaluated to determine the need for riprap.

Storm Drain Hydraulic Design Storm drains shall be designed as open channels, where there is a free water surface (just full or less than full), or for pressure or pipe flow under surcharged conditions. The design shall account for backwater effects and all energy losses in the system. A hydraulic analysis shall be performed to determine whether the pipe will operate as an open channel or as a pressure system. Backwater calculations shall be made for each run of pipe. Friction losses and form losses due to manholes, bends, enlargements, and transitions, shall be calculated. Manning’s equation shall be used for computing flow characteristics of conduits operating as open channels or under pressure:


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2/13/2486.1SR

nV (6.5)

2/13/2486.1SAR

nQ (6.6)

where: V = average velocity (ft/s) Q = flow (cfs)

R = hydraulic radius (feet), area divided by wetted perimeter S = the slope of the energy grade line (ft/ft) n = Manning’s roughness coefficient A = cross sectional flow area (square feet)

Manning’s roughness coefficients shall be in accordance with Table 6-10. The values provided in the table are recommended values. Actual field values for older existing pipelines may vary depending on the effects of abrasion, corrosion, deflection and joint conditions. Concrete pipe with poor joints and deteriorated walls may have n values of 0.014 to 0.018. Corrugated metal pipe with joint and wall problems may also have higher n values and, in addition, may experience shape changes that could adversely affect the general hydraulic characteristics of the pipe.

Table 6-10. Manning’s Pipe Roughness Coefficients for Storm Drainage

Type of Pipe Roughness Coefficient (n)

Reinforced Concrete (smooth walls) 0.010 to 0.013

Plastic Pipe

Corrugated polyethylene, smooth 0.009 to 0.015

Corrugated polyethylene, corrugated 0.018 to 0.025

Polyvinyl chloride (PVC), smooth 0.009 to 0.011

Graphical solutions (nomographs) of Manning’s equation for pipes flowing full and partially full are provided as Figures 6-1 and 6-2, respectively. Total allowable headwater (AHW) depths shall be less than 1.2 times the clear height or diameter of the pipe but shall not provide less than 1 foot of freeboard prior to overtopping the embankment or affecting other structures that could be impacted by the proposed culvert design. Deviation from this standard shall require the approval of the Town and may require special design considerations such as pressure-tight joints, restrained joints, etc. The hydraulic grade line (HGL) must be evaluated during the hydraulic design of storm drains. The HGL aids the designer in determining the acceptability of the proposed system by establishing the elevations along the system to which the water will rise when the system is operating from a flood of design frequency. In general, if the HGL is above the crown of the pipe, pressure flow hydraulic calculations are appropriate. Conversely, if the HGL is below the crown of the pipe, open channel flow calculations are appropriate. A special concern with


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Figure 6-1. Nomograph for Manning’s Equation for Flow in Storm Drains

(Flowing Full)


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Figure 6-2. Nomograph for Flow in Storm Drains

(Partial Depth Flow)


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storm drains under pressure flow conditions is that inlet surcharging and possible manhole lid displacement can occur if the HGL rises above the ground surface. As hydraulic calculations are performed, frequent verification of the existence of the desired flow condition should be made, as storm drain systems can often alternate between pressure and open channel flow conditions from one section to another. Section 11.12 of the Connecticut Department of Transportation Drainage Manual contains acceptable methods for calculating hydraulic grade lines. Profiles with the HGL must be submitted to the Town for all proposed drainage systems. The hydraulic grade line calculations must account for friction losses in the pipes and losses caused by structures due to differences in velocity, changes in direction, incoming flow volume, entrance losses, exit losses, and other applicable conditions.

Minimum Grades Storm drains should be designed such that flow velocities will not be less than 2.5 ft/s at design flow. For very flat grades the general practice is to design components so that flow velocities will increase progressively throughout the length of the pipe system. The storm drainage system should be checked to ensure sufficient velocity in all of the drains to deter settling of particles. Minimum slopes required for a velocity of 2.5 ft/s, with pipes flowing full, can be calculated by the Manning’s equation or by using values provided in Table 6-11. The maximum allowable velocity shall be 16 feet per second. A series of drop manholes may be required to reduce velocities to this level.

Table 6-11. Minimum Allowable Pipe Slopes (%) to Ensure 2.5 ft/s

In Storm Drains Flowing Full

Pipe Diameter

Pipe Material 12” 15” 18” 24” 36” 48” 60”

Reinforced Concrete (RCP) n=.013

0.3% 0.22% 0.2% 0.12% 0.07% 0.05% 0.04%

Corrugated Polyethylene (corrugated) n=.025

1.1% 0.83% 0.65% 0.44% 0.3% 0.2% 0.13%

Corrugated Polyethylene (smooth) n=.015

0.4% 0.3% 0.23% 0.2% 0.09% 0.06% 0.05%

Polyvinyl chloride (PVC) (smooth) n=.011

0.22% 0.16% 0.13% 0.09% 0.05% 0.034% 0.03%

6.3.6 Headwalls and Trash Racks

A headwall, or other approved protection, shall be required wherever an enclosed drainage system discharges into or is preceded by a ditch, stream or channel. Unless specified otherwise in this manual, headwalls shall be located within and adjacent to the road right-of-way line, except in residential acreage zones (RA-1, RA-2, and RA-4) where they may be located in the embankment slope.


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The need for trash racks shall be established based upon field conditions. An approved trash rack is required at the intake end of drainage systems that are more than 100 feet in length, unless otherwise approved by the Town. If used, they should be designed to allow for overflow when clogged. Trash racks at entrances to pipes and conduits should be sloped between 3H:1V and 5H:1V to allow trash to slide up the rack with flow pressure and rising water level – the slower the approach of flow, the flatter the trash rack angle.

6.3.7 Outlet Protection

Drainage pipe outlets are points of critical erosion potential. Stormwater which is transported through closed conveyance systems at design capacity generally reaches a velocity which exceeds the permissible or erosion resistant velocity of the receiving channel or overland area. To prevent scour at storm drainage system outlets, a flow transition structure is needed which will absorb the initial impact of the flow and reduce the flow velocity to a level which will not erode the receiving channel or overland area. Such a structure is referred to as outlet protection. The most commonly used devices for outlet protection are riprap lined aprons and preformed scour holes. In most cases, a riprap apron or preformed scour hole will provide adequate outlet protection. However where design and site conditions warrant, structurally lined outlet protection or energy dissipators may be required including check dams, drop structures, baffles,

and stilling basins. Design of storm drainage outlet protection shall be in accordance with Section 11.13 of the Connecticut Department of Transportation Drainage Manual. Energy dissipators shall be designed in accordance with FHWA, Hydraulic Engineering Circular No. 14, Hydraulic Design of Energy Dissipators for Culverts and Channels.

6.4 Culverts

Culverts shall be designed to convey discharges resulting from the storm frequencies indicated in Table 6-2 and following design procedures contained in Section 8 of the Connecticut Department of Transportation Drainage Manual. The water surface used at the inlet of the culvert to determine culvert size shall be based upon the allowable headwater (AHW), allowing for at least one foot of freeboard, unless otherwise approved by the Town. AHW is one of the critical elements of culvert design and must be indicated on the culvert computation form along with a clear description of how it was obtained. The AHW is determined or limited by both the existing and future elevations of the roadway, ramp, floor level of a building, etc., that may be impacted by the proposed culvert design. Culvert design must be based on the AHW and the topography of the area upstream and adjacent to its inlet. Special consideration must be given to the effect of the proposed water surface on abutting private property. In general, the backwater effects should be contained within the road right-of-way unless the existing topography allows for the containment of flows approaching the culvert and is approved by the Town. Sound engineering judgment must also be exercised to be certain that the AHW computation is reasonable for both the present and


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future conditions of the site. At all culvert inlets, the approximate limits of the flooded area covered by the headwater shall be shown on the plans submitted for review. Where a culvert is proposed downstream of an existing or proposed detention basin, the culvert sizing shall assume that zero upstream storage exists; i.e., the culvert is designed as if the stormwater detention facility did not exist. The locations and alignment of culverts should be consistent with the flow tendency of existing streams. New culverts or replacement culverts to convey streams shall comply with the Connecticut Department of Energy and Environmental Protection Stream Crossing Guidelines (as amended) to accommodate high flows, minimize erosion, and support aquatic habitat and wildlife passage. Where successive culverts are utilized and the flow in the upper culverts is affected by headwaters in the lower culverts, a water surface profile and appropriate computations shall be submitted for review. The effects of tidal action shall be investigated to ensure that scour or erosion resulting from high velocities due to the movement of the tidal prism will not endanger the structure, roadway, or adjacent property. Culvert design calculations shall be submitted in a clear, neat, and understandable manner and contain the information shown on the culvert design form in the Connecticut Department of Transportation Drainage Manual.

6.5 Channels

Channels shall be designed with a section and grade that will carry the design discharge, in its flattest section, under the controlling conditions, providing freeboard as needed, and with channel lining protection to prevent erosion. The channels described in this section are intended to primarily function as conveyance systems, as opposed to water quality swales which serve as both conveyance and water quality treatment. Channels shall be designed for the storm frequencies identified in Table 6-2 and following procedures set forth in Section 7 of the Connecticut Department of Transportation Drainage Manual. The analysis and design of channels shall be consistent with the type of channel and its intended purpose. Channels shall be classified as local drainage channels or as watercourse channels, depending on their use. Local drainage channels, including minor LID vegetated swales and grass drainage channels, are designed with the primary purpose of conveying urban, parking lot and road runoff from small watersheds, frequently with intermittent flow and limited ecological value and are intended to convey their design flow within their banks. Watercourse channels are designed as natural perennial watercourses or man-made channels planned to simulate a natural watercourse. The following criteria apply to local drainage channels:


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Channel side slopes shall not exceed the angle of repose of the soil and/or lining and shall be 1V:2H or flatter in the case of rock-riprap lining and 1V:3H in the case of vegetative lining.

The use of flexible linings (grass, other vegetation, riprap, keyed riprap, and gabions) is preferred over rigid linings such as riprap revetments. Flexible linings shall be designed according to the method of allowable tractive force. Preference should be given to vegetation-lined channels where possible.

The use of impervious linings (concrete, asphalt, etc.) is discouraged except for very high velocity flow and steep slopes.

The design discharge for permanent roadside ditch linings shall have a 10-year frequency while temporary linings shall be designed for the 2-year frequency flow.

Channel freeboard shall be a minimum of 1 foot or two velocity heads, whichever is larger. Freeboard allowances shall be provided in proportion to the potential damages that could occur in the event of overtopping.

Where possible, channels should have vegetative buffers.

Where velocities in swales, ditches or channels exceed 5 fps, riprap or other protective treatment must be installed to prevent erosion.

The following criteria apply to natural watercourse channels:

If relocation of a natural channel is unavoidable, the cross-sectional shape, meander, pattern, roughness, sediment transport, capacity, channel slope, and side slope shall conform to the existing conditions insofar as practical. Some means of energy dissipation may be necessary when existing conditions cannot be duplicated.

Streambank stabilization shall be provided, when appropriate, as a result of any stream disturbance or encroachment and shall include both upstream and downstream banks when required, as well as the local site.

Shall have minimum flow capacity of a flood equal to at least 25 year frequency flood.

Shall have an inner channel to concentrate low flows with a capacity of a 2 year frequency flood.

Shall have water surface profiles prepared for the 2, 25, and 100 year frequency floods.

Shall consider the hydraulic capacity of floodplains.

Shall have a sediment transport capacity similar to upstream and downstream channels.

Shall be designed to minimize the use of artificial linings for flows in excess of the two year frequency flood.

Shall encourage ecological productivity and variety.

Shall be visually compatible with its surroundings.

The alignment and slope shall be compatible with natural channels in similar site conditions.

Variations in width, depth, invert elevations, and side slopes are encouraged for aquatic and visual diversity.

Straightening channels and decreasing their length is discouraged.

The cross sections used to define the channel and floodplain geometry for water surface profile computations shall be located upstream and downstream of hydraulic structures,


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at changes in bed slope or cross section shape, and generally at intervals of not more than ten times the width of the 100 year floodplain.

The friction coefficients used in the hydraulic analysis are to assume maximum seasonal vegetation conditions, and should be adjusted to the depth of flow.

6.6 Bridges

The hydraulic design of bridges shall be in accordance with Section 9 of the Connecticut Department of Transportation Drainage Manual.

6.7 Storage Facilities

The traditional design of storm drainage systems has been to collect and convey storm runoff as rapidly as possible to a suitable location where it can be discharged. As areas urbanize this type of design may result in major drainage and flooding problems downstream. The permanent or temporary storage of some of storm runoff can decrease downstream flows and often the cost of the downstream conveyance system. Urban stormwater storage facilities are often referred to as either detention or retention facilities. Detention facilities are those that are designed to reduce the peak discharge and only detain runoff for a short period of time. These facilities are designed to completely drain after the design storm has passed. Examples of stormwater management practices that can serve as detention facilities include surface basins; vegetated swales; subsurface chambers, pipes or reservoirs that discharge to a surface water or drainage system; and infiltration facilities. Retention facilities are designed to contain a permanent pool of water. Stormwater is temporarily stored above the normal water surface elevation during and immediately after runoff events. Examples of retention facilities include ponds, wetlands, galleries, and other underground storage. Storage facilities can be designed to provide both water quality and quantity benefits, including satisfying the requirements of Stormwater Management Standard 5 (Peak Flow Control) and Standard 6 (Pollutant Reduction). An analysis of storage facilities consists of comparing the pre-existing discharges with the proposed design flow at a point or points downstream of the proposed storage site with and without storage. In addition to the design flow, other flows in excess of the design flow that might be expected to pass through the storage facility shall be included in the analysis. The design of storage facilities shall consider:

Release rate,

Storage volume,

Grading and depth requirements,

Outlet works and location,

Evaluation of the increase of total volume of runoff on downstream facilities. Storage facilities shall be designed in accordance with the following general procedures:

1. Develop an in-flow hydrograph for the detention or retention facility for the design storm using the Modified Rational Method, U.S. Army Corps of Engineers HEC


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computer modeling methods, the NRCS Tabular Hydrograph Method, or the NRCS Unit Hydrograph Method.

2. Determine the allowable peak discharge rate. This shall not be greater than the undeveloped peak discharge rate. Peak discharge rates shall be calculated using the point of discharge at the downgradient property boundary, prior to discharge to the receiving water body. The topography of the site may require evaluation at more than one location if flow leaves the property in more than one direction. Calculations shall include runoff from adjacent upgradient properties. A proponent may demonstrate that a feature beyond the property boundary is more appropriate as a design point.

3. Compute the storage/stage curve for the proposed storage facility indicating the volume of storage at various water surface elevations.

4. Select a specific outlet control structure, and compute the outflow discharge rates for various water surface elevations to produce an outflow/storage curve.

5. The time interval selected shall be no greater than 5 minutes for a time of concentration less than one hour and no less than 10 minutes for a time of concentration equal to or greater than one hour.

6. Route the design in-flow hydrograph through the storage facility using the Storage-Indication Working Curve Method or Modified Puls Method using standard flood routing equations.

7. The maximum outflow shall not exceed the allowable discharge rate and the freeboard at peak water surface elevation shall not be less than one foot.

8. Once the storage facility has been designed to offset peak runoff from the design frequency storm, storms of other durations and frequencies should be checked to ensure that downstream flooding does not occur for these storm events.

9. The Stormwater Management Report shall document the method of generating the in-flow hydrograph and all assumptions for the routing. The routing curves and tables shall be included. The routing output shall provide, but not be limited to, the time, in-flow rate, out-flow rate, and the maximum storage volume.

Detailed procedures for the design of stormwater detention and retention facilities are described in Section 10 of the Connecticut Department of Transportation Drainage Manual. As discussed in Section 5.6, the peak discharge stormwater management standard may be waived, at the discretion of the approving authority, for sites that discharge to a large river, lake, estuary, tidal waters, or land subject to coastal storm flows, as described in the Connecticut Stormwater Quality Manual. When detention is proposed, a downstream hydrologic analysis may be required by the approving authority to determine whether peak flows, velocities, and hydraulic effects are attenuated by controlling the 2-year, 5-year, 10-year, and 25-year, 24-hour design storms. Analysis of larger design storms may be required by the approving authority for large developments and special or sensitive situations. This analysis must be performed at the outlet(s) of the site and at critical downstream locations (stream confluences, culverts, other channel constrictions, and flood-prone areas) to a confluence point where the site drainage area represents 10% of the total drainage area above that point. When detention is not proposed, a downstream hydrologic analysis must be performed, as described above.


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6.8 Erosion and Sedimentation

Control

Projects to be reviewed by the Town of Greenwich for drainage design purposes shall include methods to minimize the harmful effects of soil erosion and sedimentation during construction. The proposed sedimentation and erosion control measures shall be included with the Stormwater Management Report (see Section 7). Erosion and sedimentation control measures shall be designed in accordance with the Connecticut Guidelines for Soil Erosion and Sediment Control (as amended). All proposed developments, regardless of the area of proposed disturbance, must implement erosion and sedimentation controls prior to and throughout the duration of construction.

6.9 Structural Design

Table 6-12 summarizes design considerations/limitations, cover requirements, and installation guidelines for various types of storm drainage pipe within the Town of Greenwich.

6.9.1 Reinforced Concrete Pipe

Storm drainage pipe and culvert installations shall use reinforced concrete pipe (RCP). The strength required for concrete pipe shall be determined by its size; the height, character and weight of fill over the pipe; the line load; the character of the foundation; the depth and width of trench in which the pipe is installed and the method of bedding and installation. The minimum strength pipe that will be permitted shall be ASTM C-76, Class IV, except that no less than Class V shall be used in a saltwater environment and as directed by the Town. When the vertical distance from top of pipe to the finished grade of the road is 2 feet or less, the pipe shall be a minimum of Class V and encased in 1:3:5 concrete with 6 inches minimum concrete thickness extending no less than 3 to 6 inches below finished road grades.

6.9.2 Plastic Pipe

Plastic pipe, including high density polyethylene pipe (HDPE) and polyvinyl chloride plastic pipe (PVC), may be used for storm drainage systems and culverts with the approval of the DPW Engineering and Highway Divisions. The design engineer must request the use of plastic pipe during the Highway Permit review process. The use of plastic pipe may be considered for the following types of storm drainage applications:

Temporary installations,

Areas remote from the traveled portions of pavements,

Medians,

Parking lots, (where vehicular traffic is light to moderate and truck traffic is light),

Longitudinal installations on local and collector routes within the shoulder areas,

Slope drains,

Areas with little or no underground utility involvement,

Where parallel underground utility work is not likely in the foreseeable future.


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Buried plastic pipe is a composite structure made up of a plastic ring and the soil envelope. Both materials play a vital part in the structural integrity of the plastic pipe. The successful performance of plastic pipe depends upon proper bedding, backfill and care in installation. Minimum cover over plastic pipes shall be established by the engineer based on an evaluation of specific site conditions. In the absence of pipe strength calculations, the minimum cover above the pipe shall be at least 2 feet or one pipe diameter (whichever is larger). The minimum cover should be maintained before allowing vehicles or heavy construction equipment to traverse the pipe trench. One to 2 feet of cover is allowed if the pipe is encased in concrete.

Table 6-12. Storm Drainage Pipe Design and Installation Guidelines

Pipe Material Common Shapes Design

Considerations and

Limitations

Cover General Installation

Guidelines

Reinforced Concrete Pipe (RCP)

Circular, arch, horizontal & vertical ellipse

Minimum Class V in a saltwater environment or in unstable foundation conditions, as directed by the Town.

1’ min. – 15’ max. Strength analysis required for cover less than 2’ or greater than 15’. 2’ min. for Class IV RCP. Concrete-encased Class V RCP if less than 2’. No less than 1’ of cover for RCP.

Cross culverts are widely used for stormwater conveyance systems. Minimum 12” diameter regardless of material within Town ROW. Minimum Class IV RCP on public property and within Town ROW.

Corrugated Structural Plate Pipe (CSPP)

Circular, arch, horizontal & vertical ellipse, box culverts, pipe arch

Larger culverts field assembled with structural plate products.

Strength calculations should be considered for all cover depths.

Generally reserved for larger culverts and where structural considerations require thicker gauge metal.

Corrugated Polyethylene Pipe (CPP) or High Density Polyethylene (HDPE)

Circular, arch (only use corrugated pipe with smooth interior)

Proper bedding and controlled backfill is crucial particularly in adverse soil conditions.

2’ min. (or one pipe diameter) to 12’ max. Otherwise strength calculations are required. 1’-2’ cover if encased in concrete. No less than 1’ of cover.

Site stormwater conveyance systems, culverts and subsurface stormwater detention/retention systems. Maximum pipe diameter in the Town R.O.W. is 24". See RCP requirements for larger pipes. Water-tight joints required for all installations.

Polyvinyl Chloride (PVC)

Circular Proper bedding and controlled backfill is crucial particularly in adverse soil conditions.

Same requirements as HDPE.

Roof drain laterals, pressure and pumped systems. SDR-35 PVC is required, otherwise same guidelines as HDPE.

Other – large spans, bridges

Rectangular (box culvert), arch, flat top 3-sided (rigid frame), 3-sided arch

Requires structural design of footings and other integral components. A geotechnical investigation is generally required.

Strength calculations based on geotechnical findings.

Bottomless culverts, waterway crossings, bridge crossings.


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Maximum cover over plastic pipe should be limited to 12 feet (measured to top of pipe). Pipe strength calculations in accordance with AASHTO Standard Specification for Highways and Bridges, Section 18 (ASD) or AASHTO LRFD Bridge Design Specifications Section 12 are required for the following installations:

Installations subject to vehicle loads,

Fills greater than 12 feet (measured to top of pipe),

Fills less than 2 feet (measured to top of pipe) unless encased in concrete,

Adverse soil conditions,

High water table. Because plastic pipe is relatively lightweight, buoyancy forces, especially at the culvert inlet, may be a concern. Anchorage in the form of a headwall, slope paving or other stabilization methods may be necessary. Since proper bedding and backfill are vital to a successful installation, diligent construction and inspection is needed. Vibratory compaction of backfill can cause plastic pipe to shift and therefore appropriate measures and monitoring during installation are necessary. Normally, visual inspections are adequate to confirm the installation is acceptable. However, a mandrel test may be requested by the engineer when it is necessary to confirm the acceptability of an installation. When specifying plastic pipe, designers must consider loads from construction vehicles as well as those experienced during construction staging operations.


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7 Submittal Requirements

All land development applications in the Town of Greenwich subject to the Greenwich Stormwater Management Standards shall include the following information to demonstrate conformance with the standards, the design requirements contained in this manual, and related land use regulatory requirements:

1. Stormwater Management Report 2. Construction Plans 3. Supporting Documents and Studies 4. Operation and Maintenance Plan 5. Erosion and Sediment Control Plan 6. Plan & Report Revisions 7. Certifications

Specific requirements for each of the above items are summarized in the checklists provided in Appendix I and outlined below.

7.1 Stormwater Management Report

The Stormwater Management Report shall describe how the proposed project has addressed the following LID and stormwater management elements:

Project Narrative: o Project description and purpose o Site description, including a description of on-site and off-site resources o Proposed non-structural BMPs (source controls and LID site planning and design

measures) o Proposed structural BMPs o How the proposed development complies with the Stormwater Management

Standards o Soil evaluation o Site plans o Construction schedule

Stormwater Management Standards Narrative

Credits for LID BMP’s

Comparison Table for Pre-&-Post Development Peak Flow, Volume, and Percent Difference

Identify Applicable Land Use Regulations

Site Inventory & Evaluation o Topography o Soil Evaluation (Soil Evaluation Test Results (Form SC-101) Shall Be Used)

Initial Feasibility Evaluation (NRCS Web Soil Survey and similar sources of information)

Concept Design Testing (test pits/borings and saturated hydraulic conductivity testing, as per Appendix B)

o Hydrologic patterns and features


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o Generalized forest types (old field/shrub; early succession and mature hardwood) as determined in the field by a qualified professional

o Dominant deciduous and evergreen tree species by class and the density of the trees and canopy coverage. The average type of the forest cover shall be discussed.

o Discuss efforts made to protect wetland areas and riparian corridors (as applicable)

o Discuss efforts made to protect floodplains and water bodies (as applicable) o Discuss efforts made to protect natural drainageways

Define Development Envelope o Consider construction techniques o Determine and protect sensitive areas o Retain and protect mature trees o Minimize disturbance of steep slopes (over 25%) o Minimize clearing and grading for buildings o Access and fire prevention, and other construction activities, including

stockpiles and storage areas o Delineate preferred areas for infiltration (A & B soils) o Confine envelope to areas to be permanently altered

Develop LID Control Strategies o Evaluate Pre-Development Site Hydrology (Runoff Volume and Peak Flow

Rate – 1, 2, 5, 10, 25, 50 and 100-Year Storms)

Watershed Map Pre-Development

NRCS Runoff Curve Numbers Pre-Development

Time of Concentration Pre-Development o Minimize Total Site Impervious Area o Minimize Directly Connected Impervious Areas o Minimize Site Disturbance o Modify Drainage Flow Paths to Increase Time of Concentration o Evaluate Post-Development Site Hydrology (Runoff Volume and Peak Flow

Rate – 1, 2, 5, 10, 25, 50 and 100-Year Storms)

Watershed Map Post-Development

NRCS Runoff Curve Numbers Post-Development

Time of Concentration Post-Development o Compare & Summarize Pre-&-Post Development Site Hydrology to Evaluate

Benefits of Non-Structural Site Planning Techniques o Identify Remaining need for Structural BMPs to Satisfy:

Water Quality Volume/Flow

TSS Removal Computations

Runoff Reduction Volume: 1-Year Storm

Groundwater Recharge Volume (if necessary)

72-Hour Drawdown Computations

2-Year Over Control Channel Protection (Required on a Site by Site Basis)

Peak Runoff Attenuation: 2, 5, & 10-Year Storm (and 25-Year Storm for projects that do not rely solely on LID BMPs), 50-Year & 100-Year Peak Runoff Attenuation Required on a Site by Site Basis


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Remaining Structural BMPs to Meet Water Quality Volume/Flow (WQV)/(WQF) and TSS Removal

Remaining Structural BMP’s to Meet 80% TSS Removal

Remaining Structural BMPs to Meet Runoff Reduction Volume (RRV): 1-Year Storm

Remaining Structural BMPs to Meet Groundwater Recharge Volume (GRV), if necessary

Remaining Structural BMP’s to Meet 72-Hour Drawdown

Remaining Structural BMPs to Meet Stream Channel Protection: 2-Year Frequency (―Over-Control‖ of 2-Year Storm) – Required on Site By Site Basis

Remaining Structural BMPs to Meet Peak Runoff Attenuation: 2, 5, & 10 Year; 25-Year for Projects that Do Not Rely Solely on LID BMPs; 50-Year & 100-Year Required on Site by Site Basis

Conveyance Protection: 10, 25, 50 & 100-Year Depending on Peak Flow Rate for Which Downstream Stormwater Facilities are Designed

Emergency Outlet Sizing: Safely Pass the 100-Year

Culvert Capacity Calculations

Outlet Protection Calculations – Based on Conveyance Protection

Downstream Analysis – Required on Site by Site Basis and When No Detention Proposed

Town Storm Drain Analysis – Required on Site by Site Basis

Gutter Flow Calculations – Required on Site by Site Basis

Supporting Documents

Sealed and Signed By a Professional Engineer A Stormwater Management Report checklist is included in Appendix I.

7.2 Construction Plans

7.2.1 Plan Set Standards

General

All plans shall be black and white

Submittals shall be a bound (four staples on bound edge), full set of plans – no individual sheets

Required Size (no larger than 36‖x48‖ and no smaller than 24‖x36‖)

Required scale (maximum scale of 1‖ = 40’, larger scales up to 1‖ = 100’ may be used to depict overall site development plans or conceptual plans)

Certifications: o Surveys – Licensed Land Surveyor o Development Plans – Professional Engineer o Wetland Location – Certified Soil Scientist


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Cover Sheet with Sheet Index

Title block – Title shall include submittal type from Site Development Review Request

Legend

North arrow

Property boundary of subject property (including parcels, or portions thereof, of abutting land and roadways within one hundred feet of the property boundary)

Site locus map (recommended scale 1‖ = 1,000’) with north arrow

Survey Plan (Existing Conditions Survey)

Prepared according to the Minimum Standards for Surveys and maps in Connecticut

The class of survey shall be A-2 and T-2 and represented on the plan

Depict topography at contour intervals of two feet for the property and 10 feet beyond the property limits, if possible. Depict topography at a minimum 100 feet beyond the limits of the subject property using available GIS data.

Topography flatter than 2% requires additional spot elevations and contour intervals of one foot

Spot elevations

The referenced or assumed elevation datum (the FEMA datum shall be used for sites located within a Flood Hazard Zone)

One (1) permanent benchmark on the site within one hundred feet of the proposed construction

Plan references

Shall include the entire Town of Greenwich Right-of-Way for the property frontage (drainage, curbs, sidewalk, trees, walls, contours, etc.)

If a new curb cut is required, then the entire Town of Greenwich Right-of-Way is required in both directions for the minimum sight distance

Storm drainage, sewer, water, etc.

Roads, buildings, driveways, patios, walks, walls and other structures

Utilities and easements

Sealed and signed by a Professional Land Surveyor

Low Impact Development and Soil Tests Plan

Depict existing and proposed topography at contour intervals of two feet for the site and at a minimum 100 feet beyond the limits of the subject property


All slopes (existing and proposed) greater than 25% (4H:1V slope) as measured over a minimum distance of fifty (50) feet

Depict the site’s soil type and associated Hydrologic Soil Groups (HSG)

Wetland soils and watercourses (both intermittent and permanent), delineated/flagged wetland areas, riparian buffer areas (as applicable)

100-year flood boundaries (as taken from flood hazard mapping prepared by the Federal Emergency Management Agency for the Town of Greenwich). If necessary,


February 2014

the limits of 100-year flood boundaries shall be verified in the field by a licensed land surveyor.

Deep test pits and hydraulic conductivity tests (include circular influence zone of each test)

Structural and non-structural (e.g., source controls) BMPs

Delineated roof areas with a callout specifying which BMP receives runoff

Include areas of amended soils

Include areas of disconnected roofs

Sealed and signed by a Professional Engineer

Site Plan (Use multiple plan sheets to keep legible)

Depict existing and proposed topography at contour intervals of two feet for the site and at a minimum 100 feet beyond the limits of the subject property


Spot elevations

Storm drainage, sewer, water, etc.

Locations of stormwater discharges

Wetlands, perennial and intermittent streams

Deep test and infiltration test locations

Vegetation and proposed limits of clearing and disturbance

Resource protection areas such as wetlands, lakes, ponds, and other setbacks (stream buffers, drinking water well setbacks, septic setbacks, etc.)

Roads, buildings, driveways, patios, walks, walls and other structures

Utilities and easements

Temporary and permanent conveyance systems (grass channels, swales, ditches, storm drains, etc.) building grades, dimensions, and direction of flow

Location of floodplain and floodway limits and relationship of site to upstream and downstream properties and drainage systems

Location, size, maintenance access, and limits of disturbance of proposed structural stormwater management practices (treatment practices, flood control facilities, stormwater diversion structures, etc.)

Final landscaping plans for structural stormwater management practices and site revegetation (if no structural items are proposed, this portion of the plan may be signed by a licensed landscape architect or other environmental professional)

Locations of non-structural stormwater management practices (i.e., source controls)

Excavation and fill quantities in a table – Note: An Excavation and Fill Permit from the Engineering Division is required for an excavation or fill of 500 cubic yards or greater for all projects not submitted to Planning & Zoning, IWWA, or the Building Division for approval.



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Driveway Profile & Sight Distance Plan

Sight distance for existing driveway

Sight distance for proposed driveway

Driveway dimensions for remaining and proposed driveways o Width at property line o Width at roadway o Distance to intersection o Distance between driveways o Distance from property line to driveway o Distance from edge of road to proposed driveway gate (minimum 25 feet)

Proposed and remaining modified driveway profile from edge of road to garage o Slope of Proposed Driveway for first five feet on Profile (+3% to 6% include on

profile) o Slope of Proposed Driveway for next twenty feet on Profile (Maximum Slope of 4%

when remaining slope ≥ 10%, include on profile) o Slope of Proposed Driveway for the remaining distance to Garage on Profile

(Maximum Slope of 8% for Commercial, 12% Residential (Two or More Family), and 15% for Residential (include on profile))


Turning Movement Plan

Turning movements for SU-30, SU-50, School Bus, and Fire Apparatus


Traffic Signage, Pavement Markings, and Parking Space Layout Plan

Include all traffic signs

Include all pavement markings (stop bar, arrows, etc.)

Include all parking space pavement markings

Include all parking space and travel lane dimensions


Erosion and Sediment Controls Plan

The Erosion and Sediment Control plan shall comply with the requirements of the current version of the Connecticut Guidelines for Soil Erosion and Sediment Control

Site plan showing controls

Construction fence delineating the limit of disturbance

Construction fence delineating areas not to be disturbed

Construction fence delineating areas of BMP’s to be protected from compaction

Construction phasing and erosion & sediment controls sequencing plan

E&S Details

Computations if required

Operations and maintenance of erosion & sediment controls



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Construction Details

Catch basins, manholes, chambers, control structures, etc.

Structural BMP’s (bioretention, drywells, infiltrators, permeable pavement, dry/wet swales, wet pond, etc.)

Trench section

Retaining wall cross-section

Curbs, sidewalks, driveway entrance, etc.

Pavement cross-section

Road cross-section, profile, etc.

Pipe cross-section, profile, etc.

Special details as needed for a particular project

Town of Greenwich – DPW Engineering Division Standard Notes (included in Appendix J)– Scan of notes is required

Town of Greenwich Standard Construction Detail Sheets

Building/House Section or Elevation Plan

Include one section or elevation of the building/house

Include all elevations to the deepest footings

Include existing and proposed grade elevation

Include existing mottling elevation

Include existing groundwater elevation

Include existing ledge elevation

Sealed and signed by a Professional Engineer A Construction Plans checklist is included in Appendix I.

7.3 Supporting Documents and

Studies

Other supporting documentation should be provided, including but not limited to the following types of information:

Soil textural analysis (soil maps, borings, and test pits)

Soil boring and test pit results

Saturated hydraulic conductivity test results

Groundwater mounding analysis

Information on wetlands (function and values) and surface waters (impairments and Total Maximum Daily Loads)

Flood studies


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7.4 Operation and Maintenance Plan

Stormwater Management Standard 12 requires that a long-term Operation and Maintenance (O&M) Plan be developed and implemented to ensure that stormwater management systems function as designed. At a minimum, the O&M Plan shall identify:

Stormwater management system(s) owners15

The party or parties responsible for operation and maintenance including how future property owners will be notified of the presence of the stormwater management system and the requirement for proper operation and maintenance

The routine and non-routine maintenance tasks to be undertaken after construction is complete and a schedule for implementing those tasks

Log form for recording operation and maintenance activities

Estimated operations and maintenance budget

The maintenance declaration in place

Plan that is drawn to scale and shows the location of all stormwater BMPs in each treatment train along with the discharge point

Sealed and signed by a Professional Engineer An Operations and Maintenance Plan checklist is included in Appendix I. The project proponent shall also provide a legal mechanism for implementing and enforcing the O&M Plan (i.e., stormwater maintenance declaration), which shall be filed on the Town of Greenwich Land Records. A copy of the stormwater maintenance declaration is provided in Appendix H. The maintenance declaration shall reference as an attachment of those activities that must be carried out to maintain compliance with the declaration (Exhibit A)., and a map showing the location of each stormwater management practice affected by the declaration (Exhibit B), Exhibits A and B of the declaration shall be prepared by the applicant’s engineer. In the event that the stormwater BMPs will be operated and maintained by an entity, municipality, state agency or person other than the sole owner of the lot upon which the stormwater management facilities are placed, the proponent shall provide a plan and easement deed that provides a right of access for the legal entity to be able to perform said operation and maintenance functions, including inspections.

7.5 Erosion and Sediment Control

Plan

Land development projects in the Town of Greenwich shall include methods to minimize the harmful effects of soil erosion and sedimentation during construction. The land development application submittal shall include an Erosion and Sediment Control Plan that describes the proposed sedimentation and erosion control measures. Erosion and sediment control measures shall be designed in accordance with the Connecticut Guidelines for Soil Erosion and Sediment

15 The stormwater management system owner is generally considered to be the landowner of the property, unless

other legally binding agreements are established.


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Control, as amended. All proposed developments, regardless of the area of proposed disturbance, must implement erosion and sediment controls prior to and throughout the duration of construction. Erosion and Sediment Control Plans shall be signed and sealed by a Professional Engineer licensed in the State of Connecticut.

7.6 Plan and Report Revisions

7.6.1 Plans

All plans must include the revision date and a CAD revision symbol on all changes. Each review period shall start with a clean slate and no revisions (e.g., Preliminary Site Plan Approval, Final Site Plan Approval, Construction Site Plan Approval, etc.). The approved Construction Site Plan with revisions must be submitted to the Building Division and future revisions updated as necessary. A letter shall be submitted with each revised set of plans and it shall discuss all revisions.

7.6.2 Reports

All reports must include the revision date and a symbol, special text, or other identifying formatting on all changes. Each review period shall start with a clean slate and no revisions (e.g., Stormwater Management Report Part One, Stormwater Management Report Part Two, Preliminary Operation & Maintenance Plan Report, Final Operation & Maintenance Plan Report, etc.). A letter shall be submitted with each revised report and it shall discuss all revisions.

7.7 Certifications

The following signed certifications are required to be submitted for all land development projects that are subject to the Greenwich Stormwater Management Standards prior to the issuance of a Building Permit:

Stormwater Management Standards – Drainage Report Exemption (as applicable)

Soil Evaluation Test Results

Sight Distance Certification

Engineer of Record

The following signed certifications are required to be submitted for all land development projects that are subject to the Greenwich Stormwater Management Standards prior to the issuance of a Certificate of Occupancy:

Site Inspection Certification

Drainage Certification

Field Inspection Record

Bioretention Certification (as applicable)

Copies of the above certification forms are provided in Appendix K.


February 2014

8 References

American Association of State and Highway Transportation Officials. 2002. Standard Specifications for Highway Bridges. Washington, D.C. American Association of State and Highway Transportation Officials. 2007. AASHTO LRFDBridge Design Specifications. Washington, D.C. Center for Watershed Protection. 2003. Impacts of Impervious Cover on Aquatic Systems. Ellicott City, Maryland. Center for Watershed Protection. 2007. Urban Subwatershed Restoration Manual No. 3, Urban Stormwater Retrofit Practices, Version 1.0. Ellicott City, Maryland. Center for Watershed Protection and Chesapeake Stormwater Network. 2008. Technical Memorandum: The Runoff Reduction Method. Connecticut Department of Energy and Environmental Protection. 2004. Connecticut Stormwater Quality Manual. Hartford, Connecticut. Connecticut Department of Energy and Environmental Protection. 2007. A Total Maximum Daily Load Analysis for Eagleville Brook, Mansfield, CT. Hartford, Connecticut. Connecticut Department of Energy and Environmental Protection. 2008. Connecticut Department of Energy and Environmental Protection Stream Crossing Guidelines. Connecticut Department of Energy and Environmental Protection, Inland Fisheries Division, Habitat Conservation and Enhancement Program. Hartford, Connecticut. Connecticut Department of Energy and Environmental Protection. 2008. 2008 State of Connecticut Integrated Water Quality Report. Connecticut Department of Energy and Environmental Protection, Bureau of Water Protection and Land Reuse. Hartford, Connecticut. Connecticut Department of Energy and Environmental Protection and Connecticut Council on Soil and Water Conservation. 2002. 2002 Connecticut Guidelines for Soil Erosion and Sediment Control. Hartford, Connecticut. Connecticut Department of Transportation. 2000. Connecticut Department of Transportation Drainage Manual. Newington, Connecticut. Connecticut Department of Transportation. 2000. Connecticut Department of Transportation Highway Design Manual. Newington, Connecticut. Connecticut LID Inventory, Nonpoint Education for Municipal Officials, Website. Greenwich Municipal Code. Chapter 6. Land Use. Greenwich, Connecticut.


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Hirschman D. and Kosco, J. 2008. Managing Stormwater in Your Community: A Guide for Building an Effective Post-Construction Program (EPA Publication No: 833-R-08-001). Center for Watershed Protection. Ellicott City, Maryland. Low Impact Development Hydrologic Analysis. 1999. Prince George’s County, Maryland Department of Environmental Resources, Program and Planning Divisions. Largo, Maryland. Low Impact Development Strategies: An Integrated Design Approach. 1999. Prince George’s County, Maryland Department of Environmental Resources, Program and Planning Divisions. Largo, Maryland. Low Impact Development – Technical Guidance Manual for Puget Sound (PSAT 05-03). 2005. Puget Sound Action Team, Office of the Governor. Olympia, Washington. Massachusetts Department of Environmental Protection. 2008. Massachusetts Stormwater Handbook. Boston, Massachusetts. Massachusetts Low Impact Development Toolkit, Metropolitan Area Planning Council, Website. Massachusetts Smart Growth/Smart Energy Toolkit, Massachusetts Executive Office of Energy and Environmental Affairs, Web. New Hampshire Department of Environmental Services. 2008. New Hampshire Stormwater Manual. New Hampshire Department of Environmental Services, Watershed Assistance Section. Concord, New Hampshire. New Jersey Department of Environmental Protection. 2009. New Jersey Stormwater Best Management Practices Manual. New Jersey Department of Environmental Protection, Division of Watershed Management. Trenton, New Jersey. Rawls, Brakensiek and Saxton. 1982. Estimation of Soil Water Properties, Transactions American Society of Agricultural Engineers 25(5): 1316 - 1320, 1328. Schueler, T. 2000. The Importance of Imperviousness, Article 1 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, Maryland. Schueler T. In review. Implications of the Impervious Cover Model: Stream Classification, Urban Subwatershed Management and Permitting. Chesapeake Stormwater Network Technical Bulletin No. 3. Southeast Michigan Council of Governments. 2008. Low Impact Development Manual for Michigan: A Design Guide for Implementers and Reviewers. Detroit, Michigan. University of New Hampshire Stormwater Center, Website.


February 2014

U.S. Department of Commerce, U.S. Weather Bureau. 1961. Rainfall Frequency Atlas of the United States, Technical Paper No. 40. Washington, D.C. U.S. Environmental Protection Agency. 2008. Managing Wet Weather with Green Infrastructure Municipal Handbook. Washington, D.C. U.S. Department of Transportation, Federal Highway Administration. 2006. Hydraulic Engineering Circular No. 14, Third Edition, Hydraulic Design of Energy Dissipators for Culverts and Channels (FHWA-NHI-06-086). Arlington, Virginia.


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Appendix A

Water Quality – Town of Greenwich


February 2014

Appendix B

Stormwater Infiltration/Recharge Requirements


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Appendix C

Credits for Low Impact Development

Best Management Practices


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Appendix D

Suggested Sources of Information

on the Effectiveness

of Proprietary Stormwater BMPs


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Appendix E

Recommended Process for Evaluating

the Proposed Use

of Proprietary Stormwater BMPs


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Appendix F

TSS Removal Efficiency Calculations


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Appendix G

Design Guidance for Structural Stormwater BMPs


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Appendix H

Stormwater Maintenance

Declaration and Easement


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Appendix I

Checklists


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Appendix J

Standard Notes


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Appendix K

Certification Forms


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Appendix L

24-Hour Design Storm Rainfall Amounts


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Appendix M

Glossary