rhode island stormwater design and installation standards...

Manual Overview and Minimum Standards

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LID for Linear Transportation Projects: RI Stormwater Manual 1

Rhode Island Stormwater Design and Installation

Standards Manual

Horsley Witten Group, Inc.Horsley Witten Group, Inc.

RIDOT WorkshopManual Overview and Stormwater Minimum

Standards and Performance Criteria

August 25, 2011

1993 Manual • Peak rate attenuation (2 & 25 year storms, often

100 yr as well);• Proper conveyance of 10-yr storm;• 80% TSS Removal Rate with 1” / impervious area;• Additional controls for impaired waters, drinking


supply reservoirs, ONRWs, etc.• Acceptable Water Quality BMPs

– Wet Ponds;– Extended Detention Ponds;– Infiltration Practices;– Pretreatment Devices;– Grassed Swales with higher infiltration soils;– Vegetative Filter Strips as last resort.

Why was the manual updated?

• Technical/scientific advances since 1993;• Earlier methods/techniques fell short in

adequately protecting water resource quality;• 80% TSS removal requirement has not prevented

resources from degrading;


• To address hydrologic alteration of new development (runoff reduction and recharge); and

• Widely recognized that a more holistic management approach is necessary.


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Stormwater Impacts

At < 10% impervious we begin to see: • Water quality issues• Impacts to biological

communities


• Increased flooding• Stream erosion• Loss of recreational uses• Shellfish bed closures• Reduced baseflow and

recharge


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Horsley Witten Group, Inc.Horsley Witten Group, Inc.Photo Copyright 1999, Center for Watershed Protection



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Evaporation

Precipitation

Transpiration

Evaporation

EvaporationEvaporation


Groundwater

GroundwaterGroundwater

Soil MoistureSoil MoistureInfiltrationInfiltration Surface StorageSurface Storage

Surface WatersSurface Waters

Evaporation

Direct RunoffDirect Runoff


Most significant changes since the 1993 manual?

• Low impact development (LID) required to the MEP;• Recharge criteria to infiltrate runoff from up to

0.6″/Imp acre;• Revised design precipitation rates based on latest

rainfall data;• Expanded water quality pollutant removal targets


• Expanded water quality pollutant removal targets (85% TSS, 60% Pathogen, 30% TP, 30% TN);

• Changed the type of practices acceptable for water quality treatment (extended detention, wet basins,and filter strips no longer acceptable as stand-alone practices), flow splitters now required for some practices;


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Continued

• Changes to infiltration practice application – must fully pre-treat for direct discharge at a high rate, limitations on fill applications, significant dewatering requires water budget analysis;

• Special design requirements for discharges in cold-water fisheries;


• Extended detention of the 1-year storm required;• Changed how sediment load is addressed;• Pollutant loading approach and analysis updated

(when req’d); and • New approach and criteria for redevelopment and

infill projects.

Minimum Standard 1: LID Site Planning and Design Strategies

Objective - to provide a process by which LID is considered at an early stage in the planning process to prevent stormwater impacts rather than mitigate them.

LID site planning and design strategies must be used to


• LID site planning and design strategies must be used to the maximum extent practicable.

• Projects must include a completed Stormwater Management Checklist (Appendix A) for review that shows compliance with this standard.

• If full compliance is not provided, an applicant must document why key steps in the process could not be met and what is proposed as mitigation.


MANAGEMANAGE


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Linear Project Considerations

• Is LID Site Planning and Design Criteria relevant to linear projects?– There are certain constraints based on

limited space and ensuring public safety– However, most of the criteria are doable for


,many linear projects!

LID Site Planning and Design Criteria

A Protect undisturbed open space; A Maximize the protection of natural drainage areas,

streams, surface waters, wetlands, and buffers; A Minimize land disturbance, locate disturbances in less

sensitive areas;A/R Minimize the decrease in the "time of concentration"

f t ti t t t ti


from pre-construction to post-construction;A/R Minimize soil compaction; R Minimize impervious surfaces; M Provide vegetated conveyance and treatment systems; M Provide low-maintenance landscaping; M Break up or disconnect runoff over impervious surfaces; M Provide source controls to prevent / minimize the

release of pollutants into stormwater runoff.

Minimum Standard 2: Groundwater Recharge

Objective - to protect water table levels, stream baseflow, wetlands, and soil moisture levels.

• Stormwater must be recharged within the same subwatershed to maintain baseflow at predevelopment levels to the maximum extent practicable. Recharge may be achieved by both structural and


• Recharge may be achieved by both structural and nonstructural practices (Stormwater Credit)


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Recharge Criteria, Rev

Requires that the following volume of stormwater be recharged:

Rev = (1”) (F) (I) / 12

where: Rev = recharge volume (in acre-feet)


v g ( )I = impervious area in acres

HSG Recharge Factor (F)

A 0.60 B 0.35C 0.25D 0.10


• Linear projects may cross subwatershed boundaries – must recharge accordingly

• Can be difficult in areas with high groundwater• Specific waivers may be allowed if:

– Physical limitation that would make implementation


Physical limitation that would make implementation impracticable

– Unusual geological or soil features exist where recharge does not currently occur (clay deposits or ledge)

– Fill soils– Areas of documented slope failure.

Minimum Standard 3: Water QualityObjective - to reduce the water quality impacts from stormwater on downstream waters.

• Stormwater runoff from a site must be adequately treated to achieve the following minimum pollutant removal requirements at each discharge location: – 85% of total suspended solids (TSS),

60% removal of pathogens and


– 60% removal of pathogens, and – either 30% removal of TP for discharges to sensitive freshwater

systems, or 30% removal of TN for discharges to saltwater systems.

• May need to achieve higher pollutant removal efficiencies for other conditions such as: impaired waters, SAMPs, or TMDLs.

• Structural BMPs in Chapter Five can be assumed to meet these standards when designed for all 11 Standards.


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Water Quality Criteria, WQv

The WQv is calculated using the following equation:

WQv =(1”) (I) / 12

where: WQv = water quality volume (in acre-feet)

I = impervious area in acres


I = impervious area in acres

• Non-structural practices can be used to reduce treatment volume (Stormwater Credit);

• Pretreatment is required as a percentage of WQv –varies based on BMP.

• Often, calculation of a Water Quality Flow rate (WQf) is also required.

BMP Pretreatment Requirements

BMP Group Required %WQv

Notes

WVTS 10% • Provided at each inlet, unless inlet provides <10% of inflow

I filt ti 25%

• Grass channel, filter strip, sediment forebay, proprietary device

• Deep sump catch basin combined with one of the following:


Infiltration 25% following: – Upper sand layer; or – Washed pea gravel (1/8” to 3/8”)

• Not required for permeable pavements (unless there is “run-on”) or drywells

Filtering Practices 25% • Deep sump catch basins may not be used as sole

pretreatment. Green Roofs Not Applicable. No pretreatment required for direct rainfall.

Open Channels 10%

• forebays/checkdams at pipe inlets and/or driveway crossings.

• filter strip

WQf vs. WQv

• Standard 3 is volume-based. Why would a corresponding WQf be necessary? – To ensure that the full WQv reaches an off-line BMP– Some proprietary devices are flow-based

A Modified Curve Number (CN) is needed:


• A Modified Curve Number (CN) is needed:– Traditional TR-55 CNs can lead to significantly

underestimated runoff from small storm events– Used to calculate WQf

– Used to correctly model the WQ event


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To Water Quality

Flow Diversion Structure


Q yFacility

Bypass Pipe


• Need to be creative with limited space and/or unique geometry– Shallow WVTS require a lot of space, but can be

perfect at large interchanges– Most other practices can be designed to fit the

geometry of the project


geometry of the project


• Acceptable pretreatment included in Chapter 6:– Grass channels– Filter strips– Sediment forebays


Sediment forebays– Deep sump catch basins*– Proprietary devices

* Only catch basins designed per Section 6.5 meet the pretreatment requirement.


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* Must be catch-basin-to-manhole configuration to count as pretreatment

Minimum Standard 4: Conveyance and Channel Protection

Objective - to prevent erosive flow within natural channels and drainage ways.

• Conveyance systems must be designed for the 10-year, 24-hour design storm event.


• Channel protection must be supplied by providing 24-hour ED for the one-year24-hour design storm event.

• If a stormwater discharge is to a coldwater fish habitat, surface detention practices have additional restrictions.

Cpv Waivers

Cpv not required for:• discharges to a large river (i.e., 4th-order stream),

surface water body > 50 acres (lake, pond reservoirs), estuary, or tidal waters.

• small sites with impervious cover ≤ 1 acre • Sites where peak flow < 2 cfs


Sites where peak flow 2 cfs


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Minimum Standard 5: Overbank Flood Protection

Objective - to prevent an increase in the frequency and magnitude of overbank flooding and to protect downstream structures from flooding.

• Downstream overbank flood protection must be provided by controlling the post development peak discharge rate to the predevelopment rate for the 10-


discharge rate to the predevelopment rate for the 10year and 100-year, 24-hour design storm events.

• The Qp criteria can be waived for sites that:– Discharge to a large river (i.e., 4th order stream),

surface water body > 50 acres (lake, pond reservoirs), estuary, or tidal waters.

– A downstream analysis indicates that peak discharge control is not necessary.

Downstream Analysis• Required for projects of certain size AND impervious

cover (see table)• Determine if peak flow impacts attenuated by

controlling the 10- and 100-year events • “10% rule” used for the limit of the downstream

analysis


• Analyzed per subwatershed

Area of Dist.(acres)

Impervious Cover (%)

>5 to10 >75>10 to 25 >50>25 to 50 >25

>50 all projects

Minimum Standard 6: Redevelopment and Infill

Objective - to allow flexibility to meet the goals of improved water quality and channel protection to receiving waters while still promoting redevelopment and infill development.

• Redev. is any construction, alteration, or improvement that disturbs a total of 10 000 sf or more of existing impervious


disturbs a total of 10,000 sf or more of existing impervious area

• Redev. sites with < 40% impervious coverage must meet new development criteria. For sites ≥ 40%, requirements are based on 50% improvement.

• In no case shall levels of treatment and recharge be less than the levels prior to initiation of proposed project!


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Redevelopment Options• Reduce existing impervious area by at least 50%; or• Use LID techniques for at least 50% of area; or• Use BMPs (Chapt. 3) to provide recharge and water quality

management for at least 50% of area; or• Any combination of impervious area reduction, other LID

techniques, or BMPs for at least 50% of area.


q ,• If none of the above are feasible, alternatives may be

proposed that achieve an equiv. pollutant reduction (e.g., treating more of redev. area by BMPs with lesser pollutant removal efficiency than stipulated in Standard 3).

*Off-site management MAY be allowed at an approved off-site location if all of the above have been used to the MEP.

Infill DevelopmentA development site that meets all of the following:

– Predominately pervious (<10,000 sq ft);– Surrounded by existing development;– Served by an existing network of infrastructure (no

utility line extensions); and– Site is ≤ 1 acre


– Site is ≤ 1 acre.

• Requirements for Stds 2 and 3 same as new development except can comply with either on-site or at approved offsite location within same watershed.

• Must demonstrate compliance with Std 1 to MEP.

Linear Project Considerations• What constitutes redevelopment?

– Removal of roadway materials down to erodible soil surface; simply resurfacing roadway surface is not. Incidental excavation/patching for non-roadway projects is not.

• What constitutes redevelopment site?


– Project limits

• What constitutes redevelopment area?– Disturbed impervious area

• Are Infill projects likely?– Parking lots? – Any associated buildings?


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Linear Project ConsiderationsWhat about existing detention basins?• Do not provide recharge, but may be used

to meet WQv in redevelopment cases if:1. Achieve Equivalent Pollutant Reduction

− Standard 3 requires 85% TSS, 30% TP/TN, 60% Bacteria.


TP/TN, 60% Bacteria. − Under Redev., only need to treat 50%

of total load. Thus, 43% TSS, 15% TP/TN, 30% Bacteria.

− Dry Basin for 100% of site – 50% TSS, 20% TP/TN, 35% Bacteria (Table H-4).

2. Designed to meet extra criteria on pg 7.4.


• Many road projects may be a mix of redevelopment and new development

• Recharge and WQv

– 50% management for existing impervious100% g t f i i


– 100% management for new impervious

• LID, Cpv, and Qp

– Only for new impervious– Qp compares proposed to existing conditions,

not natural conditions

Minimum Standard 7: Pollution Prevention

Objective - to prevent, to the maximum extent practicable, pollution from entering water resources.

• All development projects must implement source control and pollutant prevention measures outlined in a stormwater pollution prevention plan (SWPPP).



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SW Manual SWPPP vs. CGP SWPPP??

• The RIPDES CGP requires the site owner (≥1 acre disturbance) to develop a SWPPP as a stand-alone document which must be kept on-site during the active construction phase of the project.


of the project. • DEM has modified the Appendix A Checklist

to include a list of elements needed to create a SWPPP that complies with both the Stormwater Manual AND the RIPDES CGP.

Minimum Standard 8: Land Uses with Higher Potential Pollutant Loads (LUHPPLs)Objective - to prevent, to the maximum extent practicable, pollution from entering water resources.

• Stormwater discharges from LUHPPLs require the use of specific source control/pollution prevention measures and BMPs approved for such use.


o Industrial sites subject to RIPDES Multi-Sector General Permit o Auto fueling facilitieso Exterior vehicle service, maintenance, and equipment cleaning

areas o Road salt storage and loading areas (if exposed to rainfall)o Outdoor storage and loading/unloading of hazardous materials

Linear Project Considerations• What about off-site LUHPPLs that drain to roadway

system where redevelopment is proposed?


1. Propose to manage portion of project that does NOT include LUHPPL runoff


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2. Work with LUHPPL landowner to treat runoff on-site

- Pollution prevention techniques


- Stormwater BMPs suitable for LUHPPL

*Benefits – more cost-effective to deal with LUHPPL runoff atsource.

3. Treat LUHPPL runoff in ROW as close as possible to source


Other Options?

4. If you cannot treat the LUHPPL close to the source and you cannot keep the LUHPPL runoff out of your proposed BMP(s), then that BMP must meet LUHPPL requirements.


LUHPPL requirements.


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Minimum Standard 9: Illicit Discharges

Objective - to prevent pollutants from being discharged into MS4s and Waters of the State, and to safeguard the environment and public health, safety, and welfare.

• All illicit discharges to stormwater management systems are prohibited, including discharges from onsite wastewater treatment systems (OWTS) and sub drains


wastewater treatment systems (OWTS), and sub-drains and French drains near OWTSs that do not meet the State’s OWTS Rules.


• Illegal discharges to road drainage system:– Residential pipes– Any non-stormwater, non-

permitted discharge to the


permitted discharge to the system

Minimum Standard 10: Construction Erosion and Sedimentation Control

Objective - to prevent erosion and sedimentation from construction site runoff.

• Erosion and sedimentation control (ESC) practices must be utilized during the construction phase as well as during any


utilized during the construction phase as well as during any land disturbing activities. – temporary sediment trapping practices must be sized for 1

inch of runoff, and – temporary conveyance practices must be sized to handle

flow from the 10-year, 24-hour design storm. • ESC practices must be designed according to the guidelines in

the most recent edition of the “Rhode Island Erosion and Sediment Control Handbook.”


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Minimum Standard 11: Stormwater Management System Operation & Maintenance

Objective - to ensure that stormwater BMPs continue to function as designed.

The stormwater management system must have an O&M Plan that shall at a minimum include:

• Stormwater management system(s) owners;


• The party(ies) responsible for operation and maintenance;• Routine/non-routine maintenance tasks and a schedule;• Plan showing location of all stormwater BMPs and discharge

points;• Description/delineation of public safety

features; • An estimated budget; and• The funding source.

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