restoring and protecting the cahaba river watershed and it

Restoring and protecting the Cahaba River watershed and it’s rich diversity of life.

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th Avenue South, Suite 205, Birmingham, Alabama 35233-3421 • Tel 205 322-5326 • Fax 205 324-8346

www.cahabariversociety.org 1

February 4, 2013

Mr. Mike Wood, Chairman City of Hoover, Planning & Zoning Board 100 Municipal Drive Hoover, AL 35236 Re: Signature Homes PRD proposal for the O’Neal property. Dear Mr. Wood, The Cahaba River Society is a 501 (c) 3 non-profit river conservation group located in Birmingham, Alabama. Our mission is to restore and protect the Cahaba River watershed and its rich diversity of life. The diverse lives depending on the Cahaba include the 600,000 people and numerous businesses in the watershed and Birmingham Water Board service area relying on the River as a major source of drinking water as well as its internationally significant diversity of freshwater wildlife. In sum, with this letter we raise important issues concerning low impact development (LID) post-construction stormwater design that we recommend to be considered as part of Planning & Zoning review of the Signature Homes PRD proposal for the O’Neal property. We ask the City of Hoover to implement the post-construction aspects of your MS4 storm water permit in the stormwater plan for this project. Below we describe our recommendations regarding stormwater management and offer potential solutions. Mr. Jonathan Belcher with Signature Homes has been generous to meet with Dr. Haddock of our staff to discuss this project. We have also met with concerned citizens who would be neighbors if this proposal is approved. In both meetings we discussed the issues outlined below. We have been encouraged by the fact that in discussions about previous Signature Homes developments, some LID and sound stormwater management elements have been implemented. We hope to stimulate even greater adoption of these cost-effective stormwater management and LID techniques in this and future projects.

Board of Directors

Officers Jim Barton, President Dick Pigford, Immediate Past President Dan Monroe, VP/President Elect Lawrence Conaway, Secretary Dr. Elizabeth Turnipseed, Treasurer

Scot Duncan, PhD, VP Conservation

Eleanor DelBene, DMin, Chair, Stewardship Colin Coyne, Chair, Policy Bob Shepard, Chair, Education

Board Members

Rob Angus, PhD Michelle Blackwood Frazier Christy Betsy Dobbins, PhD John English Ben Erdreich Will Goodwyn Arlen Lewis Nancy Long Sonja Lother Lea Ann Macknally Angela Pewitt Jim Proctor Robbey Stanford Merrill Stewart Troy Wallwork Chris Williams

Emeritus Board

Tim Blair David Cunningham Bob Tate Beth Maynor Young Frank Young, III Staff

Beth Stewart Executive Director Tricia Sheets Director of Administration Randall Haddock, PhD Field Director Monica Carmichael Director of Development Gordon Black Education Director Kim Adams Office Manager


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Background

The Cahaba River Society has been supportive of development that is protective of the Cahaba River Watershed. Each year since 2007, our annual meeting has awarded projects and firms in the Cahaba River Watershed and the Birmingham area for water-smart projects that exemplify sound Low Impact Development (LID) practices that help manage stormwater, employ energy saving techniques that save water, and employ

water efficiency techniques (see Attachment A, awarded project list). So far, nineteen projects and 83 firms have been recognized through our “Watershed Conservation Development” awards. CRS is a resource promoting wider adoption of LID and water efficiency practices in development.

The Business and Municipal Case for LID Stormwater Management Our optimism about the success of wider use of LID comes in part from the economics of its adoption. Multiple studies have demonstrated that LID approaches are typically less

expensive for developers. Attachment B lists a variety of LID information resources available online that have proven the economic advantages of adopting LID practices. LID practices to reduce the negative impacts of post-construction stormwater protect water quality, help maintain water supply in creeks and streams during dry weather, and reduce damage to downstream properties from bank erosion, localized flooding, and sedimentation of lakes, ponds and stormwater conveyances. When infrastructure costs to municipalities are considered, these approaches significantly reduce the long-term expense of managing stormwater and environmental degradation. So, adoption of LID techniques is not merely a good idea for developers, but to an even greater degree, a good idea for municipalities.

What’s the Problem: Increased Post-Construction Stormwater Runoff Volume While most people recognize that mud washing off a construction site causes sediment problems in streams, relatively few people understand that a larger and more important source of stream sedimentation comes from post-construction stormwater runoff. Part of the reason post-construction stormwater runoff has such out-sized impact is that it is not limited to the time interval of project construction. Instead, those impacts extend for the entire decades-long lifetime of the project or development. Post-construction stormwater runoff has been shown to be the source of two thirds of the total sediment loading for Shades Creek1, due to instream erosion and collapse of stream banks. This is a growing source of Cahaba River pollution also. The DRAFT Cahaba Sediment TMDL2 also acknowledges the significance of better management of post-

1 US EPA Region 4. 2004. Total Maximum Daily Load (TMDL) for Siltation, Turbidity, and Habitat Alteration in Shades Creek, Jefferson County, Alabama. 2 http://adem.alabama.gov/programs/water/tmdls/7cahabatmdl.pdf


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construction stormwater to achieving the sediment TMDL goals. All MS4 permittees, including Hoover, will be directly responsible under the upcoming MS4 permit revision to improve stormwater programs to help meet the sediment reduction targets of the Cahaba TMDL.

Attachment C describes how and why there is such a significant increase in the volume of post-construction stormwater runoff. While this increased stormwater discharge volume has potential negative impacts for the development itself, the greatest potential impacts are for water quality, stream bank stability, downstream neighbors, and to the City of Hoover’s stormwater infrastructure. The Alabama Department of Environmental Management (ADEM) and the Environmental Protection Agency (EPA) have recognized the necessity of managing this significant water quality degradation problem. These regulatory agencies are beginning to require municipalities to give post-construction stormwater management much more attention. The next revision of Hoover’s MS4 permit will have a detailed section outlining requirements to establish a post-construction stormwater program. The City of Hoover has already experienced the difficulties that arise from the activities of developers who fail to consider this important environmental impact3. It is in the City of Hoover’s interests to require developers to adequately retain and infiltrate as much stormwater on-site as possible. For example, protecting the City’s existing stormwater conveyance system will require careful management of the magnitude of volume increases that come with additional development.

A Post-construction Stormwater Management Standard & Upcoming MS4 Permit

Requirements The question arises, ‘what stormwater management standard should be applied?’ The federal government has provided a well-researched model in the new federal facility construction requirements under Section 438 of the Energy Independence and Security Act. The Act has the following performance standard: rains from up to and including the 95th percentile storm events must be infiltrated on-site 4. Similarly, ADEM’s current draft MS4 permit for the Alabama Department of Transportation, issued for public comment in December, includes a requirement to match pre-development hydrology to post-development hydrology, to the maximum extent practicable, for stormwater from rain events up to the 95th percentile event. This is the first Phase I MS4 permit in Alabama that includes this standard, and it is likely to be a model for the upcoming revision of the other Phase I MS4 permits, including the Phase I

3 Post-construction stormwater impacts from additions to the Riverchase United Methodist Church have caused significant problems for downstream neighbors, resulting in a lawsuit against the church that might

have been avoided had the City of Hoover required better construction stormwater control and better evaluation of post-construction stormwater runoff management. 4 US EPA. 2009. Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act. Available at

http://www.wbdg.org/ccb/EPA/epa_841b09001.pdf, see page 12.


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MS4 Permit for the City of Hoover. So, in all likelihood, this standard will be in the City of Hoover’s next MS4 stormwater permit. The requirement to infiltrate stormwater from up to and including the 95th percentile storm event highlights a change in thinking about stormwater management. While it is essential to manage runoff from large rain events for flood management purposes, to avoid excessive in-stream bank erosion and ongoing post-construction degradation to water quality, it is also essential to manage runoff from the more frequent but smaller rain events. Based on these existing and developing NPDES MS4 requirements, and based on our understanding of stream hydrology, we strongly recommend the City of Hoover require all new developments to strive to meet a post-construction stormwater runoff management standard of infiltrating on-site, to the maximum extent practicable, rainfall from up to and including the 95th percentile storm event. Our calculations estimate that value for the Birmingham area to be a 1.6” per day storm event. For rain events of greater magnitude, stormwater runoff obviously cannot be avoided. Conventional stormwater detention facilities continue to be needed to minimize flooding potential. However, use of LID stormwater practices can allow smaller pipes and ponds for overflow detention, which is one reason that LID typically saves money over conventional stormwater infrastructure.

Ways to Achieve a Post-Construction Stormwater Performance Standard In our meetings with Signature Homes and with the adjacent neighbor homeowners, we

discussed the importance of sufficient stormwater infiltration. Attachment B lists a variety of LID information resources available online, highlighting both the economic advantages of adopting LID techniques and providing examples of how to achieve better stormwater management results. Here, we will describe specific ways to achieve that goal of reducing the volume of stormwater runoff. There are a variety of ways to infiltrate or reuse stormwater. Each of these approaches may at first seem inconsequential. Yet the cumulative benefits of adopting multiple strategies for small scale infiltration and capture/reuse throughout a project can significantly diminish the negative impacts of post-construction stormwater runoff. The aim of LID is to minimize runoff from hard surfaces, conserve open and forested areas that naturally infiltrate and filter rain, and infiltrate or capture/reuse rain as close to where it falls as possible, rather than to convey and collect stormwater into a few large facilities. The first site design strategy is to minimize the project’s hard surface pootprint as muxch as is feasible. Allowing cluster development and narrower streets in exchange for protected open space and forests will improve stormwater management and reduce development costs.


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Stormwater management approaches that get runoff into the ground help avoid in-stream erosion problems. Some examples are:

• Directing the flow of roof downspouts to a grassy area, rather than to a paved area that concentrates flow into a storm drain, is beneficial.

• Alternatively, collecting roof runoff in “rain barrels” for subsequent irrigation needs provides a dual benefit of cutting potable water demand.

• Minimizing street widths and paved parking is beneficial.

• Using two paved “tracks” rather than paving the entire driveway surface helps minimize paving and provides in increment of earth that can infiltrate stormwater.

• Using pervious paving such as pea gravel, pervious pavers, pervious concrete and pervious asphalt where feasible in individual lots, sidewalks and common parking areas reduce runoff from paved areas. Conventional paved surfaces can be combined with small areas of pervious paving in low spots to minimize costs.

• Bioswales located in public or common areas, including beside streets, in parking areas, and in common stormwater open space, improve the infiltration potential of a development significantly and add landscape value.

• Placing Rain Gardens on as many individual lots as possible would significantly diminish the volume of stormwater runoff.

• French drains collect and infiltrate roof runoff or runoff from paved areas.

• Infiltration swales are often used to infiltrate runoff from paved areas.

• Dedicating specific areas within the development as infiltration ponds and/or created wetlands.

We point out that if a sufficient number of these measures are adopted for each building site, then perhaps no building sites need be given over completely to stormwater management. We urge Signature Homes to recognize that there can be very significant savings in the cost of stormwater conveyances (pipes and other stormwater infrastructure) that result from adopting a sufficient number of these small-scale infiltration measures. Traditional curb and gutter street design concentrates stormwater flow to the Municipal Separate Storm Sewer System (the MS4). Concentrating the flows makes stormwater infiltration more difficult. Luckily, there are many examples of street design that avoid this concentration problem by, as much as possible, directing stormwater to bioswales, filter strips, or other stormwater infiltration features that manage the ‘first flush’5 of stormwater6. How such a variety of stormwater runoff management features are integrated onto the landscape is very dependent on the local conditions. Thus the best approach is to specify a performance standard for matching pre-development and post-development hydrology to the greatest extent feasible, while ensuring that the community’s development regulations give sufficient flexibility for minimizing paving and the project footprint and for using LID stormwater design.

5 The ‘first flush’ phenomena refers to the highly concentrated pollutant content of the initial flush of stormwater. Infiltrating the first flush of stormwater thus achieves both volume control and significant pollutant control. 6 http://www.lowimpactdevelopment.org/pubs/LID_Hydrology_National_Manual.pdf or Appendix A.


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It is essential that far greater thought and effort be given over to mitigating what has become the most important source of the excessive sediment impairment of the Cahaba River and, cumulatively, a major cause of bank erosion, property damage, filling of private lakes with sediment, and localized flooding. It serves the interests of the City of Hoover, the adjacent and downstream neighbors, and the developer to have a thoughtful post-construction stormwater management plan. We would be eager to continue to serve as a resource to the City and Signature Homes for achieving the challenging goal of managing post-construction stormwater runoff. We hope you will consider these complex issues. We would like the opportunity to meet with the appropriate staff and leadership of the City of Hoover to explore solutions that will meet the goals of both the development and the citizens of Hoover. Sincerely,

Randall C. Haddock Beth K. Stewart Field Director Executive Director CC: Gary Ivey, Mayor, City of Hoover Allen Pate, Operations, City of Hoover

Margie Handley, City Clerk, City of Hoover Trent Lott, City Council, City of Hoover Vanessa Bradstreet, Secretary, Hoover Planning and Zoning Commission Jonathan Belcher, Signature Homes Steve Castleman, Spectrum Environmental Services, Inc. Bob House, House Consultants, Inc.

Lindsay Mardick, Inverness Master Homeowners Association Noel Chamblis, Inverness Master Homeowners Association

Henry Hager, President, The Sanctuary at Caldwell Crossings Homeowner Association Steve Land, President, Mill Springs Homeowner Association Steve Goldman, President, Caldwell Crossings Homeowner Association Randy Johnston, President, Altadena Woods Homeowner Association Courtney Shea, Birmingham Field Office, US Army Corps of Engineers


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The Shops of Grand River was

awarded in 2011 for excellence in

sediment & erosion control under

extremely challenging site and

weather conditions

Attachment A

Watershed Conservation

Development Awards

About CRS’s Watershed Conservation Development Awards The Cahaba River Society believes that restoring the Cahaba and safeguarding our drinking water supplies can be

achieved through transforming the way developments are designed and built. Our annual Watershed

Conservation Development awards demonstrate the value of collaborative relationships with many partners to

guide and promote development that protects and restores the Cahaba watershed.

Since 2007 CRS has given awards recognizing projects in our region that

conserve water resources through environmental site design, LID (low

impact development) post-construction stormwater practices, effective

construction best management practices, water harvesting and reuse,

drinking water efficiency, and watershed restoration.

These awards highlight model projects and prove the feasibility, cost-

effectiveness, and value of these practices to the triple bottom line –

people, planet and prosperity. The awards also honor the stewardship

commitment of businesses, governments, and institutions that invest in

these solutions, as well as the capability of project team members to

deliver quality water-smart development.

Award Criteria

Project demonstrates innovative design to restore, protect, or conserve water resources through Low Impact

Development post-construction stormwater management, environmental site design, excellence in construction

stormwater management, drinking water efficiency, water reuse, and/or watershed restoration.

Project is completed, with proven performance. A completed phase of a larger project is eligible if water-smart value

can be demonstrated.

Located within the Cahaba watershed or drinking water area (Jefferson, Shelby, St. Clair, Bibb Dallas and Perry County,

generally).

We are especially interested in projects that are influential in leadership for green building for our region, that have

educational value, and that can demonstrate the economic value of water-smart approaches.

Application & Selection Process

CRS circulates a Call for Nominations each year, due in mid-January. Contact Beth K. Stewart, Executive Director,

[email protected] and 205-322-5326 ext 411 for more information and to be on the notification list.

Selections are made by a committee of CRS Board members and staff. Awards are given at the Cahaba River

Society Annual Meeting typically on the last Thursday of January or first Thursday of February, from 5:30 to


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Hewitt-Trussville High School used

many LID and environmental site

design practices, including woodland

parking.

McWane, Inc. solved compliance issues

with an innovative stormwater &

process water reuse system.

Alabama Power Co. reuses groundwater

and stormwater for cooling and landscape

irrigation and installed water efficient

fixtures. This retrofit saves money, energy

& enough water for 100 homes per year.

LID bioswales, parking and roads saved money

for Bass Pro and reduced its footprint,

preserving forests and providing park areas to

complement and increase the store’s

commercial appeal.

Water features that are a part of

Railroad Reservation Park’s appeal

also cleanse and percolate rain runoff.

8:00pm. Awardees are notified in advance and are our honored guests at the meeting. CRS seeks media

coverage to celebrate the awardees.

Past Watershed Conservation Development Award Winning Projects

Over the past 6 years, CRS has honored 83 firms, institutions and governments for their role in designing and

delivering great projects that meet growth goals while also protecting and restoring water resources.

2012

Railroad Reservation Park Stormwater System

Alabama Power Company Water Efficiency Retrofit

Birmingham-Southern College Lakeview Residence Hall

Samford University Shades Creek Restoration

2011

The Shops of Grand River

Vestavia Hills Library in the Forest

Little Shades Creek Restoration

St. Vincent’s Birmingham Water Efficiency Retrofit

2010

Ruffner Mountain Nature Center

Birmingham-Southern College Urban Environment Park

2009

Bass Pro Shops

Stewart Perry Company Headquarters

Brasfield and Gorrie Headquarters Expansion

2008

Hewitt-Trussville High School

McWane, Inc. Stormwater & Process Water Reuse

Social Security Administration Building

Protective Life Headquarters (“Classic”)

2007 St. Vincent’s 119 Health and Wellness and Shoppes at River Run


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Attachment B INFORMATION RESOURCES ON THE WEB:

LOW IMPACT DEVELOPMENT, GREEN INFRASTRUCTURE,

& COST SAVINGS January 2012

ECONOMIC VALUE OF LID & GREEN INFRASTRUCTURE

Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices December 2007 EPA 841-F-07-006

EPA LID case studies comparing LID with conventional stormwater infrastructure, proving that LID usually saves money.

http://www.epa.gov/owow/nps/lid/costs07/documents/reducingstormwatercosts.pdf http://www.epa.gov/owow/nps/lid/costs07/

American Society of Landscape Architects Stormwater Case Studies 2011 http://www.asla.org/ContentDetail.aspx?id=31301

ASLA collected 479 case studies from 43 states, the District of Columbia, and Canada on projects that successfully and sustainably manage stormwater. These projects demonstrate to policymakers the value of promoting green infrastructure

policies. Green infrastructure and low-impact development (LID) approaches, which are less costly than traditional grey infrastructure projects, can save

communities millions of dollars each year and improve the quality of our nation’s water supply. Each case study is summarized and cost factors are reported. Combined results found that green infrastructure practices reduced costs in 44.1%

of cases, did not influence costs in 31.4% of cases, and increased costs in 24.5% of cases.

The Value of Green Infrastructure: A Guide to Recognizing its

Economic, Environmental and Social Benefits Center for Neighborhood Technology and American Rivers, Inc. 2010 http://www.cnt.org/repository/gi-values-guide.pdf A cumulative assessment of the multiple benefits of LID and GI as a municipal or

private investment. Since methods and tools for assessing benefits have been lacking, municipalities more easily can assess gray infrastructure cost-benefits and

favor those solutions. This guide provides simplified ways to assess the full benefits of LID and GI to aid decision-makers in evaluating options for water management.


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Integrating Valuation Methods to Recognize Green Infrastructure's

Multiple Benefits Center for Neighborhood Technology (lead author), 2010

http://www.cnt.org/repository/CNT-LID-paper.pdf This research paper reviews current methods, tools and case studies of valuation of the

economic and social benefits produced by green infrastructure practices, particularly in urban settings. It begins to define a framework for assessing the economic

benefits of LID practices at the site and community scale.

National Green Values™ Calculator Center for Neighborhood Technology

http://greenvalues.cnt.org/national/calculator.php The National Green Values™ Calculator is a tool for quickly comparing the performance, costs, and benefits of Green Infrastructure, or Low Impact

Development (LID), to conventional stormwater practices. The GVC is designed to take you step-by-step through a process of determining the average precipitation at

your site, choosing a stormwater runoff volume reduction goal, defining the impervious areas of your site under a conventional development scheme, and then

choosing from a range of Green Infrastructure Best Management Practices (BMPs) to find the combination that meets the necessary runoff volume reduction goal in a cost-effective way.

The Economics of Low-Impact Development: A Literature Review ECONorthwest

November 2007 Download the report at:

http://www.econw.com/reports/ECONorthwest_Low-Impact-Development-Economics-Literature-Review.pdf or www.econw.com provides a link directly to the report

Also see http://www.econw.com/casestudies/casestudy?study=low-impact-development

For links to other ECONorthwest publications on LID and green infrastructure practices


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LID & GREEN INFRASTRUCTURE ~ GENERAL

1. American Rivers, Inc.

Local Water Policy Innovation: A Road Map for Community Based

Stormwater Solutions A Joint Publication Of American Rivers, Inc.

Midwest Environmental Advocates, Inc. September 2008

http://www.americanrivers.org/library/reports-publications/local-water-policy-innovation.html

Putting Green to Work: Economic Recovery Investments for Clean and

Reliable Water American Rivers

http://www.americanrivers.org/our-work/global-warming-and-rivers/infrastructure/funding-green-infrastructure.html www.americanrivers.org

2. Storm Water Manager’s Resource Center www.stormwatercenter.net Maintained by the Center for Watershed Protection. Extensive reference library, sample ordinances, specs and performance data for innovative storm water BMP’s

and design, including techniques for volume reduction. Especially helpful to local government storm water staff and consultants.

3. U.S. EPA Links to many U.S. EPA reports and presentations, LID & Green Infrastructure: www.epa.gov/nps/lid

www.epa.gov/npdes/greeninfrastructure http://water.epa.gov/infrastructure/greeninfrastructure/

Green Infrastructure Case Studies: Municipal Policies for Managing

Stormwater with Green Infrastructure (EPA-841-F-10-004) - 09-08-2010 This report presents the common trends in how 12 local governments developed and

implemented stormwater policies to support green infrastructure. http://www.epa.gov/owow/NPS/lid/gi_case_studies_2010.pdf

Managing Wet Weather with Green Infrastructure Municipal Handbook

Water Quality Scorecard The Municipal Handbook is a series of documents to help local officials implement green infrastructure in their communities. August 2009, EPA- 833-B-09-004

http://www.epa.gov/npdes/pubs/gi_municipal_scorecard.pdf


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EPA's Water Quality Scorecard was developed to help local governments identify opportunities to remove barriers, and revise and create codes, ordinances and incentives for better water quality protection. It guides municipal staff through a

review of relevant local codes and ordinances, across multiple municipal departments and at the three scales within the jurisdiction of a local government (municipality,

neighborhood, and site), to ensure that these codes work together to protect water quality goals.

5. William F. Hunt, Ph.D., PE, NC State University

Dr. Hunt is knowledgeable about the effectiveness of post-construction storm water BMP’s in the southeast. Various storm water and LID publications, research reports, and online courses and workshop information are at:

www.bae.ncsu.edu/people/faculty/hunt/


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Attachment C The following is excerpted from Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act. (EPA 841-B-09-001, December 2009, available at www.epa.gov/owow/nps/lid/section438 .), beginning on page 3. The excerpt provides an explanation for why it is important to infiltrate stormwater. Following that excerpt, we briefly describe our understanding of why the 95th percentile rain event was selected as a basis for the stormwater treatment standard. The following text and images have been slightly reformatted:

Part I: Implementation Framework

A. BACKGROUND

This section contains background on the causes and consequences of stormwater discharges, solutions

that can be used to address the causes and consequences of stormwater discharges and how to

implement those solutions to comply with Section 438 of EISA. Alterations to Natural Hydrology and the Impact on Stormwater Runoff In the natural, undisturbed environment rain that falls is quickly absorbed by trees, other vegetation,

and the ground. Most rainfall that is not intercepted by leaves infiltrates into the ground or is returned

to the atmosphere by the process of evapotranspiration. Very little rainfall becomes stormwater runoff

in permeable soil, and runoff generally only occurs with larger precipitation events. Traditional

development practices cover large areas of the ground with impervious surfaces such as roads,

driveways, sidewalks, and buildings. Under developed conditions runoff occurs even during small

precipitation events that would normally be absorbed by the soil and vegetation. The collective force

of the increased runoff scours streambeds, erodes stream banks, and causes large quantities of

sediment and other entrained pollutants to enter the water body each time it rains (Shaver, et al., 2007;

Booth testimony, 2008).

As watersheds are developed and impervious surfaces increase in area, the hydrology of the

watersheds fundamentally changes over time which results in degraded aquatic ecosystems. In

recognition of these problems, stormwater managers employed extended detention approaches to

mitigate the impacts of increased peak runoff rates. However, wet ponds and similar practices are not

fully adequate to protect downstream hydrology because of the following inherent limitations of these

conventional practices (National Research Council, 2008; Shaver, et al., 2007):

• Poor peak control for small, frequently-occurring storms;

• Negligible volume reduction; and

• Increased duration of peak flow.

Detention storage targets relatively large, infrequent storms, such as the two and 10-year/24-hour

storms for peak flow rate control. As a result of this design limitation, flow rates from smaller,

frequently-occurring storms typically exceed those that existed onsite before land development

occurred and these increases in runoff volumes and velocities typically result in flows erosive to


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stream channel stability (Shaver, et al., 2007). Section 438 is intended to address the inadequacies of

the historical detention approach to managing stormwater and promote more sustainable practices that

have been selected to maintain or restore predevelopment site hydrology.

A 2008 National Research Council report on urban stormwater confirmed that current stormwater

control efforts are not fully adequate. Three of the report’s findings on stormwater management

approaches are particularly relevant (National Research Council, 2008).

1. Individual controls on stormwater discharges are inadequate as the sole solution to stormwater

in urban watersheds;

2. Stormwater control measures such as product substitution, better site design, downspout

disconnection, conservation of natural areas, and watershed and land-use planning can

dramatically reduce the volume of runoff and pollutant load from new development; and

3. Stormwater control measures that harvest, infiltrate, and evapotranspire stormwater are critical

to reducing the volume and pollutant loading of small storms.

Pre-development Hydrology. Courtesy of C. May, Post-Development Hydrology. Courtesy of C. May, University of Washington. University of Washington.

Figure 1. Pre-Development and Post-Development Hydrology. (USDA).

Figure 1 contains two sets of diagrams depicting the water balances at undeveloped and developed

sites. Runoff patterns will vary based on factors such as geographic location, local meteorological

conditions, vegetative cover and soils. The first set of figures represents conditions in the Pacific

Northwest where storms have a long duration and low intensity, i.e., the volume of rain in an

individual storm is small. The second set of figures from the U.S. Department of Agriculture


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represents a more generalized set of conditions, but was included to illustrate that heavily urbanized

areas typically cause large increases in runoff.

Land cover changes that result from site development include increased imperviousness, soil

compaction, loss of vegetation, and loss of natural drainage patterns, which result in increased runoff

volumes and peak runoff rates. The cumulative impacts of the land cover changes result in alterations

of the natural hydrology of a site, which disrupts the natural water balance and changes water flow

paths. The consequences of these impacts include:

1. Increased volume of runoff. With decreased area for infiltration and evapotranspiration due to

development, a greater amount of rainfall is converted to overland runoff which results in

larger stormwater discharges.

2. Increased peak flow of runoff. Increased impervious surface area and higher connectivity of

impervious surfaces and stormwater conveyance systems increase the flow rate of stormwater

discharges and increase the energy and velocity of discharges into the stream channel.

3. Increased duration of discharge. Detention systems generate greater flow volumes and rates.

These prolonged higher discharge rates can undermine the stability of the stream channel and

induce erosion, channel incision and bank cutting.

4. Increased pollutant loadings. Impervious areas are a collection site for pollutants. When

rainfall occurs these pollutants are mobilized and transported directly to stormwater

conveyances and receiving streams via these impervious surfaces.

5. Increased temperature of runoff. Impervious surfaces absorb and store heat and transfer it to

stormwater runoff. Higher runoff temperatures may have deleterious effects on receiving

streams. Detention basins magnify this problem by trapping and discharging runoff that is

heated by solar radiation (Galli, 1991; Schueler and Helfrich, 1988).

The resulting increases in volume, peak flow, and duration are illustrated in the hydrograph in Figure

2, which is a representation of a site’s stormwater discharge with respect to time. The hydrograph

illustrates the impacts of development on runoff volume and timing of the runoff. Individual points on

the curve represent the rate of stormwater discharge at a given time. The graph illustrates that

development and corresponding changes in land cover result in greater discharge rates, greater

volumes, and shorter discharge periods. In a natural condition, runoff rates are slower than those on

developed sites and the discharges occur over a longer time period. The predevelopment peak

discharge rate is also much lower than the post-development peak discharge rate due to attenuation and

absorption by soils and vegetation. In the post-development condition there is generally a much shorter

time before runoff begins because of increased impervious surface area, a higher degree of

connectivity of these areas and the loss of soils and vegetative cover that slow or reduce runoff.


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The Solution: Preserving and Restoring Hydrology A new approach has evolved in recent years to eliminate or reduce the amount of water and pollutants

that run off a site and ultimately are discharged into adjacent water bodies.

The fundamental principle is to employ systems and practices that use or mimic natural processes to:

1) infiltrate and recharge, 2) evapotranspire, and/or 3) harvest and use precipitation near to where it

falls to earth.

GI/LID practices include a wide variety of practices that utilize these mechanisms. These practices can

be used at the site, neighborhood and watershed/regional scales. In this document the focus is on site-

level practices, which is most consistent with the terms used in Section 438: “project,” “facility,” and

“property.” Although these performance requirements apply at the project site-level, flexibility exists

to utilize nearby areas or areas directly adjacent to the facility to manage the runoff, i.e.,


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evapotranspirate, infiltrate or harvest and use. Where justifiable, it also may be appropriate to

evapotranspirate, infiltrate or harvest and use an equivalent or greater amount of runoff offsite as long

as the runoff is discharged or used in the same receiving subwatershed or watershed.

The purpose of EISA Section 438 is to replicate the pre-development hydrology to protect and

preserve both the water resources onsite and those downstream. For example, if prior to development,

twenty five (25) percent of the annual rainfall runs directly into the stream and the remainder infiltrates

into the ground or is evapotranspired into the air, then the post-development goal should be to limit

runoff to twenty five (25) percent of the annual precipitation while maintaining the correct aquifer

recharge rate. This has the benefit, in most cases, of delivering water to the stream at approximately

the same rate, volume, duration and temperature as the stream had naturally evolved to receive prior to

development. The result will be to eliminate or minimize the erosion of streambeds and streambanks,

significantly reduce the delivery of many pollutants to water bodies, and retain historical instream

temperatures.

Restoring or maintaining pre-development hydrology has emerged as a control approach for several

reasons. Most importantly, this approach is intended to directly address the root cause of impairment.

Current control approaches have been selected in an attempt to control the symptoms (peak flow, and

excess pollutants), but this strategy is not fully adequate because of the scale of the problem, the

cumulative impacts of multiple developments and the need to manage both site and watershed level

impacts. With current approaches, it is also difficult to adequately protect and improve water quality

because the measures employed are not addressing the main problem which is a hydrologic imbalance.

Designing facilities based on the goal of maintaining or restoring pre-development hydrology provides

a site specific basis and an objective methodology with which to determine appropriate practices to

protect the receiving environment.

Using pre-development hydrology as the guiding control principal also allows the designer to consider

climatic and geologic variability and tailor the solutions to the project location. Thus the need for a one

size fits all approach is rendered unnecessary since the design objective is dictated by the pre-

development site conditions and other technicalities of the project site and facility. Instead of

prescribed approaches dictating discharge volumes or flow rates, site assessments of historical

infiltration and runoff rates will inform the designer and provide the basis for a suitable design. The

use of this approach will minimize compliance complications that may arise from prescriptive design

approaches which do not account for the variability of precipitation frequencies, rainfall intensities and

pre-development land cover and soil conditions that influence infiltration and runoff.

B. BENEFITS AND OUTCOMES OF THE NEW STORMWATER PERFORMANCE REQUIREMENTS

Implementation of these new stormwater performance requirements in EISA Section 438 provides

numerous environmental and economic benefits in addition to reducing the volume of stormwater

runoff:


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Benefits to Water Resources:

Cleaner Water. The use of plants, soils and water harvesting

and use practices can reduce stormwater runoff volumes and

pollutant loadings and the frequency and magnitude of

combined sewer overflows (volume and pollutant loading

reductions). These practices are part of a larger set of

practices called green infrastructure/low impact development.

Clean and Adequate Water Supplies. GI/LID approaches

using soil based vegetated infiltration systems can be used to

recharge ground water and maintain stream base flow. By

recharging ground water aquifers, aquatic ecosystem health is

maintained and base flows are increased which helps ensure

more constant flows for drinking water withdrawals.

Harvesting and reusing rainwater also reduces the need to use

potable water for all uses and can reduce both the

infrastructure and energy needed to treat and transport both

drinking water and stormwater.

Source Water Protection. GI/LID practices provide pollutant

removal benefits, thereby providing some protection for both

ground water and surface water sources of drinking water. In

addition, GI/LID provides ground water recharge benefits.

Other Social and Environmental Benefits:

• Cleaner Air. Trees and vegetation improve air quality by filtering many airborne pollutants and

can help reduce the amount of respiratory illness (Vingarzan and Taylor, 2003).

• Reduced Urban Temperatures. Summer city temperatures can average 10ºF higher than nearby

suburban temperatures (Casey Trees, 2007). High temperatures are also linked to higher

ground level ozone concentrations. Vegetation creates shade, reduces the amount of heat

absorbing materials and emits water vapor – all of which cool hot air (Grant, et al., 2003).

Reductions in impervious surface and the use of light colored pervious surfaces (e.g.,

permeable concrete) also can mitigate urban temperatures.

• Moderate the Impacts of Climate Change. Climate change impacts and effects vary regionally,

but GI/LID techniques can provide adaptation benefits for a wide array of circumstances. They

can be used to conserve, harvest and use water, to recharge ground waters and to reduce

surface water discharges that could contribute to flooding. In addition, there are mitigation

benefits such as reduced energy demand and carbon sequestration by vegetation.

GI/LID approaches are a set of

management approaches and

technologies that utilize and/or

mimic the natural hydrologic cycle

processes of infiltration,

evapotranspiration and use. GI/LID

practices include green roofs, trees

and tree boxes, rain gardens,

vegetated swales, pocket wetlands,

infiltration planters, porous and

permeable pavements, vegetated

median strips, reforestation and

revegetation and protection of

riparian buffers and floodplains.

These practices can be used almost

anywhere soil and vegetation can

be worked into the urban or

suburban landscape. They include

decentralized harvesting

approaches such as rain barrels and

cisterns that can be used to capture

and re-use rainfall for watering

plants or flushing toilets.


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• Increased Energy Efficiency.

Green space helps lower

ambient temperatures and,

when incorporated on and

around buildings, helps shade

and insulate buildings from

wide temperature swings,

decreasing the energy needed

for heating and cooling.

Diverting stormwater from

wastewater collection,

conveyance and treatment

systems can reduce the amount

of energy needed to pump and

treat the water. Energy

efficiency not only reduces

costs, but also reduces

generation of greenhouse

gases.

• Community Benefits. Trees

and plants improve urban

aesthetics and community

livability by providing

recreational and wildlife areas.

Studies show that property

values are higher when trees

and other vegetation are

present. Increased green space

also has public health benefits and has been shown to reduce crime and the associated stresses

of urban living.

(This is the end of the excerpt from the EPA technical guidance.)

Why the 95th percentile was recommended as design standard for stormwater

management The feasibility of using LID techniques to achieve post-construction stormwater management of five different design standards was examined by Horner and Gretz, 20117 for the Natural Resources Defense Council. According to the authors, “The study assessed five urban land use types (three residential, one retail commercial, and one infill redevelopment), each placed in four climate regions in the continental United States on two regionally common soil types.” The Southeast was one of those four climate regions evaluated.

7 http://water.epa.gov/infrastructure/greeninfrastructure/upload/gi_NRDC-EPA-standard-asssessment-report-2.pdf


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The authors examined case studies from five types of urban land uses: (1) multi-family residential, (2) small-scale single-family residential, (3) large-scale single-family residential, (4) large-scale commercial, and (5) infill redevelopment. Building permit data from each region were consulted to determine typical distributions of site features for each (e.g., land cover by buildings, parking areas, roadways, walkways, driveways, landscaping). The authors described the five different design standards as follows: The potential regulatory standards investigated were capture and retention of, at minimum:

• Standard 1—The runoff produced by the 85th percentile, 24-hour precipitation event8, a standard commonly used in California;

• Standard 2—The runoff produced by the 95th percentile, 24-hour precipitation event, the standard adopted under Section 438 of the Energy Independence and Security Act;

• Standard 3—90 percent of the average annual post-development runoff volume;

• Standard 4—The difference between the post- and pre-development3 average annual runoff volumes; and

• Standard 5—The difference between the post- and pre-development runoff volumes for all events up to and including the 85th percentile, 24-hour precipitation event.

They also investigated the influence of Soil Type and found that for B and C soils, the LID techniques evaluated were generally successful in meeting the various design standards (i.e., successful for 113 of 125 evaluations). D soils were more challenging. These authors concluded the following:

In summary, standards 2 and 3 are clearly superior to the other three options. Standard 3 is entirely consistent from place to place in degree of environmental protection, and standard 2 does not deviate much. Analysis of the five development cases on two soil groups in each of four regions demonstrated the two standards are virtually identical in the runoff retention and pollutant loading reduction they would bring about.

Based on these studies, the Technical Guidance has adopted both Standards 2 and 3 as optional ways to satisfy the requirements of Section 438 of the Energy Independence and Security Act.

8 The 85th percentile, 24-hour event represents the precipitation quantity in a 24-hour period not exceeded

in 85 percent of all events in an extended record.

restoring and protecting the cahaba river watershed and it

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