Schools-2009 SSc6.1: Stormwater Design—Quantity Control

  • NC CS Schools SSc6.1 Credit Requirements Diagram
  • Cost-effective with an integrated approach

    Development usually comes with increased stormwater runoff due to impervious surfacesSurfaces that promote runoff of precipitation volumes instead of infiltration into the subsurface. The imperviousness or degree of runoff potential can be estimated for different surface materials. like roofs and parking lots. To earn this credit with previously undeveloped sites, you’ll need to avoid any increase in runoff, while on mostly impervious developed sites, you’ll need to reduce runoff. You may need to go beyond standard practice to achieve this credit, and you might see increased costs, although an integrated approach can make this credit cost-effective.

    If you're planning to pursue this credit, make sure your civil engineer is aware of it and on board, in order to achieve the credit without added steps.

    Encourage natural infiltration when possible

    Many project teams are reluctant to attempt this credit because engineers typically use conventional methods that might not meet requirements. Although it's readily achievable, this credit can be challenging, particularly in areas with compacted soil, no landscaped area, large parking areas, or water laws that preclude rainwater harvesting. Green roofs will be helpful in these cases, but the simplest and cheapest option, whenever available, is to simply encourage natural infiltration of stormwater into the ground. Reducing hardscapes, designing a smaller building footprintBuilding footprint is the area on a project site used by the building structure, defined by the perimeter of the building plan. Parking lots, parking garages, landscapes, and other nonbuilding facilities are not included in the building footprint., increasing landscaping area, using porous paving materials, using natural swales and other low-impact development strategies, and preserving natural site features are all cost-effective methods for promoting natural infiltration. Although natural infiltration may decrease the cost of maintenance compared to other structural and packaged stormwater control systems, keep in mind that it will still require regular maintenance.

    Similar to SSc6.2

    The two stormwater credits, SSc6.1 (stormwater rate and quantity), and SSc6.2 (stormwater quality) involve similar calculations and can be addressed by similar strategies, such as promoting natural infiltration. Keep in mind, however, that each credit requires different calculations and methodologies. Reducing the quantity of stormwater runoff for SSc6.1 does not always equate to a quality improvement for SSc6.2. Both credits focus on smaller, more frequent storms, not the larger ones that are more likely to cause flooding.

    Many of the benefits of this credit are indirect and can be difficult to calculate. These include issues like reducing the burden on the municipal stormwater system; reducing contaminants in waterways; reducing peak runoff, which makes stream habitats more consistent; reducing the temperature of runoff, which improves the conditions for aquatic life; and reducing erosion.

  • FAQs for SSc6.1 and SSc6.2

    Why do the requirements focus on 1-year and 2-year, 24-hour storms?

    The 2-year, 24-hour design stormA 2-year, 24-hour design storm is a nationally accepted rate that represents the largest amount of rainfall expected over a 24-hour period during a 2-year interval. The rate is the basis for planning and designing stormwater management facilities and features. is a storm that has a high probability of happening and contributing to stormwater pollution. A 2-year storm has a 50% chance of happening in a given year, whereas a 1-year storm has a 100% chance.

    It should be noted that most state or local programs only require projects to meet regulatory requirements related to flooding and/or water quality.  This type of stormwater management program is designed to control the large, infrequent storm events that cause flooding, but not to manage smaller storm events that we now know cause the majority of the overall erosion and quality concerns because of their much higher frequency. The criteria of SSc6.1 are designed to ensure that both concerns are addressed in LEED projects that achieve this credit.

    Why include the 1-year storm in the credit requirements? Won't management practices for the 2-year storm be effective?

    It depends on how you look at it. Here's how LEEDuser Expert Michael DeVuono describes it: Think about it in terms of a simple pre>post analysis. Your one year "pre" number will be smaller than your 2-year "pre" number. Sometimes that 1-year number is so small that you have to choke back a lot of water, to ensure the "post" 1-year is smaller. This raises the required storage volume for the BMPBest Management Practice. So if you're looking at both the 1- and 2-year events, you may have a greater storage need than if you simply looked at the 2-year event. The 2-year "pre" number will be bigger, so you can let more out in the "post."

    How can green roofs count as a stormwater control measure?

    There are different approaches to this. One approach is to ensure that green roof soil depth and retention capacity allows for the 2-year, 24-hour design storm.

    However, simply taking a “CN credit” for a green roof is usually beneficial enough. (The Curve Number or CN provides a number characterizing the runoff properties for a particular soil and ground cover.) Instead of the roof being modeled as impervious (with a CN of 98 which produces a high rate of runoff) some projects with extensive green roofs have used a lawn CN—usually around 61. In the calculations this results in a lower overall rate of runoff for the site, and is usually a more feasible option that providing stormwater storage in the roof media itself. If you can model your site so there is less runoff, there is less runoff volume that needs to be stored.

    How can I achieve compliance if my project's stormwater control measures are outside the LEED project boundary?

    Projects with stormwater control measures outside the LEED project boundary may be accepted if the measures appropriately take into account neighboring facilities by demonstrating that the existing stormwater management systems that serve the LEED project boundary meet the LEED requirements for all areas within the site serviced by those systems. LEED 2009 campus projects are required to reference USGBC's AGMBC guidance, which has specific guidelines for stormwater. For more on this see, for example, LI#2275 from 08/22/2008.

    I have 100-year data—how do I convert to 2-year?

    Storm intervals don’t convert. These numbers represent specific storm event probability. A 100-year storm has a 1% chance of happening in a given year, while a 2-year storm has a 50% chance of happening in a given year. The best resource for rainfall intensity data is NOAA’s Hydrometeorological Design Studies Center Precipitation Frequency Data Server. Further guidance on interpolating 2-year, 24-hour storm event can be found in LEEDuser's EBOM SSc6 Guidance.

    Is it an acceptable strategy to capture the rainwater into tanks and discharge it into the public sewers after the rainstorm reducing the peak discharge?

    This is a common strategy for reducing peak rate, which will help you comply with SSc6.1, but you'll need to add onsite reuse or infiltration to meet SSc6.2 requirements.

    A sample graph illustrating the 95th percentile rainfall event

    Are there special considerations for international projects?

    In 2012, an additional compliance option was added to SSc6.1 that was specifically written with international projects in mind. This can be found in the credit language, and is fully supported on the most recent LEED Online forms. Projects in some countries can have trouble finding the stormwater data they're looking for. Some useful sites are posted in LEEDuser's Resources tab.

    What performance threshold do I need to achieve for an Exemplary Performance point?

    LEED Interpretation #10108 dated 11/01/2011 gives guidance in achieving Exemplary Performance. Achievement of the exemplary performance point encompasses both quantity and quality measures, and includes a comprehensive approach to capture and treat stormwater runoff.

    The calculations for this credit are always a headache! While our projects are usually awarded the credit, the equations in the LEED Reference Guide are helpful mostly for sizing a reservoir or cistern, but don't help you get to the final results. Does USGBC provide any step-by-step guidance that would make submitting these credits more predictable?

    No. USGBC has indicated that providing step-by-step instructions for this entire calculation process within the context of LEED reference documents is not possible. Various methods and computer-based software programs are available to estimate stormwater runoff rates and volumes, and the exact methods used for a particular project will depend upon the data available for a given site and the preferences of the qualified professional (typically a civil engineer) performing the calculations.

    LEEDuser has heard from LEED project teams that the LEED expert on the project is sometimes expected to do the calculations for these credits, even if that person isn't a stormwater expert. We recommend a more integrated process in which the civil engineer documents this credit.

Legend

  • Best Practices
  • Gotcha
  • Action Steps
  • Cost Tip

Pre-Design

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  • Consider low-impact development (LID) strategies such as bioretention, vegetated swales, a green roof, rainwater cisterns, and porous pavement. LID strategies can have a wide-ranging impact on decisions including site selection, landscaping, addressing off-site drainage onto the site, space and structural requirements, flood protection, and stormwater discharge locations. Consider this full range of factors in creating the stormwater management plan.


  • You will probably need to go beyond standard practice to achieve this credit, requiring deliberate design and the potential for up-front cost increases. Strategies going beyond standard practice but not likely to incur additional costs include infiltration swales and bioretention areas.


  • Overlapping strategies and technologies address both stormwater credits, SSc6.1 (stormwater rate and quantity), and SSc6.2 (stormwater quality). Vegetative swales, for example, can contribute to both credits—integrate the requirements of both for best results. Keep in mind, however, that each credit requires different calculations and methodologies. Reducing the quantity of stormwater runoff for SSc6.1 does not always equate to a quality improvement for SSc6.2.


  • Approach this credit with an integrated design strategy that incorporates the input of the entire site team, including the civil engineer, landscape architect, and architect.


  • The easiest way to achieve credit compliance is by decreasing impervious area. You can do this by reducing the building footprint and hardscape area, and establishing rain gardens or other bioretention areas.


  • Using site space for stormwater management is often a must. Architects and owners may see stormwater best management practices (BMPs) as wasting valuable land—a mentality that can make this credit difficult. It may help to stress that stormwater BMPs can act as aesthetic features that enhance the quality of the site and add value to the project. Creative, integrated approaches can even reduce space-hogging, unattractive strategies like detention ponds while adding amenities with multiple benefits, like green roofs.


  • Most credit compliance problems are due to stormwater volume reduction, in part because many municipalities are more interested in runoff rate and do not require volume calculations. A civil engineer must run calculations for pre- and post-development runoff rate and quantity, for the one- and two-year, 24-hour design storm. Most jurisdictions don’t require calculations for these specific storm designs.


  • Creative stormwater management techniques such as open channels, eliminating curbs and gutters, and depressed parking islands may reduce construction costs by reducing runoff and the need for more costly infrastructure.


  • Indirect benefits of stormwater systems are just as real as direct costs to the project, but can be harder to quantify. These include issues like reducing the burden on the municipal system; reducing contaminants in waterways; reducing peak runoff, making stream habitats more consistent; reducing the temperature of runoff, which improves the conditions for aquatic life; and reducing erosion.


  • Most municipalities require stormwater documentation. In these cases, the documentation for LEED requirements should not represent a significant soft-cost premium.

Schematic Design

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  • A reliable source for rainfall intensity data is NOAA's Hydrometeorological Design Studies Center Precipitation Frequency Data Server.


  • Having trouble calculating the 2-year, 24-hour storm event? See LEEDuser's guidance on interpolation of rainfall intensity values


  • The owner and civil engineer determine the feasibility and rough costs of appropriate stormwater management techniques. Identifying cost tradeoffs for complementary strategies is a crucial component of the decision process. For example, a rooftop runoff collection system may be more cost-effective when combined with a graywater collection and reuse system. Fully explore the potential for LID strategies such as rainwater cisterns, green roofs, and bioswales.


  • A site visit and tests are integral to understanding the natural hydrology, site topography, and soil infiltration rates.


  • Research local regulations on stormwater reduction requirements, as well as regulations on the collection, storage, and reuse of rainwater. (See Resources for examples.)


  • Determine the imperviousness of the existing site. The Rational Method (see Resources for more information) is most commonly used to determine the weighted runoff coefficient. Then multiply by 100 to get the percent imperviousness. The imperviousness of the site determines which compliance path the project must take.

    • If the average imperviousness of the pre-development site is 50% or less (typically, most of the site is vegetated or permeable), follow requirements for Case 1.
    • If the average imperviousness of the pre-development site is more than 50% (typically, most of the site is hardscape or impermeable), follow requirements for Case 2. Redevelopment projects often fall into this category.

  • The Rational Method is the most common for determining peak discharge rate and runoff volume. It requires the runoff coefficient for each surface type, the total area for each surface type, and the total project area. Runoff rate and volume are directly proportional to landscape or hardscape porosity or perviousness. Undeveloped land has little imperviousness, while previously developed land will have more. However, many materials that seem to be impervious do not necessarily have 100% imperviousness. For example, asphalt will absorb and evaporate some rainfall and has an imperviousness of 85%–95%.


  • Develop a project-wide water budget and a landscape irrigation water budget. This helps teams decide if reusing rainwater may be appropriate and where to use it—typically either in irrigation or toilet flushing.


  • We recommend that the civil engineer conduct a cost-benefit analysis of stormwater-reduction strategies, including cisterns, porous pavement, rain gardens, parking garages (instead of parking lots), detention ponds, green roofs, sand filters, or detention tanks.


  • Research historical climate records to understand the frequency, intensity, and duration of the design storm event. A longer record of daily rainfall events (rather than monthly rainfall averages) will result in more accurate sizing of components like cisterns.


  • Some jurisdictions may have stormwater standards that are similar to the LEED requirements. For example, Portland, Oregon's Title 17, Public Improvements, Chapter 17, 17.38.030 Section C, states that the quantity and flow rate of stormwater leaving the site after development shall be equal to or less than the quantity and flow rate of stormwater leaving the site before development, as much as is practicable.


  • Quantity of stormwater is typically the more difficult measurement for project teams to reduce. Detention basins can help reduce peak flow rate, but they may not reduce overall stormwater quantity. This is a common municipal requirement, and you may need to take additional measures to meet the credit requirements.


  • Integrating the stormwater plan into the design at an early stage and calculating the stormwater reduction percentages significantly decreases additional costs. This way, landscaping and building infrastructure can be designed with stormwater reduction in mind.

Design Development

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  • All Cases


  • Explore potential synergies and tradeoffs with other LEED credits or green building strategies. Items to discuss can include the use of parking lots versus parking garages for stormwater management, trees for shading hardscapes, and avoiding impervious surfaces (SSc7.1), trees for passive solar design (EAc1), plantings with native or adapted plants (WEc1), water reuse (WEc3), and rainwater capture (WEc1).


  • The civil engineer and landscape architect collaborate to design the stormwater systems to meet project goals, using the civil engineer's assessment of how much stormwater may be reduced through nonstructural means, such as increased landscape area or bioswales, and how much must be treated through engineered systems such as rainwater cisterns or green roofs.


  • The civil engineer typically uses a computer program or in-house spreadsheets to calculate the current rainfall and infiltration rates, which helps to determine the best practices and best systems for an individual site. Many projects measure peak flow rates and volumes with the National Resource Conservation Service unit hydrograph method outlined in TR-55. (See Resources.)


  • Existing stormwater management systems can be used to demonstrate credit compliance, provided that the system meets the requirements.


  • Involve the whole project team in integrating stormwater strategies with the site design and structure. For example, calculate a cistern size appropriate for water reuse needs and for rainfall patterns, being sure to allocate proper space. If using a green roof, incorporate structural considerations, planting decisions, and energy impacts


  • In place of elevated planters, grade parking lots and walkways to direct runoff to depressed swales or bioretention areas with perforated pipes and other slow-release infiltration mechanisms. This design is better for stormwater management than typical elevated or impervious planters.


  • Soil type, planting medium and plant species must be considered for their capacity to promote infiltration. For example, clay soils do not allow for good infiltration rates and an engineered soil or compost could be added to allow for better absorption.


  • Detention ponds with controlled release structures only help to reduce the rate of runoff, not the volume. If a detention pond is going to be used onsite, other means of facilitating infiltration must also be used to meet the credit requirements.


  • In urban areas and sites with little land, use a variety of features to achieve project goals. For example, green roofs and rainwater cisterns may be effective in these situations. Capturing rainwater for irrigation reduces the amount of stormwater runoff leaving the site as well as outdoor potable water use. Reusing captured rainwater for toilet flushing has similar effects, in addition to reducing potable water use indoors. In some cases, cisterns with open bottoms may be effective in storing stormwater runoff, encouraging infiltration and reducing the peak flow rate discharge. These cisterns may be incorporated under parking areas or other hardscape.


  • Porous pavement can be incorporated into many sites and climatic conditions. Proper design, installation, and maintenance is important. Work with an experienced contractor, and verify that porous paving will work with your site’s climate and soil conditions. For example, snowplowing, sanding, and salting can damage porous paving.


  • Green roofs can reduce peak runoff rates on developed sites. However, the volume reduction potential of any green roof will depend on its moisture-retention capacity, which depends on the soil profile. One storm may saturate the soil, leading to a conventional amount of runoff resulting from a second storm in close succession.


  • Mitigate cost premiums by getting the most from stormwater strategies. Onsite treatment and retention strategies like green roofs and rainwater cisterns can be costly, but may serve additional purposes and contribute to other LEED credits, including open space requirements (SSc5.2), mitigating the urban heat island effect (SSc7.2), and reducing potable water use for landscaping (WEc1). Features such as constructed wetlands, green roofs, and bioswales can also increase property value. Mitigate cost premiums by designing strategies for multiple purposes.


  • The most cost-effective stormwater management strategies are those that preserve or restore natural site features and promote natural infiltration: reducing hardscapes, designing a smaller building footprint, increasing landscaping area, using porous paving materials, natural swales, and other low impact development strategies. Natural infiltration may also decrease the cost of maintenance compared to other structural and packaged stormwater control systems.


  • Bioinfiltration strategies on streets and parking lots such as bioswales and grass filter strips are alternatives to typical curb and gutter design that allow for infiltration of stormwater, as opposed to conveying the runoff to storm drains. Reducing the number of curbs, storm drains, and piping systems can substantially reduce construction costs.


  • Consider maintenance costs in choosing stormwater strategies. Check with the product manufacturer, designer, or engineer for cost details.


  • Case 1: Existing imperviousness is less than or equal to 50%


  • The civil engineer calculates the pre- and post-development peak rate and total volume of stormwater runoff for the one-year and two-year, 24-hour design storms.


  • The civil engineer verifies that post-development rate and quantity are equal to or less than pre-development.


  • If the stormwater reduction goals are not met, the civil engineer must adjust the design to meet them.


  • If post-development rate and quantity are not equal to or less than pre-development, the option exists for the civil engineer to design stormwater improvements to enable discharge channels from the site to the receiving stream channels to be protected from erosion. The stormwater management narrative must detail the strategies used and how they protect receiving stream channels from excessive erosion. In this plan the civil engineer verifies that post-development stormwater runoff is below critical values for the receiving waterway.


  • Projects using stream protection to achieve the credit must provide a detailed narrative describing how the stormwater management plan protects the receiving waterway from erosion and keeps runoff below critical levels.


  • Projects implementing a stream protection plan must calculate the pre- and post-development runoff rate and quantity for the one- and two-year design storms. The requirements for this plan are fairly vague and dependent on the specifics of the project. Including the percent reductions for rate and quantity along with a description of the project design will help buttress the plan with specifics.


  • Projects using the stream channel protection option need to also implement strategies to reduce the quantity of stormwater runoff, where possible. Typical strategies could include reduced building footprint, reduced hardscape, infiltration areas, or rainwater harvest and reuse. These stategies need to be described in the stormwater management plan and narrative in order to meet the credit requirements.


  • Projects may use a combination of Option 1 (rate and quantity calculations) and Option 2 (stream protection) to meet the requirements of this credit.


  • Case 2: Existing imperviousness is greater than 50%


  • The civil engineer calculates the post-development runoff volume for the two-year, 24-hour design storms.


  • Verify that post-development volume is at least 25% less than pre-development, using site-appropriate stormwater strategies.


  • If the stormwater reduction goals are not met, the civil engineer needs to adjust the design to meet them.


  • Case 2 requires calculating just the volume for the two-year, 24-hour design storm, not the rate or the one-year storm.

Construction Documents

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  • Remember to provide stormwater calculation results in the LEED Online credit form, showing stormwater rate and quantity.


  • If following Option 2 - Stream Channel Protection, don’t forget to provide a narrative describing the project’s site conditions, measures taken, and controls implemented to prevent excessive stream velocities and associated erosion.


  • The civil engineer provides final calculations for the stormwater design. Verify that volume and discharge flow rate reduction goals are met. Be sure that any items removed through value-engineering do not impact stormwater calculations.


  • Maintenance is usually needed for stormwater quantity reduction systems. The civil engineer should develop a maintenance plan shortly after design completion.


  • On the project plans, include all stormwater quantity reduction strategies. Indicate where BMPs are located and what areas they serve.


  • For LEED documentation, the civil engineer needs to fill out the LEED Online credit form, including the pre-development rate and quantity of stormwater runoff, the post-development rate and quantity, and a stream-protection narrative (as applicable). The civil engineer should also provide a copy of the project plans with designated stormwater strategies. (See Documentation Toolkit for samples.)

Construction

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  • Compacted soil from high vehicle traffic prior to or during construction can severely limit natural infiltration of stormwater. Avoid site compaction during construction as much as possible (This also helps compliance with SSc5.1). Aerating soils is not a substitute for avoiding compaction, but can be used to improve infiltration rates.


  • Commission any water reuse systems to ensure that they operate as designed. Include this in the commissioning credits EAp1 and EAc3.

Operations & Maintenance

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  • If relying on natural infiltration in landscaped areas, keep the plants in those areas healthy and avoid soil compaction from vehicle use.


  • Implement a maintenance plan to ensure ongoing, as-designed performance of stormwater systems and equipment. Doing so will also contribute to LEED-EBOM SSc6 compliance.


  • If using porous paving, implement a plan to maintain its porosity. Vehicle use, sand and organic matter, and snowplowing can all damage or reduce the effectiveness of porous paving.


  • Provide maintenance personnel with plans and operations manuals for the operation of all structural control systems.

  • USGBC

    Excerpted from LEED 2009 for Schools New Construction and Major Renovations

    SS Credit 6.1: Stormwater design - quantity control

    1 Point

    Intent

    To limit disruption of natural hydrology by reducing impervious cover, increasing on-site infiltration, reducing or eliminating pollution from stormwater runoff and eliminating contaminants.

    Requirements

    Option 1. Design storms
    Case 1. Sites with existing imperviousness 50% or less
    Path 1

    Implement a stormwater management plan that prevents the postdevelopment peak discharge rate and quantity from exceeding the predevelopmentPredevelopment refers to before the LEED project was initiated, but not necessarily before any development or disturbance took place. Predevelopment conditions describe conditions on the date the developer acquired rights to a majority of the buildable land on the project site through purchase or option to purchase. peak discharge rate and quantity for the 1- and 2-year 24-hour design storms.

    OR

    Path 2

    Implement a stormwater management plan that protects receiving stream channels from excessive erosion. The stormwater management plan must include stream channel protection and quantity control strategies.

    Case 2. Sites with existing imperviousness greater than 50%

    Implement a stormwater management plan that results in a 25% decrease in the volume of stormwater runoff from the 2-year 24-hour design storm.

    OR

    Option 2. Percentile rainfall events
    Case 1. Non-zero lot line projects

    In a manner best replicating natural site hydrology1 processes, manage onsite2 the runoff from the developed site for the 95th percentile of regional or local rainfall events using Low Impact Development (LID)3 and green infrastructure4.

    Use daily rainfall data and the methodology in the United States Environmental Protection Agency’s Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act to determine the 95th percentile amount.

    OR

    CASE 2: zero lot line projects

    For zero lot line projects located in urban areas with a minimum density of 1.5 FAR (13,800 square meters per hectare net), in a manner best replicating natural site hydrology processes, manage onsite the runoff from the developed site for the 85th percentile of regional or local rainfall events using LID and green infrastructure.

    1Natural Site Hydrology is defined as the natural land cover function of water occurrence, distribution, movement, and balance.

    2Manage Onsite refers to capturing and retaining the specified volume of rainfall to mimic natural hydrologic function. This includes, but is not limited to, strategies that manage volume through evapotranspiration, infiltration, or capture and reuse.

    3Low Impact Development (LID) is defined as an approach to managing stormwater runoff that emphasizes on-site natural features to protect water quality by replicating the natural land cover hydrologic regime of watersheds and addressing runoff close to its source. Examples include better site design principles such as minimizing land disturbance, preserving vegetation, minimizing impervious cover, and design practices like rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, and soil amendments. These are engineered practices that may require specialized design assistance.

    4Green Infrastructure is a soil and vegetation-based approach to wet weather management that is cost-effective, sustainable, and environmentally friendly. Green infrastructure management approaches and technologies infiltrate, evapotranspire, capture and reuse stormwater to maintain or restore natural hydrologies (US EPA).

    Potential Technologies & Strategies

    Design the project site to maintain natural stormwater flows by promoting infiltration. Specify vegetated roofs, pervious paving and other measures to minimize impervious surfacesSurfaces that promote runoff of precipitation volumes instead of infiltration into the subsurface. The imperviousness or degree of runoff potential can be estimated for different surface materials.. Reuse stormwater for non-potable uses such as landscape irrigation, toilet and urinal flushing, and custodial uses.

    FOOTNOTES

    1 Natural Site Hydrology is defined as the natural land cover function of water occurrence, distribution, movement, and balance.

    2 “Manage Onsite” refers to capturing and retaining the specified volume of rainfall to mimic natural hydrologic function. This includes, but is not limited to, strategies that manage volume through evapotranspiration, infiltration, or capture and reuse.

    3 Low Impact Development (LID) is defined as an approach to managing stormwater runoff that emphasizes on‐site natural features to protect water quality by replicating the natural land cover hydrologic regime of watersheds and addressing runoff close to its source. Examples include better site design principles such as minimizing land disturbance, preserving vegetation, minimizing impervious cover, and design practices like rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, and soil amendments. These are engineered practices that may require specialized design assistance.

    4 Green Infrastructure is a soil and vegetation‐based approach to wet weather management that is cost‐effective, sustainable, and environmentally friendly. Green infrastructure management approaches and technologies infiltrate, evapotranspire, capture and reuse stormwater to maintain or restore natural hydrologies (US EPA).

Technical Guides

Stormwater Best Management Practices Design Guide, Vegetated Biofilters (EPA/600/R-04/121A)

This guide provides design strategies and techniques on incorporating biofilters in projects.


Bioretention (Rain Gardens) – EPA’s Stormwater Menu of BMPs

This website gives designers and planners information on the appropriate application of bioretention areas.


Model Low Impact Development Strategies for Big Box Retail Stores

This report describes low-impact development approaches to stormwater management for big-box stores.


Urban Drainage and Flood Control District

Technical manuals on stormwater BMP’s as they relate to Denver and surrounding counties.


Clean Water Ways, Stormwater Implementation BMP Guidelines

http://www.cleanwaterways.org/professionals/index.html


Hydraulic Design Manual

The Texas Department of Transportation offers this guide to the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size., which, it notes, is appropriate for estimating peak discharges for small drainage areas of up to about 200 acres in which no significant flood storage appears.


Effectiveness of Urban Stormwater BMPs in Semi-Arid Climates

This design manual provides stormwater information specific to semi-arid climates, including Denver, Colorado.


Denver Water Quality Management Plan, Stormwater Quality BMP Implementation Guidelines, Chapter 6

This design manual provides stormwater information specific to Denver, Colorado.


City and County of Denver, Department of Public Works Wastewater Management Division Storm Drainage Design and Technical Criteria

This design manual provides stormwater information specific to Denver, Colorado.


Stormwater Best Management Practices Design Guide, Vegetated Biofilters (EPA/600/R-04/121A)

This guide provides design strategies and techniques on incorporating biofilters in projects.


Stormwater Management Manual (Portland Bureau of Environmental Services)

This website provides stormwater information specific to the Portland, Oregon area.


Rain Garden Handbook for Western Washington Homeowners: Designing your Landscape to Protect our Streams, Lakes, Bays and Wetlands

A guide to low-impact development for residences.


Maryland Stormwater Design Manual

This design manual provides stormwater information specific to Maryland.


Massachusetts Stormwater Technology Evaluation Project

This website provides stormwater information specific to Massachusetts.


Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act

This technical manual from the U.S. EPA contains background on documenting stormwater requirements through capturing the 95th percentile storm using onsite management practices.


International Stormwater Best Management Practices (BMP) Database

Features a database of over 500 BMPBest Management Practice studies, performance analysis results, tools for use in BMP performance studies, monitoring guidance and other study-related publications.

Articles

EPA Urban Stormwater BMP Study

This article provides a preliminary study on BMPs.

Organizations

EPA Low Impact Development Website

EPA provides valuable information on low-impact development through fact sheets, design guides and cost estimates for low-impact development strategies that reduce stormwater runoff.


National Pollutant Discharge Elimination System (NPDES)

EPA offers help on managing stormwater, including fact sheets on the six minimum control measures for best management practices.


Stormwater Manager’s Resource Center

This site for practitioners and local government officials provides technical assistance on stormwater management issues.

Web Tools

International Stormwater Management Best Practices Database

This database provides studies and analysis on BMPs and is intended to improve design.


Technology Acceptance and Reciprocity Partnership

This website provides information on the performance of technologies in a number of states across the U.S.

Publications

Stormwater — A Journal for Surface Water Quality Professionals

Online magazine for stormwater professionals.

Stormwater Management Report

All Options

A stormwater management and drainage report covering both SSc6.1 and SSc6.2 can document all aspects of credit compliance.

Design Submittal

PencilDocumentation for this credit can be part of a Design Phase submittal.

6 Comments

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Nicole Ferrini via Greengrade LEED Management Software
Aug 01 2013
Guest
306 Thumbs Up

Peak Discharge Rate

We are working on a LEED for Schools 2009 project on a greenfield site and are targeting this SS 6.1. We are retaining 100% of stormwater on site through various methodologies including stormwater cisterns, vegetated swales and on site ponding. ZERO water will run off our site. Despite this fact, our civil engineer has told me that when he runs the Pre / Post Discharge Rate Calculations the Post is more than the Pre. I am not a civil engineer, but this does not make sense to me. We feel as though we've met the intent of the credit. Can someone explain if it is our strategy that is wrong or are we missing something in the calculations? Thanks!

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Michael DeVuono Senior Staff Designer, T&M Associates Aug 02 2013 LEEDuser Expert 1853 Thumbs Up

Something is not correct with this, but I would need to see the calcs and the drainage plans. What storms have 0 discharge? All of them? Are you using Case 1, Option 1?

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Mary Wohnrade
Sep 24 2010
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SSc6.2 - Calculate % of Annual Rainfall Treated by BMP

I have not found a good example of how you calculate the % of Annual Rainfall Treated by BMPBest Management Practice. I'm guessing that it has to do with the tributary area draining to each particular BMP. So if 5.0 acres of runoff is draining to a bioretention area, and the total area of the project LEED boundary is 10.0 acres, then the % of Annual Rainfall treated by the bioretention area is 50%. Can anyone confirm thIs is correct? Thanks.

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Gregory Hurst Office Manager, Robert Peccia & Associates Oct 08 2010 LEEDuser Expert 930 Thumbs Up

The response to your question is a little more complicated than just calculating areas of watershed.

First, the effectiveness of the BMPBest Management Practice must be determined and the percentage of annual rainfall. LEED provides some guidance concerning the average annual rainfall depth, where LEED suggests that the depth of treatment for a humid area is 1-inch, semi-arid is 0.75 inch and arid is 0.5 inch. The IMP (Integrated Management Practice) treatment train must be designed to remove 80% of TSSTotal suspended solids (TSS) are particles that are too small or light to be removed from stormwater via gravity settling. Suspended solid concentrations are typically removed via filtration. for this run-off volume (rainfall depth x area x run-off coefficient x BMP effectiveness). Table 2, page 104 of the LEED Reference Guide provides some guidance on the average TS removal for different BMP’s. This process will provide you with the level of conformance that you can reasonably expect to meet for your project for the BMP’s that you have selected.

If a portion of the project site is not providing any treatment, then the reduction in TSS that you are achieving is reduced by the portion of the site for which you have no treatment. If the portion of the site that is being treated is being treated to a higher level than 80% TSS removal, you can claim some credit against the portion of the site that is not being treated.

So, you are correct that if 5.0 acres of runoff is draining to a bioretention area and the total area of the LEED boundary is 10.0 acres, then the % of Annual Rainfall treated by the bioretention area is 50% assuming a similar run-off coefficient for all of the site boundary. However, it is possible that you are not meeting the requirement for 80% TSS reduction for 50% of the site, depending on the size and effectiveness of your bioretention area, or it is possible that you are achieving more than 80% reduction for 50% of the site.

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Gregory Hurst Office Manager Robert Peccia & Associates
Feb 24 2010
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Soil Infiltration

You bring up a good discussion. Typically when SSc6.1 is evlauated, you can use the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size. to determine the peak rate and volume of runoff for the 1 and 2-year, 24 hour storms or the NRCS method is used for volume of runoff. Both of these methods consider infiltration in developing their respective results, NRCS through soli types, and Rational Method through the runoff coefficient.

However, your question seems to imply that you would like to consider the use of a BMPBest Management Practice or LID strategy to help meet the peak flow and volume reduction criteria in your discharge calculations. Certainly, the soil infiltration will have an effect on how effective these BMP's are in minigating the flow and volume; if the soil is a clay that does not infiltrate, the BMP will still be mostly full when the next storm hits and the BMP will not be effective in reducing the flow or volme. On the other hand, if the soil allows stormwater to infiltrate quickly, it will be very effective. Many times, the soil can be modified to provide greater infiltration.

The end result is that the only way that I know to determine the iniltration rate for a BMP is to have a permeability test completed using standard procedures. Our experience is that even this approach may only be useful during the early life of the BMP, and the permeability of the soil may change basee on the sediment that the BMP is collecting. Maintenance is the key to long term BMP effectiveness.

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Landon Chapman Graduate Architect, LEED AP MSKTD & Associates, Inc.
Feb 23 2010
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Soil infiltration

Can someone direct me toward appropriate calculations to determine infiltration rates for various soil types? It seems that this calculation would be critical in determining whether the proposed low-impact strategies will be sufficient, however I can't seem to find the needed information.

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