CS-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.

  • 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.)

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

Construction

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

  • 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.

Operations & Maintenance

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

  • 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.

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

  • USGBC

    Excerpted from LEED 2009 for Core and Shell Development

    COPYRIGHT © 2009 BY THE U.S. GREEN BUILDING COUNCIL, INC. ALL RIGHTS RESERVED

    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.
    23 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).




    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

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.

ASHRAE 90.1 - 2007 Energy Standard for Buildings Except Low Rise Residential

This is a widely used standard for the design of HVAC systems.

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.

Effectiveness of Urban Stormwater BMPs in Semi-Arid Climates

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

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.

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.

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.

Articles

EPA Urban Stormwater BMP Study

This article provides a preliminary study on BMPs.

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.

LEED Online Forms: CS-2009 SS

The following links take you to the public, informational versions of the dynamic LEED Online forms for each CS-2009 SS credit. You'll need to fill out the live versions of these forms on LEED Online for each credit you hope to earn.

Version 4 forms (newest):

Version 3 forms:

These links are posted by LEEDuser with USGBC's permission. USGBC has certain usage restrictions on these forms; for more information, visit LEED Online and click "Sample Forms Download."

Design Submittal

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

21 Comments

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Mohammed Magdy LEED AP Al Yamama Construction
Jun 17 2013
LEEDuser Member

Discharging the rainwater to municipality facilities.

Does discharging the collected rainwater to municipality facilities will help with achieving the requirements for credit 6.1 & 6.2?

we are following Case1 - path 1,
we will reclaim the rainwater from the building roof but we can't reclaim it from the whole site due to it's huge capacity and also we can't infiltrate it through the soil due to it's type.

Post a Reply
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0
Ronald Dean Sumac Inc.
Feb 28 2013
LEEDuser Member
177 Thumbs Up

Low Anual Rainfall Average

Our project is located in a city with a very low annual rainfall average (8.3mm that is about 0.327 inches per year). The project will have a green roof in about 60% of the roof areaRoof area is the area of the uppermost surface of the building which covers enclosed Gross Floor Area, as measured when projected onto a flat, horizontal surface (i.e. as seen in Roof Plan view). ‘Roofs’, or portions of roofs, covering unenclosed areas (e.g. roofs over porches and open covered parking structures) are not included in the areas used to evaluate compliance with SSc7.2, though they may be applicable to SSc7.1.. It is possible to direct the slopes of the other 40% of impervious area of the roof to the green roof. These will be sufficient to control runoff from all the rain water. It is possible to apply to the credit under these conditions.
Thanks.

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Luis Miguel Diazgranados Green Factory
Jan 15 2013
LEEDuser Member
736 Thumbs Up

Undefined Outdoor Spaces

Greetings Everyone

I'm working on a office project with outdoor spaces within the LEED Boundary. The project's construction phase is almost finished. Originally this was going to be the only building in the project, but during construction, the owner bought the adjacent land and decided to build a second building which is still on early design phase.

Originally a design for the outdoor spaces was made by the architects, but the owner idea is that both building will be part of a same development, and will share some of their outdoor spaces. Therefore, the design and construction of some of these spaces have been halted and will resume when the second building's construction begins.

Since the spaces I'm speaking about are inside my project's LEED project boundary, but their design will not be defined until some time in the future I don't know how to account for it in this and other credits. Should I exclude this spaces form the boundary? Should I assume a totally impervious surface (performing calculations with the worst case scenario would assure that any other material installed will comply)? Any ideas?

Thanks in advance for your help.

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Tristan Roberts LEED AP BD+C, Editorial Director – LEEDuser, BuildingGreen, Inc. Mar 22 2013 LEEDuser Moderator

Luis, I think you'd need to either change the current project's LEED boundary, or pause your documentation until the design is clarified.

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Brian Wolfe Sustainable Coordinator HKS
Aug 24 2012
LEEDuser Member
20 Thumbs Up

Pond use outside LEED boundary

Is it acceptable to utilize an existing detention pond that is outside the proposed LEED boundary? The stormwater runoff will be piped under the existing road to the pond.

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Emily Catacchio Sustainability Specialist, Wight and Company Aug 26 2012 LEEDuser Moderator

Yes, as long as you can show that it is designed to handle the stormwater from your site.

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Noriko Yasuhara CSR Design & Landscape Co., Ltd.
Jun 19 2012
LEEDuser Member
398 Thumbs Up

Pump up stored rainwater to local drainage system

Our project has a big rainwater collecting tank which has enough capacity to store 2-24hr design storm, but the tank doesn't have function of infiltration and all the stored rainwater will be pumped up and discharged to local sewage system. Does it meet this credit's requirement?

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Tristan Roberts LEED AP BD+C, Editorial Director – LEEDuser, BuildingGreen, Inc. Aug 30 2012 LEEDuser Moderator

Noriko, I would say this fits with Case 1, Path 1, but not with the other credit options.

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Therese Malm WSP Environmental
Mar 07 2012
LEEDuser Member
55 Thumbs Up

Calculation of drawdown rate Qr

How is design storm interval determined? I looked into the LEED BD+C Reference Guide on page 97 in the example (last calculation) at the top of the page. From where do the 3 days / 72 hrs come?
I have an infiltration pond and it can hold the volume that is calculated, but it needs to be emptied before the next storm arrives. Is 3 days a default value or how can I determine the time to use between two storms? The actual project is in Warszawa, Poland.

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Amy Rider Sustainability Manager, KEMA Services Aug 31 2012 LEEDuser Expert 1410 Thumbs Up

Therese,
The days is not a default value, but an example from the Maryland Stormwater Design Manual.
Also in this manual is this definition, "The detention lag time (T) for the one-year storm is defined as the interval between the center of mass of the inflow hydrograph and the center of mass of the outflow hydrograph." (p. 48). Does that help?

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Angela Saggin
Feb 24 2012
Guest
235 Thumbs Up

Calculations

Hi!

I'm having some difficulties with the calculations, specially with the meaning of each variable in each equation.

In equation 1, the 'P' (average rainfall event), should we consider the year average or the day average? The building is located at northeast region from Brazil, here it rains a lot during january to may, but the rest of the year it doesn't rain, so the day average isn't a real data.

And in equation 2, what is the "rainfall event interval"? Is it the average duration of the rain or the interval between two different rain events?

Thanks!

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Amy Rider Sustainability Manager, KEMA Services Aug 31 2012 LEEDuser Expert 1410 Thumbs Up

Angela,
I would use the seasonal or monthly average for your rainy season. The rainfall event interval is the period of time between two rain events.

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Markus Henning M.Eng Facility Management Alpha Energy & Environment
Feb 10 2012
Guest
77 Thumbs Up

Existing Imperviouness

My question is. The Guide says choose Case 1 Sites with Existing ImperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground. 50%or less or Case 2 Sites with Existing Imperviousness greater than 50%.
Refers the Existing Imperviousness [%] to Pre or Post development of the site?
Our project is located on a green space with large parts of meadow. Must we use Case 1 or Case 2.

Thank you for any suggestions.

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Emily Catacchio Sustainability Specialist, Wight and Company Feb 10 2012 LEEDuser Moderator

Existing means "pre," as in, before your project. Does that help?

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Citlali Mora Bello Architect, LEED Green Associate SYASA - México
Oct 08 2011
LEEDuser Member
1654 Thumbs Up

Using a water table tank as a retaining method for compliance

Our project is capturing rainwater and sending it to a water table tank (I don't know if this is the correct term, it is the tank that recieves water from the water table below foundations).

The tank is big enough to comply with credit compliance, but I am wondering if LEED requires a specific tank dedicated to captured rainwater or if we can use this water table tank to fulfill credit requirements.

Thanks

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David Posada Sustainability Manager, GBD Architects Oct 10 2011 LEEDuser Expert 11746 Thumbs Up

David,
As long as the tank is sized big enough for both functions it should be fine. We've seen storage tanks that have multiple functions being allowed - for example, a tank that originally was only to be used for storing fire protection water was increased in size and used also to store rain water.
In our area, we've used the term "de-watering" for ground water that is pumped out from below a foundation, but it sounds like the same thing.

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Jiri Dobias
Jun 22 2011
LEEDuser Member
477 Thumbs Up

Retention tank

Hi all,
We are working on a project in Prague with imperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground. >50% (Case 2). We would like to build a retention tank which will collect rainwater and than slowly release the collected water to a public drainage system. The collected water will be used for irrigation (WEc1a and WEc1b) as well but the irrigation system will use LESS than 25% of the collected rainwater.
Are we eligible for this credit if we decrease stormwater runoff more than 25% during a storm (and therefore decrease burden on public drainage system) but afterwards slowly release the water from the tank in order to be ready for the next storm?
Thank you for any suggestions.

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Susann Geithner Director of Sustainability, HSB Architects & Engineers Jun 22 2011 LEEDuser Expert 7979 Thumbs Up

Delaying the discharge of the stormwater means you reduce the rate. To use it for irrigation means you will reduce the volume. Since you have to use case 2, you will have to reduce the volume by 25%. If the irrigation isn't enough maybe you can improve the imperiousness of your site. More vegetated areas, open grid paving. If that's not an option you could also use infiltration wells or other underground infiltration systems to achieve 25% reduction. We did a project in Munich, which was using an infiltration system (rigole).

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Citlali Mora Bello Architect, LEED Green Associate SYASA - México
Mar 23 2011
LEEDuser Member
1654 Thumbs Up

Retaining rainwater for future uses

Will capturing all rainwater and keep it in a tank for future uses such as cooling towers and irrigation? Our project will not infiltrate it but reuse it instead.

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David Posada Sustainability Manager, GBD Architects Mar 25 2011 LEEDuser Expert 11746 Thumbs Up

Yes, capturing and storing rain water for irrigation, cooling towers, etc is a great way to meet the storm water quantity reduction. The most common problem is not having a storage tank that is big enough to prevent over flowing in typical conditions.
You will want to show calculations of the 1- and 2- year design storms, but also show that you are using the water from that tank quickly enough that it will have room for the next storm event. There's a related conversation about a similar situation over at NC 2009 SSc6.1 from this month, and also one from Jun 15 2010.
Hope that helps!

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Citlali Mora Bello Architect, LEED Green Associate, SYASA - México May 11 2011 LEEDuser Member 1654 Thumbs Up

I just checked the NC2009 discution and it is really good, the one from june. The case of having some use for the water that has been just collected before the next event is perfectly clear.

I'm thinking about this:

Our project is a mixed used with several buildings in it. Only one bulding (with its related grounds) will apply for certification.

The rainwater collected within the LEED boundary will be used for cooling towers and irrigation. However, the real demand for cooling towers will be supplied almost in its entirety by treated water so I am almost 100% sure that some rainwater collected will remain at the water collection tank. I was wondering if use this remaining water for other buildings' needs will satisfied the intention of this credits. The water will leave the LEED boundary however it will not reach the municipal sewage nor any other municipal system.

I don't know if I explained myself properly.

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