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.
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.
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.
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.
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."
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
Having trouble calculating the 2-year, 24-hour storm event? See LEEDuser's guidance on interpolation of rainfall intensity values.
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.
A reliable source for rainfall intensity data is NOAA's Hydrometeorological Design Studies Center Precipitation Frequency Data Server.
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.
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.
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.
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.)
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.
Maintenance is usually needed for stormwater quantity reduction systems. The civil engineer should develop a maintenance plan shortly after design completion.
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.
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.
Excerpted from LEED 2009 for New Construction and Major Renovations
To limit disruption of natural hydrology by reducing impervious cover, increasing on-site infiltration, reducing or eliminating pollution from stormwater runoff and eliminating contaminants.
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.
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.
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.
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.
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).
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.
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 manuals on stormwater BMP’s as they relate to Denver and surrounding counties.
This guide provides design strategies and techniques on incorporating biofilters in projects.
This website gives designers and planners information on the appropriate application of bioretention areas.
This report describes low-impact development approaches to stormwater management for big-box stores.
This design manual provides stormwater information specific to Denver, Colorado.
This website provides stormwater information specific to the Portland, Oregon area.
A guide to low-impact development for residences.
This design manual provides stormwater information specific to Maryland.
This website provides stormwater information specific to Massachusetts.
This design manual provides stormwater information specific to semi-arid climates, including Denver, Colorado.
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.
This technical manual from the U.S. EPA contains background on documenting stormwater requirements through capturing the 95th percentile storm using onsite management practices.
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.
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.
EPA offers help on managing stormwater, including fact sheets on the six minimum control measures for best management practices.
This site for practitioners and local government officials provides technical assistance on stormwater management issues.
This database provides studies and analysis on BMPs and is intended to improve design.
This website provides information on the performance of technologies in a number of states across the U.S.
Online magazine for stormwater professionals.
This article provides a preliminary study on BMPs.
A stormwater management and drainage report covering both SSc6.1 and SSc6.2 can document all aspects of credit compliance.
Create documentation quantifying stormwater volume and peak rate mitigation strategy.
The following links take you to the public, informational versions of the dynamic LEED Online forms for each NC-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."
Documentation for this credit can be part of a Design Phase submittal.
My project features raised green roof beds (6" deep soil) located directly downstream from a tin roof. There are no gutters and for the most part runoff from the roof runs directly into the roof beds, the excess runoff from the green roof then runs into an order flat roof where it is directed to standard storm drain. My project also has a roof terrace surrounded on the perimeter by green roof bed as well. The roof terrace rain scuppers are located in the green beds, thus the runoff from the flat terrace must pass through as well. For the purpose of calculating my BMPBest Management Practice area for capture, storage, and treatment, can I consider the sloped roof as an addition to my 1st green roof bed, and the roof terrace as addition to my 2nd bed? The EPA National stormwater Calculator doesn't have a capture ratio for green roofs, although it does have a capture ratio for rain planters. But the rain planter model requires a permeable surface beneath something I don't have.
I'm not sure I follow your question completely ....
But area is simply = (L)(W)
Volume = (L)(W)(6/12)(some void ratio dependent on the media)
Your rooftop will runoff into the green roof, which I would model as a storage basin when doing your calcs.
You really should not be using the EPA calculator for actual design purposes.
I wasn't using it for design purposes, but simply to evaluate the performance of the existing design in regards to its ability to capture the 2-year, 24-hour storm. Incidentally, the EPA stormwater caculator doesn't give results for the 2-year, 24 hour storm (silly I know), so I ended using the web-based CMT calculator (the national version, not the Chicago version). But I will follow your suggestions, and treat my green roof beds as something else, see what happens. Thanks.
your comment above states: 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.
can you provide more guidance on this? Our Civil Engineer does not agree with this as an acceptable strategy.
This is a common stormwater management approach that has been in practice for the past 40+ years. Retain stormwater in something (basin, tank, cistern, milk jugs, whatever) ... and slowly release into something, whether it is a storm drain, stream channel, river, etc. so that the rate does not exceed that before development.
The key word here is rate, this does nothing for volume, however.
Depending on the option being pursued you also need to reduce the QUANTITY... slowly releasing it reduces the rate but not necessarily the quantity...
thanks for your quick reply!
In case 1, we are required to perform a site run off calculation for 1 yr-24 hr design storm and 2 yr-24 hr design storm for both pre- and post- development period. can you pls clarify how this actually determines the cf/storm quantity? We are having difficulty understanding the difference.
Stormwater runs off with a rate and volume.
How this rate and volume is determined requires about 9 semester hours of hydrology courses, and is beyond the scope of this forum. I recommend you consult with a civil engineer to perform this work for your project.
Hi, there has been some previous discussion in this topic, but no definite answer has been given. Example 2 in the ref. guide uses 1" storm, referring to it as 90% of the average rainfall event. However, the credit requirement for SSc 6.1 requires the usage of the 24hr storms. It is quite confusing whether this calculation would meet the requirement, which they give as an example. Can we use the 90% of the annual rainfall events??
This credit uses the 1 and 2-year 24-hour design storm for your project area.
That example simply shows that the raingarden being designed, holds the volume of 1" runoff, which coincidentally is the 90th percentile storm. The examples do not set the credit requirements, the requirements page does that.
We are working on previously undeveloped land project, which apply for case 1 “site 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”. With the new project, the imperviousness will increase, and all the rain water from terraces, roofs and hardscapeHardscape consists of the inanimate elements of the building landscaping. Examples include pavement, roadways, stone walls, concrete paths and sidewalks, and concrete, brick, and tile patios. will be collected and filtrated (to remove suspended solids) and at the time, the water will be directed to an adjacent river as the only strategy in the stormwater management plan. Is this last point allowed to achieved the credit, as we are directing the water to a natural drain system and not to the public sewer?
Thanks and regards!
My project site is composed of sand,gravel.
How do I define the run off coefficient?
Where can I get it?
I'm not experienced enough to work LEED project.
Might be a poor question..sorry.
Curve numbers (CN) are obtained from TR-55
In my region, the DP requires that the major storm system be designed to convey runoff from the 1:100yr event (and minor system to convey the 1:5yr event). Acknowledging that 1:100yr is a statistic probability of that storm, the systems are nonetheless designed to manage the amount of precipitation that would exceed any lesser events.
Would our design, then, meet the SSc6.2 requirement to prevent post-development peak discharge rate and quantity for the smaller 1 and 2yr storms? Or, does LEED require that we manage two(2), 1:100 year back to back events?
Lindsey, that is impossible to say, as these storm events are not linear in nature.
You need to have whomever did your stormwater design for the 5 and 100-year storms submit calculations for the 1 and 2 year storms in order to qualify for this credit.
Would a 100 year storm not drop more precipitation than a 1 and 2 year storm?
Yes it is a higher rainfall depth. But Stormwater design is not a linear relationship. Frankly, if I were given the requirements to only reduce rate for the 5 and 100 year storms, the 1 and 2 would likely show a rate increase, because I would not detain those storms at all.
You need to submit calcs for the 1 and 2 year storms. Anything else will cause you to be denied for this credit.
As part of NYC's blue roof requirement, roofs must provide 24 hour detention detention for the 2-yr design storm and an equivalent peak runoff rate equal to a curve # of approx 50 for projects discharging into a combined sewer. However the template only looks at runoff volume pre versus post. How do I convert my runoff rate into a runoff volume? Multiply by 24?
Rate does not convert to volume. Have your CE develop a hydrograph which will give you the volume.
I am trying to document a project under option 2 for this credit for a zero lot line project. Does anyone know what objective data compliance for this option is based on? Do we have to be able to keep all of the 85th percentile rainfall event on site through storage, vegetated roof or some other design strategy? OR Do we just have to document the strategies that we are using? Any experience that can be shared would be greatly appreciated. Thanks!
You need to have your CE document that you are not blowing out the receiving channel's capacity or causing an erosive condition due to increased velocity and rate.
Look below for a more detailed response.
Why is equation 1 divided by 12" ?
P is in inches and you're solving for cubic feet.
To be clear, you are not dividing by 12" either.
If P = 2 inches 2/12 is the same number in feet.
We certify a project that will have two extensions in the future. Because of this we cannot earn crédits SSc5.1 and SSc5.2 as per the MPR. But I was wondering if this is the case for SSc6.1?
Do I have to calculate post-development run-off by considering the future extensions (this would obviously make my compliance much harder but I could understand the logic of such requirement...)?
Thank you for your help,
Are you referring to MPR 2? Building in its entirety?
If your answer is yes, then yes, your stormwater calcs must be based on full build out. You can't show compliance for half the building, then build the remaining half down the road without improving the site SWM.
This isn't just a LEED thing either, it is protection for the health, safety, and welfare of those downstream.
Did I get this right? The building will be certified as a whole, but built in phases?
The building and its site will be certified as a whole. 2 future phases are planned but they are not in the scope of the present certification work, they will be coming probably years after the start of operation of this first phase. Therefore there are 2 large areas of green spaces waiting for this to happen much later.
The MPR I refer to is found in supplemental guidances of MPR, in the section dedicated to phased projects. There, you can find that credits SSc5.1&5.2 are not elligible in such projects and it makes sense.
I assume that SSc6.1 is elligible but that the calculation conditions of the post development runoff must include the future buildings in order to make sense. But I was not entirely sure. I agree with you on the point that apart from LEED perspective it should be done this way to ensure health and safety. But the credit language is sometimes another thing.
What do you think in light of these explanations?
Valentin, I am not sure how this should play out in regards to LEED. It is my opinion, that you should account for the future development, if you can do so reasonably, and if you are positive that these future phases will be constructed.
I do not think it would be unreasonable to prepare two SWM designs, one that addresses the current project, and a second that is a "retrofit" that addresses the expanded impervious surface, and is to be implemented at the time of the future expansion. This sounds like a great question for the GBCI technical staff so you do not waste too much time in design going down the wrong path.
Thank you for your advises!
Wish you great new year,
Sorry, might be a stupid question:
in our case the pre-development is a demolished building complex.
Do we calculate the post-development against the previous development or the undeveloped conditions?
As far as LEED goes, your pre-development is based on what is on the ground right now, before you start construction. However, check with your local regs, as well as NPDESThe National Pollutant Discharge Elimination System (NPDES) is a permit program that controls water pollution by regulating point sources that discharge pollutants into waters of the United States. Industrial, municipal, and other facilities must obtain permits if their discharges go directly to surface waters. requirements, which usually make you take a portion or all of existing impervious or other cover types back to meadow.
Can we obtain this credit for a site that already has natural drainage systems surrounding it, i.e. outside the site boundary?
The water that falls on the site is taken care of in a natural way. There are currently ditches around the site that lead runoff water to nearby ponds. The stormwater runs down to the pond and is infiltrated there. The site area is de-watered via ditches and culverts to a stream as well as a lake.
Rainwater that falls on e.g. car parks and sources of pollution (in this case diesel aggregates and tanks) will be 'cleaned' on site, i.e. only cleaned water will run off to the ponds, streams etc. It is then let out into the stormwater system. The stormwater flow will be delayed to ensure that the current rate will not increase in a 10 year storm event
The offsite pond is dimensioned for a 10 year storm. The site area is included [in the area that the system can take care of] and the amount of runoff will be accounted for, by including detention depositories in the design.
Will this be acceptable for obtaining this credit?
So are you implementing swm controls on site or not? I'm sorry, but your statement above is not very clear.
Be advised, 6.1 deals with only runoff rate and volume, not WQ.
The flow rate will be dealt with onsite, but the volume will be dealt with offsite.
If you are using an offsite location to control volume, but the increased volume is being carried to this offsite location via an existing drainage swale, you do not comply with this credit. You will be exceeding the pre-development flow through the channel.
A project I'm working on is factory on a rural setting. All roads and parking lots are gravel. Does this account as pervious paving? I'm certain it should, and it should count towards reducing peak discharge (as opposed to a paved road) and also as a water quality BMPBest Management Practice. Am I right? Thanks!
In regards to 6.1, you are really only worried about the CN for gravel, which is between 76-91 depending on the underlying soil. The lower CN (as opposed to 98 for impervious) will result in lower discharge rates and volume, this is the fundamental concept behind SWM.
A water quality BMPBest Management Practice is a feature that treats your surface runoff, a gravel road by itself is not treating anything, especially if it exists in the pre-development. You could do a trench under your parking lot, but without paving above you will have no structural support for vehicles.
We are pursing Option 2 for this project. Our stream channel protection will handle a 10 year storm event. How does the credit handle excessive storms, 50 or 100 year events? Or are they not a consideration for this credit?
One of the most vague options in the LEED rating system.
You need to demonstrate that you are not exceeding the critical capacity of the receiving stream channel, nor are you causing an erosive condition by increasing velocity in the channel.
SSc6.1 Case 1-Option 2 requires the project to 1) include stream channel protections that prevents excessive erosion of the stream bed and banks and 2) pursue stormwater quantity control strategies, so a project must make a concerted effort to reduce both rate and quantity of stormwater runoff from the project site to the receiving water body, similar to the requirements in Option 1, but with obviously less restrictions. The LEED Rating System doesn’t define how a project meets Case 1 - Option 2 because the Rating System can’t define the appropriate level of runoff reduction that is required in order to maintain the in-stream habitat/channel protections for any given water body, which can be defined as the critical capacity value. Oftentimes a municipality can provide this information or the stormwater designer/civil engineer can calculate the critical capacity for a receiving water body.
Honestly, this is/should be a very complex analysis of pre and post development stream morphology and aquatic habitat, but I just don't think GBCI is getting that deep with this analysis. If you have your CE prepare a report that shows some rate and volume reduction, a capacity analysis of the channel, show that it works....you should be good.
In your scenario, if you are causing an increase in runoff rate and volume for the 50 and 100 year storms, you are causing a downstream impact that does not exist today.
Hi, We are willing to use an innovative type of green roof that's based on compacted plastic waste board and textile (1,3cms thickness + textile). This substrate replaces the need of soil.
Can this count for Storm water credits, quantity and quality?
As it has vegetation, that can grow Sedums and grass...
I would take a CN credit for the green roof to reduce your overall peak rate and runoff volume totals. A green roof is not really a water quality measure though, and I'd be willing to bet the pollutant load is very similar to that of a roof.
In simple terms, is there a difference between 'discharge rate' and 'run off'? We are currently assessing our site to determine which case and path to follow. Discharge rate is the terminology used in case 1 option 1, run off is used in case 2.
Run off is simply describing the stormwater that "runs off."
Stormwater runs off with a rate (cf/sec), and a quantity or volume (cf).
Hello, We have a project in Sri Lanka where we try to attempt this credit via percentile rainfall event option. However we find difficulty of getting continuous 30 year rainfall data. There are some months that the rainfall data are not available. So in that case can we still calculate the percentile storm event using the available data or attempt to consider about extra couple of years (say 32 years instead of 30 years) in order to compensate for the missing months of data?
This seems reasonable to me, provided it is not the same time period missing every year.
If it is just random months, I think you will be able to zero in on a value even without going past 30 years, but if you have the additional data, you may as well use it.
Thanx Michael. I mean I wasn't sure if there is a minimum number of data points required for this calculation to be accurate on statistical basis. Do you know about such requirement?
As my statistics professor used to say "the more, the better."
As far as LEED is concerned, I do not believe there is specific number of data points required.
I will look at the EPA document (which will be the standard for v4 rainwater) they have a methodology outlined in there, they may specify a certain number of data points, but I think they just say 30 years as well.
This is the EPA Section 438 document. It has methodology for calculating percentile storms, with links to other docs to assist with the methodology. LEED v4 will be based on this document,
When following the ACP, the LEED manual states that attenuation must be undertaken through Low Impact Development (LID) and green infrastructure strategies. In our development we are using large attenuation tanks which serve the adiabatic cooling for the data centre and the toilets flush and which discharge into an existing attenuation pond designed for the whole of the business park where the development is located. Would these measures qualify like LID’s.
Thank you very much
So runoff from the site is reused on site for toilet flushing and cooling towers?
Any unused water is slowly discharged over time?
I would consider this applicable, provided you meet all the peak rate and volume calculations.
I am working on a project in the UK and we are struggling to get the 1 year 24 hours rainfall. I have been told by the Environmental Agency in the UK that the software used by the classes a 1 year event as ‘commonplace’, i.e. is likely to occur each year, and so doesn’t calculate a return for it. The forecasting team normally deals with return periods of 10 years or more, with the software having a lower limit of 2 years for calculations. I understand that both 1-year-24-hour and 2-year-24-hour are required and I wonder if anyone has experience in getting this data for the UK. We are pretty confident that our site will comply with the credit requirements, but demonstration of compliance is proving complicated so any advice would be much appreciated.
Alicia, there is an FAQ above which addresses international projects. I don't know if it will help your situation specfiically, but it's worth reviewing.
My project is using Option 1: Design stroms,Case 2 :Site 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. mote than 50%
LEED online template V 5.0 Table 6.1-3 Site runoff: Two year,24 hour Design storm quantity is requested to fill in cf/storm
Just wish to re-confirm that the cf/storm stand for cubic feet per storm?
Civil engineer had provided me a value in cubic meter 8900m3 per storm.
Am I correct to fill that in form as 314300 cf/storm (using 1 cubic foot = 0.02831685 cubic meter)
Thank you Michael .
LEED 2009 NC online template version 5 mentions cf/storm and not cfs.
am i missing something?
How to convert m3 into cubic feet/sec now?
It appears they are looking for total volume (quantity), not the rate, if that is the case the units are simply CF, convert your m^3 to f^3. I would put the total volume in this block and move on.
CF/sec measures the rate, the volume that runs off per second.
I am working on cleaning up a lot of this language for v4, so it is in actual engineering terminology.
We have a new construction project that is to build 30 ha factory, however the owner decided only to pursue LEED certification for the office building site, a small portion of the factory site which only has +/- 9000 m2 (building area to LEED site area ratio is 43%).
We decided to calculate the pre- and post-development site runoff only at the LEED office building site, not the entire project, since the LEED project boundary includes the office building site area only.
And the result is that the post-development site runoff is higher than the pre-dev, so we should opted the Stream Channel Protection right?
In this factory site, an underground drainage system are already built. This drainage will lead the stormwater from roofs and hardscapes (internal roads, parking) into an artificial retention pond located outside of the site project which also owned by the same owner of this project. So, the stormwater from the LEED site area will also flow there. No water will flow directly into public drainage system.
For Stream Channel Protection, we have an idea to install an underground tank that will collect stormwater from office building's roof and LEED site area's hardscape (road), and later be used or infiltrate on LEED site area (landscaping). Is this stormwater management plan can be enough for the Stream Channel Protection? Besides this description, what other aspects do we have to input to the stormwater management plan?
And in the leedonline form, it indicates that calculation of the "Critical capacity values for receiving streams demonstrating that waterways can accommodate the runoff values" should be included in the plan. Do you have any ideas on how to measure this? Thanks
Refer to previous posts on how others have handled SWM in a campus setting.
Hi Michael, thanks for your quick response. Unfortunately, I only found a thread in this page related to campus (Lisa Sawin's post), and I did not get the idea of SWM on her project. Actually, I get confused by the term campus as I'm kind of new in LEED. Can you enlighten me with this?
By the way, what do you think about our SWM idea of installing underground tank for Stream Channel Protection? and lastly, how to measure the critical capacity values? I'm truly sorry for this too many questions.
It is right above, in the FAQ.
An underground tank will assist in solving your peak rate runoff, but it does not control the volume. You need to control the volume through infiltration or reuse. If you have good infiltration rates where you want to put the tank, I suggest looking at a pre-fab arch system, they are open bottom, and allow for infiltration.
As for how do you size this, I would suggest you seek the assistance of a civil engineer or other qualified SWM professional, as the methodology for this is outside of the scope of this forum.
To control the volume, we do intend to reuse the water for landscape irrigation and if overflow, drain it to the pond outside the leed project. What i'm confused is, if the pond is properly made to retain stormwater runoff volume, then what's the need to calculate critical capacity value for the receiving stream?
There is no anlaysis of the receiving stream channel required. Simply reduce peak rate and volume at the point of interest.
We have a tall office building that has a cistern for rainwater harvesting. I have two related questions:
1. The amount of runoff reduced by stormwater harvesting system is based, I understand, on its storage volume, the rate at which the system is emptied, and the interval between storm events. How can I calculate the interval between the storm events? In the reference guide, example 2 "volume of captured runoff" the design storm interval is 3 days. Can the team use this as the default value for the project?
2. The volume of captured runoff is based on the area of collection surface. Considering that the building has terraces, does the team count the horizontal surfaces of the terraces in this calculation?
A lot of discussion about time intervals today (see below). I do not necessarily agree with the 72 hour dewatering for a cistern, an above-ground basin, sure (its a mosquito thing), and as far as I know, the ref manual does not require all facilities to drain within 72 hours.
Recent research has shown that the chance of back to back 2-year storms within 3 days is less than 1%. If this system is for reuse, can you use all of the water that quickly? If you can, then sure use the 72 hours as your interval. But if you can not, I would make the argument that the interval between 1 and 2 year storms is 1 and 2 years. If the cistern is simply to control the runoff rate, I'd use the 72 hours.
Yes, you need to account for the terraces (or the paving below, whichever is greater). Just develop an impervious footprint, if the terraces are stacked above one another, i would not count all of them, just the upper most.
Michael, can you please direct me towards the recent research that you referenced above? I have searched the Villanova Urban Stormwater Partnership for information pertaining to the likelihood (less than 1%) of a 2-year storm occurring twice within a 3 day period. I need an excerpt of the research along with the source. Thank you for your assistance.
I'm not too sure exactly what presentation this came from, but this is simply a math problem.
You are seeking the probability that a 2 year storm will occur twice within 3 days.
Pn = 1-(1-P)^N
P = Chance of 2-year storm occuring any year - 0.50
N = 0.0082 years
Pn = 0.006
Hi, We are trying to pursue both storm water quality control and quantity control credits. When we calculate the 2 year - 24 hour design storm it comes to about 6.75 inches and 25% of that we are going catch in a catchment area.
Then there will be an overflow from the catchment area and that we intend to release through a silt trap.
The confusing part is that we are in the wet zone and the quality control credit requires us to design the silt trap for 1 inch rainfall per 24 hours. But from the catchment area there will be overflow in the range of 75% of 6.75 inch 2 year - 24 hour design storm. That is about 5 inches and only 1 inch of that will be handled effectively by the silt trap. If we try to make the silt trap to handle a 5 inch storm then it becomes so large.
Am I making the correct reasoning here? Or how should I handle this situation? Please advise. Thank you.
Are you sure about the 6.75 inch depth? That is a 50-year storm depth here in the Mid-Atlantic.
Thank you Michael for the reply. This project is in SL and usually we get quite heavy rain all through the year. So given that this 6.75 inch is correct, am I reasoning this in the correct direction?
That is a lot of rain.
Let me see if I understand this correctly. You are using the same BMPBest Management Practice to manage both peak rate and volume (6.1), and water quality (6.2). The 6.75" depth is the controlling factor, and is producing a BMP which is much larger than that which would be required if you were simply going after 6.2???
In short, to answer your question to the best of my ability (again, I am not sure I understand this correctly), if you are going to use the same facility to meet both 6.1 and 6.2, you need to size this accordingly, i.e. based on the 6.75" depth, and the requirements of 6.1, then 6.2 should simply fall into place.
Let me know if this helps, or if I completely missed the mark on this. I wish we could upload images here, it would sure make my life easier!
Green roofs help retain stormwater and reduce peak flow.
Pavement that allows stormwater infiltration reduces stormwater quantity.
Senior Staff Designer
Control stormwater quantity can also improves stormwater quality. Promoting infiltration is one way to contribute to both credits.
Native plantings can function as natural stormwater controls, and reducing site disturbance also protects natural infiltration.
Open space can function as natural stormwater controls. In dense neighborhoods, vegetated roofs can count as open space while also benefiting stormwater reduction.
Using porous pavement can contribute to stormwater reduction and heat-island mitigation.
Installing a green roof can contribute to stormwater treatment and heat-island mitigation.
Capturing and reusing stormwater for irrigation reduces the need for potable water for irrigation.
Collecting rainwater for toilet flushing can be part of a stormwater management plan.
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