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).
Achievement of this credit can be documented via a LEED ND v2009 submittal. For more information check out this article.
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).
This database provides studies and analysis on BMPs and is intended to improve design.
This tool is very useful for determining the percentile for rainfall information for a site. However, it should be used for planning purposes only, and should not be a substitute for a site-specific hydrology study performed by a qualified civil engineer or stormwater professional.
This website provides information on the performance of technologies in a number of states across the U.S.
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.
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.
Sample LEED Online forms for all rating systems and versions are available on the USGBC website.
Documentation for this credit can be part of a Design Phase submittal.
This is just a suggestion.
To achieving this credit the pre-development condition is no more the "existing site" we were talking about in v2009, but is now(LEED v4) the "natural land cover condition". It sounds similar but the treshold to reach is now very higher. Am i wrong?
I think it will be very expensive for the Contractors to send back the site's rainwater impacts to the Age of Stone.
Our project is based on the case 2. Existing Site 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. is greater than 50%.
We implemented several strategies for improvement:
a) Reduced runoff place
b) Accumulation of rainwater for reuse in toilet flushing
We calculate reducing stormwater runoff by reducing runoff coefficients. The result is an improvement of 12%:
Pre-development = 508,5 cf
Post-development = 447,2 cf
Then, we calculate the water reuse for toilet flushing in 72 hours. The result is 500 cf.
(The tank capacity is 1550 cf)
So, can we prove that our post-development runoff is 0 cf (447,2 cf - 500 cf)?
That is what the math looks like to me.
In our project outside the US and in the rainy tropics our site pre-development is >50% impervious (only 10% site is natural green) therefore we need to go for 25% reduction in both volume and rate.
The new project design is built up 100% (no natural infiltration), but does have 30% of site area as an intensive 300mm deep green roof. Therefore we aim to manage the target water volume by reducing the runoff by the green roof, capturing the stormwater and reusing it on site for cooling towers, flushing and irrigation. Questions are:
1. Compliant rainfall data
The prefered 2 year design storm option (to achieve a 25% reduction from pre-development) we do not understand clearly how to extract from available 5 year local data as this is an international project. From the LEED2009 ACPs guide we understood that we are are allowed to manage the 100% volume of the percentile storm from 5 year data, or a 100% volume of a 5 and 10 year design storm ?
2. Back to back storm time to use the volume
If we are capturing & reusing the water volume on site, how much time we have available to reuse the volume it until the next storm hits and the tank needs to be empty? How can we calculate the back to back storm frequency of a local percentile storm, or an fair assumption needs to be made by observing the local data? For example, if its possible that a percentile storm occurs 2 days in a row, this means we need to utilise 100% of that storm volume in 1 day? This might be very difficult to achieve due to large rain volume in the tropics.
3. Green roof retention
The retention capabilities of the dry green roof are expressed by the run-off coefficient. If the 2 percentile storms fall within few days, the green roof might be quite saturated and the retention capability to capture new volume will be reduced?
We would really appreciate your view on our the issues above, if possible.
Let me do my best to get you started, but answers to all of your questions can be found in various threads here.
1. You can not get 2-year rainfall data from 5-year rainfall data. It is not a linear relationship. But if you are telling me that you are managing (meaning 0 runoff) for the 5-year storm you would comply with the 1 and 2-year requirements. If you tell me that your 5-year runoff meets the 25% reduction only, you can not be certain that this is the case for the 1 and 2-year storms.
2. There is no hard and fast rule to the reuse thing. See other threads, and your engineer should use appropriate judgement. If your BMPBest Management Practice is above ground, like say a raingarden or wet pong, then 72-96 is pretty much the norm to prevent West Nile. If it is conveyed to a tank for reuse, guidance gets a little wishy washy. 2-year storms within 3 days of each other have a statistical probability of happening of like 0.5% so you decide.
3. Again, have your engineer use appropriate judgement. in this case.
Thanks for the answer.
1. We aim to go for the option 1 Design Storms (>50% impervious, 25% reduction) if possible. Our water engineer informed us that the local rainfall data up to 100 years is available and he proposes to use that data with IDF curves where the intensity of the 2 year storm event is obtainable. Would this approach be accepted by LEED?
You should really be using a DDF curve, depth-duration. IDF doesn't really give you a volume because it is based off 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.. But this is not a LEED requirement. If you engineer is comfortable with the design, it is up to his/her judgement.
We are having a hard time determining how to calculate the flow quantities for our site when it has been designed so all of the run off is captured and conveyed to on-site retention ponds. Is this a case where we can say that 100% of the runoff is infiltrated on-site?
Pre or post?
To reduce runoff peak and quantity our project team used an embedded underground chambers system. In this practice, the only chambers waterflow outlet is water exfiltrationExfiltration is air leakage through cracks and interstices and through the ceilings, floors, and walls. on surrounding soil.
As far as you know, Is the underground Exfiltration cosidered as runoff, or as a water management system to reduce and prevent it?
According to your scenario, whatever gets into the underground chambers is infiltrated, and therefore, is not considered runoff.
The only runoff you would have on your site is that which bypasses your system, if any.
From the data i modeled on Hydrocad, it results that the soil is so pervious ( 2,880 mm/h - 0,0008 m/s ) that water don't even storage into chambers but directly infiltrates underground.
Is it possible to achive a zero runoff in a teorethical model as the runoff calculation?
I think GBCIThe Green Building Certification Institute (GBCI) manages Leadership in Energy and Environmental Design (LEED) building certification and professional accreditation processes. It was established in 2008 with support from the U.S. Green Building Council (USGBC). is gonna reject it..
How can i do to demonstrate compliance?
That infiltration rate doesn't seem realistic for any soil that is suitable for construction. I would check that rate first. While LEED doesn't require it, I never use more than 10-15 in/hr as an infiltration rate, and always apply a FOS=2.
But to answer your question: "Is it possible to achive a zero runoff in a teorethical model as the runoff calculation?"
Yes, you just did it.
Thank you for all of the information provided on this forum. I am confident in all of our calculations up to the Storm Interval and Drawdown calculations. If I understand things correctly, the calculations are fairly simple:
Storm volume: 119,000 liters
Storage Capacity: 148,000 liters
Dawdown Rate (reuse): 2,300 liters/day
Storage + (drawdown x 3 days) - Storm volume = capacity remaining (or runoff)
148,000 + 6,900 - 119,000 = 35,900 storage remaining after three days.
Storm volume - Capacity remaining - drawdown (1 day) = Runoff
119,000 - 35,900 - 2,300 = 80,800 liters of runoff (post-dev)
Assuming a fairly impervious Pre-dev site condition let´s assume a calculated runoff of 115,000 liters per 2 year 24 hour storm:
Pre-dev runoff: 115,000 liters
Post-dev runoff: 80,800 liters
Runoff reduction: 34,200 liters
Runoff reduction: 30%
Is it that easy? Am I missing something?
I am not following your post at all, but simply put, you need a minimum discharge rate of 0.02 cfs to drain that tank full in 72 hours.
Your theoretical 2 year storm, at that same drawdown rate will drain in 56 hours.
Thanks, Michael, I will try to clarify. In case two, you must reduce runoff by 25% as compared to pre-development conditions.
From what I understand, the tank doesn't need to be empty. The storage capacity and drawdown rate must be sufficient to reduce post-development runoff by 25%. You have the two year 24 hour storm and, in our case, there is no runoff due to the tank capacity and drawdown rate, however three days later you have the same storm event and this is where you measure post-development runoff? Or due you continue with the deluge every three days? I guess it is the storm interval that has me confused. Thanks again.
You need to reduce "volume" by 25% not rate.
You are not doing that if you are simply capturing runoff and slowly releasing it. Your site hydrology, the pre vs post analysis, should be completed by a civil engineer with experience in stormwater management.
You need to show that the tank has sufficient volume to handle a 2nd 2-year storm after a pre-determined time (it is usually 72 hours, but this isn't actually specified in LEED).
You need to come up with a plan for reuse of the captured water, and you also need to pass a straight face test. This thing needs to serve a purpose.
You need to show how you are reducing volume by 25% through reuse, infiltration, etc.
Thank you, please bear with me.
We are demonstrating a 25% reduction in Post-development runoff. This does not necessarily require capturing and using all of the storm water volume. Our drawdown is achieved through reuse of rainwater to flush toilets and urinals (based on WEp1 performance case flush fixture use).
119,000 liters, storm volume
148,000 liters, storage capacity
6,900 liters, drawdown per storm interval (2,300 per day x 3 days)
148,000 + 6,900 - 119,000 = 35,900 storage volume remaining after storm 1.
3 days later, Storm 2
119,000 liters, storm volume
35,900 liters, storage remaining
2,300 drawdown (1 day)
Storm Volume - Capacity Remaining - Drawdown = Runoff (in this case)
119,000 - 35,900 - 2,300 = 80,800 liters of runoff after Storm 2.
Pre-development runoff 108,000 liters
Post-development runoff 80,800 liters
Runoff reduction is 25.18%
Please let me know if this is correct and, again, thanks for your time.
Michael, can you comment on my last post, please.
I guess not. I wonder, what am I paying for?
I'm sorry I missed your post. Regardless, I feel I have provided you enough guidance to attempt the credit or have your civil engineer attempt the credit. I will offer guidance and my opinion, but I will not provide engineering direction, check your math, or do the work for you.
Best of luck with your project.
Michael, I am probably not describing my doubt clearly and am certainly not asking you to check my math or do any of our work. My doubt concerns what the LEED NC reference guide says on page 97 in the example given. It says that "the captured rainwater must be drained within three days... for the tank to be emptied before the next storm. If the drainage rate is slower, full capacity cannot be assumed to be available during the 2-year 24-hour design storm." My doubt is whether or not the system capacity truly needs to be emptied before the next storm
The requirement for case 2 is, as you said, to reduce runoff volume by 25% compared to pre-development conditions and I probably confused things with the example calculations I gave.
My question remains if, at the time of the Second Storm Event, the following it true:
Storm volume - Storage capacity remaining - Drawdown = Runoff in the case that the system in not completely empty.
In a nutshell, your storage volume needs to accommodate a 2nd 2-year storm that occurs in 72 hours.
Does the cistern need to be empty? If it was only sized to handle the volume of the 2-year storm, then yes it needs to be empty. If you oversized the thing, then it would not need to be empty because you have supplemental storage volume built in.
If you only sized for the 2-year volume, and the cistern does not drain within 72 hours (lets say you have 50% volume remaining on Day 3), then an equivalent volume would miss the cistern at the next storm event (because the cistern would be full), you would thereby have 50% of the storm volume leaving your site.
I personally would model this in HydroCAD as a back to back event, but your approach with a detailed description could work.
The first thing I would want you to prove is that fact that you can use all the water you are drawing down within 3 days, so be sure to check that.
That helps a lot, thanks very much.
Thiago, FYI, the LEEDuser forum is a free part of our site and our experts volunteer their time. Please be kind to them. See our FAQ for more info.
Which formula do you use to calculate peak flow rate (1 and 2-year, 24hours). I am using NRCS (SCS) TR-55 Graphical Discharge Method. Is that recommanded?
While the actual methodology is not specified, you'll never go wrong using SCS.
How to calculate 2 year 24 hour design storm?
The rainfall used for a design storm is based on historical rainfall amounts for the region in question. This is typically a "lookup" from a local weather agency (NOAA in the US), and not typically something you would be calculating.
The Project is in Bangladesh...Not U.S.
The Bangladesh Metro-logical Department lists month-wise Average Normal Rainfall in mm .
For Dhaka following are the Values
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
7.7 29 66 156 339 340 373 316 300. 172 34.4 12.8
Kindly tell us how to calculate 2 year 24 hour storm from above values?
You do not have enough data to discern anything from what you have posted. What you are trying to do is not easy, and is well outside the scope of this forum.
I suggest you read this: http://www.eng-tips.com/viewthread.cfm?qid=399896
Depending on the rainfall data available in your country you might prefer to use the v4 methodology (95% percentile)
The requirements for an alternative compliance for projects outside of the United States can be found in the LEED BD+C v2009 Reference Guide with Gobal Alternative Compliance. You will also need to update your LEEDOnline form to be able to use the Alternative Compliance Path (ACP).
My project is two underground basement parking floors, that are built on the total land area, while the above-grade floors are built on 30% of the total land area.
Should I consider this project as case -1 (non-zero lot line); accounting for the run-off that may occur on the area outside the above-grade building, or case-2 (zero lot line); as I did practically build 100% of the project area for basements.
Case 1 and Case 2 apply to existing impervious, not proposed.
Hello, we are working on a previously developedPreviously developed sites are those altered by paving, construction, and/or land use that would typically have required regulatory permitting to have been initiated (alterations may exist now or in the past). Previously developed land includes a platted lot on which a building was constructed if the lot is no more than 1 acre; previous development on lots larger than 1 acre is defined as the development footprint and land alterations associated with the footprint. Land that is not previously developed and altered landscapes resulting from current or historical clearing or filling, agricultural or forestry use, or preserved natural area use are considered undeveloped land. The date of previous development permit issuance constitutes the date of previous development, but permit issuance in itself does not constitute previous development." 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. less than 50% (Option 1, case 1). Our engineer has made the calculations for the 1 and 2 year, 24 hr design storms for 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. conditions:
1 year, 24hr = 0.15 cfs and 2,497.8 cf/storm
2 year, 24hr = 0.19 cfs and 4,425.4 cf/storm
100% of the Post development Stormwater coming from the site and roofs will be infiltrated on site, so the results are zero for both rate and quantity. However the leedonline template does´t recognise any points if 0 value is entered for the post development condition, if you enter any number higher than 0 (example: 0.01) it does recognise the points.
How can we get credit for 100% infiltration on site??
Rodrigo, this seems like a problem with the LEED Online form. Are you using the most up to date form? I would contact GBCIThe Green Building Certification Institute (GBCI) manages Leadership in Energy and Environmental Design (LEED) building certification and professional accreditation processes. It was established in 2008 with support from the U.S. Green Building Council (USGBC). for assistance.
I am fielding a question for our civil engineer. We are trying to determine if the existing conditions (previously developedPreviously developed sites are those altered by paving, construction, and/or land use that would typically have required regulatory permitting to have been initiated (alterations may exist now or in the past). Previously developed land includes a platted lot on which a building was constructed if the lot is no more than 1 acre; previous development on lots larger than 1 acre is defined as the development footprint and land alterations associated with the footprint. Land that is not previously developed and altered landscapes resulting from current or historical clearing or filling, agricultural or forestry use, or preserved natural area use are considered undeveloped land. The date of previous development permit issuance constitutes the date of previous development, but permit issuance in itself does not constitute previous development.") are actually hurting us on this credit, and if there's any alternative way we'd be able to meet requirements and document our situation. Here is his breakdown:
The stormwater drainage design for the project collects and treats 98% of the runoff generated from the site. The existing site soils are environmentally impacted and per the environmental consultant, based on the soils conditions, areas of concentrated infiltration into the subsurface is not feasible. This is a hardship for the project as the proposed design for stormwater outflow from the site cannot be reduced for the 1 and 2 year storm events to be at or below 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. conditions. Further, the site historically was previously developed and included industrial/manufacturing uses on the site, which we understand included a significant amount of impervious area on the site. However, detailed record plan information is not available to confirm the exact limits and runoff that was once generated from the site.
Any suggestions for meeting compliance? We will be performing site mitigation based on the previous industrial uses, and extensive mitigation to a waterfront on one side of the project site, including restoration to shorelines and shellfish beds, among other aspects.
If you have an existing site, with compacted soils (and you document this), your CE would be correct in assuming a higher pre-development CN, thus yielding a higher pre-development runoff rate and volume.
This actually makes this credit easier from a SWM perspective.
How does LEED SSc6.1 calculate "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."? I have been told that "imperviousness" means the % of the site that is impervious (paved vs green).
However, that would mean that a 1 acre site with 0.5 acre of building and asphalt and 0.5 acre of mowed lawn would be considered a "largely undeveloped site" by LEED.
I find that very hard to believe. It seems to me that they are actually looking at it like a composite C value from 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. (i.e. C for lawn is 0.2 and C for pavement is 0.9, so a composite C=0.55 (or 55%). Or maybe as a runoff value from SCS method which I think could yield an even higher number if the soils are bad.
This credit compares pre vs post runoff rate and volume.
Using SCS methodology a CN of 98 is used for impervious areas and the appropriate CN for all other cover types based on the underlying soil condition.
This is a fairly straight forward calculation and analysis.
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. should not be used for determining runoff volume, because it doesn't determine runoff volume.
Thanks, but that was not my question.
In order to determine whether a site falls into Case 1 or Case 2, we have to calculate the 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.". I cannot find where we are told exactly how to calculate "imperviousness". I have been told that "imperviousness" simply means the % of the site that is impervious:
If that is true, then a 1 acre site with 0.5 acre of roof and pavement and 0.5 acre of mowed lawn would be a "Case 1" site and considered a "largely undeveloped site" by LEED. Really?
In Case 1 you just have to show there is no increase in runoff between existing and proposed (keep it as good as you found it).
Is that correct? It seems too "easy" compared to what LEED usually demands of us.
(On the other hand, if the existing site is actually 0.51 acres of pavement, it would then switch to Case 2 and we now have to make the proposed runoff 25% less than the existing runoff. That is SO much harder).
Percent impervious is calculated:
Impervious Area/Total Area
Case 2 you are only managing volume, and only for the 2-year storm, not the 1-year. A design for a redevelopment project that shows a volume reduction equal to a 20-25% reduction in impervious area is a fairly common approach to stormwater.
Remember, these are Site Credits, it isn't a stormwater ordinance.
I have to assume you meant to say that "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." is equal to "Percent impervious" which is calculated: Impervious Area/Total Area.
Please let me know if there is a different definition of Imperviousness.
Our project is a 30 year old building undergoing a Major Renovation and will be registered as LEED-NC v2009.
In calculating pre- and post-development runoff, do we consider the pre-development conditions of the site 30 years ago when the building was originally constructed (greenfield) or the site conditions before the renovation (existing building and grounds)?
LEED does not establish criteria for determining pre-development runoff.
Calc the pre based on today's footprint, the post after development.
Thank you Michael.
We are the construction materials manufacturers. We wish to determine the runoff coefficient for our porous concrete product.
Can anyone advise what is acceptable testing stadard for defining the runoff coefficient for porous concrete product that can be adopted for the storm water run off evaluation as per the requirements of SSc6.1 and SSc6.2, LEED NC Version 2009.
I typically use the underlying cover type ...... just pretend the porous isn't even there. You could always bump the CN up a little bit to be conservative.
Thank you for your advise. Pleas let me confirm my understanding. Do you suggest that the runoff coefficient of the porous concrete can be ignored and we use the runoff coefficient of the substance lay beneath the porous concrete for the storm water runoff calculation.
There is no hard and fast guidance to this, but this has been the trend in stormwater modeling recently.
Thank you very much for your clarification
I got a question about the calculataion of Volume of captured run off;
In the formula,(V= P.A.R/12) P value is taken according to the location of project ; humid ,semi arid and arid.which in his case can be 0,5 0,75 or 1 inch.
my confusion starts right at that point: the avarage rainfall i have had calculated in my project zone is 26 inches; so does that qualify me as a semi arid watershedWatershed that receives less than 20 inches of rainfall per year. where my p value is 0.75 inches? or would my P value be different ?
Going from LEED-EB 2.0 SS credit 5.1 to LEED 2009, O&M, what is the number for "Volume Captured"? Is it
a) "Stormwater management practices (structural detention facilities)" (gallons converted to cubic feet)
b) "Total Mitigation" (gallons converted to cubic feet)
I have a NC project in Colombia and the Environmental Authority just have IDF curves since 3 years. I don't have information since 1 and 2 years. Can I use the value for 3 years in order to calculate the runoff or should I use an alternative methodology to calculate IDF curves for 1-2 years?
You want to be using depth, not intensity.
IDF curves are typically used for 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 does not yield an accurate volume. What methodology are you using? Use NOAA for site specific 24-hour rainfall depths.
But to answer your original question, no, you can not back into a 1 and 2 year depth from a 3-year intensity. The relationships are not linear.
How did you manage to document this credit? We have a project in Colombia and we do not know how to find the rainfall data to make the calculations (either v3 or v4 methodology).
For international projects, seek out an agency that is the equivalent of NOAA in the United States. I recommend contacting a local engineering university with a program in hydraulics and hydrology for direction.
Thanks Michael! I've found this document which will be useful for other project teams: http://www.bdigital.unal.edu.co/2467/4/98671272.2009_4.pdf
page 95 of the LEED manual (description of case 1 - option 2) states that post development runoff rate and quantity must be below "critical values" for the relevant receiving waterways.
It isn't defined objectively anywhere (as far as I can tell) what this means/how to determine what is considered critical value for any given type of waterway. I'll ask our civil engineer - but thought I'd pose the question here also.
Michael, this is answered at length below.
site is on the river. I assume this doesn't change anything, but civil engineer asked I check to see if LEED has exceptions for this. I guess the thought was that it might be strange to retain stormwater on site so it can be slowly released - essentially right back on site (property line is technically in the river)
There are several discussions on direct discharge districts below. But essentially, there is no exception for this credit. You either meet the rate/volume reductions or you do not.
thanks Michael. I did read below but I guess I must have not understood.
Suppose we pursued the stream/channel protection option? All water from the site that isn't absorbed into ground will be cleaned by Snoit filters, then directed to a outlet pipe in the bulkhead, where it will dump into river (which Houston is fine with stormwater into river). Since the only "stream"/waters edge is the actual river - would a steel bulkhead be considered erosion control? I mean, there certainly will not be any erosion.
again, sorry if these are answered below just not real clear to me
thanks very much...
We have found a sample calculation to determine storm water rate and quantity for 1 yr-24 hr design storm and 2 yr-24 hr design storm from www.illinoisfloods.org/.../4D_Stormwater%20Volume%20and%20Treat.... We would like to know about the process followed in this website is correct for the solution or not?
The calculations required to determine the stormwater rate and runoff volume should be completed by a qualified professional experienced in this field, it is not a process you can learn on a website.
TR-55 is one of the more commonly used guidance documents.
Thank you for your reply.
The requirements for Option 2 (case 1) of this credit refer to the protection of receiving stream channels. If my project receiving body is not a stream channel, but a bayA bay is a component of a standard, rectilinear building design. It is the open area defined by a building element such as columns or a window. Typically, there are multiple identical bays in succession. area (ocean), is it eligible for this approach?
Great question! This is definitely one I would take to the GBCIThe Green Building Certification Institute (GBCI) manages Leadership in Energy and Environmental Design (LEED) building certification and professional accreditation processes. It was established in 2008 with support from the U.S. Green Building Council (USGBC). for a definitive answer, but here is my gut feeling:
You are most likely located in some kind of "direct discharge" watershed district if you are this close to an ocean, correct? If this is the case you are likely providing no (or very little) in the way of stormwater management, correct? You are just letting the post-development rip off, thus this would make you ineligible for Option #1.
Option #2 requires you to show that a receiving "channel" can handle your increase in rate and volume without causing a downstream impact. You definitely will not be creating a downstream impact since you are discharging directly into the ocean.
My gut feeling (and again, this is not official guidance), is you need to pass the straight face test. Can you really say that you are meeting the intents of this credit?
There was a discussion (on here I believe) about direct discharge districts, and the consensus was that under direct discharge, due to the location of your site, this credit just may not be attainable.
If you take this to GBCI, I'd be curious to hear their response.
Green roofs help retain stormwater and reduce peak flow.
Pavement that allows stormwater infiltration reduces stormwater quantity.
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