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Cost-effective with an integrated approach
Development usually comes with increased stormwater runoff due to impervious surfacesSurfaces that promote runoff of precipitation volumes instead of infiltration into the subsurface. The imperviousness or degree of runoff potential can be estimated for different surface materials. like roofs and parking lots. To earn this credit with previously undeveloped sites, you’ll need to avoid any increase in runoff, while on mostly impervious developed sites, you’ll need to reduce runoff. You may need to go beyond standard practice to achieve this credit, and you might see increased costs, although an integrated approach can make this credit cost-effective.
If you're planning to pursue this credit, make sure your civil engineer is aware of it and on board, in order to achieve the credit without added steps.
Encourage natural infiltration when possible
Many project teams are reluctant to attempt this credit because engineers typically use conventional methods that might not meet requirements. Although it's readily achievable, this credit can be challenging, particularly in areas with compacted soil, no landscaped area, large parking areas, or water laws that preclude rainwater harvesting. Green roofs will be helpful in these cases, but the simplest and cheapest option, whenever available, is to simply encourage natural infiltration of stormwater into the ground. Reducing hardscapes, designing a smaller building footprintBuilding footprint is the area on a project site used by the building structure, defined by the perimeter of the building plan. Parking lots, parking garages, landscapes, and other nonbuilding facilities are not included in the building footprint., increasing landscaping area, using porous paving materials, using natural swales and other low-impact development strategies, and preserving natural site features are all cost-effective methods for promoting natural infiltration. Although natural infiltration may decrease the cost of maintenance compared to other structural and packaged stormwater control systems, keep in mind that it will still require regular maintenance.
Similar to SSc6.2
The two stormwater credits, SSc6.1 (stormwater rate and quantity), and SSc6.2 (stormwater quality) involve similar calculations and can be addressed by similar strategies, such as promoting natural infiltration. Keep in mind, however, that each credit requires different calculations and methodologies. Reducing the quantity of stormwater runoff for SSc6.1 does not always equate to a quality improvement for SSc6.2. Both credits focus on smaller, more frequent storms, not the larger ones that are more likely to cause flooding.
Many of the benefits of this credit are indirect and can be difficult to calculate. These include issues like reducing the burden on the municipal stormwater system; reducing contaminants in waterways; reducing peak runoff, which makes stream habitats more consistent; reducing the temperature of runoff, which improves the conditions for aquatic life; and reducing erosion.
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FAQs for SSc6.1 and SSc6.2
Why do the requirements focus on 1-year and 2-year, 24-hour storms?
The 2-year, 24-hour design stormA 2-year, 24-hour design storm is a nationally accepted rate that represents the largest amount of rainfall expected over a 24-hour period during a 2-year interval. The rate is the basis for planning and designing stormwater management facilities and features. is a storm that has a high probability of happening and contributing to stormwater pollution. A 2-year storm has a 50% chance of happening in a given year, whereas a 1-year storm has a 100% chance.
It should be noted that most state or local programs only require projects to meet regulatory requirements related to flooding and/or water quality. This type of stormwater management program is designed to control the large, infrequent storm events that cause flooding, but not to manage smaller storm events that we now know cause the majority of the overall erosion and quality concerns because of their much higher frequency. The criteria of SSc6.1 are designed to ensure that both concerns are addressed in LEED projects that achieve this credit.
Why include the 1-year storm in the credit requirements? Won't management practices for the 2-year storm be effective?
It depends on how you look at it. Here's how LEEDuser Expert Michael DeVuono describes it: Think about it in terms of a simple pre>post analysis. Your one year "pre" number will be smaller than your 2-year "pre" number. Sometimes that 1-year number is so small that you have to choke back a lot of water, to ensure the "post" 1-year is smaller. This raises the required storage volume for the BMPBest Management Practice. So if you're looking at both the 1- and 2-year events, you may have a greater storage need than if you simply looked at the 2-year event. The 2-year "pre" number will be bigger, so you can let more out in the "post."
How can green roofs count as a stormwater control measure?
There are different approaches to this. One approach is to ensure that green roof soil depth and retention capacity allows for the 2-year, 24-hour design storm.
However, simply taking a “CN credit” for a green roof is usually beneficial enough. (The Curve Number or CN provides a number characterizing the runoff properties for a particular soil and ground cover.) Instead of the roof being modeled as impervious (with a CN of 98 which produces a high rate of runoff) some projects with extensive green roofs have used a lawn CN—usually around 61. In the calculations this results in a lower overall rate of runoff for the site, and is usually a more feasible option that providing stormwater storage in the roof media itself. If you can model your site so there is less runoff, there is less runoff volume that needs to be stored.
How can I achieve compliance if my project's stormwater control measures are outside the LEED project boundary?
Projects with stormwater control measures outside the LEED project boundary may be accepted if the measures appropriately take into account neighboring facilities by demonstrating that the existing stormwater management systems that serve the LEED project boundary meet the LEED requirements for all areas within the site serviced by those systems. LEED 2009 campus projects are required to reference USGBC's AGMBC guidance, which has specific guidelines for stormwater. For more on this see, for example, LI#2275 from 08/22/2008.
I have 100-year data—how do I convert to 2-year?
Storm intervals don’t convert. These numbers represent specific storm event probability. A 100-year storm has a 1% chance of happening in a given year, while a 2-year storm has a 50% chance of happening in a given year. The best resource for rainfall intensity data is NOAA’s Hydrometeorological Design Studies Center Precipitation Frequency Data Server. Further guidance on interpolating 2-year, 24-hour storm event can be found in LEEDuser's EBOM SSc6 Guidance.
Is it an acceptable strategy to capture the rainwater into tanks and discharge it into the public sewers after the rainstorm reducing the peak discharge?
This is a common strategy for reducing peak rate, which will help you comply with SSc6.1, but you'll need to add onsite reuse or infiltration to meet SSc6.2 requirements.
A sample graph illustrating the 95th percentile rainfall eventAre there special considerations for international projects?
In 2012, an additional compliance option was added to SSc6.1 that was specifically written with international projects in mind. This can be found in the credit language, and is fully supported on the most recent LEED Online forms. Projects in some countries can have trouble finding the stormwater data they're looking for. Some useful sites are posted in LEEDuser's Resources tab.
What performance threshold do I need to achieve for an Exemplary Performance point?
LEED Interpretation #10108 dated 11/01/2011 gives guidance in achieving Exemplary Performance. Achievement of the exemplary performance point encompasses both quantity and quality measures, and includes a comprehensive approach to capture and treat stormwater runoff.
The calculations for this credit are always a headache! While our projects are usually awarded the credit, the equations in the LEED Reference Guide are helpful mostly for sizing a reservoir or cistern, but don't help you get to the final results. Does USGBC provide any step-by-step guidance that would make submitting these credits more predictable?
No. USGBC has indicated that providing step-by-step instructions for this entire calculation process within the context of LEED reference documents is not possible. Various methods and computer-based software programs are available to estimate stormwater runoff rates and volumes, and the exact methods used for a particular project will depend upon the data available for a given site and the preferences of the qualified professional (typically a civil engineer) performing the calculations.
LEEDuser has heard from LEED project teams that the LEED expert on the project is sometimes expected to do the calculations for these credits, even if that person isn't a stormwater expert. We recommend a more integrated process in which the civil engineer documents this credit.
Legend
- Best Practices
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- Cost Tip
USGBC
Excerpted from LEED 2009 for New Construction and Major Renovations
SS Credit 6.1: Stormwater design - quantity control
1 Point
Intent
To limit disruption of natural hydrology by reducing impervious cover, increasing on-site infiltration, reducing or eliminating pollution from stormwater runoff and eliminating contaminants.
Requirements
Option 1. Design storms
Case 1. Sites with existing imperviousness 50% or less
Path 1
Implement a stormwater management plan that prevents the postdevelopment peak discharge rate and quantity from exceeding the predevelopmentPredevelopment refers to before the LEED project was initiated, but not necessarily before any development or disturbance took place. Predevelopment conditions describe conditions on the date the developer acquired rights to a majority of the buildable land on the project site through purchase or option to purchase. peak discharge rate and quantity for the 1- and 2-year 24-hour design storms.
OR
Path 2
Implement a stormwater management plan that protects receiving stream channels from excessive erosion. The stormwater management plan must include stream channel protection and quantity control strategies.
Case 2. Sites with existing imperviousness greater than 50%
Implement a stormwater management plan that results in a 25% decrease in the volume of stormwater runoff from the 2-year 24-hour design storm.
OR
Option 2. Percentile rainfall events
Case 1. Non-zero lot line projects
In a manner best replicating natural site hydrology1 processes, manage onsite2 the runoff from the developed site for the 95th percentile of regional or local rainfall events using Low Impact Development (LID)3 and green infrastructure4.
Use daily rainfall data and the methodology in the United States Environmental Protection Agency’s Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act to determine the 95th percentile amount.OR
CASE 2: zero lot line projects
For zero lot line projects located in urban areas with a minimum density of 1.5 FAR (13,800 square meters per hectare net), in a manner best replicating natural site hydrology processes, manage onsite the runoff from the developed site for the 85th percentile of regional or local rainfall events using LID and green infrastructure.
1Natural Site Hydrology is defined as the natural land cover function of water occurrence, distribution, movement, and balance.
23 Low Impact Development (LID) is defined as an approach to managing stormwater runoff that emphasizes on-site natural features to protect water quality by replicating the natural land cover hydrologic regime of watersheds and addressing runoff close to its source. Examples include better site design principles such as minimizing land disturbance, preserving vegetation, minimizing impervious cover, and design practices like rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, and soil amendments. These are engineered practices that may require specialized design assistance.
4 Green Infrastructure is a soil and vegetation-based approach to wet weather management that is cost-effective, sustainable, and environmentally friendly. Green infrastructure management approaches and technologies infiltrate, evapotranspire, capture and reuse stormwater to maintain or restore natural hydrologies (US EPA).Potential Technologies & Strategies
Design the project site to maintain natural stormwater flows by promoting infiltration. Specify vegetated roofs, pervious paving and other measures to minimize impervious surfacesSurfaces that promote runoff of precipitation volumes instead of infiltration into the subsurface. The imperviousness or degree of runoff potential can be estimated for different surface materials.. Reuse stormwater for non-potable uses such as landscape irrigation, toilet and urinal flushing, and custodial uses.
FOOTNOTES
1 Natural Site Hydrology is defined as the natural land cover function of water occurrence, distribution, movement, and balance.
2 “Manage Onsite” refers to capturing and retaining the specified volume of rainfall to mimic natural hydrologic function. This includes, but is not limited to, strategies that manage volume through evapotranspiration, infiltration, or capture and reuse.
3 Low Impact Development (LID) is defined as an approach to managing stormwater runoff that emphasizes on‐site natural features to protect water quality by replicating the natural land cover hydrologic regime of watersheds and addressing runoff close to its source. Examples include better site design principles such as minimizing land disturbance, preserving vegetation, minimizing impervious cover, and design practices like rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, and soil amendments. These are engineered practices that may require specialized design assistance.
4 Green Infrastructure is a soil and vegetation‐based approach to wet weather management that is cost‐effective, sustainable, and environmentally friendly. Green infrastructure management approaches and technologies infiltrate, evapotranspire, capture and reuse stormwater to maintain or restore natural hydrologies (US EPA).
Technical Guides
Urban Drainage and Flood Control District
Technical manuals on stormwater BMP’s as they relate to Denver and surrounding counties.
Clean Water Ways, Stormwater Implementation BMP Guidelines
http://www.cleanwaterways.org/professionals/index.html
Stormwater Best Management Practices Design Guide, Vegetated Biofilters (EPA/600/R-04/121A)
This guide provides design strategies and techniques on incorporating biofilters in projects.
Bioretention (Rain Gardens) – EPA’s Stormwater Menu of BMPs
This website gives designers and planners information on the appropriate application of bioretention areas.
Model Low Impact Development Strategies for Big Box Retail Stores
This report describes low-impact development approaches to stormwater management for big-box stores.
City and County of Denver, Department of Public Works Wastewater Management Division Storm Drainage Design and Technical Criteria
This design manual provides stormwater information specific to Denver, Colorado.
Stormwater Best Management Practices Design Guide, Vegetated Biofilters (EPA/600/R-04/121A)
This guide provides design strategies and techniques on incorporating biofilters in projects.
Stormwater Management Manual (Portland Bureau of Environmental Services)
This website provides stormwater information specific to the Portland, Oregon area.
Rain Garden Handbook for Western Washington Homeowners: Designing your Landscape to Protect our Streams, Lakes, Bays and Wetlands
A guide to low-impact development for residences.
Maryland Stormwater Design Manual
This design manual provides stormwater information specific to Maryland.
Massachusetts Stormwater Technology Evaluation Project
This website provides stormwater information specific to Massachusetts.
Effectiveness of Urban Stormwater BMPs in Semi-Arid Climates
This design manual provides stormwater information specific to semi-arid climates, including Denver, Colorado.
Denver Water Quality Management Plan, Stormwater Quality BMP Implementation Guidelines, Chapter 6
This design manual provides stormwater information specific to Denver, Colorado.
Hydraulic Design Manual
The Texas Department of Transportation offers this guide to the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size., which, it notes, is appropriate for estimating peak discharges for small drainage areas of up to about 200 acres in which no significant flood storage appears.
Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act
This technical manual from the U.S. EPA contains background on documenting stormwater requirements through capturing the 95th percentile storm using onsite management practices.
International Stormwater Best Management Practices (BMP) Database
Features a database of over 500 BMPBest Management Practice studies, performance analysis results, tools for use in BMP performance studies, monitoring guidance and other study-related publications.
Organizations
EPA Low Impact Development Website
EPA provides valuable information on low-impact development through fact sheets, design guides and cost estimates for low-impact development strategies that reduce stormwater runoff.
National Pollutant Discharge Elimination System (NPDES)
EPA offers help on managing stormwater, including fact sheets on the six minimum control measures for best management practices.
Stormwater Manager’s Resource Center
This site for practitioners and local government officials provides technical assistance on stormwater management issues.
Web Tools
International Stormwater Management Best Practices Database
This database provides studies and analysis on BMPs and is intended to improve design.
Technology Acceptance and Reciprocity Partnership
This website provides information on the performance of technologies in a number of states across the U.S.
Publications
Stormwater — A Journal for Surface Water Quality Professionals
Online magazine for stormwater professionals.
Stormwater Management Report
All Options
A stormwater management and drainage report covering both SSc6.1 and SSc6.2 can document all aspects of credit compliance.
Credit Documentation
Case 2
Create documentation quantifying stormwater volume and peak rate mitigation strategy.
LEED Online Forms: NC-2009 SS
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)
- SSp1: Construction Activity Pollution Prevention
- SSc1: Site Selection
- SSc2: Dev. Density & Community Connectivity
- SSc3: Brownfield Redevelopment
- SSc4.1: Alt. Trans.—Public Trans. Access
- SSc4.2: Alt. Trans.—Bicycle Storage
- SSc4.3: Alt. Trans.—Low-Emitting Vehicles
- SSc4.4: Alt. Trans.—Parking Capacity
- SSc5.1: Site Dev.—Protect or Restore Habitat
- SSc5.2: Site Dev.: Maximize Open Space
- SSc6.1: Stormwater Design—Quantity Control
- SSc6.2: Stormwater Design—Quality Control
- SSc7.1: Heat Island Effect—Non-Roof
- SSc7.2: Heat Island Effect—Roof
- SSc8: Light Pollution Reduction
Version 3 forms:
- SSp1: Construction Activity Pollution Prevention
- SSc1: Site Selection
- SSc2: Dev. Density & Community Connectivity
- SSc3: Brownfield Redevelopment
- SSc4.1: Alt. Trans.—Public Trans. Access
- SSc4.2: Alt. Trans.—Bicycle Storage
- SSc4.3: Alt. Trans.—Low-Emitting Vehicles
- SSc4.4: Alt. Trans.—Parking Capacity
- SSc5.1: Site Dev.—Protect or Restore Habitat
- SSc5.2: Site Dev.: Maximize Open Space
- SSc6.1: Stormwater Design—Quantity Control
- SSc6.2: Stormwater Design—Quality Control
- SSc7.1: Heat Island Effect—Non-Roof
- SSc7.2: Heat Island Effect—Roof
- SSc8: Light Pollution Reduction
These links are posted by LEEDuser with USGBC's permission. USGBC has certain usage restrictions on these forms; for more information, visit LEED Online and click "Sample Forms Download."
Design Submittal
Documentation for this credit can be part of a Design Phase submittal.
LEEDuser is produced by BuildingGreen, Inc., with YR&G authoring most of the original content. LEEDuser enjoys ongoing collaboration with USGBC.
116 Comments
Rainfall Event Interval
Hello,
I was wondering if someone could clarify how is the Rainfall Event Interval required to calculate the minimum drawdownrate for a tank calculated. Is this something we need to calculate? and if so, what should be the assumptions?
Many thanks
Reagrds
Alicia
The draw down would be calculated usi g the actual demand on the tank. You rainfall intensity should have nothing to do with this. Your intensity would allow you to calculate the amount of water getting to your tank for reuse.
Design Respons from GBCI C6.1
Hej,
We got feedback from our design submittal with the following technical advice:
“Rainfall intensity is normally dependent on the time of concentration. Confirm that time of concentration is approximately the same for both pre- and post-development conditions to document that the rainfall intensity does not change.”
As I interpret the feedback they want us to show if the rainfall intensity changes pre and post development? How do we do that? The area has pretty much the same ground conditions post development, just more buildings with roof instead of asphalted areas. I do not believe that it will have an impact on the weather.
Or have I interpreted it wrong?
Note our project is located in Sweden, so we cannot use existing American data.
Thanks in advance ! /Veronika
Tc can vary wildly between pre and post. It can be 5 hours in pre and 5 minutes in post. Your stormwater management practice is what compensates for this.
Your rainfall intensity should not vary. You need to use the same depth in both pre and post models.
Your stormwater model and calculations should be prepared by a civil engineer or stormwater professional.
Okej, this is confusing to me, because in my opinion we have shown the conditions and made calculations pre and post according with the requirements. But maybe we need to clarify something. The professional level is not an issue on our project.
Thanks for your quick respond! /Veronika
The amount of rain that falls on the site per calculated storm event does not change . What changes is the amount of run-off. The credit requires that you show a reduction in stormwater running off of 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. and leaving the project site. If your impervious surfaces are the same in your pre and post development cases then you will need to demonstrate how the project retains/infiltrates the water on site.
Veronika, is that the entire comment?
Are the ground cover areas the same in the pre and post?
If areas were the same, I think I see where the reviewer was going with the Tc comment....the Tc for the post would need to be slower than the pre in order to generate a lesser peak-rate runoff, but it is just a really cumbersome way of stating this.
Did you submit Tc calculations?
Waiver for Quantity Requirement
We have a project where infiltration is not possible as the soils are D type. Can the quantity requirement be waived and we still get credit as we meet the rate requirement or is the credit out of reach? No reuse is planned for the project.
You need to reduce the volume. There is no way around that. A D soil is not necessarily the kiss of death. Look into BMPs that promote evapotranspiration, such as raingardens and ammended soils in the lawn area.
Ambiguity re: detention as an acceptable solution
Folks:
We have a project (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%) where we have calculated that through a combination of green roof, pervious pavers, landscaped open space, and a rainwater cistern we can reduce the peak discharge rate and quantity by 25% per LEED reqs for SSc6.1. Based on rainfall patterns, we assume that we need to draw down the cistern in 2 days to accommodate potential back-to-back storms. It's really unclear to us whether or not it is acceptable to draw down the cistern by draining into the storm sewer or whether we must infiltrate, evapotranspire, or otherwise reuse all of the water on site. We'll use the cistern for irrigation in summer, but otherwise it is effectively serving as a detention tank.
Is detaining stormwater and then discharging it to the stormwater acceptable, provided you reduce the peak 24 hr discharge rate per LEED requirements? This certainly has benefits (we are in a CSO area) but does not seem to meet the full intent. My confusion is compounded by the language on LEED User (above):
"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."
I don't see why onsite reuse or infiltration are required to meet SSc6.2 (other methods remove TSSTotal suspended solids (TSS) are particles that are too small or light to be removed from stormwater via gravity settling. Suspended solid concentrations are typically removed via filtration.), and so it makes me think that this should say "you'll need to addd... to meet SSc6.1 requirements."
Thanks for your help,
Michael
Michael, if you are using the volume reduction of the reuse within the cistern, you can not drawdown the cistern into the storm drains. While this slow-release method would reduce peak rate (this is what the above reference was referring to), it does not reduce runoff volume or quantity.
Unless there are some local regs at work here, I would be a little more aggressive with the drawdown time. Recent studies by the Villanova University Urban Stormwater Partnership show that the chance of back to back 2-year storms occurring within 3 days is around 2%.
I have recently completed 2 projects that mitigate the 2-year volume increase using capture/reuse. I have successfully argued that the water really only needs to drawdown within 2 years, because it is a theoretical "2-year volume."
To meet in the middle, we design our capture/reuse systems to use the required volume within the 7 month irrigation season in the Northeast.
Hope that helps.
Cistern volume
Could you please give me an advice regarding question 1?
Our cistern is designed to store the volume that is required in this credit – the predevelopmet runoff is 90 m3, the post development runoff is 130 m3 and the difference 40 m3 we would like to store in the cistern (60 m3). The captured rainwater is reused for flushing – the need of water for flushing is 4 m3 per day that means, we could empty the cistern in 10 days after design storm. I think this assumption is not sufficient in order to meet the credit requirements.
We must declare that we reuse all the captured rainwater as states upwards. Is it sufficient to calculate annual balance of rainwater that is captured by the cistern and the amount of water that is reused for flushing in monthly steps? Which time step do you recommend?
Another question: which part of the monthly rainwater volume I must reuse for flushing to meet the requirement of SS6.1? I think it is not necessary to reuse all the rainwater because the credit requires retaining 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 (90 m3 during design storm). Is it for example possible to reuse 30% of the monthly rainwater volume to meet the content of this credit (30 % results from the design storm – we must capture 30 % of rainwater = 40m3/130 m3 to meet the requirement).
Could you describe briefly how it is possible to declare sufficient volume of the cistern and water for reuse?
Thanks a lot!
I would (and have) done simple hand calculations that show the Delta-2 year volume is 1412.59ft^3 (sorry I need to do this in English units), and this is how I intend to solve this:
1412.59 ft^3 = 10,566.1732 gallons.
Toilet = 1.6 gallon/flush
(100 people)(4 flush/day) = (400 flush/day)(1.6 gpf) = 640 gpd
10,566.1732/640 = 16.5 days
Of course, substitute your own numbers.
You are dealing with 1 and 2-year runoff volumes, not monthly volumes. Don't make this more complicated than it needs to be....simply calculate the delta and go from there.
Hope this helps you on your way.
Computer-based software: Estimating SW runoff rates and volumes.
Hi everyone,
While studying the GBD+C Guide, I noticed that under the Calculations section, says: "various methods and computer-based software programs are available to estimate stormwater runoff rates and volumes".
I'd like to know if some of you have ever used this kind of software (and if it is acceptable for LEED certification purposes), and of course, which program. I haven't been able to find any reference to this.
Thank you all.
Yes and Yes.
common programs used are: HydroCAD, PondPack, SWMM, VTSUHM, HEC-HMS.
1-year and 2-year 24-hour Design Storm
Michael DeVuono,
I've digged through both forum 6.1 and 6.2 but didn't found international data for Design Storm. I am doing a project in Russia. We have sites with existing imperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground. 50% or less and need this data for SSc6.1-1 and SSc6.1-2.
Thank you for halping.
Liubov, there are several good resources posted in the Resources tab above that should help. (You'll need to log on as a paid member to access this and other LEEDuser, guidance, though.)
A building on a campus
We currently working on a project that will be pursuing LEED certification. The building resides on a campus but we are only certifying one building as part of our contract. We are currently pursuing SSc6.1 and SSc6.2 based on our LEED boundary. Future projects on the campus will be LEED certified however they are not part of the scope of work for this project. My question is, can SSc6.1 and SS6.2 be achieved if they are based on the area within our LEED boundary or are we required to document compliance for the entire campus?
You do not have to show compliance for the entire campus. If the stormwater retention/detention and quality controls are within the LEED boundary of your current project then you need only demonstrate compliance for that area. Future projects on the campus can use the already installed system, as long as they can demonstrate that there is enough capacity for the additional site area(s) regardless of the location of the system. It is expected in a campus setting that much of the infrastructure will be shared or centralized.
Drainage Pipe Capacity
Can we count the capacity of underground drainage pipes as the retention capacity? The pipe capacity will then add to the stormwater retention capacity. This method will significantly reduce the need for a large retention tank.
Does anyone try this method in a simulation program and what about the response from the reviewer?
thanks
You certainly can use the volume of your drainage system when attempting this credit.
2-year 24-hour Design Storm in Indonesia
Hi, I'm currently doing a project in Indonesia and having trouble to find the 2-year 24-hour design storm data. Anybody has any insights on how to get these values in countries outside the US?
If no such data exists, is there a method to interpolate historical precipitation data to get the 2-year 24-hour design storm value?
Thank you.
Ivan, if you dig through this forum and the 6.2 forum, I know I posted a link at one time that gave international rainfall totals. I don't have it bookmarked.
I just googled "international rainfall data" from the US, and found a ton of sites.
Good luck.
Pre Development & Post Development runoff rate & quantity zero
We are working on a Core & Shell project on Cayman Islands and would like to pursue SS Credit 6.1 & 6.2. The project is 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.There are deep wells existing on the property where currently all the rainwater is captured and therefore there is no runoff from the site at all. These wells will be retained in the project and used for the same purpose thus resulting in zero site runoff. How do we go about documenting this credit since both pre development & post development site runoff are zero.
You have on-site runoff or the wells would not capture rainwater. Therefore the calculation is in the pre-development vs post-development scenario. If the amount of building is increased it will add to the 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. on the site. So you will need to demonstrate that those same wells can capture the additional run-off from roofs, sidewalks, roadways, etc.
Thank you Michele.
We plan to add more underground wells on the site to accomodate all the excess runoff.I hope that we can achieve this credit.
SS 6.1 Case 2 options?
Has anyone been successful applying the impounding/receiving stream protection approach for a previously >50% impervious site? Or will we absolutely have to live with reducing the storm event runoff the 25%?
I want to second Karen's question. Has anyone out there used the receiving stream approach at all? Information on successful use of this approach has been hard to come by. If you don't feel like posting experiences on the forum, please contact me through LEEDuser.
a few questions on credit 6.1
I have a few questions on this credit that are buggering me off. Since I'm working on a site with 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. higher than 50%, the main objective of this credit is to create appropriate infrastructure in order to reduce the total runoff volume of a 2-yr 24-hr design storm in 25%. The project site is largely dominated by a clay pan, so even though I'm increasing perviousness through the use of permeable pavements and rain gardens, infiltration is not feasible, so I'm retaining filtered water (from pavers and bioswales) and excess water in a wet pond, including excess from cisterns. I have two questions:
1. The cisterns are capable of holding approximately 25% of the design storm, however, they won't be empty the whole time, and therefore won't guarantee its retention throughout the year. Should I apply a given measure (say, on average), and compensate for the excess in the wet pond?
2. How do I account for water retained in planters, gardens and bioswales?
Greetings from Brazil
Question 1:We deal with this all the time on capture/reuse projects. Basically you can not store more than you have the ability to use. You need to have sufficient volume in the cistern to capture back to back lower level storms.
The problem I always have with capture and reuse, is not the capture part, it's finding a way to use all the water!
Question 2: Ideally I would have a geotech get infiltration rates for the underlying soil, then convert the flow rate to a constant flow (cfs) applied across the surface area of the BMPBest Management Practice. This will allow you to "net out" some of the runoff volume, thus reducing peak rate as well. Additionally, you can take credit for the storage volume in the BMP. The ammended soils and mulch in your BMP should have (on average) a void ratio of about 17% So take 17% of the total depth of the BMP, and net out that volume as well.
Could you please give me an advice regarding question 1?
Our cistern is designed to store the volume that is required in this credit – the predevelopmet runoff is 90 m3, the post development runoff is 130 m3 and the difference 40 m3 we would like to store in the cistern (60 m3). The captured rainwater is reused for flushing – the need of water for flushing is 4 m3 per day that means, we could empty the cistern in 10 days after design storm. I think this assumption is not sufficient in order to meet the credit requirements.
We must declare that we reuse all the captured rainwater as states upwards. Is it sufficient to calculate annual balance of rainwater that is captured by the cistern and the amount of water that is reused for flushing in monthly steps? Which time step do you recommend?
Another question: which part of the monthly rainwater volume I must reuse for flushing to meet the requirement of SS6.1? I think it is not necessary to reuse all the rainwater because the credit requires retaining 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 (90 m3 during design storm). Is it for example possible to reuse 30% of the monthly rainwater volume to meet the content of this credit (30 % results from the design storm – we must capture 30 % of rainwater = 40m3/130 m3 to meet the requirement).
Could you describe briefly how it is possible to declare sufficient volume of the cistern and water for reuse?
Thanks a lot!
Pre Development Determination
Our site was recently demolished, filled with dirt and left for some time. The demo company left the foundation behind, so there currently exists approximately 10' feet of soil in what is essentially a concrete bathtub. Water cannot return to the aquifer, but it is able to absorb storm water in the short term. I would consider this site to be impervious even though at grade it appears to be pervious. Any thoughts?
Hi,
I deal with this all the time with infill jobs in our city. The building is gone, but the basement floor is still there. For 6.1, quantity control, you would not model this as impervious, you are modelling the runoff. Just go by surface condition.
The basement would come into play in 6.2. Quantity control.
Thanks Mike, Are you in NYC? Can you describe how the basement matters in 6.2- Quality?
Thanks Again.
I apolologize. I deal with so many different regs on a daily basis, I got confused. Thank you for your follow up question. Water quality is typically associated with volume control, this the reason for my confusion.
The basement would come into play on any water quality BMPBest Management Practice that uses infiltration. You don't have infiltration (unless you excavate the basement, I've done this before).
I forgot that 6.1 deals with volume control as well. Again I apologize.
You may or may not get this approved, but I have done raingardens on this exact type of site. You take a volume credit for the raingarden, the idea is that the volume will be lost due to evapotranspiration and some infiltration. I do not know the particulars on NYC regs though.
Of course there is always capture and reuse, soil amendments,and landscape restoration and many other techniques to solve for volume. Without getting into specifics it is hard for me to add anything further.
Frankly, most of the SWM regs required by a local municipality meet or exceed LEED standards. Especially if you need an 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. permit
I am not in NYC, I'm in Philadelphia.
I'm typing this on my phone, so I do apologize for any typos, I hope this helps.
Calculating Runoff to a Rain Garden
Hello,
Our project is an adaptive reuseAdapted reuse is the renovation of a space for a purpose different from the original. on an urban site that was previously about 95% impervious. We are increasing the area of vegetated space (the old parking lot is being demolished), collecting rainwater from about 6700 SF of the roof areaRoof area is the area of the uppermost surface of the building which covers enclosed Gross Floor Area, as measured when projected onto a flat, horizontal surface (i.e. as seen in Roof Plan view). ‘Roofs’, or portions of roofs, covering unenclosed areas (e.g. roofs over porches and open covered parking structures) are not included in the areas used to evaluate compliance with SSc7.2, though they may be applicable to SSc7.1., and creating a rain garden out of a vegetated parking island that is about 1800 SF. We had already put together the stormwater volume reduction calculations using the NRCS method before the team decided to add the rain garden (we had previously been considering pervious pavement for a section of the front driveway), and now we are having trouble determining the proper way to quantify the runoff that will now be directed to the rain garden in the calculations. We plan to slope a section of the paving which surrounds the rain garden towards the rain garden, but how should this be quantified? Should the volume of water which will be draining to the rain garden be subtracted from the Runoff Volume Total in the design case (assuming that we have properly accounted for the rain garden being able to handle that 2-year 24-hour design storm volume based on the inches/hour infiltration rate for the soil), or should the CN value for that section of the paving be changed?
You take a volume reduction based on the dead storage of the raingarden.
Hi Mike, thanks! So we would subtract the dead storage of the rain garden after the runoff volume subtotal has been calculated using a CN value of 98 for the 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. and a lower CN value for the rain garden that is based on the infiltration rate of the soil? Have you previously used this strategy on a SSc6.1 submission?
Hi Christine,
I have not personally submitted any credits, ever. I'm a CE, specializing in stormwater. Many of my projects have attained various LEED status, using my SWM calcs.
Use this equation to solve for the runoff that will drain to the raingarden:
Q={((P-.2S)^2)/((P-.2S)+S))}
P=2yr 24/hr depth (get it from NOAA's website)
S=(1000/CN)-10
Use a CN of 98 for impervious....61 for Lawn areas.
Then solve for the raingarden volume to make sure the 2-year storm fits:
Dead Storage = {(Bed Area ft^2)(h)}+{(Bed Area)(Dsoil)(0.17)}
h=height of the inlet grate in the RG (should be no more than 9"
Dsoil = depth of amended soil. Should be no deeper than 4'
peak flow calculations for 24-hr duration
Hello -
My question is in regards to the 2-yr, 24-hr requirement for calculating peak runoff rate. As far as I understand, by requiring the 24-hr duration to be used with 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. for both pre- and post-development, any design that increases the site's time of concentration (thus reducing the peak runoff rate) is not accounted for. It seems to me that the rainfall duration used in the Rational Method calcs should be based on the site Tc, just as they are with traditional stormwater design. Using a 24 hr duration for peak runoff doesn't make much sense to me....even the local stormwater criteria in my area does not provide rainfall IDF curves for such long durations.
Perhaps I am misinterpreting the language. Either way, I would appreciate any insights as to how this has been used and/or interpreted by others in the past.
Thanks,
Chris
In my experience you are not limited in your calculation methodology, but do not need to account for time of concentration per the reference guide. I suggest uploading your calculations with an accompanying explanation for whichever approach you use and your results.
You really can not compare pre vs post using on the 24 hour intensity unless your Tc's remain the same in the the pre and post.
If you have a farm field with a 2-hour Tc in the pre, the pave the entire site for the post and get a 5 min tc, your peak rate runoff will obviously be much greater in the post.
Your runoff coefficient will take care of some of this, but the rate will not be correct.
SSc6.1 Pond with Flow Restrictor and Total Stormwater Volume
I am having the same issue as Lucy (October 18, 2010) on one of our current projects, and the comments by Gregory and Jennifer were helpful but I still have some questions.
Scenario:
-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. is less than 50%
-new buildng footprint will increase imperviousness
-we will build a new detention pond/swale with an outlet to city storm system
-there will be a restrictor to control the rate of outflow from the pond/swale
-we do not plan to use the water for reuse in the building or landscaping, etc.
1.) I thought that showing the restrictor outflow rate would be enough for the calculations but we got the following comment from the GBCI: "peak rate from a detention pond is typically a function of the outlet structure configuration, outliet pipe size, and the depth of water in the pond. Peak rate does not necessarily decrease at a constant rate with a decrease in runoff volume. Please revise and resubmit calculations for the peak rate leaving the pond..." So, can someone please illustrate how to calculate the peak rate if we have a restrictor (at the outlet from the pond) which allows a maximum flow of 0.5cfs/acre?
2.) The second issue is as follows: if we are catching storm water in the pond, the restrictor will solve the issue of the "rate", but not the problem of the "volume". Does someone know how to overcome this issue?
Thank you in advance!
Even though I haven't done such calculations, I might be able to shed some light on this.
1.The reviewer is arguing that your outlet will allow for more water to be discharge depending on the height of the water table in your pond. So lots of water in the pond --> higher pressure at the outlet --> higher water flow.
You will have to account for this or explain, why your outlet will only allow for a certain maximum
2. Besides reduction of the flow rate, you will have to reduce your volume. So the question is how much of that water, which is sitting in the pond, will infiltrate into the ground. Your civil engineer should be able to answer that.
I hope that helped at all.
Thank you, this is helpful. But I think we might be losing this credit, since the soil in our property has proven uncapable of absorbing as much water as we need. It seems like the only way to get this credit--at least in terms of quantity--are to use the water for something else, or to have the project on a sandy beach where the water can be absorbed by the site. :(
Is it possible to get post development run off in negative?
Our project is located in Kathmandu, Nepal and I have a few questions regarding the following points.
1. As per the Table 1 page # 104 of 'LEED Reference Guide for Green Building Design & Construction', the Annual Rainfall in Kathmandu is more than 40", which is under Humid WatershedWatershed that receives at least 40 inches of rainfall each year. area. To design the system, to capture the runoff, the average rainfall event can be taken as 1" (one inch) of rainfall per 24 hour.
2. But in 'SS Credit 6.1: Stormwater Design - Quantity Control Form' it is mentioned that 'Site Runoff: Two-Years, 24-Hours Design Storm.
Question -1:
"Which value should I take to calculated the site Stormwater Runoff?"
Question - 2:
"While calculating, if the Stormwater Runoff reduction is in negative values or less than 25. What procedure should we follow?"
In response to question #1: The credits are different and require a different set of calculations. Credit 6.2 is addressing water quality and the requirment is to treat the first 1-inch (in this case) of a rainstorm. The intent is that most pollution is contained in the small frequent storms, and that developing a pollution prevention strategy to treat stormwater runoff in the smaller storms will provide an adequate and cost effective level of protection to receiving steams and lakes to minimize the effect of sediment. The requirement is actually to treat all the stormwater from 90% of the annual runoff volume. This is a cumbersome calculation so LEED has provided a guideline based on conditions in the watershed (humid, semi-arid, and arid) to help make the calculations easier.
Credit 6.1 has been developed for a different reason and is more concerned with flood control and downstream flooding. For this reason, a two year-24 hour storm (which can have a rainfall depth greater than the 1-inch suggested for evaluation of credi 6.2) is the chosen basis for design, and, depending on the location, is intended to reduce the peak flows leaving a site that can cause downstream soil erosion, sediment transport, and a negative impact on drainage infrastructure.
Depending on your solution, usually detention and infiltration for volume reduction for 6.1 or 6.2, the solution may serve for both credits, but the requirements for both credits must be checked independently.
Question #2: I am not entirely clear on your question, if the reduction is negative, that means that there is an increase in runoff. If there is an increase in runoff, you will have to develop strategies to reduce the volume and/or flow rate to meet the LEED criteria for the credit. Again, the reduction may be different, depending on which credit you are evaluating. If you want to clarify the question, I may be able to provide a better response.
Option 2: Stream Protection Plan
I apologize if the requested information is included on this site but I have not seen it to this point. Can anyone give me a definition of what constitutes a stream channel? We are looking at a 5.0 acres site that is unimproved and the proposed development will create an 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. area of approximately 4.0 - 4.5 acres of the total 5.0 acres. With this in mind there does not appear a way that Option 1: Quantity can be met without creating a large wet pond on the site to accommodate the tremendous increase in storm water that will be generated from this site. Detaining for the peak discharge is not a problem but the total volume is the issue with Option 1 for this site. The proposed site has a concrete lined drainage ditch running the entire length of the property that the site currently sheet drains to. This ditch eventually routes to an existing detention pond before discharging to the outfall stream. We will maintain all existing drainage patterns on the proposed site. If we are wanting to achieve Option 2 are we to implement the stream protection plan on our site even though technically the stream it will be discharged to is not on our site? If we slow the water down with some design techniques, swales, rain gardens, rip rap, etc. will this be sufficient to achieve the point for this credit?
Robert, I don't think the credit language intends to restrict your efforts to streams on your site. I think you would want to be as comprehensive as possible in looking at both the drainage on your site and ther receiving stream.
Calculation of drawdown rate Qr
I have a question for a Czech colleague of mine. The have a project and want to install a stormwater storage tank. The water shall be used for toilet flushing in the building. The problem is, that he does not know how to determine the design storm interval. I looked into the LEED BD+C Reference Guide on page 97 in the example (last calculation) at the top of the page. From where do the 3 days / 72 hrs come. I understand that the tank must be emptied, but I cannot help him identifying the design storm interval. Can anybody please help?
Christian, if I understand the question properly, proper sizing of this tank should be based not on LEED compliance data but on meteoroligical data from the area and calculations of how much water will be required for flushing.
Hello Tristan,
I don't fully understand your answer. If we want to reuse the stormwater for flushing for example, I understand that we have to take in account the flushing quantity of water expected each day and increase the tank's dimension. Indeed, without this consideration, the storage tank would be always empty because of the necessity to prevent 2 years design storm. But what is the design storm intervall between two events? 24 hours? (I realize that having two 24 hour storms back to back is rare but it could happen). In LEED exemple p97, they use a design storm interval of 3 days and that make no sense for me. You said it is based on meteoroligical data but I don't understand the link.
Thanks
Benjamin, I see you're a student, so I'm wondering if your question is hypothetical or about a specific project. The reason I ask is that hypotheticals can sometimes be more confusing than real examples.
The point I was making in my earlier post is very basic: LEED doesn't tell you how to size a cistern. You have to work with the meteroology of the site and the credit requirements you are trying to meet. Those constraints will dictate your calculations. And yes, some of those requirements are imposed by LEED. But it's about your choice of how to meet the LEED credit, not LEED telling you how to design your tank.
The data I have to work with
The data I have to work with for filling this credit out are pre- and post-development calculations for a 100 year storm with a duration of 1 hour. How does this relate/convert to 1-yr, 24 hr and 2-yr, 24 hour design storms?
Thank you!
It doesn't. You will have to obtain the 1yr, 24hr and 2 yr 24 hour storm data form your local weather service. It's a totally different value. In general if the systems were designed to handle the 100 year storm than they are very much capable to handle the others. I did a project in German with the exact same question.
Case 2 Site with little decrease in Impervious Ratios
I am in the processing of accessing the feasibility of attaining this credit with a site that is currently 88% impervious and the proposed site is roughly 81% impervious. While i will be able to show a slight reduction in site runoff via a retention swale (+/- 4%), i am wondering if an exfiltrationExfiltration is air leakage through cracks and interstices and through the ceilings, floors, and walls. system would be a means of achieving the additional runoff decrease. I know this goes against the standard runoff reduction calculations, but has this credit been achieved via structured stormwater system?
We had an infiltration system to reduce the storm water runoff to zero for two projects with very limited site area. Here is a link to a similar system. http://www.intewa.com/drain_max_tunnel_infiltration_ditch_system_1258504... not sure if that's what you are planning on using, but you can install these system under neath the surface parking. It's pretty much common practice in Europe.
How to quantify a evaporation rate
I was hoping I could get some advice on obtaining the quantity requirement of SS CR 6.1. We are trying to obtain this credit on an approximate 19 acre site. The site was originally a multi-office complex that will be removed and replaced with a much larger building. The impervious area of the site will be increased slightly (69% to 71%) but we are increasing the quality of the landscaping so the runoff curve number is actually lower than the existing conditions (CN 89 to 87). To achieve this credit the volume of runoff must be 25% lower than the existing conditions for the 2-yr 24hr storm event, which we determined will be 155,347 gal of runoff to remain on-site. There will be 2 underground rainwater cisterns on-site; a 20,000 gal tank to be re-used as graywaterGraywater is untreated household waste water which has not come into contact with toilet waste. Graywater typically includes used water from bathtubs, showers, bathroom wash basins, and water from clothes-washer and laundry tubs, though definitions may vary. Some states and local authorities also allow kitchen sink wastewater to be included in graywater. Project teams should comply with the graywater definition established by the authority having jurisdiction in the project area. within the building and a 105,000 gal tank to be re-used for irrigation purposes. My first question is when calculating the volume for the underground tank, can I only use the total (125,000 gal) or can I determine a draw down and get some additional volume. Secondly, due to the volume shortage we are proposing to create a constructed wetland with enough storage volume to achieve this credit. Unfortunately the soil conditions are not great and we can only expect an infiltration rate around 0.04 in/hr. Although, with the wetland we will see some evaporation and evapotranspiration rates that would increase the volume of removed from the wetland. Will this rate need to be quantified for this credit, if so is there some documentation out there to help determine this. Also, in general do you think what we are proposing will satisfy the intent of the credit or do you think we are out of luck, other than adding more pervious surface? Thanks
Question 1: Tank Volume.
Some tank drawdown assumptions seem reasonable in this case given the greywater reuse, but the effort to prove a minimal amount of added volume may prove to be more trouble than it is worth given the addition of the constructed wetland.
Question 2: Constructed Wetland Evaporation Rate.
In general I think that any surface water retention system that is well designed and therefore adequately sized can help achieve this credit without the need to account for evaporation rates insofar as LEED is concerned. That being said it isn't entirely clear from your description that the constructed wetland will actually discharge water. If water will leave the constructed wetland the rate of infiltration and evaporation will be key to showing your project's net reduction of stormwater quantity.
Thanks for your response Amy. The wetland has been designed to be anywhere from 2' to 6" deep. An overflow wier on a riser pipe will be installed to take the storrmwater out once it reaches an elevation over the 2' depth. Also, could I include some assumption of the infiltration rate of the approximately 5.5 acreas of landscaping (sod / native plantings, etc.) and green roof on site. I would have plenlty of volume by doing this...
Thanks again.
I've been using 0.17"/hr for evapotranspiration in wetlands, raingarden, etc to justify meeting the delta 2 requirements for 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. submissions. If the DEP is accepting it, I think you should be ok using it.
FYI that is just a number I came up with from research across the web.
How to show 100% Infiltration
Our project is the only development on a small, flat, island. The surface is sand or sandy topsoil and is extremely porous. After development, the only 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. will be roofs, some of which will harvest rainwater. Rainfall from the other roofs will infiltrate naturally into the sand. There is no runoff into the surrounding lagoon as all rainfall infiltrates into the water table. Given that no specific stormwater design is needed, will it be sufficient for documentation to show that 100% infiltration remains post development? Thanks
Yes, 100% on site infiltration is an acceptable means to obtaining this credit. The reviewers will likely want to see some calculations or supporting documentation showing that the soil types and grading can adequately infiltrate the additional rainwater from the 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. that aren't being captured, but this shouldn't have to be very extensive. Be sure to describe your rainwater harvesting system as well to further support your claim.
runoff coefficient
what the value of runoff coefficient for gravel area (2% gradient)?
While this is somewhat dependent on the design (material thickness, subgrade permeability, etc), 0.5 is a good rule of thumb for low slope gravel areas.
Multiple Drainage Patterns
Can you tell me how to calculate for multiple draingage sites, i have Three on this particular project. The TC is based on the length and travel and the slope towards the discharge point . Do i take a seperate calc for each then average?
There are two scenarios that depend on the basin layout and interaction and it is not clear from your question if you have multiple discharge points, one from each basin, or if all three basins combine to a single discharge point..
The most simple scenario is that all three basins are separate, with no connection or flow through from basin to basin. In this case, you would analyze each basin separately. If one basin does not quite meet the requirements for LEED, but another basin can more than meet the requirements and make up for the deficient basin, then you may still be able to meet the LEED requirements as you add the results together.
The more complicated analysis is if the basins flow through each other (they are essentially sub-basins) in which case, you may want to analyze the sub-basins and determine if there is the opportunity to reduce the peak flow rate based on timing of the discharge from the separate sub-basins. Of course, the volume will be additive from basin to basin.
In any case, it is not appropriate to average the separate calculations, unless you developed a weighted average.. It is more probable that you will analyze each basin and add the reults together to get the combined result.
Which point in time do I take for ascerting imperviousness?
Hello,
following "problem": we have a project which will be built on a site which was an abandoned industrial site up to 2009. Then all buildings and surfaces were demolished and the site lied idle. So which situation do I take for the calculation of 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. discharge rate? The long-time situation when it was used as an industrial site with a high 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. until around 2009 or the situation we had for the last year, when there were nearly no impervious areas on the site?
Thanks for your advice!
I would take the condition at the start of your LEED project, with no impervious areas.
Is Imperviousness = Runoff coefficient ?
Hello,
Can I presume that the 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. of a site is less than 50% if the overall runoff coefficient is less than 0.5 ?
Apologies if the answer is obvious but I am not an export in this field.
Thanks,
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