NC-v4 SSc4: Rainwater management

  • Understand your site’s natural hydrology

    This credit is a combination of stormwater quality control and quantity control, and includes site-specific criteria for more frequent, low-intensity storm events. There’s even a pathway for zero-lot-line urban projects.

    Executing this credit may be expensive but it can also be economical; it varies greatly depending on site, soil, and project. Additionally, teams should take some time to better understand the historical conditions of the site; this is important as you’ll need to use low-impact developmentAn approach to managing rainwater 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 (e.g., minimizing land disturbance, preserving vegetation, minimizing impervious cover), and design practices (e.g., rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, soil amendments). These are engineered practices that may require specialized design assistance. (LIDLow-impact development: an approach to managing rainwater 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 (e.g., minimizing land disturbance, preserving vegetation, minimizing impervious cover), and design practices (e.g., rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, soil amendments). These are engineered practices that may require specialized design assistance.) and green infrastructure strategies that mimic the site’s natural pre-development hydrology.

    What’s New in LEED v4

    • This credit combines the LEED 2009 stormwater quality and quantity management credits.
    • A specific credit path has been added that accommodates zero lot line projects with reduced rainwater management opportunities.
    • Am expanded multi-tenant complex path has been added for all types of projects in addition to just retail projects.
    • Projects must use onsite low impact development and green infrastructure rainwater management techniques.
    • The credit now uses a metric for testing compliance that calculates the total volume of runoff for the 95th percentile of regional or local rainfall events, instead of one and two-year storm events.

    FAQs

    What calculations will I need to complete?

    You’ll need to calculate (1) the volume of rainfall for the 95th percentile storm, (2) the total stormwater runoff from impermeable surfaces on your site, and (3) the volume of runoff that gets infiltrated or captured for reuse by your green infrastructure.

    95th percentile storm

    USGBC’s calculator helps teams calculate the 95th percentile storm, but you first need to collect at least 10 years of historic rainfall data from the National Climatic Data Center and paste it into the USGBC calculator. The rainfall data will likely be in the format of daily rainfall amounts over historic years. So, expect to input a lot of data points into the calculator.

    Calculating runoff from impermeable areas

    LEED InterpretationLEED Interpretations are official answers to technical inquiries about implementing LEED on a project. They help people understand how their projects can meet LEED requirements and provide clarity on existing options. LEED Interpretations are to be used by any project certifying under an applicable rating system. All project teams are required to adhere to all LEED Interpretations posted before their registration date. This also applies to other addenda. Adherence to rulings posted after a project registers is optional, but strongly encouraged. LEED Interpretations are published in a searchable database at usgbc.org. #100001950 outlines updated protocol for calculating the total runoff. Generally you need to know the rainfall depth (mm) for the 95th percentile storm, the area of the impermeable spaces, and the runoff coefficient, which is described as “Small Storm Hydrology Method Runoff.” See the interpretation for more details.

     

  • SS Credit 4: Rainwater management

    Intent

    To reduce runoff volume and improve water quality by replicating the natural hydrology and water balance of the site, based on historical conditions and undeveloped ecosystems in the region.

    Requirements

    Option 1. Percentile of rainfall events
    Path 1. 95th percentile (2 points except Healthcare, 1 point Healthcare)

    In a manner best replicating natural site hydrology processes, manage on site the runoff from the developed site for the 95th percentile of regional or local rainfall events using low-impact developmentAn approach to managing rainwater 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 (e.g., minimizing land disturbance, preserving vegetation, minimizing impervious cover), and design practices (e.g., rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, soil amendments). These are engineered practices that may require specialized design assistance. (LIDLow-impact development: an approach to managing rainwater 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 (e.g., minimizing land disturbance, preserving vegetation, minimizing impervious cover), and design practices (e.g., rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, soil amendments). These are engineered practices that may require specialized design assistance.) and green infrastructure.

    Use daily rainfall data and the methodology in the U.S. Environmental Protection Agency (EPA) 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
    Path 2. 98th percentile (3 points except Healthcare, 2 points Healthcare)

    Achieve Path 1 but for the 98th percentile of regional or local rainfall events, using LID and green infrastructure.

    OR
    Path 3. Zero lot line projects only – 85th Percentile (3 points except Healthcare, 2 points Healthcare))

    The following requirement applies to zero lot line projects in urban areas with a minimum density of 1.5 FARFloor-area ratio is the density of nonresidential land use, exclusive of parking, measured as the total nonresidential building floor area divided by the total buildable land area available for nonresidential structures. For example, on a site with 10,000 square feet (930 square meters) of buildable land area, an FAR of 1.0 would be 10,000 square feet (930 square meters) of building floor area. On the same site, an FAR of 1.5 would be 15,000 square feet (1395 square meters), an FAR of 2.0 would be 20,000 square feet (1860 square meters), and an FAR of 0.5 would be 5,000 square feet (465 square meters).. In a manner best replicating natural site hydrology processes, manage on site the runoff from the developed site for the 85th percentile of regional or local rainfall events, using LID and green infrastructure.

    OR

    Option 2. Natural land cover conditions (3 points except Healthcare, 2 points Healthcare)

    Manage on site the annual increase in runoff volume from the natural land cover condition to the postdeveloped condition.

    Projects that are part of a multitenant complex only

    The credit requirements may be met using a coordinated approach affecting the defined project site that is within the master plan boundary. Distributed techniques based on a watershed approach are then required.

    SITES-LEED Equivalency

    This LEED credit (or a component of this credit) has been established as equivalent to a SITES v2 credit or component. For more information on using the equivalency as a substitution in your LEED or SITES project, see this article and guidance document.

Design Submittal

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

116 Comments

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Jimmy Carchietta LEED AP CEO The Cotocon Group
Feb 15 2017
LEEDuser Member

Rainwater Management Calculation

Dear all,

I'm working on v4 NC project. In the rainwater management Credit, I'm following the modified 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.. In this method, What is APF?
How to take value of APF into calculation?

Or suggest another way to calculate runoff volume.

Thanks!
Jimmy

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Feb 15 2017 LEEDuser Expert 6003 Thumbs Up

APF is the Antecedent Precipitation Factor. It is a variable that that accounts for soil moisture prior to the storm you are analyzing. The more moist the soil, the more runoff.

It is highly localized, and variable dependent on local precipitation depths and the soil type(s) on your site.

Modified 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. is not an acceptable mean to estimate runoff volume. I know it's in the reference manual, but it shouldn't be used to estimate volume or you will grossly underestimate the storage volume required for your BMPBest Management Practice.

You should really be using the SCS method for all your calculations.

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Theresa Backhus Sites Technical Specialist, LEED, USGBC Feb 15 2017 Guest 1011 Thumbs Up

Hi Jimmy, I think you're referencing an older version of the Reference Guide. We published addenda in October 2014 that revises the "Calculations" section (#100001950: http://www.usgbc.org/content/100001950). You can view the latest v4 Reference Guide online in the Credit Library (http://www.usgbc.org/credits), under the "Guide" tab of each credit.

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Aaron Luthien
Feb 08 2017
Guest
384 Thumbs Up

95th 98th percentile rainfall data

Hello,

Where can we access online 95th 98th percentile rainfall data?

Thank you!

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Feb 15 2017 LEEDuser Expert 6003 Thumbs Up

You need to compile this from local weather data.

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Feb 17 2017 LEEDuser Expert 6003 Thumbs Up

Let me see if we can't get the calculator up on the site too, because I know this question is going to a popular one.

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Rita Haberman Brightworks
Feb 08 2017
LEEDuser Member
370 Thumbs Up

Vegetated Space as LID

Hi all,

My project is utilizing bioretention areas to treat the majority of runoff, but we also have vegetated areas that are not, by definition, and LIDLow-impact development: an approach to managing rainwater 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 (e.g., minimizing land disturbance, preserving vegetation, minimizing impervious cover), and design practices (e.g., rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, soil amendments). These are engineered practices that may require specialized design assistance., just planted areas/green space that will receive some of the runoff. Water will flow from the sidewalk into the planters for absorption. Can we list this methodology under the "LID/Green Infrastructure" table within the calculator, or does treatment need to occur from specifically a bioretention area or rain garden?

Thank you.

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Feb 08 2017 LEEDuser Expert 6003 Thumbs Up

Extended time of concentration promotes infiltration, absorption, evapotranspiration and evaporation. It is the essence of the natural hydrologic cycle. You aren't going to get more LIDLow-impact development: an approach to managing rainwater 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 (e.g., minimizing land disturbance, preserving vegetation, minimizing impervious cover), and design practices (e.g., rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, soil amendments). These are engineered practices that may require specialized design assistance. than this.

I would just describe in detail what you're doing, and how the Tc and Tc Path lead to all runoff being treated on site.

As for "planters" ..... a raingarden is essentially the same thing, just on a larger scale.

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Rita Haberman Brightworks Feb 08 2017 LEEDuser Member 370 Thumbs Up

Thanks! The civil engineer working on this wanted clarification, and I wasn't exactly sure.

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Risto Beatty
Jan 27 2017
LEEDuser Member
11 Thumbs Up

Stormwater Reuse for Cooling Tower Makeup

Project Location: United States

Hi, I am working on a project located in Manhattan where infiltrating water is not possible due to the amount of below grade infrastructure. To reduce the amount of runoff from the site, the building has been designed with extensive green roofs and vegetation at grade. All the remaining water will be piped into a stormwater retention tank that is sized to handle the entire volume of a 98th percentile rainfall event. Rather than releasing this water back to the sewer, we are proposing a pipe connection the the buildings central plant where 100% of the water will be used to offset the makeup water required by the cooling towers.

Will this strategy qualify us to earn Option 1 of this credit? The Reference Guide seems to stress that the runoff must be managed in a way that mimics natural conditions, but in our case, this simply is not possible. However, the credit intent says. “Rainwater [should be] treated as a resource rather than a waste product”, and I believe that our proposed methodology does just that. Is this an acceptable approach?

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Jan 27 2017 LEEDuser Expert 6003 Thumbs Up

There haven't been many test cases on this, and I have a feeling you may be in the forefront here. I think you have a solid approach, it's definitely a "green" approach. But I just don't know if this is going to fly. I also don't believe the intent of this credit was to exclude all projects within urban areas either, because that is what it is doing.

But why are you going after Option 1? Look at Option 2, all you'll need to manage is the difference in volume between Pre and Post. It will be less to manage, and you may hit it with just the green roof.

Keep me posted on this one.

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Risto Beatty Jan 27 2017 LEEDuser Member 11 Thumbs Up

Michael,
Thank you for the timely response. I had not looked into Option 2 since I interpreted 'natural land cover condition' to mean without any development, not the pre-development condition. Do you really just need to show that the developed site produces less runoff than it did when you found it? I will investigate this.

As for Option 1, I agree with you. It may be worth submitting a CIRCredit Interpretation Ruling. Used by design team members experiencing difficulties in the application of a LEED prerequisite or credit to a project. Typically, difficulties arise when specific issues are not directly addressed by LEED information/guide to see if they will offer an alternative compliance path. Beyond managing the site runoff, using the water to offset cooling tower reuse is favorable since it will decrease the fees attributed to sewage conveyance. I will let you know how this develops.

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Jan 27 2017 LEEDuser Expert 6003 Thumbs Up

"Natural Condition" or Pre-Development definitely means the stone age. What was the cover condition before development?

But we're talking Manhattan, it was either woods, meadow, or marshland right? It's easy enough to figure out.

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Carmen Mielecke CEO LCEE Life Cycle Engineering Experts
Jan 11 2017
Guest

Rainwater Events Calculator v4

Project Location: Germany

Dear all,

my 2 question refer to the rainfall events calculator.
1) the percentil is calculated on the historical data sheet. The amount is expressed in mm. On the next page "strategies" the "runoff volume required to be managed on site" has to be filled in in cu m. Does it have to be calculated in the following manner? (inteded percentil value - Depression storage- infiltration loss)*365 days * A? I am a bit confused by the use of the 2 different units "mm" and "cu m"...
2) is it correct to multiply the surface area for each Strategie with ist runoff coefficient to obtain the runoff volume managed?

Thank you very much for your answer in advance!

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Jan 11 2017 LEEDuser Expert 6003 Thumbs Up

Hi Carmen,

mm is a depth of rainfall, cubic meter is volume.

There is a lot more to this credit than multiplying a few numbers together. Runoff volume is calculated via any number of generally accepted methodologies, but again, more involved than a simple math problem.

You really should have a civil engineer or hydrologist look at the calcs for this credit.

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Carmen Mielecke CEO, LCEE Life Cycle Engineering Experts Jan 12 2017 Guest

Thank you very much for your answer! The question came from the hydrologist. The Problem is not the calculation in general. He is just unclear about the fill out of the tool.

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Florian Schmidtchen EGS-plan International GmbH
Dec 08 2016
LEEDuser Member
655 Thumbs Up

Detaining the rainwater runoff -an accepted solution?

Project Location: Germany

Dear all,
In our project we are planning to follow option 1 path 2. Therefor we provide underground-trench systems and a technical ditch on our project site, which gets inserted creatively into the landscape in the course of the final outdoor facilities planning. Is this an accepted solution for achieving this credit even if the approaches and techniques applied just pursue the aim of detaining the rainwater runoff?
Does an evaporation or an infiltration has to take place?
What do you think?
Thanks!

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Dec 08 2016 LEEDuser Expert 6003 Thumbs Up

Are you using the runoff in some way?

You need to actually use the runoff onsite, somehow (infiltration, evapotranspiration, reuse, etc.)

Simply routing it through a BMPBest Management Practice and discharging will not meet the intent of the credit. You need to reduce the volume somehow.

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Florian Schmidtchen EGS-plan International GmbH Dec 08 2016 LEEDuser Member 655 Thumbs Up

The current planning provides an underground trench-system, which let the runoff infiltrate partially. That means that the system cuts the peak of the runoff by infiltrating it partially and the rest will be discharged in the public wastewater system delayed.
Does the complete runoff have to infiltrate for achieving this credit?

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Dec 08 2016 LEEDuser Expert 6003 Thumbs Up

Option 1, Path 2 requires you to manage ALL of the volume from the 98th percentile storm. So you can't let anything go.

Now if you look at Option 2, you only need to manage the increase between the pre and post, so this may better suit your project.

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Florian Schmidtchen EGS-plan International GmbH Dec 08 2016 LEEDuser Member 655 Thumbs Up

Thanks Michael, that helped a lot!

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Pedro Ribeiro Director of Sustainability Edifícios Saudáveis Consultores
Nov 25 2016
LEEDuser Member
1581 Thumbs Up

Amount of rainwater treated with a storage tank

Hi all,

I'm working on a project that is pursuing LEED certification. Stormwater management strategy will be based on the collection of runoff from the roofs in order to reuse it later for irrigation, WC flushes, etc (the water will be stored in cisterns).

My doubt is how can the contribute of the collected water be accounted for the intent of this credit. I think that the way to do this is to estimate the amount of harvested water volume and compare it with the water needs for irrigation and flush discharges. If water needs are higher than the collected volume is it reasonable to consider that all the harvested water will be retained? Or is it needed to ensure that a permanent detention volume is available, what can lead to water discharge for the municipal drainage system, not accounted as retained?

Kind regards,

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Filipe Boni 2030STUDIO
Nov 22 2016
LEEDuser Member
99 Thumbs Up

Green roof compliance on runoff

Project Location: Brazil

Hi!

I'm working on a green roof over a building in IP and want to understand what system (extensive, etc)would be better.

I was able to calculate the 95th percentile of my site (46mm) and the Small Storm Method (14,80, considering a 30% impervious area).
Then I've developed a complicated spreadsheet showing the green roof depth, substract weight dry and wet, drainage type, etc, in order to understand how much runoff the green roof can handle, but I think I have a big chance to lose some point in here. My question is: it's there a standartized procedure, since I haven't found anything on the reference guide? Thanks!

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Nov 22 2016 LEEDuser Expert 6003 Thumbs Up

I would model using the SCS method, develop and appropriate CN for the roof, develop a Tc, and figure the storage/voids in the media and model it as a storage BMPBest Management Practice.

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Filipe Boni 2030STUDIO Nov 22 2016 LEEDuser Member 99 Thumbs Up

Hello Michael, thank you for your reply. Will try to develop in the steps you mentioned. Have a nice day!

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Rita Haberman Brightworks
Nov 14 2016
LEEDuser Member
370 Thumbs Up

Stormwater discharged to the Bay

Hi all,
I am working on a project located close to the 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.. Our site will have bioretention ponds, rain gardens, etc., but for the stormwater not infiltrated on site, it will redirect to campus retention and drain directly to the bay. How can we qualify for this credit? Is directing drainage to the bay enough, or does the water have to be treated in some way (similar to SSc6.2 v3 quality control)? Thank you.

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Nov 14 2016 LEEDuser Expert 6003 Thumbs Up

If you are doing Option 1, I can't think of a way you'd meet this credit while discharging anything from the design storm, regardless of location.

Option 2, you are only managing the delta Pre vs Post, and could conceivably let some go. As long as your required volume under this option is treated on-site, I can't see an objection to your scenario.

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Reema Nagpal Green Building Coordinator DES Architects + Engineers
Sep 06 2016
Guest
98 Thumbs Up

Bioretention Area Calculation

I was wondering about how to calculate the Area of bioretention needed to manage expected runoff on the Reference Guide v4 example. I believed it uses the Direct Determination method, however the example it does not show how it come up with the bioretention sizes in detail.
Could someone point out an example or provide a step by step guidance on how these calculations should be made?

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Nov 14 2016 LEEDuser Expert 6003 Thumbs Up

Dead storage = LxWxH

Storage in Media = LxWxHx(void ratio of media/100)

Your provided volume should meet or exceed the required volume to treat (and you also need to ensure that volume actually gets there).

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Robert Hink Principal, LEED Faculty The Spinnaker Group
Sep 06 2016
LEEDuser Member
946 Thumbs Up

Campus application

Our LEED project site falls within a larger medical campus which has a campus-wide stormwater management plan which meets the credit requirements. However, the post-development volume within our LEED project boundary is not managed internally, but instead is discharged to the broader campus system of retention areas and swales etc. In v2009 this was an acceptable approach for campus applications. Does this logic carry through into v4 or are the requirements for onsite management strictly interpreted as within the LEED project boundary?

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Sep 06 2016 LEEDuser Expert 6003 Thumbs Up

Tough to say how this will turn out, there isn't enough project data and experience at this time.

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Daniel Hartsig Sustainability Analyst Transwestern
Aug 31 2016
LEEDuser Member
186 Thumbs Up

Only bowls and castles need apply?

Full disclosure: I'm not a civil engineer but I have to try to explain the LEED requirements too them. In the case of this credit, I'm trying to figure out if I'm missing something major here.

The online guide added an exclusion of protected natural areas from the calculations and recommends different calculations than the print guide. Both of these changes help significantly but the rest of the language still implies (and at least one reviewer has stated) that every drop of water that isn’t absorbed or pooled on site must be prevented from leaving (for design rainfall X).

It seems that the only possible approach is to have the entire site slope inwards, or have a wall or trench run along the LPB to stop any water from running off the edges of the site.

Discrepancies between the language and examples occur in online and in print. These examples focus only on hardscapeThe inanimate elements of the building landscaping. It includes pavement, roadways, stonewalls, wood and synthetic decking, concrete paths and sidewalks, and concrete, brick, and tile patios. and roof runoff, and ignore new and existing vegetated areas and any inconveniently sloped areas near the LEED Project Boundary where rainwater could leave the site. At least in SSc6.2 in v2009, you only had to treat 90% of all runoff, but I don't see the same flexibility in v4.

There seems to be either a LEED subtext or professional design convention that I’m missing that ignores runoff from vegetated areas and the edges of a site, or the discussions and examples to date haven't met the LEED requirements.

Can anyone point me in the right direction? I'm quite lost.

Thank you,
Daniel Hartsig

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Aug 31 2016 LEEDuser Expert 6003 Thumbs Up

I like the bowl analogy, and in a sense you are correct if you are looking into Option 1, this is why I steer everyone to Option 2. It is a simple (for your CE) pre vs post analysis, and it will award you points in the end.

If you are going to do Option 1, you need to consider your LEED boundary, and exclude any undisturbed areas, because as you stated, you will never be able to solve this. This is a known issue with this credit, and I've been trying to get it corrected.

Do not use the print version of this credit, use the online.

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Daniel Hartsig Sustainability Analyst, Transwestern Aug 31 2016 LEEDuser Member 186 Thumbs Up

Thank you for the confirmation Michael, I thought I was really missing something.

We're assisting on a v4 Platinum in Haiti that needs every point it can get, so they're trying Option 1. They're lucky enough to have a bowl shaped site surrounded by a wall, and they can contain and infiltrate more than the 98th percentile rainfall volume (if you only look at the calculated volume), but there will be some runoff from the top of the wall and some from the edge of the driveway that must slope towards the road. The requirement for 100% capture just doesn't seem logical.

I will update this post if we find a way forward. Please let us know if you get any traction on your end, and if anyone else has seen this issue and has ideas they'd be appreciated. Thank you!

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Aug 31 2016 LEEDuser Expert 6003 Thumbs Up

This has been my gripe since the beginning, if you put say a sidewalk around your site, and if it drains to the road (which it should) you can't meet this credit.

Good luck, and let me know how you make out.

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Maria Glädt White Arkitekter AB
Aug 24 2016
LEEDuser Member
204 Thumbs Up

Rainwater Management

Project Location: Sweden

 We are planning to follow option 2 (Natural Land Cover Conditions). The area consists mainly in clay with very low infiltration capacity. The rainfall data are 78,50 mm (100 percentile) and 18,40 mm (95th percentile) if calculated with runoff coefficients for clay of 0,2, the runoff from the area before developtment is 12 m3 (using the 95th percentile as rainfall event).  The runoff coiefficent after development is 0,76, which results in a runoff of 44 m3.
Moreover, there is a pumping station to drainage a train tunnel located nearby the area.
The municipality is planning a flooding park just downstream the building for managing stormwater from this part of the city. Due to infiltration is not a feasible option in this case, we are working with the alternative of installing an underground detention tank. The tank will have capacity for the 95th percentile and just a natural flow will leave the area (the volume that is not required to be managed according to the credit). When the rainfall ends, the stormwater will be discharged from the tank as a natural flow and will not be reused as graywaterUntreated household waste water which has not come into contact with toilet waste. Graywater includes used water from bathtubs, showers, bathroom wash basins, and water from clothes-washers and laundry tubs. It must not include waste water from kitchen sinks or dishwashers (Uniform Plumbing Code, Appendix G, Gray Water Systems for Single-Family Dwellings); waste water discharged from lavatories, bathtubs, showers, clothes washers and laundry sinks (International Plumbing Code, Appendix C, Gray Water Recycling Systems). Some states and local authorities allow kitchen sink wastewater to be included in graywater. Other differences can likely be found in state and local codes. Project teams should comply with the graywater definition established by the authority having jurisdiction in the project area. for flushing toilets or watering plants.  In this way, 95th percentile is being retained on site and discharged in a natural way. Does this approach meet the credit definition? Does it help the fact that the tank will be a gravel-filled-tank, which replicates in a better way a natural retention? Do you have any recommendation to a better approach in this case to meet the credit requirements? Is the 95 percentile the right rainfall value to be used in the calculations according option 2?
Would it for example be ok if we retained the water and then discharged it to the flooding park that is designed in order to handle some of the runoff volumes from the area and to also show and display LIDLow-impact development: an approach to managing rainwater 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 (e.g., minimizing land disturbance, preserving vegetation, minimizing impervious cover), and design practices (e.g., rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, soil amendments). These are engineered practices that may require specialized design assistance. measures to the public.

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Aug 24 2016 LEEDuser Expert 6003 Thumbs Up

If I understand your explanation, you are not reducing volume at all, only rate (if you are even doing that). It doesn't appear that you are meeting the intent of this credit.

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Maria Glädt White Arkitekter AB Aug 29 2016 LEEDuser Member 204 Thumbs Up

In other words, we need to manage the water onsite. Though infiltration through the underlaying sediments it's not possible (due to the deep layers of clay, we would have to reuse it on site?
We could easy in this layers where alteration to ground structure is made use other types of soil and gravel to infilitrate in this layers, but it will eventually reach clay and the water is therafter be discharged to closest recepient, some small brooks and the sea, Öregrund.
As mentioned earlier, the property line borders to a flooding park. The flooding park and our property are the only to propreties within the local zoning plan. Could it be possible to state that this is a multitenant complex. As earlier mentioned, the flooding park is designed to handle water runoff volumes and to in an educational way show these measures.

Best regards

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Anastasia Makarenko EcoStandard
Aug 02 2016
Guest
420 Thumbs Up

n/a

Dear all,
we're planning to install tank for collecting 180% of stormwater from 95 percentile rainfall event. Collected rainwater will be used for irrigation, but there won't be any sprinkler system at the site - it's planned that there will be special vehicle that will take in water and deliver it to the vegetated area needed to be irrigated. In other words irrigation will be performed manually. How do you think, is it OK with the intent of the credit or do we still need to arrange direct irrigation system on site?

Thank you all in advance.

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Aug 02 2016 LEEDuser Expert 6003 Thumbs Up

I don't know about the 180% part, but your approach seems reasonable. I would document what you are doing in a clearly written narrative, that explains your approach, the amount of runoff collected, and the amount used for irrigation.

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Anastasia Makarenko EcoStandard Aug 02 2016 Guest 420 Thumbs Up

Michael, thank you very much for your reply.
Could you please explain what's wrong with 180%? Do you think the tank should be larger?

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Aug 02 2016 LEEDuser Expert 6003 Thumbs Up

Oh I get it now. The volume of the tank is 180% of the required volume. I got it now.

Should it be larger? Only you can know, I would make sure the volume is such that it actually satisfies your irrigation requirement without requiring you to finish the job with potable waterWater that meets or exceeds U.S. Environmental Protection Agency drinking water quality standards (or a local equivalent outside the U.S.) and is approved for human consumption by the state or local authorities having jurisdiction; it may be supplied from wells or municipal water systems..

Then do you want to plan for a dry period? That's entirely up to you.

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Anastasia Makarenko EcoStandard Aug 03 2016 Guest 420 Thumbs Up

Yes, exactly, the tank will be sized for 180% of the required volume.

Well, actually we're planning to install drought-tolerant landscaping that will not require irrigation at all. We decided to install tank for harvesting rainwater mostly to comply with requirements of the Rainwater Management credit.
So the irrigation is not necessary in that case, it's an additional measure required simply to reduce runoff volume from the project site.
So, every time the tank is filled up with rainwater we will discharge it by watering green areas of the site. How do you think, does this approach seem logic?
I'm not sure about it as it seems kind of unreasonable to harvest water for irrigation of plants that do not need to be irrigated. :/

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Manuel Martin Delgado Buro Happold Polska Sp. z o.o
Mar 23 2016
LEEDuser Member
138 Thumbs Up

manage (rainwater) on site

Hi,

Is reusing rainwater for flushing toilets an accepted solution for achieving this credit?

Or is reusing for irrigation the only solution accepted?

Thanks.

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Ashwini Arun Sustainability Manager, YR&G Mar 23 2016 LEEDuser Expert 200 Thumbs Up

BH,
The intent of the credit is to reduce run-off from the site using LIDs and green infrastructure. In your case, I suppose the rainwater is captured in a rain barrel or a cistern in order to be reused for the toilet? If that's the case, this qualifies as an LID control and reduces run-off, thus meeting the intent of the credit. I believe you should be able to achieve the credit based on this.

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Marco Valerio Ceccotti Environmental Economist and LEED Consultant
Mar 21 2016
Guest
76 Thumbs Up

LEED NCv4 getting harder

Hi All.

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 set back the site's rainwater impacts to the Age of Stone.

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Mar 21 2016 LEEDuser Expert 6003 Thumbs Up

LEED v4 is actually a much more achievable SWM target than in previous versions of LEED, and some may argue, as step backwards.

As for cost, if the project is located in the US, there are most likely much more stringent SWM requirements in place at the local level. But modeling a site in a Pre vs. Post condition is hardly some groundbreaking engineering requirements. We have been doing this for decades.

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henry nguyen
Jan 20 2016
Guest
30 Thumbs Up

Option 2

Project Location: United States

Hello, I wonder how should I understand Opt.2 BD+C
Rain water Mgmt credit.
"Manage on site the annual increase in runoff volume from the natural land cover condition to the post-developed condition."
Does that mean that I should design the site so that it can help increase annual runoff volume and stays in such condition all the way thru post-developed condition, given the base line is "natural land cover condition" ?

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Jan 20 2016 LEEDuser Expert 6003 Thumbs Up

Option 2

Post Development Volume - Pre Development Volume = Volume to be Managed

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henry nguyen Jan 20 2016 Guest 30 Thumbs Up

Thank you Michael, it's much clearer the way you said it.

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Manuel Martin Delgado Buro Happold Polska Sp. z o.o Mar 10 2016 LEEDuser Member 138 Thumbs Up

Hi,
I am not sure about the definition of "natural land cover condition" and which 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 should be used.
If the pre Development site, for example, is a parking lot, the Pre Development Volume is the volume from parking lot or from the same site before any land modification was undertaken "Natural"?
Thank you.

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Mar 10 2016 LEEDuser Expert 6003 Thumbs Up

Natural Condition = What your site was in the Stone Age, before it was touched by man.

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Noriko Yasuhara Woonerf Inc.
Jan 05 2016
LEEDuser Member
3526 Thumbs Up

natural land cover conditions in a portion of project site

Hi,

Our project site contains a portion of natural land cover conditions site and previously developed siteA site that, prior to the project, consisted of at least 75% previously developed land..Shall we use option 1, or shall we use combined option 1 and 2, such as using option 1 for previously develped portion of the site and option 2 for natural land portion?

Thank you,
Noriko

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Jan 05 2016 LEEDuser Expert 6003 Thumbs Up

If this were my project I would be using Option-2 for the entire project, as this yields more points. You already know what the natural site conditions were, as portions of your site are still in this condition.

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Noriko Yasuhara Woonerf Inc. Jan 05 2016 LEEDuser Member 3526 Thumbs Up

Hello Michael,

Thank you very much for your prompt reply. There is a green field area along the project boundary. The green field area in our project would be consistent with the definition of green field area and would be almost half of the entire site. Do you think it's the project team's discretion which option to choose?

Thank you,
Noriko

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Jan 06 2016 LEEDuser Expert 6003 Thumbs Up

It is always your option to choose. However, Option-2 is weighted as such to award more points, as this method will most accurately replicate the natural hydrologic cycle.

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Noriko Yasuhara Woonerf Inc. Jan 06 2016 LEEDuser Member 3526 Thumbs Up

Thank you very much. It's very helpful.

Best,
Noriko

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Emmanuel Pauwels Owner Green Living Projects s.l.
Dec 04 2015
LEEDuser Member
4396 Thumbs Up

compliant strategies

We have a project with an underground parking which covers the whole site. On top of that underground parking we have a 6 floor building and an open space with a green roof (on top of the underground parking structure)
Question 1: If rainwater is captured and filtered through the green roof, can it then be discharged?
Question 2: If we install a deposit, sized for the 95th percentile, would it be ok if, in case of a torrential rain event, that part of the water is discharged directly (without going to the tank) The project engineer is afraid the deposit would not be able to manage such an intense rainfall event and therefore suggests some way to discard the excessive rain.
Question 3: If the water that is captured in the tank is then pumped to the green roof where it filters through the substrate and is then discarded from the site. Would that be compliant? Or does the water have to be used on the site (for toilets for instance)

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Dec 06 2015 LEEDuser Expert 6003 Thumbs Up

1.This all sounds like a viable approach. You probably want to be looking at Option 2 which only requires you to manage the delta Pre vs Post, that way you can probably let some of the green roof go. The other Options require you to manage on site, the rate and volume from the 95/98th percentile storms.

2. Installing a bypass for higher level storms is acceptable, and very common.

3. You need to be able to pass the straight face test on this. Is the runoff being reused for irrigation (sprinklers, etc). Just quantify the water needed vs water pumped back onto the roof. Show that you are re-using it. What we don't want is this simply being pumped back to a mulch bed passing through, as discharging, thereby your runoff volume is completely unchanged.

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Emmanuel Pauwels Owner, Green Living Projects s.l. Dec 06 2015 LEEDuser Member 4396 Thumbs Up

Michael,
Thanks for your feedback. Very useful. The existing site consists mostly of a hardscapeThe inanimate elements of the building landscaping. It includes pavement, roadways, stonewalls, wood and synthetic decking, concrete paths and sidewalks, and concrete, brick, and tile patios. foundation which is part of a previous project that was abandoned. That means that the pre-development situation is 25% permeable soil and 75% impermeable hardscape. It would be easier to comply with option 2 in that case, I agree. The only issue is that we have a value for the 2year 24h design storm but we do not have value for the 1 year 24h design storm. It seems this value is never used/calculated in the country where the project is located. Would it be possible to use only the 2year 24h design storm value? That would help a lot in demonstrating compliance with option 2. Thanks for your (sunday) advice.

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Dec 06 2015 LEEDuser Expert 6003 Thumbs Up

In theory, if you're managing the 2 you "should" be managing the 95/98 in most climates. But this isn't a certainty. I am quite sure your reviewer is going to tell you to compile 10 years worth of local rainfall data and plug it into the rainfall calculator that is found in LEED online.

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Petr Vogel Specialist Consultant EkoWATT CZ
Nov 24 2015
Guest
484 Thumbs Up

What is to be considered LID + measures outside LEED Site

Project Location: Czech Republic

Dear all,
we have a project site situated in the rural mountains where infiltration is not even possible due to geological conditions. The site consists of very low permeable soil, with high groundwater level (only 2 m below grade). The site is currently drained off into existing water surface streams situated 1) on site and 2) at the boarder of the site. One of the streams is drained into underground pipe also situated on site that releases water outside the LEED Site. This pipeline was constructed in the past to prevent the whole area from flooding. Both of the streams are then leading to a local dam (situated roughly 100 m from the LEED Site).
We would like to approach the credit SSc4 LEED NCv4 ideally through option 2: to manage all the incremental runoff volume on site compared the project status after the construction to full natural undeveloped land cover.
But the project team is suggesting not to design on site bioretention ponds but rather to use the already built dam to catchup the incremental runoff from the LEED Site. This sounds very logical to utilize the already built dam nearby that was designed specifically for the flooding prevention purpose.
Is it possible to include retention dam outside the LEED Site to fulfill the requirements of the credit nevertheless what option is used? This approach was possible in LEED v3 by assuming all the credit requirements to be applied for the whole water collection areas including the LEED Site. What approach should be used within LEEDv4 for option 1 vs. option 2?
Is it possible to consider the proposed solution as a low-impact developmentAn approach to managing rainwater 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 (e.g., minimizing land disturbance, preserving vegetation, minimizing impervious cover), and design practices (e.g., rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, soil amendments). These are engineered practices that may require specialized design assistance. strategy? The dam is natural based structure – regional soil and stone based bearing structure (layers of stones and soil with different granularity) but of course made by hard mechanization. Dam is equipped with 16 kW output hydro plant. The top of the surface is surrounded by paved concrete road. Also the overflow bypass is concrete based structure. The rest of the embankments is pedestrian based with natural slope into the water.
Thanks in advance for any kind of help

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Nov 24 2015 LEEDuser Expert 6003 Thumbs Up

You are increasing both volume and rate to the receiving streams, and piping. You are not restoring your site to the natural hydrologic state. I don't agree with your approach, and your ability to meet the intent of this credit.

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Petr Vogel Specialist Consultant, EkoWATT CZ Nov 25 2015 Guest 484 Thumbs Up

Michael, thanks for your answer. The investor is pushing us not to invest to bioretention onsite since it will only retent water, there is nearly no infiltration possibility + it is really hard to argue if the already built dam is nearby. Besides the described above we have designed green roofs + permeable hardscapes. Can you suggest some other ideas please because this is not enough to restore the natural conditions?
Thanks, Petr

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Nov 25 2015 LEEDuser Expert 6003 Thumbs Up

Do you have any ability to do capture and reuse? That is really what you need to be looking into when infiltration isn't possible.

If you are in an area with no infiltration, a mountanous area, your pre-development runoff is already going to be pretty high. Looking at Option 2, pre vs post, your post development runoff that needs to be managed proobably isn't all that much, especially if you are doing green roofs and porous paving.

Add some ammended soils to lawn areas as well, and model with a 17% void ratio. Constructed wetlands are always great, and look great, if you have the room.

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Petr Vogel Specialist Consultant, EkoWATT CZ Nov 25 2015 Guest 484 Thumbs Up

Yes we are considering to capture and reuse rainwater. This is not yet certain due to high potential of greywater to be reused + to recover heat (hotel with welness and swimming pool center).
If I can I do have 2 more questions since LEEDv4 is really fresh for us...
If we take the capture and reuse approach does it have to be also in the logic of LIDLow-impact development: an approach to managing rainwater 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 (e.g., minimizing land disturbance, preserving vegetation, minimizing impervious cover), and design practices (e.g., rain gardens, vegetated swales and buffers, permeable pavement, rainwater harvesting, soil amendments). These are engineered practices that may require specialized design assistance.? Having in mind solution of underground accumulation tank vs. surface accumulation pond to be naturally landscaped.
I would also say that if we add imported soil with higher infiltration parameters it would be in conflict with the SSc2 topsoil compliance " imported topsoils may not include the following: soils from other greenfield sites, unless those soils are a byproduct of a construction process." Am I right?

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Petr Vogel Specialist Consultant, EkoWATT CZ Nov 30 2015 Guest 484 Thumbs Up

Michael or other experts,

some new thoughts?

Thanks for clarifications,
Petr

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Hana Raitrova
Nov 13 2015
Guest
8 Thumbs Up

Modified rational method - rainfall intensity

Dear All,

I´m still wondering how to get a value of rainfall intesity in inches/hour or mm/hour for the 95th percentile (option 2 - modified rathional method) based on time concentration. I understand I simply use a different design storm with LEED v4 (the 95th percentile as opposed to 1- and 2-year 24-hour design storm), but to get the amount of rainfall intensity I need to know if I should count with 1- or 2- or x- year storm (to choose the right amount from the graph). Is it correct to choose the value for 2-year storm event? Or am I completely wrong?

Thanks in advance

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Michael D. DeVuono Senior Staff Engineer, T&M Associates Nov 13 2015 LEEDuser Expert 6003 Thumbs Up

You aren't going to get a runoff volume from the Modified 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 know it's in the guide, but it just isn't correct. The 95th percentile storm is a depth, its not an intensity.

I recommend using SCS methodology.

As for the 95th percentile depth, the EPA National Stormwater calculator is pretty good. You can also get from most local DOTs or weather agencies. Or collect local data from NOAA or your local equivalent and plug into the rainfall calculator spreadsheet which should be in LEED online.

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Valentin Grimaud Thermal Engineer TERAO Green Building Engineering
Nov 06 2015
LEEDuser Member
1331 Thumbs Up

Is green space above underground parking previous or impervious?

Dear all,

We are currently wondering what is th ruling in the case of an underground parking being below a green space area, which we would tend to account for in the pervious area contribution for post-development runoff. Considering the soil layer might be more than 2m thick is it acceptable?

Many thanks,
Aymeric

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