Run-off calculations for a site with future extensions

December 19, 2013

Valentin Grimaud

Thermal Engineer
TERAO Green Building Engineering

LEEDuser Basic Member

43 thumbs up

December 19, 2013

Dear community,

We certify a project that will have two extensions in the future. Because of this we cannot earn crédits SSc5.1 and SSc5.2 as per the MPR. But I was wondering if this is the case for SSc6.1?
Do I have to calculate post-development run-off by considering the future extensions (this would obviously make my compliance much harder but I could understand the logic of such requirement...)?

Thank you for your help,

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Michael DeVuono

Regional Stormwater Leader
Arcadis North America

LEEDuser Expert

186 thumbs up

December 19, 2013 - 1:50 pm

Are you referring to MPR 2? Building in its entirety?

If your answer is yes, then yes, your stormwater calcs must be based on full build out. You can't show compliance for half the building, then build the remaining half down the road without improving the site SWM.

This isn't just a LEED thing either, it is protection for the health, safety, and welfare of those downstream.

Did I get this right? The building will be certified as a whole, but built in phases?

Valentin Grimaud

Thermal Engineer
TERAO Green Building Engineering

LEEDuser Basic Member

43 thumbs up

December 22, 2013 - 7:53 am

Dear Michael,

The building and its site will be certified as a whole. 2 future phases are planned but they are not in the scope of the present certification work, they will be coming probably years after the start of operation of this first phase. Therefore there are 2 large areas of green spaces waiting for this to happen much later.

The MPR I refer to is found in supplemental guidances of MPR, in the section dedicated to phased projects. There, you can find that credits SSc5.1&5.2 are not elligible in such projects and it makes sense.
I assume that SSc6.1 is elligible but that the calculation conditions of the post development runoff must include the future buildings in order to make sense. But I was not entirely sure. I agree with you on the point that apart from LEED perspective it should be done this way to ensure health and safety. But the credit language is sometimes another thing.

What do you think in light of these explanations?

Many thanks,

Michael DeVuono

Regional Stormwater Leader
Arcadis North America

LEEDuser Expert

186 thumbs up

December 23, 2013 - 8:26 am

Valentin, I am not sure how this should play out in regards to LEED. It is my opinion, that you should account for the future development, if you can do so reasonably, and if you are positive that these future phases will be constructed.

I do not think it would be unreasonable to prepare two SWM designs, one that addresses the current project, and a second that is a "retrofit" that addresses the expanded impervious surface, and is to be implemented at the time of the future expansion. This sounds like a great question for the GBCI technical staff so you do not waste too much time in design going down the wrong path.