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 DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
187 thumbs up
December 6, 2015 - 7:39 am
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.
Emmanuel Pauwels
OwnerGreen Living Projects
137 thumbs up
December 6, 2015 - 8:18 am
Michael,
Thanks for your feedback. Very useful. The existing site consists mostly of a hardscape 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.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
187 thumbs up
December 6, 2015 - 9:37 am
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.