Forum discussion
NC-2009 SSc6.2: Stormwater Design—Quality Control
We will install an extensive green roof in our project. Does anyone know what the runoff rarte is and how much TSS are captured by 2.7 inches of substrate?
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Why do the requirements focus on 1-year and 2-year, 24-hour storms?
How can green roofs count as a stormwater control measure?
I have 100-year data—how do I convert to 2-year?
Are there special considerations for international projects?
What performance threshold do I need to achieve for an Exemplary Performance point?
Tristan Roberts
RepresentativeVermont House of Representatives
LEEDuser Expert
11477 thumbs up
March 31, 2011 - 5:32 pm
Emmanuel, I am not a stormwater expert so I wouldn't be able to tell you how to get this value. However, I would be cautious about assuming standard values for green roofs. This article in Environmental Building News shared some interesting data on this: Green Roof Study Finds Big Variation in Stormwater Retention.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
187 thumbs up
March 3, 2012 - 9:34 am
This is a credit that the CEs should worry about, and unless you have PondPack, HydroCAD, or something similar in your office (and know how to use it) you should pass this work on to the CE.
You can get a VERY rough estimate using the rational method (this method isnt really accepted for SWM any more because it is so rough), but it will get you in the ballpark.
Q=CIA
Q=Rate of runoff
C=Runoff Coefficient
I=Rainfall Intensity
A=Area
C=I would use .6 for your green roof, .95 for any impervious areas (walks, parking, etc). Get their areas and calculate a weighted C value.
I=Local rainfall intensity can be obtained from NOAA, use 5min Tc
A=Area of the site/building/whatever