In the Volume Captured equation the variable "I" is said to stand for percent impervious of surface. Is this the same thing as the coefficients or is this different? I thought it could mean 2 other things; one being that if 68% of a certain area was impervious then you would put .68. The other option would be that the surface type is 68% impervious so it rejects 68% of all water...which this could actually be the same theory behind the coeffecients. Anyone know?
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Gregory Hurst
Office ManagerRobert Peccia & Associates
66 thumbs up
November 22, 2011 - 11:33 am
Generally, when we calculate runoff, we use a runoff coefficient, or in the case of a site with multiple types of surfaces (pavement, grass, gravel, etc.) we calculate the areas of each surface and use a table that provides us with runoff coefficients for each type of surface. Then we develope a combined run-off coefficient value for the site or drainage that can be used with the Rational Method. In the case of storage volume, LEED provides us with an emperical method that essentially calculates the runoff coefficient (Volume of captured runoff) and the "I" stands for the percent of impervious area, as a whole number in the equation. In my opinion, I would rather use a combination of runoff coefficient values and their associated areas to develop a more accurate combined ronoff coefficient volume. This is the same runoff coefficient that is used for the Rational Method. The difference is that you are just calculating a simple volume for storage based on a given rainstorm over an entire area, assuming the surface is impervious, or it isnt, using the LEED storage equation. Runoff coefficients can be easily obtained for various surfaces from the internet.