The key for volume calculation is knowing the 1 and 2-year, 24 hour storm depth. This is calculated fordifferent locations and it is a long process and it is not the same as calculating runoff rate using the Rational Method and multiplying by time. For this reason, most jurisdictions publish a series of 24-hour storm depths (1,2,10,50, and 100 year depths). LEED asks that we calculate volumes for the 1 and 2-year 24 hour storm to show that there is no net increase in the volume of runoff from the site after development..

For example, a 2-year 24-hour storm depth for Baltimore, Maryland is 3.2 inches.

When the 2-year 24 hour storm depth is known, it can be multiplied by the run-off coefficient (a dimensionless number) and the area of the site to calculate a total volume. The only variable in that calculation from predevelopment conditions to post development conditions is the runoff coefficient. Therefore, the purpose behind the credit is to develop a stormwater management program that does not change the combined runoff coefficient. Pavement will have a higher coefficient, bioswales will have a lower coefficient, detention ponds may have a zero coefficient for a 2-year, 24 hour storm, depending on your overall design. The runoff coefficients from all the various sub-basins, multiplied by the associated areas for the sub-basins can be combined to develop a combined run-off coefficient for the entire study are of the site..