I have a general stormwater question with respect to quality. I've read and heard that the general convention is to treat the runoff from the first inch of rainfall. This dictates the sizing of BMP's as far as volume is concerned. As far as the effective treatment rate of the BMP's, how do you know what the intensity should be? That is, the intensity of the first inch of a 25 yr. storm would be different than the intensity during the first inch of a 2 yr. storm.

In determining these quantities, I'm assuming I would initially generate a 1 hr unit hydrograph. Then upon determining/deciding what the duration should be, I would develop the unit hydrograph for that duration (using the S-curve method for example).

Again, I'm new to the stormwater management world and I appreciate any and all help even remotley related to this question. Also, if someone has a good Excel spreadsheet for hydrographs and the convolutions that you are willing to share, feel free to do so. I have one from my hydrology class but I'm not sure how good it is. Thanks.

Jeff Thompson
JThompson@HOBBSUPCHURCH.COM