Hello -

My question is in regards to the 2-yr, 24-hr requirement for calculating peak runoff rate. As far as I understand, by requiring the 24-hr duration to be used with the rational method for both pre- and post-development, any design that increases the site's time of concentration (thus reducing the peak runoff rate) is not accounted for. It seems to me that the rainfall duration used in the Rational Method calcs should be based on the site Tc, just as they are with traditional stormwater design. Using a 24 hr duration for peak runoff doesn't make much sense to me....even the local stormwater criteria in my area does not provide rainfall IDF curves for such long durations.

Perhaps I am misinterpreting the language. Either way, I would appreciate any insights as to how this has been used and/or interpreted by others in the past.