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.
Thanks,
Chris
Amy Rider
Sustainability ManagerKEMA Services
161 thumbs up
December 19, 2011 - 5:49 pm
In my experience you are not limited in your calculation methodology, but do not need to account for time of concentration per the reference guide. I suggest uploading your calculations with an accompanying explanation for whichever approach you use and your results.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
March 5, 2012 - 8:23 am
You really can not compare pre vs post using on the 24 hour intensity unless your Tc's remain the same in the the pre and post.
If you have a farm field with a 2-hour Tc in the pre, the pave the entire site for the post and get a 5 min tc, your peak rate runoff will obviously be much greater in the post.
Your runoff coefficient will take care of some of this, but the rate will not be correct.