Maximum Outflow rate requried

[feng123]

Please helpo, I am designing a detention and I need to know how to calculate the maximum outflow rate required. This is my data:

Detention Calculations- The Barns at the Depot

Existing Impervious Cover= 0.118 AC
New Impervious Cover= 0.308 AC
Increased Impervious Cover= 0.190 AC

Detention Summary

Area Serviced New Impervious 0.190 AC
Detention Storage Rate Cs=1 AC-FT/AC
Detention Storage Volume Required 0.190 AC-FT
Detention Storage Volume Provided 0.208 AC-FT
Maximum Outflow rate Required ????

 

[beej67]

How do you know your detention storage provided if you haven't designed your pond's outlet control structure yet?

In many municipalities, the maximum allowable discharge rate equals the existing discharge rate. To determine that, you'll need not only to know your existing impervious cover, but also your total watershed broken down by all different kinds of cover, your time of concentration of your drainage basin, and event storm rainfall data. Then you'll need to select which method you're using to calculate it, (NRCS/Rational) and stick to that method throughout the process.

In some municipalities they do not let you take credit for existing impervious cover. In some they require a percentage reduction beyond existing. Some both. Some make you manage multiple storms. In some they do not go by existing discharges at all, and simply set an allowable based on the capacity of downstream structures. In some they make you model all the downstream structures between you and a major point of discharge to prove you're not impairing them. In some you have to provide other functions with your stormwater pond as well, such as water quality protection. In some regions of the country they're getting very particular about that as the EPA starts twisting the knife over MS4's TMDLs.

Your question is complicated, and if you don't know what you're doing you can get yourself or your client sued. Consider contracting with a professional.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com