Grate inlets may be evaluated using the following formula (with restrictions)
Q cfs = 3.0(PH^3/2)
P = permiter of the grate (ignoring bars)
H = head over grate
this can only be used for h=0.4 or less
If it is not a combination inlet them multiply by 75% to account for clogging
this assumes...
You have not defined the problem ... if you are trying to control quantity or quality of discharge. If it is quality the Maryland SWM Design Manual has a whole chapter on nonstructural devices you can use (credits). There are also some low impact development techniques such as rain gardens...
Sheet flow is usually 0.1' or less and shallow concentrated flow is usually between 0.1' and 0.5'. Muncipalities can usually define when a floodplain study is to be performed in any natural low. This information should be contained in a manual or ordinance. Floodplains are not usually defined...
