Serious answer:
What you need, to answer the question, is the following:
100 year rainfall data
10 year rainfall data
3 year (?) rainfall data (good luck with that by the way, report back if you're able to find it)
watershed area
watershed land cover (may include soil types, depending on method selected)
watershed time of concentration
unit hydrograph (SCS? Something else?)
regional hyetograph shape, depending on method (Type I? Type II?)
stage-storage relationship for your detention pipes
stage-discharge relationships for your selected outflow control structure
Then you apply the storage-indication method for reservoir routing, which of course you learned in your senior level civil hydrology class. (right?)
tl;dr:
I'm going to go out on a limb, here, and say you're in way over your head, you need to go back to school to be able to answer this question, and your boss needs to be slapped for thinking he can hand this to someone without the proper training. He needs to be slapped
with a lawsuit if he stamps it.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -