How are stormwater detention basins designed?

Detention basin design: (1) Determine inflow hydrograph for design storm using rainfall-runoff methods, (2) Set allowable peak outflow (often pre-development rate), (3) Route inflow through basin using storage-indication method to find required storage, (4) Size basin for required volume with adequate freeboard, (5) Design outlet structure (orifice, weir combinations) to achieve target outflow rate, (6) Include emergency spillway for larger storms. Multi-stage outlets can control multiple frequencies. Extended detention for water quality provides settling time before release.