Detention, Routing & Outlet Structures

Worked: a subdivision pond

After development the design storm produces a triangular inflow hydrograph peaking at 60 cfs with a 90 minute base. The ordinance caps release at the pre-development peak, 25 cfs. V_s ≈ ½ × 5,400 s × (60 − 25) = 94,500 ft³ = 2.17 acre-ft.

A 4 ft weir passes 25 cfs at H = (25/(3.33 × 4))^(2/3) = 1.52 ft, which sets the top of the pond above the crest.