A 40-acre development site case study — worked pre/post-development peak flow with the Rational Method, TR-55 detention pond volume sizing, and an orifice-equation outlet control structure design that matches post-development peak flow to the pre-development target.
The project brief: a 40-acre meadow is being developed into mixed-use commercial and residential space with 60% impervious cover, and the municipal stormwater ordinance requires that post-development peak runoff not exceed the pre-development peak for the design storm. Every number in this module traces back to a single comparison — the pre-development peak flow versus the post-development peak flow — and the detention pond and outlet structure exist only to close that gap.
This module works the full design the way a real site-civil engineer would: Rational Method peak flow calculations for both pre- and post-development conditions, an NRCS/TR-55 storage estimating equation sized from the ratio between those two peaks, an SCS curve-number runoff volume calculation, and an orifice equation solved for the outlet structure's exact diameter. The full worked numbers, complete bill of materials, and finished design reasoning are part of the unlocked module below.