EngineerHub.orgWhat this calculator does
Compares pre- and post-development runoff volumes, subtracts controlled release during the event and adds a planning contingency.
Inputs
Results
Worked example: 1.2 ha development site
This example uses the calculator's default inputs so the arithmetic can be checked against the live results above.
Vrain = 0.050 m × 12,000 m² = 600.0 m³.
Pre-development runoff = 0.25 × 600.0 = 150.0 m³.
Post-development runoff = 0.75 × 600.0 = 450.0 m³.
Excess runoff = 450.0 − 150.0 = 300.0 m³.
Released volume = 10 L/s × 2 h = 0.010 m³/s × 7,200 s = 72.0 m³.
Net storage = 300.0 − 72.0 = 228.0 m³.
Planning storage = 228.0 × 1.15 = 262.2 m³.
Background
Optional reading — open any section below.
The page estimates post-development runoff volume, subtracts a pre-development runoff allowance and the controlled discharge volume, then applies a planning contingency.
Real detention sizing depends on the timing of inflow and outflow, not only total volumes. Hydrograph routing can produce a different peak storage than this simplified balance.
Use a recognized hydrologic/hydraulic model when outlet controls, multi-stage storage, infiltration, connected drainage networks or permit calculations matter. EPA SWMM is designed for drainage-system and detention analysis.
Frequently Asked Questions
Practical questions about assumptions, inputs and limitations.
No. This percentage is a volume allowance. Physical freeboard is a vertical elevation difference and must be designed from basin geometry and local criteria.
Yes mathematically, in which case this simplified balance returns zero net storage. Real inflow timing and outlet hydraulics still need routing.
No. Treat infiltration, evapotranspiration and soil storage separately or incorporate them through a calibrated runoff coefficient or detailed model.