What this calculator does
Detention basins exist because a site cannot discharge its peak runoff as fast as it arrives. This calculator takes the peak inflow from the Modified Rational Method, subtracts the permitted discharge, and gives the storage volume and the pond footprint at your chosen depth.
How to use it
- Enter the catchment area in hectares and the composite runoff coefficient.
- Set the design storm duration and the rainfall intensity for that duration from your local IDF curve.
- Enter the allowable release rate set by the approving authority or downstream capacity.
- Enter the pond depth; the footprint is the storage volume divided by that depth, before allowing for side slopes.
Governing formula
Q = 2.78·C·i·A ; V = (Q_in − Q_out)·t Where:
-
C= Composite runoff coefficient [–] -
i= Rainfall intensity [mm/h] -
A= Catchment area [ha]
Derived Equations:
Q (m³/s) = 2.78·C·i·A / 1000 A_pond = V / d Worked example — 2.5 ha site, 45 mm/h for 30 minutes, 0.1 m³/s release
The figures below were produced by running this page's own engine with the example inputs (Catchment area (ha), Composite runoff coefficient (0–1), Design storm duration (min), Rainfall intensity for that duration (mm/h), Permitted discharge rate (m³/s), Pond design depth (m)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Peak inflow | 0.19 m³/s |
|---|---|
| Required storage | 157.77 m³ |
| Pond footprint at that depth | 131.48 m² |
| Assessment | Peak inflow 0.19 m³/s vs 0.10 m³/s release → ≈ 158 m³ storage; at 1.2 m depth the pond footprint is ≈ 131 m². |
- Q = 2.78 × 0.6 × 45 × 2.5 ≈ 188 L/s = 0.188 m³/s.
- Storing the excess above 0.1 m³/s for 30 minutes needs roughly 150 m³.
- At 1.2 m deep that is a water surface of about 125 m² — add freeboard and side slopes to the excavation.
Assumptions & limitations
Screening-level estimate. Modified Rational Method: a constant-intensity storm over a single duration. It does not build a hydrograph, so it does not model outflow during the storm or true peak storage.
- The Modified Rational Method assumes the inflow stays at its peak for the whole duration, so storage is over-estimated for long storms.
- No outflow routing, no infiltration into the pond base and no evaporation or overflow weir behaviour.
- An orifice or weir outlet must be designed separately to release the permitted rate across the water-level range.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- Local stormwater management manual — storage volume requirements.
- BS EN 752-4 — drainage pipework systems.
- Department of Transportation storm drainage design manuals.