How much stormwater storage a site actually needs
Comparing peak inflow against the permitted discharge and turning the difference into a storage volume.
The engineering question
The site cannot discharge as fast as rain arrives. How much storage has to be provided, and how big does the pond need to be?
Worked example
The figures below were produced by running the Detention Basin Calculator with the same inputs — during the build, from its authoritative engine.
| Peak inflow | 0.1877 m³/s |
|---|---|
| Required storage | 157.77 m³ |
| Pond footprint at that depth | 131.475 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. |
- Peak inflow follows the Modified Rational Method: Q = 2.78·C·i·A.
- Storage is the excess above the permitted release over the storm duration.
- Pond footprint is that volume divided by the design depth.
Sanity check
If the permitted release exceeds the peak inflow, no storage is required and the calculation will return zero.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
Engineering review required
This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.
References & standards
- Local stormwater management manual
- BS EN 752-4 — drainage systems
Related problems
When the ground can absorb the water, and when it cannot
Sizing infiltration storage and soakaways, and recognising the soils that will not drain.
Also: Geotechnical
Sizing culverts and storm drains without silting them up
Choosing a barrel that passes the design flow, and the self-cleansing velocity that stops it silted up within a few seasons.
Also: Environmental