Engineering calculators · Drainage
Storm attenuation storage calculator
Estimate the attenuation storage a site needs to hold back storm runoff to an allowable discharge rate. Enter the site area, a runoff coefficient, rainfall depths for a range of storm durations, the climate change uplift and urban creep, and the calculator finds the storage for each duration and the critical duration that governs. A helper estimates the greenfield runoff rate from the IH124 QBAR formula. It is a screening tool for feasibility and for sense-checking model results, not a replacement for FEH rainfall and hydraulic modelling.
Inputs
Results
- Screening estimate only. The method ignores the hydrograph shape, outlet control behaviour, storage above the outlet and rainfall profile. Final design needs FEH rainfall for the site and a hydraulic model (for example InfoDrainage, MicroDrainage or Causeway Flow).
- Allowable discharge is 5.0 L/s/ha of total site area.
- Critical duration is 12 h, inside the range of durations tested.
- Climate change uplift of 40% is within the EA 2022 peak rainfall allowance range.
Storage by storm duration
| Duration | Depth, mm | With uplift, mm | Intensity, mm/h | Inflow, L/s | Inflow, m³ | Outflow, m³ | Storage, m³ |
|---|---|---|---|---|---|---|---|
| 15 min | 20.0 | 28.0 | 112 | 445 | 400 | 9 | 391 |
| 30 min | 27.0 | 37.8 | 75.6 | 300 | 541 | 18 | 523 |
| 1 h | 35.0 | 49.0 | 49.0 | 195 | 701 | 36 | 665 |
| 2 h | 45.0 | 63.0 | 31.5 | 125 | 901 | 72 | 829 |
| 4 h | 57.0 | 79.8 | 20.0 | 79.2 | 1,141 | 144 | 997 |
| 6 h | 65.0 | 91.0 | 15.2 | 60.2 | 1,301 | 216 | 1,085 |
| 12 h | 80.0 | 112 | 9.33 | 37.1 | 1,602 | 432 | 1,170 |
| 24 h | 100 | 140 | 5.83 | 23.2 | 2,002 | 864 | 1,138 |
Sensitivity to the allowable discharge
| Allowable discharge, L/s | L/s/ha | Storage, m³ | Critical duration | Drawdown, h |
|---|---|---|---|---|
| 5.00 | 2.5 | 1,570 | 24 h | 87 |
| 7.50 | 3.8 | 1,354 | 24 h | 50 |
| 10.0 | 5.0 | 1,170 | 12 h | 32 |
| 15.0 | 7.5 | 977 | 6 h | 18 |
| 20.0 | 10 | 869 | 6 h | 12 |
| 30.0 | 15 | 709 | 4 h | 6.6 |
How the calculation works
- Effective area. Aeff = C × A × (1 + creep). Urban creep adds paving and extensions over the life of the development; Defra and most authorities take 10% [Defra].
- Design rainfall. For each duration D the depth is uplifted for climate change: dcc = d × (1 + uplift) [EA]. Intensity i = dcc ÷ D in mm/h.
- Inflow volume. Vin = Aeff × dcc ÷ 1000 in m³ (m² × mm ÷ 1000 = m³). Average inflow rate = Aeff × i ÷ 3600 in L/s, the Rational Method Q = C i A [CIRIA].
- Outflow volume. Vout = Qallow × D × 60 ÷ 1000 in m³, assuming the outlet discharges at the allowable rate for the whole storm.
- Storage. S(D) = Vin − Vout. The required storage is the largest S over all durations and the duration that gives it is the critical duration [CIRIA].
- Greenfield helper. QBAR (m³/s) = 0.00108 × AREA0.89 × SAAR1.17 × SOIL2.17 with AREA in km² [IH124] [EA 2013], shown in L/s and used as the allowable discharge when selected.
Worked example with the default inputs
A 2 ha mixed site with C = 0.65 and 10% creep has an effective area of 0.65 × 20,000 × 1.1 = 14,300 m². In the 4 hour storm the depth is 57 mm, uplifted by 40% to 79.8 mm, so inflow = 14,300 × 0.0798 = 1,141 m³. The outlet passes 10 L/s × 240 min × 60 ÷ 1000 = 144 m³, leaving 997 m³ to store. The 6 hour storm gives 14,300 × 0.091 − 216 = 1,085 m³ and the 12 hour storm 14,300 × 0.112 − 432 = 1,170 m³, while the 24 hour storm gives 14,300 × 0.14 − 864 = 1,138 m³, so the critical duration is 12 hours and the storage is about 1,170 m³. The 15 minute storm at 20 × 1.4 = 28 mm is 112 mm/h, an average inflow of 14,300 × 112 ÷ 3600 = 445 L/s.
What this cannot tell you. It uses block rainfall, so it cannot see the peak within a storm, and it assumes the outlet runs at its full allowable rate from the first minute, which a vortex control or orifice does not. It ignores infiltration, interception and the storage in pipes and on the surface, and it does not check half-drain times, exceedance routes or the 1 in 30 year no-flooding test in the Defra standards. Rainfall depths must come from FEH for the site, not from the indicative defaults. Treat the result as an order of magnitude for land take and cost, then model it.
Sources
- Environment Agency (2022) Flood risk assessments: climate change allowances (peak rainfall intensity allowances by management catchment, central and upper end)
- Defra (2015) Sustainable drainage systems: non-statutory technical standards for sustainable drainage systems (greenfield runoff, 1 in 100 year storage, climate change and urban creep)
- CIRIA C753 (2015) The SuDS Manual, ch. 24 (design storms, runoff coefficients, attenuation storage estimation)
- Marshall, D.C.W. and Bayliss, A.C. (1994) Flood estimation for small catchments. Institute of Hydrology Report No. 124 (QBAR = 0.00108 AREA^0.89 SAAR^1.17 SOIL^2.17)
- Environment Agency (2013) Rainfall runoff management for developments, Report SC030219 (greenfield runoff estimation, QBAR and small-site methods)
Frequently asked questions
What is the critical storm duration?
The storm duration that needs the most storage. Short storms are intense but deliver little volume; long storms deliver more volume but the outlet has longer to drain it. Somewhere between, the difference between inflow and outflow volume peaks, and that is the critical duration. It gets longer as the allowable discharge gets smaller. A design must test a range of durations, usually from 15 minutes to 24 hours or more, rather than a single storm.
What allowance should I make for climate change?
The Environment Agency 2022 guidance gives peak rainfall intensity allowances by management catchment for the 2050s and 2070s epochs, with central and upper end values. For the 1 in 100 year event over the lifetime of a development the upper end allowance, which is 40% or more in most catchments, is normally required. Check the figure for your catchment on the EA climate change allowances page and record which epoch and percentile you used.
How do I estimate the greenfield runoff rate?
For sites of 50 hectares or less the Institute of Hydrology Report 124 formula gives the mean annual flood QBAR from the catchment area, the standard average annual rainfall (SAAR) and the SOIL index from the Flood Studies Report soil map. QBAR is roughly the 1 in 2.3 year flow, and growth factors (about 1.2 for 1 in 10, 1.7 to 2 for 1 in 100) are applied for other return periods. Many authorities limit discharge to QBAR or to the 1 in 1 year greenfield rate. The helper applies the formula to the whole site area and lets you apply a multiplier.
Can I use this instead of a drainage model?
No. It is a screening estimate for early feasibility and for checking a model output. It treats the storm as a flat block of rain, assumes the outlet discharges at the full allowable rate from the start and ignores the storage above the outlet, so it tends to overestimate the volume a well-controlled system needs and underestimate it where the outlet is a small orifice with low head. Use FEH rainfall and a hydraulic model for design and for any submission to a lead local flood authority.