Critical duration storm and attenuation volume

The critical duration storm is the rainfall duration that, at a given return period, demands the largest attenuation volume on your site. It is not something you can look up for a postcode. You find it by running the same return period across a spread of durations, from a quarter of an hour to several days, and recording which one leaves the widest gap between the water arriving on the drained area and the water the flow control is allowed to release. That gap, at its worst, is the storage you have to build.

Most refusals and most late redesigns in drainage come from getting this one step wrong, usually by sizing a tank from a single duration because it was the duration in the local guidance note.

Why several durations are tested

A short storm is intense and brief. A long storm is gentler and far bigger in total volume, and the outlet has hours rather than minutes to work. Required storage is simply the inflow volume minus whatever has discharged by that point, so both halves of the sum grow as the duration lengthens, at different rates. The difference between them rises, peaks, then falls away.

Where the peak sits depends on three things: the area positively drained to the system, the permitted discharge rate, and the rainfall depth-duration-frequency relationship at that location.

A standard duration set for a residential site runs 15, 30, 60, 120, 180, 240, 360, 480, 600, 720 and 1440 minutes. Sites with very low permitted discharge rates need longer durations on top, out to 2,880 or 10,080 minutes, because the outlet is so throttled that multi-day events still accumulate storage. Summer and winter rainfall profiles also give different answers, and winter profiles usually govern volume because the assumed antecedent wetness is higher.

How the critical duration is identified

Run the model once per duration at the design return period, read off the maximum volume held in the system during each run, and tabulate the results. The largest figure is your critical duration and your design volume.

Two things are worth separating in that output. The volume that fills a downstream tank and the depth that surcharges an upstream pipe often arise in different storms, so a system can have one critical duration for storage and another for node flooding. Check both. The Defra non-statutory technical standards for sustainable drainage expect no internal property flooding in the 1 in 100 year event including an allowance for climate change, alongside the peak flow rate controls, and those are separate tests rather than one.

Then there is the rainfall model itself. Older strategies were built on the Flood Studies Report relationship; current practice uses the Flood Estimation Handbook depth-duration-frequency model, and the two give different depths at the same duration and return period. State in the report which model you used and the date of the extraction, because a reviewer who re-runs it on a newer model and gets a different number will ask why.

What the flow control does to the answer

The permitted discharge rate is usually the binding constraint, and it is set by the lead local flood authority rather than by the designer. On a greenfield site it is normally tied to the estimated greenfield run-off rate for the equivalent event. On a brownfield site it is often a betterment figure, a stated percentage reduction on the existing rate.

Lower that rate and two things happen at once. Storage grows, and the critical duration gets longer. A site that was critical at three hours at 10 litres per second can be critical at ten hours or more at 2 litres per second.

There is a practical floor on how low you can usefully go. Vortex and orifice controls below roughly 2 litres per second are prone to blockage, and many authorities will not accept a design rate under 5 litres per second per outfall on a small site, though that varies and should be checked locally. Hand calculations also flatter the design here, because a hydro-brake style control only achieves its stated rate near its design head. At low water levels it passes much less, so the real outflow over the storm is below the nominal figure and the real storage requirement is above a constant-rate estimate.

Climate change and urban creep allowances

Attenuation sizing uses the 1 in 100 year event with the peak rainfall intensity allowance for the relevant river basin district and the development's lifetime. The Environment Agency publishes those allowances and has revised them more than once, so take them from the current guidance page rather than from a previous scheme, and record the epoch you selected and why.

Urban creep is the second uplift, and it is the one that gets left out. Households pave front gardens, add patios and extend over the years, so the impermeable area at year 60 is larger than the area on the approved layout. The SuDS Manual sets out creep allowances by housing density and a 10 per cent uplift on impermeable area is widely applied to medium-density residential schemes, though your authority's local guidance takes precedence.

Apply the allowances in the right order. Rainfall uplift acts on depth, creep acts on area, and the two multiply.

Half drain time and recovery

Storage has to empty before the next event arrives. The SuDS Manual recommendation is that attenuation storage half-empties within 24 hours, which gives the system usable capacity again inside a day.

This bites on heavily throttled sites. A 2 litre per second control draining a 300 cubic metre tank cannot meet a 24 hour half drain time by arithmetic alone, so the design has to change rather than the criterion. Staged controls that pass a higher rate at high water levels, infiltration where ground conditions and groundwater levels allow it, and splitting storage across several cells with separate controls are the usual answers.

Where the strategy also has to provide long-term storage, because a brownfield site is being brought back towards greenfield volume, that volume sits on top of the attenuation requirement and is governed by its own event rather than by the critical duration.

An illustrative storage calculation

The figures below are illustrative. They use a simple mass balance to show the shape of the result, not real rainfall data, and real software routes hydrographs through the network instead.

Assume 0.30 hectares of positively drained impermeable area, so 3,000 m², a permitted discharge of 5 litres per second, and these rainfall depths for the 1 in 100 year event after a climate change allowance:

The three hour storm governs, at 111 m³. Shorter storms deliver less water. By six hours the outlet has caught up. Drop the discharge to 2 litres per second and the 360 minute row becomes the worst case instead, which is the whole point of testing the range.

What a drainage strategy should present

A reviewer at the lead local flood authority is checking assumptions, not admiring the tank. Give them the duration table with the storage figure for every duration run, the rainfall model and extraction date, the return periods tested, the climate change allowance with its epoch and the creep allowance with its source, the discharge rate and how it was derived, the control type with its head-discharge curve, a storage schedule component by component, the half drain time, the exceedance route for events beyond design, and the maintenance regime with who owns it.

That evidence belongs in the Drainage Strategy rather than scattered across drawings, and it needs to agree with the flood zone and finished floor levels in any accompanying Flood Risk Assessment. Where an authority publishes standing advice for consultees rather than commenting on each application, that advice is the checklist your strategy is being marked against.

One honest caveat: duration sets and acceptable half drain times still vary between authorities, and a design that passed in one district can be queried in the next.

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This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.