Attenuation storage, interception and half drain time

Sizing attenuation storage is a volume problem. The discharge rate sets how fast water leaves the site, the rainfall sets how much arrives, and the storage holds the difference. Get the suds attenuation storage volume wrong and the consequence is not a marginal exceedance, it is a system that surcharges in an event well below the design return period. Interception, long term storage and half drain time each change that volume, and each is checked separately by the lead local flood authority.

Reviewers see the same errors repeatedly. A single storm duration modelled. Urban creep left out. A control that empties the basin over four days. None of these show up as an obvious failure in the model output.

Sizing storage against the critical duration

Run the full range of durations. A 15 minute summer storm and a 24 hour winter storm at the same return period produce completely different volumes, and which one governs depends on the discharge rate.

The tighter the allowable discharge, the longer the critical duration becomes. On a site restricted to a greenfield rate of a few litres per second, the governing event is frequently six, twelve or twenty-four hours rather than anything shorter. Designers who model only the short durations underestimate the volume, sometimes by a wide margin.

The design event for storage is normally the 1 in 100 year event with an allowance for climate change. That allowance comes from the Environment Agency's published figures, which vary by river basin district, by epoch and by the type of assessment, and they are updated periodically. Take the current figure from the guidance rather than from a previous project file.

Interception and the first few millimetres

Interception is about the small, frequent rainfall that makes up most of the year's events. The aim is that the first few millimetres of rainfall produce no runoff to the receiving system at all, through infiltration, evapotranspiration, or retention within the construction of a permeable surface.

Five millimetres is the figure commonly used as the interception target, taken from CIRIA guidance on sustainable drainage. Meeting it usually needs source control rather than a pipe, and the features that work are permeable paving, bioretention areas, rain gardens, swales and green roofs.

Reviewers look for evidence rather than assertion. Where permeable paving is proposed, they want the sub-base depth, the void ratio and the infiltration assumption. Where a green roof is proposed, they want the substrate depth and the runoff coefficient applied.

Long term storage on greenfield sites

A developed site produces more total runoff volume than the greenfield site it replaced, not just a higher peak. Long term storage deals with that difference, holding the additional volume and releasing it at a very low rate, or removing it from the system through infiltration.

The non-statutory technical standards for sustainable drainage set the expectation that runoff volume is constrained towards the greenfield case for the 1 in 100 year, 6 hour event where that is reasonably practicable. Where it is not practicable, the shortfall has to be justified.

In practice long term storage is provided either as a separate cell with its own small orifice or as additional depth within the main basin with a two-stage control. Both work. What does not work is claiming the volume twice by counting the same storage as attenuation and long term storage in the same calculation.

Half drain time and the next storm

Storage that is still full is not storage. Half drain time measures how long the system takes to release half of its stored volume after an event, and CIRIA guidance points to around 24 hours as the target for attenuation storage.

Long drain times come from very small orifices, which designers reach for when the allowable discharge rate is low. There is a tension there, and the honest answer is that it cannot always be fully resolved on a tight site. Vortex controls, two-stage outlets and a hydrobrake with a defined head-discharge relationship are the usual responses.

Short drain times cause their own problems, because a basin that empties in three hours is probably not attenuating much.

Urban creep and the allowance to include

Residential sites change after occupation. Front gardens become parking, back gardens acquire patios and garden rooms, and side returns get paved. Urban creep allowances add a percentage to the impermeable area to reflect that.

The figure is set locally, so check the lead local flood authority's published guidance before applying anything. Applying no allowance at all on a residential scheme is one of the fastest ways to get a drainage strategy returned. Our note on what the lead local flood authority checks covers the wider review.

One further point on infiltration. Where soakaways or infiltration basins are proposed, the storage calculation depends on an infiltration rate measured on site rather than assumed from a soil map. Tests carried out to the standard method, at the depth and location of the proposed feature, with a factor of safety applied, are what a reviewer expects to see. A strategy resting on a single trial pit in the corner of the site usually comes back.

Groundwater levels matter as much as permeability. An infiltration feature needs a clear separation from the seasonal high groundwater level, and where that separation does not exist the volume has to be found elsewhere.

Exceedance routing above the design event

Every system is exceeded eventually. The design should show where water goes when it is, and that is a drawing rather than a calculation.

Show overland flow routes from each storage feature, the levels that direct water away from buildings, and the safe destination for the excess. Finished floor levels set above the adjacent external levels, kerb lines that steer water to the road, and a basin with a defined overflow weir all belong in the strategy.

The test a reviewer applies is simple. If the outlet blocks completely, does water reach habitable accommodation before it finds a route away.

Checking a model output for plausibility

Before the Drainage Strategy goes out, run four sense checks. Compare the storage volume against a rough hand calculation of impermeable area multiplied by rainfall depth minus discharge over the duration. Check that the critical duration reported is the one that actually produces the largest volume. Check the half drain time in the output rather than assuming it. And check that the impermeable area in the model matches the areas on the layout, including the urban creep uplift.

Where the site also sits in Flood Zone 2 or 3, the drainage work sits inside a wider Flood Risk Assessment that deals with flood depths, levels and safe access. And the question of who maintains the system for the next hundred years belongs in the strategy from the start, which our note on adoption and maintenance covers, alongside where Schedule 3 of the Flood and Water Management Act 2010 currently stands.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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