Sizing attenuation storage for the 100 year event

Attenuation storage on an English development site is normally sized for the 1 in 100 year event with a climate change allowance, discharging at a controlled rate, with the volume half emptying within 24 hours so the system can take a second storm. Those three requirements together decide the number, and most of the argument with a lead local flood authority is about the inputs rather than the arithmetic. A drainage strategy that shows the inputs clearly gets through consultation far faster than one that presents a single volume.

The events the storage has to handle

Three design events do different jobs, and a scheme has to satisfy all of them.

The non-statutory technical standards for sustainable drainage set the framework for these, alongside the requirement to limit peak discharge. For a greenfield site that means the greenfield run-off rate for the corresponding event. For a previously developed site it means a reduction against the existing rate, and many authorities expect a substantial betterment rather than a token one.

Note that the 1 in 100 year plus climate change event does not require a dry site. It requires that flooding, if it happens, stays out of buildings and away from people.

Inputs the calculation needs

Get these agreed before you run anything, because changing one of them changes everything downstream.

1. Impermeable area, catchment by catchment, measured from the layout rather than assumed as a percentage of the site.
2. An allowance for urban creep on residential development, covering future extensions and paved gardens.
3. Rainfall depth duration frequency data for the site from the current Flood Estimation Handbook model, with the version stated.
4. The climate change allowance for peak rainfall intensity, taken from the Environment Agency's published allowances for the relevant management catchment and the development lifetime.
5. The permitted discharge rate, either the greenfield rate calculated by an accepted method or a rate agreed with the receiving body.
6. Any infiltration rate, with the soakage test results and the factor of safety applied.

The climate change allowance is the input that moves. Allowances are revised as the projections are updated, so state the version you used and the date you took it, and check the current figures before finalising.

Then run the full range of storm durations rather than a single one. The critical duration for storage is often much longer than the critical duration for pipe capacity, and on a site with a very low permitted discharge rate it can run to many hours.

Half drain down and back to back storms

Storage that is still full when the next storm arrives is not storage. The convention, set out in the SuDS Manual and carried into most authorities' expectations, is that the volume should half empty within 24 hours of the design event.

That requirement conflicts directly with a low discharge rate. Squeeze the outflow to a very low figure and the tank will not drain in time, so you either accept a larger volume or negotiate the rate. Hydro-brakes and vortex controls have a minimum practical orifice below which blockage risk becomes the governing concern, and most authorities will not accept a control below a stated minimum diameter.

Where half drain down cannot be met, say so and explain the trade-off rather than hiding it in an appendix. Consultees read the half drain time line in the model output, and the point is covered in more depth in the note on attenuation storage, interception and half drain time.

Routing the water that exceeds the design event

Something larger than the design event will happen, and the drainage strategy has to show where that water goes.

An exceedance plan shows overland flow routes from every low point, with arrows, gradients and the eventual destination. The tests are simple: no route passes through a building, no route depends on a kerb upstand being maintained forever, and no route discharges onto third party land without a right to do so.

The practical design moves are level based. Set finished floor levels above the adjacent external ground by a stated amount. Fall roads away from buildings so the carriageway becomes the flow route. Keep the lowest point of the site in open space rather than in a parking court between two blocks.

Getting exceedance right also protects the scheme when the eventual built levels differ slightly from the drawing, which they always do.

Above ground against below ground storage

Above ground storage is less costly to build, easier to maintain, and it treats water on the way through. Below ground storage takes less land and treats nothing.

The honest position is that geocellular tanks are sometimes the only answer on a tight urban site, and that they carry a whole life cost most appraisals understate. Where a tank is proposed, expect the authority to ask what treatment sits upstream of it and how it will be cleaned.

What the storage does to the layout

Storage volume translates into area, and area comes out of the developable part of the site.

A basin holding water to an average depth of around one metre needs roughly as many square metres as it holds cubic metres, before you add side slopes, freeboard, a maintenance strip and a safety margin around the edge. Side slopes flatter than one in four for public areas, plus an access track, can easily double the footprint of the stored volume.

An illustrative example. A 2.4 hectare residential site with 60 per cent impermeable area and a greenfield discharge rate of a few litres per second needs storage in the hundreds of cubic metres, and the basin that holds it consumes a corner of the site that the layout had allocated to housing. Finding that out at the drainage design stage rather than after the layout is fixed is worth more than any refinement of the model.

So size the storage early, block it out on the concept layout, and treat the number as a constraint rather than as something the engineer will solve later.

Presenting the results so a consultee can check them

The model output alone is not a strategy. What a lead local flood authority wants is a short narrative with the inputs stated and a summary table showing, for each design event, the critical storm duration, the peak storage volume, the peak outflow and the half drain time.

Include the catchment plan with areas, the discharge rate calculation with its method named, the climate change allowance with its source and date, and the exceedance plan. Cross-reference the Flood Risk Assessment instead of repeating it.

And keep the drawings consistent with the model. A basin shown at one depth on the section and another in the calculation is the single most common reason a drainage submission is returned, as the note on what a consultee checks in a drainage model output explains.

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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.