Attenuation storage sizing and the assumptions that move it

Attenuation storage sizing for planning is arithmetic performed on a stack of assumptions, and the assumptions move the answer far more than the arithmetic does. Two engineers given the same site, the same impermeable area and the same discharge rate can produce volumes that differ by half, because one used a different climate change allowance, a different urban creep uplift, or a different view of what the pre-development runoff rate was.

That is why a drainage strategy that states its volume without stating its assumptions gets queried. The reviewer at the lead local flood authority is checking the inputs, not the output.

What the storage has to achieve

Attenuation exists to hold back the difference between the rate at which rain arrives on the site and the rate at which you are allowed to discharge it. Two separate controls apply.

Peak flow control limits the discharge rate. The Defra non-statutory technical standards for sustainable drainage expect the peak runoff rate from a greenfield development, for both the 1 in 1 year and the 1 in 100 year event, to be no greater than the greenfield rate for the same event. For previously developed sites the expectation is a rate as close as reasonably practicable to greenfield, and in no case worse than the existing rate.

Volume control is the second and less well handled requirement. Where reasonably practicable, the standards expect the volume of runoff discharged off site during a 1 in 100 year, six hour event to be constrained to the greenfield volume. Where that cannot be achieved by infiltration or reuse, the excess becomes long term storage released at a very low rate. Plenty of submissions size for peak flow, ignore volume entirely, and get a holding objection.

Design events and return periods

Sizing is done for the 1 in 100 year event with a climate change uplift, with the 1 in 30 year event used to confirm no surcharging of pipework above ground, and the 1 in 1 year event used for the frequent flow condition.

Run a full range of storm durations rather than a single one. The critical duration for a small site with a tight discharge rate is often several hours, and for a very restricted rate it can be a day or longer, which is counter-intuitive to anyone used to sizing pipes. Summer and winter rainfall profiles behave differently, and the winter profile with its longer, lower intensity events typically drives long duration storage while the summer profile drives short duration peaks.

State in the strategy which rainfall dataset you used and which storm profiles were modelled. A volume quoted without the critical duration beside it tells the reviewer nothing.

Climate change and urban creep allowances

The Environment Agency publishes peak rainfall intensity allowances for use in drainage design, varying by location and by the period the development is designed for, with central and upper end figures. These allowances have been revised more than once, and the current version is finer grained than the flat percentage uplift many older reports used. Take the figure from the current published allowances for your catchment and cite it, because a strategy using a superseded uplift is the quickest way to a resubmission.

Urban creep is the allowance for future paving over of gardens and soft areas within a residential development. It is applied as an uplift to the impermeable area, with the percentage depending on the density of the development, and the drainage authority's own guidance usually specifies which figure it expects. Commercial and industrial sites are generally treated as already fully hard surfaced, so no creep allowance applies.

Both allowances are applied to the design case, not to the existing case. Using a climate change uplift on the pre-development greenfield rate to make the comparison look better is a mistake reviewers spot immediately.

Long term storage and volume control

Long term storage is the part that surprises architects when it lands on the layout. Where the volume standard cannot be met by infiltration, rainwater reuse or evapotranspiration, the additional volume is held and released slowly, often at a rate of the order of two litres per second per hectare.

Because the release rate is so low, the volume is large and it empties slowly. That has design consequences: the storage cannot be the same space as the peak attenuation unless the modelling demonstrates it, and a basin that holds water for days needs a different planting and safety treatment from one that empties in hours. Rainwater harvesting can contribute, and the extent to which a harvesting system earns attenuation credit depends on demonstrating that the tank will be empty when the storm arrives.

Half drain time and consecutive storms

Two operational checks decide whether the volume you have calculated is actually enough.

Half drain time is the period for the storage to empty to half its volume once inflow stops, and the convention is that it should be no more than 24 hours so that capacity is restored before the next event. A tight orifice on a deep tank can fail this even when the volume is correct.

Consecutive storm analysis tests the system against a second event arriving before the first has drained. Where the model shows the storage still part full, the volume needs increasing or the discharge arrangement changing. Both checks should be reported in the strategy as results rather than as assurances.

Tank, basin and permeable paving trade-offs

Most schemes end with a combination, and the ordering matters. Source control first, then conveyance, then any remaining volume in a single manageable structure. A layout where all the storage is in one deep tank beneath the access road usually signals that drainage was considered after the layout was fixed.

The assumptions reviewers challenge most

1. The impermeable area take-off, particularly whether roof overhangs, footpaths and bin stores have been included, and whether the existing impermeable area is evidenced by survey rather than estimated from aerial imagery.
2. The claim of an existing positive drainage connection on a brownfield site, which needs a drainage survey to support a pre-development rate above greenfield.
3. The climate change allowance and the epoch it relates to.
4. The urban creep percentage and whether it matches local guidance.
5. Infiltration rates, the factor of safety applied, and whether groundwater levels were monitored over a wet period rather than measured once. Where infiltration is claimed on a site with a shallow water table, the groundwater flooding evidence needs to stand up on its own.
6. Minimum practical discharge rates, since a calculated rate below about two litres per second raises a blockage concern that the flow control manufacturer's data has to answer.

Illustrative example: a one hectare residential site with a greenfield rate of five litres per second, a 40 per cent impermeable area and a central climate change allowance might need a few hundred cubic metres for peak attenuation, and roughly a third of that again as long term storage. The figures are there to show the shape of the answer. Run your own calculation with the allowances current for your catchment.

Set the drainage assumptions out explicitly in your Drainage Strategy and cross-refer them to the Flood Risk Assessment so the two documents agree. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.