A worked attenuation volume for a half hectare site

Attenuation storage for a small site comes down to four numbers: the impermeable area, the permitted discharge rate, the design rainfall depth for a range of durations, and the volume that leaves through the control while the rain is falling. Storage is the difference between what arrives and what leaves, tested across durations until you find the worst one.

The worked figures below are illustrative. They show the method for a half hectare residential plot and should not be lifted into a submission, because rainfall depths are site specific and discharge rates are set by your lead local flood authority.

The inputs the calculation needs

Six things, and you cannot start without them.

• Site area and the impermeable area after development, measured from the layout rather than assumed
• The discharge destination, established through the drainage hierarchy: infiltration first, then a watercourse, then a sewer
• The permitted discharge rate, usually the greenfield rate for the site, subject to any minimum the authority allows
• Rainfall depths for the site, taken from a site-specific rainfall data point rather than a regional average
• The climate change allowance published for the river basin district and the development's lifetime
• An allowance for urban creep on residential schemes, typically applied as a percentage uplift to impermeable area

Where infiltration is being considered, percolation testing to the recognised method is needed before anything else, because an infiltration rate that works removes the need for most of the storage. Test early: it is the single test that changes the drainage strategy completely.

Estimating the greenfield discharge rate

Greenfield runoff for small catchments is normally estimated using the published hydrological method that relates the mean annual flood to catchment area, average annual rainfall and soil type. The output is a rate in litres per second for the site.

For half a hectare in most of lowland England the greenfield rate comes out at a few litres per second, sometimes less than two. That creates a practical problem: an orifice sized for 1.5 l/s is small enough to block on a leaf. Most lead local flood authorities therefore allow a minimum controlled discharge, commonly 5 l/s, and some accept a vortex control at a lower rate where the design manages the blockage risk.

Ask the authority for its position before sizing anything. The difference between 2 l/s and 5 l/s changes the storage volume by a third on a site this size.

Illustrative calculation for a half hectare plot

Take a 0.5 ha plot developed with eight houses, driveways and a shared access, giving 3,000 m² of impermeable area after a 10 per cent urban creep allowance. Assume the lead local flood authority has accepted a controlled discharge of 5 l/s to a ditch, and that the climate change allowance for the location and lifetime is 40 per cent. Both assumptions are illustrative.

The rainfall depths below are illustrative values for a lowland English site, already uplifted by the 40 per cent allowance.

Inflow volume is the depth multiplied by the impermeable area. Volume discharged is the controlled rate multiplied by the duration: 5 l/s is 0.005 m³/s, which is 18 m³ in an hour and 108 m³ in six.

The critical duration here is six hours, requiring about 123 m³. Round up to 130 m³ for a preliminary design and carry that figure into the layout. Note what the table shows about longer storms: by 24 hours the control has discharged more than the total inflow, so no storage is needed at that duration. The critical duration always sits where the two curves are furthest apart, and finding it is the whole point of testing a range.

Choosing between tank, basin and permeable paving

The volume is the same whichever you choose. The footprint is not.

A geocellular tank stores at a void ratio close to 95 per cent, so 130 m³ needs roughly 137 m³ of tank, which at 1 m deep occupies about 137 m². It is compact, buried and expensive, and it provides no treatment on its own.

A detention basin at an average depth of 600 mm needs a footprint of a little over 200 m² allowing for side slopes, costs a fraction of that, and provides treatment and amenity. It needs land that is not doing anything else, which on an eight-house scheme is the constraint.

Permeable paving over a granular sub-base stores at a void ratio nearer 30 per cent, so 130 m³ would need about 430 m³ of sub-base. Over 800 m² of driveway and access that is a 540 mm layer, which is close to what the construction needs anyway. This is why permeable paving is so often the answer on small residential sites. The storage costs almost nothing extra, because it sits inside a layer you were building regardless.

Most schemes end up with a combination, and the combination is also what delivers the treatment stages a surface water system needs before discharge. A Drainage Strategy sets out that management train, component by component.

Checking half drain down time

Storage that empties too slowly is not available for the next storm. The usual check is the half drain down time: how long the system takes to release half of its stored volume through the control.

At 5 l/s, half of 130 m³ is 65 m³, which takes 13,000 seconds, or about three and a half hours. That comfortably meets the expectation that storage should be substantially emptied within a day. A design that failed the check would need a larger orifice, a two-stage control, or more storage at a lower level.

Run the same check for the 1 in 1 year event as well as the 1 in 100 year plus allowance, because the technical standards deal with both frequent and extreme events, and a system tuned only for the extreme can perform poorly in ordinary rain.

Where these figures come from and what they are not

This method is a preliminary sizing tool. It assumes a constant discharge rate, ignores routing through the system, and takes no account of the shape of the storm profile. Detailed design uses simulation software with a site-specific rainfall data point, a full range of durations and profiles, and a check on exceedance flow routes for events beyond the design standard.

For a small residential scheme the preliminary figure is usually enough to fix the layout and prove the principle at application stage, with the detailed design secured by condition. That is the approach set out in a typical drainage strategy for a minor residential scheme, and it keeps the engineering cost proportionate to the risk.

Where the site sits in Flood Zone 2 or 3, or exceeds one hectare, the drainage work sits inside a Flood Risk Assessment rather than standing alone, and the sequential and exception tests come into play.

This is general guidance rather than legal advice, and discharge rates, rainfall data and climate change allowances are set locally and revised periodically, so check the current position with your lead local flood authority or a qualified drainage engineer before relying on it.

If you are working out which drainage documents your application needs, the report index groups them by category. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.