How attenuation storage volume is actually derived

Attenuation volume is whatever is left over when you subtract what the system is allowed to discharge from what the design rainfall puts on the site. Everything else is detail about which rainfall, over what duration, onto how much impermeable area, and against what discharge rate. Get the SuDS attenuation volume calculation wrong at the wrong end and you either lose developable area to a tank you did not need, or you find out at technical approval that the volume you drew does not exist.

The design events a strategy has to test

Three events sit behind almost every English drainage strategy, and they do different jobs.

The 1 in 1 year event sets the interception and small event behaviour, and it is the event the peak flow standard uses for greenfield equivalence. The 1 in 30 year event is the one the piped network has to carry without surcharging above ground within the site. The 1 in 100 year event plus an allowance for climate change is the one that must not flood any building, and it is normally the event that sizes the attenuation.

The Defra non-statutory technical standards for sustainable drainage systems frame the requirements: peak runoff rate limited to the greenfield rate on greenfield sites and as close to it as reasonably practicable on previously developed land, runoff volume for the 1 in 100 year 6 hour event limited to the greenfield volume where reasonably practicable, no flooding of any part of the site in the 1 in 30 year event, and no internal flooding of property in the 1 in 100 year event with climate change.

Two inputs then have to come from current published sources rather than from a previous job. Rainfall depths come from the Flood Estimation Handbook depth duration frequency model for the site's grid reference. Climate change uplifts come from the Environment Agency's peak rainfall intensity allowances, which are set by management catchment and by the epoch the development's lifetime falls in, so check the current figure for your catchment rather than reusing a percentage from a scheme two counties away.

Critical duration, and why the longest storm is not the answer

Attenuation volume is a balance between inflow and outflow over time. A short, intense storm delivers a high peak but a small total volume. A long storm delivers a larger volume but the controlled outflow has longer to discharge it. Somewhere between the two sits the duration that needs the most storage, and that is the critical duration.

You find it by running the design return period across a range of durations, typically from 15 minutes to 24 hours or longer, and taking the worst result. It is not a value you can assume, because it shifts with the discharge rate. A generous discharge rate produces a short critical duration. Squeeze the outflow down to 2 or 3 litres per second and the critical duration lengthens, sometimes past 24 hours, because the system cannot shed volume fast enough for a long event to drain as it fills.

Here is an illustrative example, with assumed inputs rather than real rainfall statistics. Take one hectare of impermeable area discharging at a permitted 5 litres per second. Assume a 6 hour design storm depth of 45 mm once the climate change uplift is applied. The inflow is 10,000 m² multiplied by 0.045 m, so 450 m³. The outflow over the same 6 hours is 5 litres per second multiplied by 21,600 seconds, so 108 m³. The storage required for that duration is the difference, 342 m³. Repeat that for every duration and the largest answer is the one you build. The arithmetic is deliberately simplified; a real assessment routes the hydrograph rather than treating inflow as a block.

Urban creep and the impermeable area you model

The impermeable area is not the area shown on the landscape drawing. Householders pave front gardens, add sheds and extend, and a drainage system sized on day one areas is undersized within a decade.

An allowance for urban creep is therefore added to the contributing area on residential schemes. Guidance in the CIRIA SuDS Manual bands the allowance by development density, and many lead local flood authorities publish their own figure, commonly an addition of around 10 per cent of impermeable area for lower density housing. Check the local authority's own standards, because this is one of the most variable numbers in the whole calculation.

Two related habits improve a submission. State the impermeable area schedule explicitly, plot by plot or land use by land use, so a reviewer can add it up. And say which surfaces you have treated as permeable and why, because permeable paving over a liner with a restricted outfall is attenuation, not infiltration, and it should appear in the volume calculation rather than being deducted from the contributing area.

Long term storage on previously developed sites

Volume control is the standard that most brownfield schemes cannot meet head on. The runoff volume from a redeveloped site for the 1 in 100 year 6 hour event will usually exceed the greenfield volume for the same event, and infiltration is often ruled out by ground conditions or contamination.

Where that is the case, the technical standards expect the difference to be held as long term storage and released at a very low rate, so the total volume leaving the site over the event approaches the greenfield figure. In practice that means two storage components with different discharge controls: attenuation storage on a normal restricted outfall, and long term storage draining at a rate low enough that it is still emptying long after the event.

This is where schemes get expensive, and where an honest feasibility argument matters. If long term storage is not reasonably practicable on the site, say so with evidence: infiltration testing results, groundwater levels, contamination constraints, available head to the outfall. Where groundwater flooding evidence shows a high water table, that is a proper reason why an infiltration based solution fails, and the assessment should show the testing rather than asserting the conclusion.

Half drain time and recovery between events

Storage that is still full when the next storm arrives is not storage. The design has to recover its capacity between events, which is what the half drain time criterion measures: the time for the attenuation volume to fall to half its design level once inflow stops.

The usual expectation is that half drain time falls within around 24 hours, and some authorities set a shorter figure or apply it to full recovery rather than half. Long term storage is the deliberate exception, since its whole purpose is slow release. Set out both figures in the strategy, and be explicit about which storage component each one applies to. The detail behind that criterion, including how it interacts with tank geometry and the shape of the outflow curve, is covered in attenuation half drain time design criteria.

Rainwater harvesting complicates this, because the retained volume is only available if the tank happens to be empty when the storm arrives. Any credit claimed for it needs a yield and demand analysis behind it, and the approach set out in rainwater harvesting and attenuation credit is how reviewers expect that argument to be made.

Exceedance above the design event

The design event is a threshold, not a ceiling. Rainfall exceeding the 1 in 100 year plus climate change event will happen, and the strategy has to show where that water goes.

Exceedance routing means demonstrating a safe overland flow path from each low point to somewhere that can take water without flooding a building or sending it onto a neighbour. Kerb lines, road profiles, swale freeboard, level thresholds set above adjacent ground, and deliberately depressed areas of open space all form part of it. A plan with arrows and spot levels does more for a reviewer than a paragraph of text.

Blockage scenarios belong here too. Model the outfall control partially blocked and see what happens, because a tank with a 75 mm orifice and no blockage contingency is a maintenance liability.

Presenting the calculation so a reviewer can check it

Lead local flood authorities review a great many strategies with limited time, and the ones that clear quickly are the ones where the assumptions are visible on the page rather than buried in model output.

Include the rainfall source and grid reference, the climate change allowance with its catchment and epoch, the impermeable area schedule with the urban creep allowance shown separately, the permitted discharge rate and how it was derived, the method used for greenfield runoff, the full range of durations tested with the critical duration identified, the resulting storage volume against the volume provided, the half drain times, and the maintenance regime for each component.

Then check that the drawings agree with the numbers. The commonest reason a technical consultee comes back is that the tank shown on the layout has a smaller footprint or less depth than the volume in the calculation, or that a swale crosses a level that would not drain. A Drainage Strategy and a Flood Risk Assessment also have to tell the same story, because inconsistency between the two is treated as an error in both.

PlanScope drafts both documents structured to UK planning policy, with site specific inputs flagged rather than assumed, which leaves the drainage engineer to supply the hydraulic modelling and sign off the design. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval. For specialist technical reports, qualified professional input may be required.

This is general guidance, not legal advice. Confirm the current climate change allowances and your lead local flood authority's own standards before relying on any figure here.