Attenuation tanks or basins: choosing between them

Choosing between an attenuation tank or basin on a small site comes down to three things: how much land you can give up, who will maintain it in twenty years, and whether the drainage strategy needs the water quality credit that only a surface feature provides. A below-ground geocellular tank takes no developable land and gives almost no treatment. An above-ground detention basin takes a chunk of the plot and does both jobs at once. On a tight infill plot the tank usually wins on land take and then loses on maintenance and treatment, which is why so many schemes end up with a hybrid.

What each option actually does

Both store water temporarily during a storm and release it at a controlled rate through a flow control, so that the discharge from the site does not exceed the agreed figure. That much is identical.

A below-ground tank is a void: a geocellular structure, large diameter pipes, or a precast chamber, wrapped in geotextile or an impermeable membrane and buried under a drive, parking court or garden. It is invisible once backfilled.

A detention basin is a shallow depression, dry between storms, with a grassed or planted base, gentle side slopes and a controlled outlet. Water sits in it for a few hours after rain and then drains away. A retention pond is the permanently wet version and behaves differently again, with a greater treatment benefit and a different maintenance regime.

The design standards that both have to satisfy are the same: the national technical standards for sustainable drainage, the local authority's own requirements, and a climate change allowance applied to the rainfall.

Land take and layout consequences

A basin needs area, freeboard and side slopes shallow enough to be safe and mowable, which on a small site is the whole argument. As a rough planning figure, a basin occupies several times the plan area of the equivalent tank once slopes and access are included.

Against that, basin land is rarely wasted entirely. It can double as open space, a landscape buffer, or the wet corner of a site that was never going to take a plot. Where the site has a low corner near the outfall, the basin is almost free in land terms.

A tank's land take is nominal but its position is constrained in ways people underestimate. It cannot sit beneath a building. It needs cover depth appropriate to the loading above it, which on a trafficked drive is more than people expect. It has to be clear of tree root protection areas and of service runs. And it needs a maintenance access route for a jetting vehicle, which has to appear on the layout rather than being assumed.

A Drainage Strategy should show the chosen feature on the proposed layout with its access, not as a note on a schedule.

Maintenance access and cost over time

This is where the long view changes the answer.

A basin is maintained with a mower and occasional silt removal from the inlet and the forebay. Anyone can see when it is blocked, the work is visible, and a management company or a householder can carry it out. The failure mode is cosmetic before it is hydraulic.

A tank is maintained by inspection chambers and jetting, by a contractor, on a schedule nobody remembers. Silt accumulates in the void where it cannot be seen, geocellular units can be damaged by over-pressure jetting, and the first sign of failure is often a flood. Replacement means excavating the drive.

Price the whole life, not the capital cost. A tank is frequently the lower capital figure and the higher thirty-year figure, and on a scheme with a management company the service charge is where that shows up.

Water quality treatment performance

The national approach to surface water quality uses a simple index method in which each drainage component is credited with a pollution mitigation value, and the components in the management train have to add up to at least the pollution hazard of the contributing surfaces. Roofs score low on hazard. Residential parking and roads score higher.

A buried tank contributes almost nothing to that total. A basin contributes through sedimentation, and a planted basin or a swale upstream contributes more. So a scheme relying on a tank alone will often fail the water quality test even where the hydraulic figures work, and the usual fix is to put treatment upstream: a permeable surface, a filter strip, a swale, or a proprietary separator.

That is the hybrid most sites end up with. Treatment at the surface where the dirty water is generated, then storage below ground where the land is tight. Planting choices in the surface components matter to performance, as our note on planting specifications for swales and wetland margins sets out.

Groundwater and contamination constraints

Both options are affected by the water table and by ground conditions, in opposite ways.

A tank below the seasonally high groundwater level is subject to uplift and can float, which means anchorage, ballast or a different solution. It can also take in groundwater through an infiltration design, defeating the attenuation volume. Where the site is in a source protection zone or has a history of contamination, infiltration may be ruled out entirely and a lined tank becomes the only below-ground option.

A basin in high groundwater simply becomes a wet feature, which may be acceptable and even beneficial, but it changes the storage volume available at the start of a storm, the safety case, and the maintenance regime. Model it as wet rather than assuming it drains.

Where made ground or contamination is present, excavation volumes and disposal costs drive the economics, and a shallow basin usually generates less excavated material than a deep tank. Site investigation needs to come before the choice, which is an argument for commissioning a contaminated land assessment early where the history suggests it.

Adoption prospects for each

Below-ground attenuation designed to the current sector design and construction guidance can be offered for adoption by the sewerage undertaker under a section 104 agreement, which removes the long-term liability from the management company. Whether it will be accepted depends on the detail and on the undertaker.

Surface features are more varied. Some authorities and some undertakers adopt basins and ponds, some do not, and the fallback is a management company with a sinking fund. Resolve it before the application rather than afterwards, because the answer affects the service charge disclosed to purchasers and sometimes the layout.

Note that the sustainable drainage approval regime in Schedule 3 to the Flood and Water Management Act 2010 has not been brought into force in England, so adoption still runs through existing routes. Check its status before assuming otherwise.

Choosing on a constrained plot

Work through it in this order. Establish the discharge rate and the storage volume required, with the climate change allowance applied. Test infiltration properly, because if the ground will take water the volume drops and the whole question changes. Check the groundwater level across the seasons rather than on one day. Then look at the plot and ask whether there is any land that cannot carry a building anyway.

If there is, use it for a basin and keep the tank for the balance. If there is not, accept a lined tank with surface treatment upstream, and be honest in the strategy about the maintenance burden you are creating. Flow controls are the component that fails in both options, and the modelling needs to allow for partial blockage, as our note on modelling a partially blocked flow control explains. Where the site is in Flood Zone 2 or 3, a Flood Risk Assessment sits alongside the drainage work.

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

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