Attenuation tank or attenuation basin: which to use
Both hold the same volume of water. They differ on treatment value, land take, whole life cost and who will adopt them, and the choice is easiest to make before the layout is fixed.
Choosing between an attenuation tank or basin comes down to four things: how much developable land you are prepared to give up, whether you need the feature to provide treatment as well as storage, who is going to maintain it for the next 60 years, and whether anyone will adopt it. The required storage volume is much the same either way, so volume is rarely the deciding factor.
What tips the decision in practice is when it is made. A basin designed into the layout at concept stage costs very little in developable area because it sits in ground you could not build on anyway. A basin retro-fitted after the plots are drawn costs plots.
Storage volume for the same duty
The attenuation volume needed is set by the hydrology, not the container. For the same catchment, the same discharge rate and the same design event, a tank and a basin hold the same volume of water.
Where they differ is the excavation needed to achieve it, and that depends on the void ratio of what fills the space. A geocellular system gives around 95 per cent void, so the excavation is barely larger than the storage. Clean angular stone gives roughly 30 per cent void, so a stone-filled trench needs something like three times the excavated volume for the same storage. A basin is open, so its void ratio is effectively 100 per cent above the base.
Depth is the other constraint. A basin's usable depth is limited by the outfall invert, by groundwater, and by the side slope needed for safety and mowing, which is commonly one in three or shallower. That turns a modest depth into a large footprint. A tank can go deeper, subject to groundwater and to the cover and loading design above it.
So the honest comparison is not volume against volume. It is footprint against depth, and the site's levels usually decide it.
Land take and developable area
This is the argument that wins most discussions in a development appraisal, and it is often argued from the wrong baseline.
A tank can sit beneath a car park, an access road or a public open space, which means it consumes no plot area. It cannot usually sit beneath buildings, and adopting bodies require access and easements over it, so a tank under a road needs the road alignment to cooperate.
A basin consumes surface area, and the question is whether that area was developable. On most greenfield sites the attenuation feature belongs at the low point, next to the outfall, which is frequently land constrained by flood risk, an easement, a tree buffer or a landscape setback. Where that is true the basin costs almost nothing in yield.
The secondary question is whether the basin counts for anything else. Some authorities allow an attenuation basin to contribute to open space or green infrastructure provision, and many do not. Habitat created around a basin can generate habitat units, subject to condition assessment and the management commitment, which matters where the scheme is short on units. Establish both positions locally before you claim them in a Biodiversity Net Gain Assessment or an open space calculation.
Treatment value compared
Under the approach in the CIRIA SuDS manual, surface water quality is managed by a treatment train, and components are credited according to the treatment processes they provide against the pollution hazard of the contributing surfaces.
A below-ground tank provides almost no treatment. Water passes through a void and out again, so the tank needs upstream treatment to protect both the receiving water and the tank itself. That means a filter strip, a swale, a permeable surface, a proprietary treatment device, or a combination, plus silt capture immediately before the tank.
A basin provides treatment through sedimentation and, where it is vegetated and has a forebay, through filtration and biological processes. A well-designed basin with a sediment forebay and a shallow marshy base does real work on the pollution indices, which can remove the need for a separate treatment component upstream.
That difference changes the whole train. A car park draining to a tank needs treatment before the tank. The same car park draining to a vegetated basin with a forebay may need only a gully arrangement and a filter strip. Compare the trains, not the components.
Maintenance and access
Maintenance is where tanks lose, and the losses are borne by whoever owns the asset in year 15.
A basin is visible and its maintenance is legible: grass cutting, silt removal from the forebay, inspection of the inlet and outlet, and occasional clearance of the flow control. Work is done from the surface with ordinary plant, by people who do not need confined space training.
A tank is invisible and its maintenance is easy to neglect. Silt accumulates in a void nobody inspects, and removing it means jetting, vacuum tankering, CCTV survey and in some designs confined space entry. Geocellular systems with limited internal access are the hardest, which is why access galleries, inspection chambers at every change of direction and an effective upstream silt trap matter far more than they appear to at design stage.
Write the maintenance regime into the management plan with frequencies and access arrangements for whichever option you choose. Then ask who will actually do it. A management company with a service charge behaves differently from a homeowner who has inherited a chamber cover in their driveway.
Adoption prospects
Adoption determines the long-term risk, and the position varies by undertaker and by component.
| Below-ground tank | Open basin | |
|---|---|---|
| Sewerage undertaker adoption | Possible for some designs under current sector guidance | Rarely adopted |
| Highway authority | Sometimes, where it serves highway drainage | Rarely |
| Management company | Common, with a service charge | Common |
| Local authority open space | Not applicable | Occasionally, with a commuted sum |
| Easement requirements | Access and no-build zone above | Access track and maintenance strip |
Check the undertaker's current design and construction guidance rather than relying on what was adopted on your last scheme, because the criteria have changed more than once. And note that the SuDS approval provisions in Schedule 3 to the Flood and Water Management Act 2010 have not been brought into force in England, so approval still sits with the lead local flood authority as a consultee and with the planning conditions, rather than with a separate approving body.
Where no adopting body will take the feature, say so in the Drainage Strategy and set out the management arrangement instead. An unowned attenuation feature is a planning risk, not a drainage detail.
Whole life cost and buildability
Capital cost comparisons flatter tanks more often than they should, because the comparison usually omits the treatment components a tank requires and the excavation and disposal a tank generates.
On the build side, a tank brings excavation in ground that may be contaminated, dewatering where groundwater is high, structural design for wheel loading above, cover depth constraints, and service crossings that have to be resolved. A basin brings earthworks, a liner where infiltration is not wanted, planting, and a period of establishment during which it looks like a hole.
Over the life of the asset the running costs diverge sharply. Jetting and tankering a silted tank is a different order of expense from mowing a basin, and the interval is shorter than most management plans assume.
There is one honest uncertainty here. Tank maintenance cost depends heavily on how well the upstream silt capture works, and that varies with how diligently the silt trap is emptied. A well-managed tank can run at modest cost for decades. Most are not well managed.
Choosing between them at layout stage
Make the decision with the levels drawing open, before the plots are laid out.
- Fix the outfall and its invert level, because that sets the maximum usable depth for a basin.
- Check groundwater levels and any contamination, which may rule out deep excavation or infiltration.
- Identify land already undevelopable for other reasons and test whether a basin fits there.
- Compare treatment trains rather than single components, using the pollution hazard of the contributing surfaces.
- Ask the undertaker and the lead local flood authority what they will adopt before you commit.
- Price whole life cost, including the treatment components each option requires.
- Confirm whether the basin can count towards open space, landscape or habitat requirements.
Where the site is also in Flood Zone 2 or 3, the storage design has to be consistent with the Flood Risk Assessment, particularly on the behaviour of the outfall when the receiving watercourse is high. And where the basin is doing landscape work as well as drainage work, keep it in the Landscape Strategy so the planting and the hydraulic design agree.
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Frequently asked questions
Does a tank need less storage volume than a basin?
No. The attenuation volume is set by the catchment, the discharge rate and the design event, so both need the same volume of water storage. What differs is the excavation: a geocellular system gives around 95 per cent void, clean stone around 30 per cent, and an open basin is effectively all void above its base.
Can an attenuation basin count towards open space provision?
Sometimes, and it depends entirely on local policy. Some authorities allow a basin with shallow side slopes and usable margins to contribute to green infrastructure or open space; others exclude drainage features from the calculation. Establish the position before relying on it in an area schedule, because the answer changes the developable yield.
Do I still need treatment upstream of a basin?
Often less of it. A vegetated basin with a sediment forebay provides sedimentation and filtration, which counts towards the treatment requirement for the contributing surfaces. A below-ground tank provides almost no treatment at all, so it needs a treatment component upstream as well as silt capture to protect the tank itself.
Will a sewerage undertaker adopt a geocellular tank?
Some designs can be adopted under current sewerage sector guidance, subject to access, inspection and structural criteria. The criteria have changed more than once, so check the undertaker's current design and construction guidance for the region rather than relying on what was adopted on a previous scheme.
Has the SuDS approving body regime started?
The provisions in Schedule 3 to the Flood and Water Management Act 2010 that would create SuDS approving bodies have not been brought into force in England. Surface water drainage is therefore still handled through the planning process, with the lead local flood authority as a statutory consultee and the detail secured by condition.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.