Geocellular storage and designing access for cleaning

Geocellular storage will only be accepted by most lead local flood authorities if the design shows how silt gets out again. That is the whole argument in one sentence. Modular plastic cells wrapped in geotextile give you a large void in a small footprint, which is why they appear under car parks and gardens on tight sites, but geocellular storage maintenance access is the point on which drainage strategies are returned for revision more often than any other detail.

So the design question is not whether the cells hold the volume. It is whether anyone can inspect and clean them in year 15.

Where below ground modular storage suits a site

Modular cells earn their place where the attenuation volume cannot be found at the surface. A courtyard scheme with no usable landscape, a backland plot where every square metre is building or access, a car park that has to stay flat: these are the cases where a tank is the only answer and a crate system is lighter and quicker to build than a concrete chamber.

They are a poor answer where surface options exist. The drainage hierarchy in the non-statutory technical standards for sustainable drainage expects infiltration and surface conveyance to be considered first, and a strategy that jumps to a buried tank without explaining why a swale or a basin was rejected invites a holding objection.

One honest caveat. Cells can be used for infiltration as well as attenuation, with an open base over permeable ground, and that version is harder to defend because silt reaching the base reduces the infiltration rate over time with no practical way to recover it.

Loading, cover depth and position

Every manufacturer publishes a load case table giving permitted cover depths for pedestrian, car park and heavy goods vehicle loading, together with the required bedding and surround. Those figures are system-specific and they are not interchangeable between products, so the drainage drawing has to name the system and the load class.

Three positional rules save trouble later:

1. Keep the tank out of the influence zone of building foundations, and show the offset on the section.
2. Keep it clear of mature tree root protection areas, which means coordinating with the arboricultural constraints plan before the tank position is fixed.
3. Keep it out of adopted highway land and out of easements for other services, because neither owner will accept a private structure.

Cover depth also drives the invert levels of the inlet and outlet, and those levels drive whether the system can discharge by gravity or needs pumping. Check that chain early. A tank that works on plan and fails on levels is a common late discovery.

Inspection and jetting access

A cell system without designed access is a sealed box. The access provisions that authorities look for are:

• An inspection and maintenance channel running through the cells, usually formed by proprietary modules with an open invert, large enough to accept a jetting hose and a camera.
• Access chambers at both ends of each channel, brought to the surface in a position that a maintenance vehicle can reach, not under a parking bay in constant use.
• Rodding or jetting points at changes of direction.
• A silt collection point at the downstream end so that material washed out of the channel has somewhere to settle and can be removed.

Show those on a plan and a long section in the Drainage Strategy, and state the clear internal dimension of the channel. "Maintenance in accordance with manufacturer's recommendations" is not a design.

The deeper point is that only the channel can be cleaned. Silt that migrates into the cells themselves stays there and reduces the storage volume for the rest of the system's life. Which is why pre-treatment is not an optional refinement.

Pre-treatment upstream

Treat everything entering the tank. For a car park or access road, that means an upstream treatment stage sized for the contributing area, typically a hydrodynamic separator, a filter or a sequence of catchpits with sealed sumps, selected using the pollution hazard and mitigation index approach in the industry SuDS guidance.

For roof-only catchments the hazard is lower, but leaves and grit still arrive, so a catchpit with a deep sump at the last manhole before the tank is sensible.

Set out in the drainage strategy, in a short table, the contributing area, its pollution hazard, the treatment stage proposed and the indices claimed. That is the material the lead local flood authority uses to sign off the treatment train, and it is the part most often missing.

An illustrative example: a 14-space car park draining to a cell tank beneath it. The strategy shows a vortex separator upstream, a 450 millimetre maintenance channel through the cells with chambers at each end inside the landscaped verge, and a silt trap downstream of the flow control. The maintenance schedule gives annual inspection, separator emptying every two years, and channel jetting every five. That package answers the question. A strategy with the same tank and no channel does not.

Adoption and maintenance liability

Sewerage undertakers do not generally adopt geocellular systems, and the design and construction guidance that governs adoption is written around pipes and manholes rather than modular tanks. So in almost every case the tank stays in private ownership.

That has to be resolved before determination. The options are a management company funded by estate charges, a maintenance obligation in a section 106 agreement, or, for a single dwelling, a plain obligation on the owner secured by condition. Name the responsible party, the funding mechanism and the inspection frequency in the strategy.

Schedule 3 of the Flood and Water Management Act 2010 would create a different adoption regime through SuDS approving bodies. It had not been brought into force when this was written, so check the current position before relying on it, because it would change the answer on adoption and on mandatory standards.

Failure modes worth naming

Three things go wrong with buried cell systems, and each has a design answer.

Silt accumulation reduces volume quietly, so the system passes its design check on day one and surcharges after a decade. Pre-treatment and a maintainable channel are the answer.

Geotextile blinding at the base of an infiltration system does the same thing to the infiltration rate. The answer is to avoid open-base infiltration where the catchment carries sediment, or to accept attenuation with a controlled discharge instead.

Structural failure from overloading, usually from construction traffic running over a tank built early in the programme with insufficient cover. The answer is a sequencing note in the construction method statement and temporary protection over the tank footprint.

Details to submit

The drainage strategy should carry, as drawings rather than prose: a plan showing the tank footprint, channel route and chamber positions, a long section through the channel with invert and cover levels, a typical construction detail naming the system and load class, and a hydraulic summary showing the storage provided against the volume required for the design events, with the climate change allowance stated.

Alongside that, a maintenance schedule with named responsibilities and intervals, and the pre-treatment table described above. Where groundwater is close to the invert, the position has to be evidenced in the way set out for groundwater flooding evidence, because a tank sitting in groundwater does not store anything.

If the outfall is to a ditch rather than a sewer, the separate consent regime applies and the timing is covered in the note on land drainage consent. The attenuation design also has to be consistent with the conclusions of the site's Flood Risk Assessment, which is where the design events and allowances are fixed.

Draft a drainage strategy on PlanScope, then have a drainage engineer check the levels, the load case and the maintenance route before it is submitted.

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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.