SuDS treatment trains for a car park

A SuDS treatment train for a car park exists to deal with what cars leave behind rather than with how much it rains. Attenuation volume and water quality are separate questions answered by separate components, and a scheme that sizes a tank perfectly and pipes car park runoff straight into it will still get a holding objection from the lead local flood authority.

Car park surfaces collect suspended solids from tyre and brake wear, metals including zinc and copper, hydrocarbons from drips and small spills, de-icing salt in winter, and litter. Those pollutants arrive in the first few minutes of a storm, which is why treatment happens at the surface and near the source, not at the end of a pipe.

Why car park run-off needs treatment

Two duties drive it. National policy expects development to avoid pollution of the water environment, and the non-statutory technical standards for sustainable drainage expect runoff to be managed so that receiving waters are protected.

The practical trigger is the receiving water. A discharge to a watercourse, to groundwater by infiltration, or to a surface water sewer that itself discharges to a watercourse all carry a water quality expectation. Discharge to a combined sewer is treated differently because the flow goes to a treatment works, and even then the undertaker will resist surface water entering the combined system at all.

Infiltration raises the stakes. Where car park runoff is soaked away into the ground, groundwater is the receptor, and in a source protection zone the drainage authority and the Environment Agency will both want to see treatment before infiltration rather than relying on the soil.

Pollution hazard levels and treatment indices

The method most authorities expect is the simple index approach set out in the SuDS Manual published by CIRIA. It works by comparing hazard against mitigation.

Each land use is assigned a pollution hazard level, with indices for suspended solids, metals and hydrocarbons. A domestic roof sits at the bottom. Residential parking and low-traffic areas are low. Commercial and retail car parks with frequent vehicle turnover are higher, and service yards, lorry parking and refuelling areas are higher still. Each drainage component then has mitigation indices for the same three pollutants, and the selected train has to deliver mitigation indices at least equal to the hazard indices for the contributing area.

Two features of the method matter in practice. Components in series are additive, with the contribution of the second and subsequent components discounted, which stops a designer claiming full credit for stacking four devices. And the indices are tabulated in the manual rather than derived, so the calculation is a selection exercise recorded in a table. Reproduce the table in your drainage strategy with the hazard level stated for each contributing surface, because a reviewer will check it line by line.

Do not average across a site. A car park, a service yard and a roof have different hazard levels, and the correct approach is to treat each catchment for its own hazard.

Building a treatment train from source controls

Work from the surface downwards, and prefer components that treat and convey at the same time.

• Permeable paving across parking bays, with treatment occurring in the sub-base and the geotextile, which for many general car parks does most of the work on its own
• Filter strips and kerb openings taking sheet flow off drive aisles into adjacent soft landscaping
• Bioretention areas, rain gardens and tree pits within parking islands, which give high mitigation in a small footprint
• Swales along boundaries, conveying and treating, with check dams where gradients allow
• A basin or pond at the downstream end, with a forebay for sediment
• Proprietary treatment devices where no space exists for open components, supported by independent test data rather than manufacturer claims alone

Permeable paving is the component that changes car park layouts for the better. It removes the need for gullies across most of the surface, it provides storage in the sub-base, and it delivers treatment where the pollutant load arrives. The caveats are honest ones: it needs protecting from silt during construction, it is unsuitable directly beneath areas where fuel is delivered, and it requires a sweeping regime to stay permeable.

Parking islands are the least costly treatment you will ever buy. Turning six islands into shallow bioretention cells adds treatment, reduces the piped network, and improves the car park visually, all within a footprint the layout already gave away.

Where an interceptor is and is not enough

Oil separators are widely misused as a substitute for a treatment train. They are not one.

A separator designed to the relevant European standard removes free oil in the flows it is sized for. It does little for dissolved metals, it does not handle fine suspended solids well, and a bypass separator passes higher flows untreated by design. The accepted position is that separators are appropriate where a concentrated hydrocarbon risk exists, such as fuel delivery areas, vehicle washing, or plant and maintenance yards, and that general parking areas should be treated by a SuDS train instead.

Where a separator is proposed, say what it is for and what the train behind it does. A drainage strategy offering a Class 1 separator as its only water quality measure for a supermarket car park will be rejected.

Silt, oil and maintenance access

Treatment components fail by silting up, and they silt up fastest in the first year after construction while the site settles.

Design for that. Put a forebay or a sediment trap at the head of each component so silt collects where a machine can reach it. Keep a hard access route to every structure wide enough for a jetting vehicle. Set inspection covers where a van can park beside them rather than in the middle of a parking bay. And plan for construction phase protection, because the usual cause of a failed permeable pavement is contractors running muddy plant over it before the site is surfaced.

Write the maintenance schedule as a table of tasks with frequencies: sweeping, silt removal, inlet clearance, vegetation management, separator inspection and emptying. That table is what the authority conditions.

Evidence for the drainage authority

The submission should contain five things. A catchment plan showing each surface with its hazard level. The simple index calculation with the components and the resulting mitigation indices. A drainage layout showing the train in sequence. Infiltration or discharge evidence, with groundwater levels and any source protection zone identified. And the maintenance schedule with the responsible body named.

Set all of that in the Drainage Strategy and cross-refer the flood risk elements to the Flood Risk Assessment so the two documents agree on areas and rates. Where harvesting is proposed as part of the scheme, the extent to which it earns attenuation credit is a separate calculation from the water quality one. For a very small development the whole exercise compresses considerably, and the evidence expected for a single new dwelling is a fraction of this.

Conditions on maintenance and inspection

Expect three conditions. One requiring the detailed drainage design to be approved before construction. One requiring a management and maintenance plan for the lifetime of the development, naming the responsible body. And one requiring verification after construction, usually a report confirming the system was built as approved, with photographs and as-built drawings.

The verification condition is the one people trip over years later, when a plot sale stalls because nobody discharged it. Deal with it at practical completion while the contractor is still on site and the evidence is easy to gather.

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