Permeable paving in a planning drainage strategy

Permeable paving earns its place in a planning drainage strategy when it does three jobs at once: it removes an impermeable area from the calculation, it provides storage in the subbase, and it treats runoff before it goes anywhere. It fails when it is drawn as a hatch on a layout with no section, no maintenance regime and no thought about where the silt will come from.

Reviewers at lead local flood authorities see a lot of the second kind. The difference between a scheme that gets a condition discharged first time and one that goes round twice is usually four drawings and a maintenance table.

Where it works and where it does not

It works on car parks, private driveways, courtyards, service yards with light traffic and footways. The surface can be block paving with permeable joints, porous asphalt, reinforced grass or gravel systems, and the hydraulic function sits in the subbase rather than the surface.

It does not work in several situations that come up constantly.

Steep areas are the first. Where the surface falls more than about 5 per cent, water runs across the paving rather than through it and drains to the bottom of the subbase, so the storage is only usable at one end. Baffles or check dams across the subbase fix it, at a cost.

Run-on from soft landscaping is the second. Soil, bark mulch and unbound gravel washing onto permeable paving is the fastest way to clog it, so the levels have to fall away from planted areas rather than towards them.

Heavy goods vehicle areas are the third. Permeable construction can be designed for heavier loading, but the pavement design has to be done properly by an engineer rather than by adopting a standard driveway build-up.

And infiltration is ruled out where the ground will not take it: shallow groundwater, contaminated ground where infiltration would mobilise a pollutant, made ground of unknown composition, or a Source Protection Zone constraint. A Geotechnical Assessment with in-situ soakage testing at the right depth answers that question, and a soil description alone does not.

The three system types

Say which type you are using in the drainage strategy and show it in section. Reviewers ask this question on almost every scheme that omits it, because the type determines whether the infiltration testing, the groundwater monitoring or the flow control is the thing they need to check.

Subbase, laying course and geotextiles

The storage sits in the voids of a clean, angular, coarse graded aggregate subbase. A typical void ratio is around 30 per cent, so a 400 millimetre subbase gives roughly 120 millimetres of storage depth across its area. Use the figure from the actual material specification rather than a default.

Subbase depth is set by whichever governs, the structural pavement requirement or the hydraulic storage requirement. On a car park with a modest catchment the structural depth often exceeds the hydraulic need, which means the storage is close to free. On a scheme where the paving is taking runoff from adjacent roofs, the hydraulic requirement usually wins.

Geotextile selection is where specifications go wrong. A separation geotextile above the subgrade in an infiltrating system keeps fines out of the subbase without impeding downward flow. A tanked system needs an impermeable geomembrane, welded and protected, with the outfall detail drawn.

The layer immediately under the laying course is the one to think about. Current practice generally favours letting sediment be captured at the surface, where it can be swept off, rather than trapping it in a geotextile a hundred millimetres down where nobody can reach it. Follow the manufacturer's system detail and CIRIA C753, and say in the strategy which you have followed.

Gradients, edges and distance from buildings

Keep surface gradients gentle and design the subbase as a series of level cells where the site falls. Each cell holds its share of the storage, and without them the water finds the low corner.

Edge restraint is a structural requirement rather than a detail. Permeable block paving relies on interlock, and an unrestrained edge unravels under turning traffic within a few seasons.

Keep infiltration away from structures. The conventional minimum separation between an infiltration feature and a building is five metres, and that distance should increase where there are basements, retaining walls or shallow foundations. Where the paving sits closer than that, tank it and take the flow to a controlled outfall.

Adjacent levels matter as much as the paving. A permeable car park that receives sheet flow from an adjoining impermeable road is taking a much larger catchment than the drawing suggests, and the calculation has to include it.

Silt, the failure mode nobody budgets for

Permeable paving fails through clogging far more often than through structural failure, and most of the silt arrives during construction.

Three moves prevent it. Lay the paving late in the programme, after landscaping and after the bulk of construction traffic has finished. Where that is not possible, protect it and plan to remove and replace the jointing material and the top of the laying course before handover. And keep site haul routes off it.

In service, the regime is modest but has to happen. An annual inspection, sweeping once or twice a year with equipment that lifts material out of the joints rather than pushing it in, and periodic infiltration testing to confirm performance. Where the surface has slowed, replacing the jointing material usually restores it.

Put the regime in a maintenance schedule with frequencies, tasks, responsibility and a funding route. That table is what the lead local flood authority is checking, and it belongs in the Drainage Strategy rather than in a separate document nobody reads.

Construction phase protection should also appear in the Construction Management Plan, because the two documents are read by different officers.

Adoption and who maintains it

This is the question that delays discharge of conditions.

Highway authorities are often reluctant to adopt permeable paving on adopted highway, which pushes it into private ownership or a management company. Water and sewerage undertakers can adopt some sustainable drainage components under current sewerage sector arrangements, and the criteria are specific.

The remaining routes are a management company funded by an estate charge, a residents' management company, or retention by a commercial owner. Whichever applies, name it, say who pays, and show the access arrangements for maintenance vehicles.

Schedule 3 to the Flood and Water Management Act 2010 would change this picture in England by introducing SuDS approving bodies and a separate approval process. Its commencement has been announced and deferred more than once, so check the current position before assuming it applies to your programme.

What to show on the drainage layout

1. The extent of permeable paving hatched, with the system type labelled.
2. A typical section showing surface, laying course, subbase depth, geotextile or membrane, subgrade and any perforated pipe.
3. The contributing area draining to the paving, including any adjacent hard surfaces or roofs.
4. Cell divisions and baffle positions where the site falls.
5. Edge restraint details and levels at the interface with soft landscaping.
6. The flow control or outfall location, with the exceedance route marked.
7. A maintenance schedule table with tasks, frequency and named responsibility.

Cross-refer the whole component back to the storage calculation in the Flood Risk Assessment so the reviewer can see the volume you have claimed and the volume the section delivers.

This is general guidance, not legal advice. Drainage design should be prepared or checked by a suitably qualified engineer, and for specialist technical reports qualified professional input may be required.

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