Permeable paving is a SuDS technique that allows rainwater to pass through the paved surface and into a sub-base storage layer beneath, rather than running off into conventional drains. It is classified as a source-control measure in the SuDS management train, managing runoff at the point where it falls.

There are several types of permeable paving:

Permeable block paving: Conventional concrete blocks laid with widened joints filled with permeable aggregate, or blocks with a porous structure throughout
Porous asphalt: An asphalt mix with a high void content (typically 15-25%) that allows water to drain through the surface layer
Porous concrete: Concrete mixed with reduced fines content, creating a permeable matrix
Grass-reinforcement grids: Plastic or concrete grid systems with grass-filled voids, used for overflow car parks and access tracks
Gravel surfaces: Bound or unbound gravel retained in cellular grids

The sub-base beneath permeable paving is constructed from open-graded aggregate (typically Type 3 sub-base) with a high void ratio, providing attenuation storage. Depending on site conditions, the system can be designed as:

Full infiltration: Water soaks into the underlying soil (where ground conditions permit)
Partial infiltration: Some water infiltrates, with the remainder collected by underdrains
No infiltration: The sub-base is lined with an impermeable membrane, and all stored water is released via a flow-controlled outlet

Permeable paving is particularly effective for car parks, residential driveways, access roads, and pedestrian areas. It replaces both the conventional surface and the separate drainage infrastructure, often providing a cost-neutral SuDS solution.

Aegaea specifies permeable paving systems as part of integrated drainage strategies where site constraints and traffic loadings are appropriate.

Permeable Paving