Innovative Retrofit Permeable Paving for SuDS

A deceptively simple concept, concrete block permeable paving overlays can transform old streets and other hard surfaces as part of low-intervention renewal or regeneration schemes, delivering sustainable drainage and opportunities for urban trees – as Interpave explains.

Concrete Block Permeable Paving (CBPP) is a well-established, multi-functional SuDS technique. It simply combines self-drained, safe and attractive surfaces with attenuation, storage, pollution treatment and conveyance of rainwater runoff. CBPP can also accept runoff from adjacent impermeable paving and roofs up to twice its own area.

By its very nature, CBPP requires no additional land-take for water storage or management, and no gulleys or related pipework. Its unique capabilities include source control and delivering a gradual flow of clean water to open SuDS features, for amenity or biodiversity, to drainage systems or into the ground.

More than 25-years usage has proven it to be a robust, resilient and adaptable technology, used on projects ranging from footpaths to container terminals, with the reassurance of proven design solutions. Both construction and whole-of-life costs of CBPP have been shown to be lower than for conventional paving and drainage, and it requires only limited, straightforward maintenance without clogging problems.

Innovative Overlays

Concrete Block Permeable Paving Overlay is an innovative approach to retrofitted CBPP, delivering SuDS as part of low-intervention paved asset renewal or regeneration. The CBPP upper layer is simply applied as an overlay replacing old asphalt or other road surface, on the original structural road base. In many cases, this can simply run kerb-to-kerb to form a shared surface level with the footway and flush kerb top. This approach also enables low-cost improvement works – perhaps as part of creating traffic calming, Low Traffic Neighbourhoods or Homezones.

The same blocks and grit bedding layer and jointing material as for permeable pavements generally are used. Water is attenuated, treated and conveyed within the laying course – which is key to this technology – enabling filtration of silt and retention/treatment of pollutants without clogging. A gradual flow of clean water can then be released near the surface, meeting SuDS requirements.

This approach provides numerous benefits including:

•Interception losses, managing runoff during regular rainfall events

•Attractive, popular surfaces with no puddles or potholes, for user-safety

•Maximising re-use of existing road-base and its embodied carbon

•Optimising the original drainage regime, but below the surface

•Low-intervention, low carbon, no-heat retrofit installation

•Long-life with minimal maintenance and low whole-of-life cost

•No open gulleys – wildlife-safe

•Integral water filtration, with debris/litter remaining on the surface

•Straightforward access to below-ground services

•Sustainable re-use of blocks for reinstatement or changed layouts.

Green Infrastructure

Concrete block permeable paving overlays and trees have been proven to work in synergy. CBPP can collect rainfall away from the canopy and convey it to the tree. It can then simply discharge horizontally into a raingarden with trees, perhaps with overflow into an existing adapted gulley. The raingarden stores water during heavy rain for SuDS, retains soil moisture during dry weather and provides additional water quality ‘polishing’, as well as irrigation.

Alternatively, CBPP can be used over standard tree pits, proprietary tree planters, Stockholm System or other structural soil installations – enabling irrigation and simple gas (oxygen/carbon dioxide) exchange essential to trees, without additional reservoirs or pipes. CBPP also avoids tree root disruption common with other paved surfaces.

For project case studies and guidance on all aspects of permeable paving and SuDS visit: www.paving.org.uk