Typical adopted permeable
paving in Oxfordshire

Concrete block permeable paving (CBPP) is an important technique within Sustainable Drainage Systems (SUDS). Essentially, SUDS use a range of techniques to manage surface water by reducing and slowing down run-off while removing many pollutants. Unlike other SUDS techniques, CBPPs combine functions and optimise land-use by providing a structural pavement while also allowing rainwater to infiltrate into the pavement construction for temporary storage. CBPPs are suitable for a wide variety of residential, commercial and industrial applications, handling rainwater from the pavement itself, as well as from roof drainage or other, impervious pavements.

REDUCING FLOOD RISK
Use of SUDS is now a firm planning requirement on developments of all types. Local planning authorities must give priority to SUDS when determining planning applications and developers should incorporate SUDS to reduce flood risks. CBPPs are proving acceptable for adoption by a growing number of local authorities, particularly for residential roads and shared surface areas. Here, it is advisable to organise statutory service runs within areas of non-permeable paving so that the CBPP does not have to be interfered with by service providers carrying out maintenance. These areas of impermeable paving can discharge run-off onto the CBPP for attenuation and treatment as part of the sustainable drainage system.

Oxfordshire County Council has been leading the way with a positive and pragmatic approach to adopting streets and other areas using concrete block permeable paving. Probably the most critical consideration highlighted by Oxfordshire’s experience is the involvement of the adoption officer at the earliest planning stage, as an integral member of the development team. This is essential to avoid late changes demanded to satisfy maintenance and adoption requirements, and avoid problematic design decisions that might compromise the long-term success of a project. Oxfordshire does not charge for the future general maintenance of an adopted highway but only applies a ‘commuted sum’ to developers where special constructions – such as permeable pavements – are used that may require a higher grade of maintenance than usual. But this commuted sum applied by Oxfordshire for concrete block permeable pavements is modest – based on two visits per year for 25 years by a suction sweeper and weed killing once a year over the same period.

CLEANING UP
As well as reducing flood risk, CBPPs and SUDS are particularly useful in improving water quality, as recognised in planning guidance PPS 23 – Planning and Pollution Control, which requires planners to encourage developers to incorporate them into their projects to reduce the impact of diffuse pollution from run-off and flooding. The treated water temporarily stored in CBPPs can then infiltrate to the ground, or be released to a watercourse or other drainage system. Alternatively, the water can be harvested for irrigation or toilet flushing – as demonstrated by a recent school project in Milton Keynes.

Developed by Robert Bray Associates, in association with SEL Environmental and Rainharvesting Systems, this system relies on rainwater collection from low pollution-risk surfaces and the permeable pavement collects direct rainfall, run-off from adjacent hard play surfaces and roof water.

The collected rainfall passes through the joints in the CBPP, bedded on a permeable laying course and a filter geotextile, directly into a geocellular storage box. This polypropylenegeocellular box is enclosed in a second filter geotextile, with a waterproof polypropylene geomembrane to the sides and base, to form an open topped tank. This arrangement filters and treats the water before it passes into storage or overflows to the SUDS system. Cleaned rainwater is delivered, via a pump chamber, from the storage box to a header tank for toilet flushing in the school buildings. All the available rainwater is collected at Hazeley, including the typical 15% wastage experienced with other rain-harvesting systems.

INFORMATION RESOURCE
As well as consumer and regulatory acceptability, developers have much to gain by embracing CBPP in terms of cost, as demonstrated by recent independent research. By taking into account drainage requirements, the economic advantages of CBPP over conventional paving and drainage systems – both in terms of initial construction cost and whole life costs – have been clearly demonstrated. Interpave continues to develop an information resource covering all aspects of CBPP technology, as well as precast concrete paving and kerbs in general. A wide range of publications is available free of charge via the Interpave website www.paving.org.uk including detailed guidance for specifiers on the design, construction and maintenance of concrete block permeable pavements and case studies and a straightforward guide to responsible rainwater management around the home.

John Howe writes on behalf of Interpave.

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