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GROUNDWORKS & DRAINAGE
instead on planning policies for flood protection. It is clear that the planning- based system has not worked and the Government is now, at last, moving towards implementing Schedule 3 – announced in a 2023 review from Defra – with implementation expected this year. To demonstrate the Government’s ‘SuDS approach’, Defra’s report cited one particular project as an exemplar: Lamb Drove in Cambourne, Cambridgeshire, a monitored SuDS demonstration project. Completed in 2006, Lamb Drove is a 35-homes development on a 1-hectare site, with a conventional layout but still delivering SuDS. It was compared with a similar density, neighbouring development using piped drainage.
Concrete block permeable paving helps deliver SuDS on high-density schemes. Photo: Chris Hodson
Lamb Drove is an integrated SuDS system with predictable performance and is still operating successfully today. It demonstrates various techniques to collect, clean, convey and infiltrate, or store and release water at ‘greenfield runoff’ rates (2 l/s/hectare) from developments to watercourses or sewers. These techniques include green SuDS such as swales, basins and blue/green roofs, and concrete block permeable paving – also recognised as a nature-based SuDS solution. The Lamb Drove monitoring report highlights lower capital costs – saving £314 per property at the time – and lower maintenance costs, with potential for further improvements and savings. It identifies that the scheme successfully attenuates surface water flows and significantly reduces peak flows, with reductions in concentrations of pollutants and other water quality indicators, and with an increase in wildlife species and diversity.
Action on renewal & replacement Moving towards wider use of permeable surfaces will also help to reverse the sealing-up of our towns and cities, highlighted in a November 2022 National Infrastructure Commission (NIC) report on reducing flooding from surface water. This recommends urgent action to move away from impermeable surfaces, both for new developments and in existing settings. The NIC report highlights that, currently, around 325,000 properties in England are in areas at the highest risk (with more than 60% chance of surface water flooding) in the next 30 years and that, without action, up to 295,000 more properties could also be put at risk. It also recognises that, by 2055, some 50,000 – 65,000 properties may be
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put in areas at high risk due to ‘unplanned’ increases in impermeable surfaces, such as paving over front gardens, which increase the volumes of water entering drainage. Although planning permitted development rights for replacement or new paving around existing homes and some other properties have been removed for impermeable surfaces for some time, enforcement and compliance have been limited. A review of options for managing unplanned increases in impermeable surfaces and potential policy changes to prevent it adding to the problem is recommended. Most recently, a number of other agencies are exploring a national strategy for retrofit SuDS, highlighting a catchment-based approach to rainwater management.
Key SuDS technique We should now see a growing application of permeable surfaces, reversing the sealing- up of urban areas. Recognised in The SuDS Manual as the most commonly used ‘Pervious Pavement’ typology, concrete block permeable paving has proven to be a robust, resilient and adaptable technology for all types of paving over 25 years in use in the UK. It is unique as a SuDS technique in combining proven engineering design solutions with nature-based water management near the surface. Of course, safe and attractive hard surfaces are needed in any development. Concrete block permeable paving’s multifunctionality and integration with SuDS can be done without additional land-take. It offers gradual supply of clean, treated water for safe, open SuDS features, landscapes and amenity, and real opportunities for designers to innovate. It can also meet the latest planning requirements for street trees with long- term maintenance, providing diffuse rainwater infiltration and retention, plus free air movement to roots without surface disruption. A new edition of the guide ‘Understanding Permeable Paving’ is available via the website, which offers a comprehensive review of all aspects of concrete block permeable paving.
Chris Hodson is consultant to the trade body MPA Precast (Interpave)
NB for a report on netMAGmedia’s recent round table on SuDS, visit netMAGinsights
www.insights.netmagmedia.co.uk
ADF FEBRUARY 2024
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