Floods SuDS and Permeable Paving

The strengthening link between flooding and climate change highlights the importance of sustainable drainage – or SuDS – on developments. But, as Chris Hodson of Interpave argues, it is becoming clear that well-designed, multifunctional SuDS actually cost less as well.

Development and urbanisation have increased sealed surfaces such as paving and roofs, dramatically reducing the capacity for natural, sustainable drainage. In rural areas with natural drainage, only 5 per cent of the surface water finds its way directly into watercourses, whereas in densely populated urban areas up to 95 per cent of rainfall becomes runoff, captured in deep, piped systems and placing increased pressures on already overburdened drainage systems.

This problem is exacerbated by climate change which – it is generally accepted – will result in milder and wetter winters and hotter summers, with more intense and frequent rainfall events. Since the 1960s, there has been a 50 per cent increase in the number of 3 consecutive day storm occurrences, resulting in continuing growth in the volume of runoff which we have to handle. And the rate at which it runs off is also much faster. When designing developments today, rainfall intensity should be increased to cater for climate change – typically by 30 per cent over the 1 in 100-year rainfall intensity required by the regulators.

Multifunctional SuDS

In contrast, SuDS replicate greenfield (pre-development) conditions with natural losses, slow flows and reduced discharge rates. They should also improve water quality by removing pollutants and provide amenity (including biodiversity) through multifunctionality. Well-designed SuDS incorporate exceedance routes to provide climate change resilience in extreme weather events. They also help mitigate against climate change with planting and green spaces, reducing urban heat island effects.

It’s over 3 years since the Government chose to abandon dedicated requirements for SuDS on new developments in England using the 2010 Flood and Water Management Act, despite the Committee on Climate Change’s 2014 Adaptation Progress Report. This stressed that increased flood risk is the greatest threat to the UK from climate change and called for the Act to be implemented without further delay. Instead, the Government now relies on planning policies, implemented at local planning authority level. However, a recent Landscape Institute and CIC Report highlights that this approach has been far from successful and merits rethinking.

In contrast, earlier this year the Welsh Government did implement Schedule 3 of the Act as intended, with measures including establishment of a SuDS Approving Body (SAB) within each local/unitary authority. SAB approval will be needed for all but the smallest developments, and the SAB will be required to adopt and maintain approved SuDS that serve more than one property.

The Welsh National Standards spell out that: “The SuDS approach mimics natural drainage, managing surface runoff at or close to the surface and as close to its source as practicable. Surface based sustainable drainage components are visible in their operation and performance and are generally simpler and easier to operate, monitor and maintain.”  By its very nature, concrete block permeable paving (CBPP) is uniquely placed to satisfy these requirements as an attractive, surface-based, multifunctional SuDS technique.

Avoiding land-take

Of course, developments need paving anyway, but CBPP also provides an inherent drainage system with no additional land take or dedicated constructions for water storage, treatment or conveyance. In fact, CBPP can also accept runoff from other impermeable paving and roofs, giving further savings.

It is therefore well-suited to higher-density urban schemes. For example, at a high-density (104 units/hectare) town-centre housing scheme in Stamford, most public areas between buildings are CBPP, which also accepts runoff from other hard areas and roofs. Clean water then passes gradually from the paving directly into planted rills and canals, adding interest and much-needed greenery to the courtyard environment, before passing into the nearby river.

CBPP addresses both flooding and pollution issues by storing and cleaning water runoff at source. It removes water-borne pollution and provides a gradual flow of clean water at the head of the ‘SuDS management train,’ enabling safe, open SuDS features on the surface, and enhancing landscape design and biodiversity. CBPP also eliminates the cost of pipework, gulleys and manholes.

Cost savings

These and other savings resulting from SuDS are demonstrated by the Lamb Drove SuDS Monitoring project in Cambridgeshire. The SuDS design optimises natural flow routes through the site for low and high flows, as well as for exceedance. SuDS have been integrated with landscape design adding amenity, interest and biodiversity to a conventional housing layout with no reduction in density. The scheme is also popular with residents.

The project assesses various SuDS techniques – including CBPP – in a management train, compared with a conventional piped drainage system nearby. The Monitoring Report noted that capital cost savings of £314 per home were achieved using SuDS, with further potential savings available when SuDS are integrated with layout design from the start.

Chris Hodson is architect and consultant to the trade association Interpave