Purposefully porous

As a SuDS expert, Chris Griffiths of Marshalls explains why he is committed to bringing the approach into schemes using porous solutions to manage surface water in an increasingly challenging environmental context

The latest State of the UK Climate report shows the weather has become wetter over the last few decades, despite annual variation. Between 2011 and 2020, the UK experienced 9% more rainfall than in any period between the 1960s and 1990s. Moving forwards, we’re expected to see even wetter winters and drier summers. While summer rains will occur with less frequency, they’re likely to be much more intense, according to climate projections.

So where does this leave us when designing flood resilient spaces? And how can we mitigate potential damage and build in resistance to more extreme weather events?

Managing surface water through SuDS
Sustainable Drainage Systems (or SuDS) is a holistic approach for managing excess rainfall. By controlling the passage of water and slowing down surface water run-off, SuDS can collect rainwater at the source. Here it can either infiltrate into the ground or evaporate, helping to minimise future flood events. A mix of guidance and legislation is pushing for greater inclusion of SuDS into urban developments, not just for the proven drainage advantages but for the many other benefits that the systems provide. These include improving water and air quality, supporting biodiversity, and creating more appealing spaces for people to live, work and play.

Permeable paving = source control
Well-designed drainage solutions should consider a range of measures to impact water flow rates, from soakaways and infiltration trenches to rainwater harvesting and permeable paving.

Permeable paving products offer a load-bearing surface coupled with source control attenuation for pavements and roads, car parks or public realm schemes, preventing surface water run-off by collecting rainfall where it lands. Porous paving solutions provide an ideal dual-purpose SuDS method; encouraging water to soak down through the surface to be stored in a specially prepared sub-base.

As well as cleansing the attenuated water (via both mechanical filtration and natural biological processes) the use of permeable paving also prevents water pooling, avoiding puddles and ice which can present a health and safety risk.

Tackling a myth
As with all innovations, some initial myths still surround the use of permeable block paving which causes some specifiers to hesitate. One such belief is that the voids on the surface (the specially widened joints between the blocks) will get blocked up with debris within a few years, preventing water run-off and creating a drainage issue.

In practice, this isn’t the case. Apart from the evidence of thousands of highly effective, still-functioning systems installed all over the UK, a wide range of academic studies have been published that disprove this myth. These include the 2006 study of Soenke Borgwardt, which found that although infiltration rates do indeed decline over time, after around 10 years they plateau at a level which still provides more than adequate infiltration rates to cope with adverse weather events. The 2018 research report of Luis Santano-Fonedua et al, similarly confirms these findings. This study demonstrates via live schemes that a concrete block paving permeable pavement should continue to provide infiltration rates that exceed the demands of the heaviest rainfall well beyond its design life.

Flood protection to support SuDS
While ‘designing in’ permeable paving solutions early in the process can make a significant (and very cost effective) impact on water management, specifiers shouldn’t neglect other defensive measures. The SuDS standards recognise that all water management systems, philosophies and products should be carefully considered to ensure the most appropriate solution for the job. For example, flood protection methods may be necessary in areas at risk of flooding in lower down catchments, using barrier systems to block water and prevent it from entering or damaging property.

These measures can be permanent (such as retaining walls) or temporary, in the form of flood barriers. Temporary measures should be passive, operating automatically when needed, without manual intervention.

Flooding is the biggest environmental threat facing the UK today. Used in combination, flood defence and SuDS measures bring the joined-up approach needed to tackle it successfully.

Chris Griffiths is head of product sustainability at Marshalls and a Code for Sustainable Homes assessor