SUDS usually cost around the same amount as traditional drainage solutions

Following the catastrophic floods that affected many parts of England in the summer of 2007, much of the immediate focus was on the apparent inadequacies of current flood prevention infrastructure, with much debate as to whether additional funding should be given to prevent similar events from occurring in the future. Understandably, the roles of flood barriers and the decisions allowing the urbanisation of flood plains were scrutinised, particularly as most climatologists expect extreme rainfall events to occur more frequently in the future.

It is a lesser known fact however, that although ‘targeted’ flood prevention schemes such as the deployment of flood barriers in urban centres will continue to be of prime importance, river catchment management schemes can play a significant role in improving the UK’s resistance to extreme weather events.

If flood defences are overwhelmed, then communities that have a level of resilience following floods are a pragmatic and affordable way of minimising the impact of extreme weather on the threatened area.

At the catchment scale, nothing can be done to prevent an extreme rain event from occurring. The factors in a flood that are controllable by people that directly contribute to the severity of a flood such as, increases in impervious surfaces and building schemes in high flood risk areas, usually act to maximise the impact of an extreme event on the receiving area. In the context of a holistic catchment view of flooding, flood defences such as barriers and the raising of doorsteps by several centimetres are only reactive measures.

If good catchment management, (or preventative flood protection) had been deployed upstream of the flooded area then the contribution of human activity to the extreme event could have been minimised.

 “Often it is not widely known that events that have occurred several miles away, up or downstream from a flooded area, have had a direct impact on the river catchment.” Dr Stephen Coupe, research manager at Hanson Formpave said. “In general terms, a piped drainage system or an impermeable surface promotes the rapid transit of water from source to discharge point. In contrast, SUDS delay, detain and minimise both the total volume of water discharged and the rate of flow of water from one location to another.”

If the water in a storm event can be released slowly and consistently from a pond or can be allowed to infiltrate into groundwater rather than run off, then an individually small but significant difference can be made to local drainage. When good drainage practice is adopted over many sites, the potential for a radical improvement in flood risk is realised.

The installation of SUDS is now promoted as good practice by the Environment Agency, and both PPS 25 and EU Water Framework Directive catchment management plans, are expected to make the use of SUDS ever more of a standard planning consideration when an urban development is proposed.

SUDS usually cost around the same as traditional drainage solutions when comparisons are made between them in the longer term. Although up front costs may be higher, it would be an unusual site that could not afford to incorporate some SUDS technology into a drainage system. Indeed, even the use of grassed areas to promote local infiltration, usually specified on a new development, is a good example of cheap yet effective good practice.

As with any traditional drainage system, SUDS schemes do need maintenance but the cost can be minimised by clear design guidance and correct installation. It is clear that SUDS are now a credible alternative to conventional drainage solutions and also offer a higher level of flood protection than reactive flood prevention.

As an example, a large urban car park, if surfaced by a pervious material such as permeable concrete block paving, can offer a durable, attractive running surface for pedestrians and cars whilst not exporting the problems of draining the water to an off site location. A permeable pavement allows water to infiltrate where it falls and with an overall pavement depth of 500 mm can store up to 1 m3 of water in only 10 m2 of area.

Even under extreme weather, if the storage capacity of the pavement was to be exceeded then the drainage of any standing water into groundwater would be faster in a permeable system than if the car park had been surfaced by an impermeable material, leading to a faster recovery of the site for normal services to resume and crucially, not exporting the problem downstream.

This example demonstrates some of the concepts behind flood resilience; recognising that extreme events will happen but mitigating the effect on the local population and contributing to a progressive, integrated catchment management plan to minimise wider flood risk.

Where a new development is planned and there is a concern over the possibility of flooding in the future, it may be possible to obtain planning permission for the site if SUDS are included in the design. This will hopefully permit a good level of development and economic activity where otherwise it would be impossible, something that must be considered in the light of the government’s plans to increase the supply of new housing, in potentially at risk areas of the UK.

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