Flooding: management of surface water
Long range forecast
26 September 2014
Last winter’s UK floods have raised fundamental questions over approaches to management of surface water, argues David Mitchell
The damage and mayhem caused by last winter's flooding shocked the nation. With pictures of the Somerset Levels dominating the media agenda for weeks, reporters held nightly vigils next to the swollen river while the authorities battled to stem the advance of the floodwater. These reports sparked considerable debate about the root causes of the devastation and how it can be prevented in the future.
The simple explanation for the flooding was the huge amount of rain falling over a prolonged period. And if, as some are predicting, we can expect this trend for increased rainfall to continue, it is quite right that we should ensure we are as prepared as possible in future. Many reasons were put forward on why the UK struggled to cope with the rainfall last winter, including the deforestation of high areas, reduced river dredging, hard surfacing of gardens, a lack of investment in infrastructure, and housing developments in flood-sensitive areas.
In fact, very few new homes are built on flood plains or areas at risk of flooding. Records show that just 5% of residential land use and 7% of new dwellings were in flood risk areas in 2011, the lowest on record. Considering that only around 115,000 new homes are built in the UK per year, and an estimated 5.3 million existing homes are already in flood plains, these basic facts put into context the negligible effect of new home construction on the UK's wider flooding problems.
As an industry we must continue to ask whether the design parameters set by government agencies are still sufficiently robust
In the few instances where new homes are built on flood plains, extensive mitigation measures are put in place. Flood risk has been a material planning consideration for many years and for the past decade housebuilders have been required under PPS25 to produce a flood risk assessment (FRA) for almost all new developments. Developers have also, since PPS3 was introduced in 2006, been required to produce a sustainable drainage system (SUDS) for most schemes, to minimise the rate and volume of surface water runoff. It should also be pointed out that about 75% of new homes are built on brownfield land and in many instances the redevelopment has reduced the impermeable area leading to reduced surface water runoff.
More generally, developers make significant contributions to local authorities and statutory bodies towards additional infrastructure in the development area – including upgrading and expanding drainage and sewerage systems. Since the privatisation of the water and sewerage sector in 1989, regulation authority Ofwat estimates that the housebuilding industry has paid in excess of £2bn in infrastructure charges to upgrade networks. It would be a useful and effective means of informing debate if the regulator were able to confirm where and how these significant contributions (currently £676 for every new home) have been invested.
Ultimately, it is local authorities that decide where development takes place, and in conjunction with the Environment Agency and other advisory bodies, assess the suitability of a site in flood risk terms, whether the proposed SUDS is adequate, and what other mitigation measures are required.
That said there may be cases where the balance of risk has nothing to do with potential flooding, but everything to do with local and elected member pressure against the release of compensating development land in safer but more 'sensitive' areas. The notion that flood risk can be mitigated by engineered solutions, delivered through additional planning conditions, may in such instances represent an attractive proposition.
The Environment Agency, which came in for heavy criticism during the winter floods, can oppose planning approval if it does not regard the FRA or the mitigation measures proposed as adequate. In the vast majority of cases where it lodged an objection to a development, permission was denied.
Experts will say that the evidence and data relied on in the UK is poor. But as an industry we must continue to ask whether the design parameters set by government agencies are still sufficiently robust when according to recent Met Office data average annual rainfall has increased by around 8% since 1969.
Furthermore, with SUDS we are building in a measure that will place greater reliance on the ground being a major sink for most flooding problems.
Against the background of widespread saturation of soils and rocks, together with unprecedented ground water levels, is infiltration drainage SUDS the right answer? I think it is clear that the time has now arrived for a more comprehensive review of how we measure, manage and finesse surface water management.
David Mitchell is Technical Director at Home Builders Federation