RECONCILING THE government's demand for high density housing with its requirement that rainwater runoff from new developments be managed using sustainable drainage systems (SuDS) is impossible, a leading hydraulic engineer warned this week.
There is an irresolvable conflict between government policy planning guidance documents PPG3 and PPG25, said HR Wallingford principal engineer Richard Kellagher, speaking at an Association of London Graduates & Students (ALGS) meeting this week.
PPG25, which gives guidance on building in flood plains, dictates that on all new housing developments rainwater should be drained to a piece of SuDS infrastructure (NCE 21 March).
PPG3, which sets out to combat suburbia's march on the UK's greenbelt, commands that housing is in future built at densities greater than 30-50 dwellings per hectare.
But according to Kellagher, it is impossible to shoehorn SuDS into developments of this density. HR Wallingford has been commissioned by the Department of Trade & Industry to report on the problem.
Under PPG25 regulations, runoff from roofs must typically be discharged into a soak-away or infiltration bed, and runoff from paved surfaces into swales (shallow ditches), retention basins or ponds.
The aim is to minimise the amount of water flowing into conventional piped sewers, which are often overwhelmed in periods of heavy rainfall, and into flood-prone water-courses.
SuDS also trap water-borne pollutants, improving water quality.
But 'you can't get 30 houses per hectare if you put in swales. The land take is too great, ' Kellagher said.
In the UK, retaining runoff at peak winter rainfall intensities, and achieving the settlement and filtration needed to remove pollutants such as oils, rubber dust and organic matter, calls for relatively large structures to make SuDS effective.
Meanwhile, the government's planning guidance requires SuDS to be at least 5m from buildings.
This means cutting back on garden space offered to home owners, or forcing people to have swales or ponds in their gardens, which throws up major maintenance problems. Private owners often do not understand what SuDS do and fill them in, or fail to clear them of vegetation, reducing their efficiency, Kellagher said.
It only becomes practical to incorporate SuDS at 'traditional suburban' densities, Kellagher claimed. In the South East, suburban estates average 23 dwellings per hectare.
Incorporating SuDS into low density suburban landscapes would also boost biodiversity by providing seminatural habitat, Kellagher added.