SuDS design for catchment flood protection – are current criteria appropriate?

Current criteria on the design of runoff storage for developments can result in surface water attenuation storage volumes in excess of 1000m3 or more per hectare which is of the order of 20m3 to 40m3 per property for large developments. The use of up to 40% increase in rainfall intensity, based on the latest guidance from the Environment Agency, for addressing expected future climate conditions, results in volumes being twice what they would be if designed for the present day. The cost of the attenuation storage is therefore a very significant component of development construction costs. Considering the expected investment in housing stock is of the order of 250,000 dwellings per year for many years ahead, this is a massive investment. So it is therefore reasonable to ask: Is there evidence that this investment is effective in protecting catchments from flooding? Are the design rules suitable for addressing flooding problems in the specific catchments in which the developments are located? Are methods of assessing runoff from small development sites accurate? If there is any doubt that current methods are not cost-effective in achieving the intended goal of effective catchment flood protection, then appropriate research should be carried out and a new approach should be developed to provide the appropriate control of site runoff which minimises this investment in storage, is targeted at catchment specific issues and uses suitable technology. This presentation will explain how drainage design criteria have evolved in the last 30 years, discuss the limitations these have and explore possible options for a new approach