A methodology for national-scale flood risk assessment

Risk analysis provides a rational basis for flood management decision-making at a national scale, as well as regionally and locally. National-scale flood risk assessment can provide consistent information to support the development of flood management policy, allocation of resources and monitoring of the performance of flood mitigation activities. However, national-scale risk assessment presents particular challenges in terms of data acquisition and manipulation, numerical computation and presentation of results. A methodology that addresses these difficulties through appropriate approximations has been developed and applied in England and Wales. The methodology represents the processes of fluvial and coastal flooding over linear flood defence systems in sufficient detail to test alternative policy options for investment in flood management. Flood outlines and depths are generated, in the absence of a consistent national topographic and water level data set, using a rapid parametric inundation routine. Potential economic and social impacts of flooding are assessed using national databases of flood-plain properties and demography. A case study of the river Parrett catchment and adjoining sea defences in Bridgwater Bay in England demonstrates the application of the method and presentation of results in a geographical information system.