Computing flood risk in areas protected by flood defences

Risk analysis of areas protected by flood defence systems involves probabilistic analysis of a large number of scenarios in which one or more of the defence sections that make up the system has failed. In systems with large numbers of defence sections, the computational expense of this calculation can be prohibitive. When the probability of failure of each defence section is not negligibly small, sampling approaches that are now in widespread use may not converge on a stable risk estimate in reasonable computational time. To overcome this worrying limitation, this paper reformulates the flood risk calculation in terms of the cumulative distribution function of the volume of floodwater entering a floodplain. An algorithm is presented whose computational expense scales linearly with the number of sections in the flood defence system. The approach is applied to flood risk analysis in areas protected by extensive systems of flood defences in the Thames estuary, revealing how flood risk varies depending on the characteristics of the flood defence system and floodplain topography. It opens up the possibility of more exhaustive risk-based appraisal and uncertainty analysis of flood risk management options than have hitherto been feasible.