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The costs and impacts of groundwater flooding in the UK are significant and almost certainly underestimated. Recent examples include the widespread disruption and damage for many communities in southern England in the winter of 2000/2001 and the flooding of Chichester in 1993/94. The latest estimates suggest 250,000 properties may be at risk from groundwater flooding. The most vulnerable properties are those located on the exposed Chalk aquifers of southern England. However, there are no currently available methodologies applicable to any of the major UK aquifers that link rainfall to groundwater flood risk.
Groundwater flooding occurs when a highly non-linear hydrological response is triggered by an extreme accumulation of long duration rainfall. Groundwater floods are spatially extensive and of long duration; inundation of properties and disruption of communications can persist for many weeks. However, the most severe effects occur in combination with intense short-duration rainfall, and the frequency of groundwater flooding is probably underestimated as it often coincides and interacts with surface water flooding. The fact that groundwater flooding involves both long term and short duration precipitation means that effects of future climate scenarios are particularly uncertain.
Since modelling of very extreme events cannot rely on data for validation or calibration, understanding and prediction of the groundwater flooding requires improved catchment science. Greater knowledge is required, for example, of flow processes in the unsaturated zone (particularly fracture/matrix interactions), the unsaturated zone/groundwater interface, stream/aquifer interactions, the interactions between surface expression of groundwater and surface runoff generation and the functioning of dry valleys during extreme events. In addition, new models are needed: current groundwater models fail to represent flood response; conversely surface water models fail to represent adequately groundwater systems. An integrated, spatially-distributed, modelling approach is required to represent the relevant processes, interrogate the limited available historical data, and investigate future flood risk. Such models will then be used to investigate the suitability of simpler methods for flood warning and the regional assessment of flood risk.