Estimating floods in permeable drainage basins

Basins with permeable soils and geology pose problems for flood frequency analysis. Several approaches to statistical flood estimation are reviewed which can account for the infrequency of flooding on rivers draining permeable basins. One method which makes efficient use of annual maximum flow data is based on conditional probability. This technique can be applied to single sites or to regional flood estimation, by allowing for the presence of years with no substantial flood. The approach is suited for use in cases where L-moments are used for flood frequency estimation. Examples illustrate that this can avoid underestimation of design floods. The method forms part of the new UK Flood Estimation Handbook procedures.