How reliable are design flood estimates in the UK?

Design flood estimates in the UK are routinely obtained by using the improved Flood Estimation Handbook (FEH) statistical procedure. This paper presents a practical framework for assessing the uncertainty associated with estimates of the index flood (QMED) obtained for a range of commonly encountered problems: the gauged, the ungauged and the sparsely gauged catchment. An assessment is presented of the uncertainty of design flood estimates when estimated at ungauged catchments for a range of return periods. The results show that the inclusion of data from nearby gauged catchments increases the reliability of the estimates when compared to an automated application of the improved FEH methods relying on catchment descriptors only.