Comparison of UK flood event characteristics with design guidelines

This paper compares the characteristics of observed floods from 206 UK catchments with the design guidelines expressed in the Flood Studies Report (NERL, 1975) and supplementary reports and the Flood Estimation Handbook (Institute of Hydrology, 1999). The characteristics that are compared are the catchment wetness index (CWI), the percentage runoff (PR) and the return period of the causative storm. In general terms, it is found that the observed maxima have a higher CWI and a higher range of values of PR than design events. The use of a catchment simulation model has also shown that the return period of annual maximum floods is generally associated with a storm of lesser return period. These findings provide the basis for an alternative set of design assumptions, which it is believed more closely mirror conditions occurring within observed flood events.