Advances in Regionalising Flood Probabilities

Abstract

Flood regionalisation methods are used to estimate floods of a given excedance probability, such as the 100-year flood, in ungauged catchments, i.e. catchments, where no local streamflow data are available. They can also be used to improve flood estimates from local data in gauged catchments. The main idea of flood regionalisation methods is to transfer flood information from hydrologically similar gauged catchments (i.e. donor catchments) to catchments, where flood statistics have to be estimated (i.e. target catchments). Different methods for transferring flood information from donor to target catchments have been proposed in the literature. The first type consists of pooling hydrologically similar catchments into a homogeneous group and subsequently using averages within each pooling group to estimate flood probabilities in target catchments. Examples of this type are the Index Flood Method and the Region of Influence approach (ROI). The second type of methods is the application of multiple regressions between flood statistics and catchment attributes. The rationale of this approach is that catchments with similar attributes are also likely to exhibit similar flood generation processes and hence may also behave similarly in terms of their flood frequency response. Catchment attributes include catchment size, land use, geology, elevation, soil characteristics as well as climate variables such as mean annual precipitation. The third type is the application of geostatistical methods, which use spatial proximity as a measure of hydrological similarity. This chapter summarizes the most important methods and recent findings from the literature with a focus on the ungauged catchment case for which flood frequency regionalisation is particularly difficult. For any practical application, the interest resides in how well flood statistics, such as the 100-year flood, can be estimated for a given catchment. Results from some intercomparisons of regionalisation methods are given.