Regional flood frequency estimation using the two-component extreme value distribution

Abstract

The two-component extreme value (TCEV) distribution, suggested for modelling Italian annual flood series (AFS), is shown to account both for the presence of flow outliers and for the high variability of the skewness of historical AFSs. Using such a model, it is found that the T-year flood varies with log(T) much more rapidly for large values of T than is the case for small values of T. The presence of four parameters in the TCEV distribution introduces great uncertainty in estimating the T-year flood when the parameters are estimated from a single series. Regional analysis, which exploits basin similarities, is then needed, not only at ungauged sites but also at gauged ones. Regionalization techniques, by which all flood data over a region are combined to produce a single regional flood distribution, are described and their application to Italian data is shown.