Multivariate probabilistic regional envelopes of extreme floods

Recently probabilistic regional envelope curves of extreme floods (RECS) were introduced along with an estimator of the exceedance probability of a REC that accounts for the impact of correlation among flood sequences. The dependence of traditional envelope curves on drainage area alone impacts their reliability for estimating the design-flood. We introduce multivariate regional envelopes (MVEs) of extreme floods which are envelope surfaces (or hyper-surfaces) that represent the bound on our flood experience in a region in terms of geomorphologic and climatic basin descriptors. An empirical MVE is derived for a group of 34 unregulated catchments located in northern-central Italy. A cross-validation procedure that simulates ungaged conditions at every site in the region is used to compare the reliability of design flood estimates resulting from REC, MVE and an index-flood approach. We document that MVEs outperform RECs and provide flood quantile estimates at ungaged sites that are nearly as reliable as index flood quantiles. (c) 2007 Elsevier B.V. All rights reserved.