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.