THE EFFECTS OF NON-STATIONARITY ON THE EVALUATION OF CRITICAL DESIGN STORMS

The critical storm is generally carried out to design urban drainage s ystems and other flood management works starting from the available hi storical information. Its evaluation associated with a fixed return pe riod is usually obtained by fitting the annual maxims of the rainfall depth with an extreme value distribution. This statistical procedure, however, leads to dubious results when the data present a non-stationa rity, induced for example, by a long-term variability. To assess the e ffects of non-stationarity, four daily rainfall series observed in Ita ly, with at least 90 years of continuous data, are analysed here. For each record and each year of the observation period, critical design s torms are estimated fitting the annual maxima collected in the past, s o allowing us to assess the progress of the design storm along time. F our different extreme value distributions are used. The results show t hat an analysis of non-stationarity is required when urban drainage sy stems and other hydraulic engineering works are designed. (C) 1998 Pub lished by Elsevier Science Ltd. All rights reserved.