Extreme sea levels in two northern Mediterranean areas

Abstract

The ‘calibrated’ Joint Probability Method (JPM) is used to estimate extreme sea levels at four stations in the northern Adriatic (Trieste, and three observatories in the lagoon of Venice area) and three stations in the Gulf of Lions (Port-Vendres, Sete, and Grau-de-la-Dent in the Rhone delta). The JPM, that involves empirical evaluation of the probability of tide and surge separately, can also be used for short records, but tends to overestimate return levels. A correction is therefore necessary. The correction coefficient Cc adopted is deduced from the dataset, by imposing to the maximum record height to coincide with the return period of the record length. Cc, which appears linked to tide-surge interaction and seasonal effects, is generally less than unit. It calibrates the whole series of extreme estimations to the observed maximum. If Cc is >1, it provides evidence that exceptional surges have occurred during the period considered. In this case its application would lead to overestimation of extreme sea levels and other methods of calculation should be preferred. This is the case, for example, for the Lagoon of Venice. Finally, extreme levels with estimated return times of 2, 10, 50 and 100 years, respectively, are proposed for each site.