Estimation of return periods for extreme sea level

Authors: Paolo Antonio Pirazzoli Alberto Tomasin

Publish Date: 2007/01/24

Volume: 57, Issue: 2, Pages: 91-107

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Abstract

The joint probability method JPM to estimate the probability of extreme sea levels Pugh and Vassie Extreme sealevels from tide and surge probability Proc 16th Coastal Engineering Conference 1978 Hamburg American Society of Civil Engineers New York pp 911–930 1979 has been applied to the hourly records of 13 tidegauge stations of the tidally dominated Atlantic coast of France including Brest since 1860 and to three stations in the southwest of the UK including Newlyn since 1916 The cumulative total length of the available records more than 426 years is variable from 1 to 130 years when individual stations are considered It appears that heights estimated with the JPM are almost systematically greater than the extreme heights recorded Statistical analysis shows that this could be due 1 to surge–tide interaction that may tend to damp surge values that occur at the time of the highest tide levels and 2 to the fact that major surges often occur in seasonal periods that may not correspond to those of extreme astronomical tides We have determined at each station empirical ad hoc correction coefficients that take into account the above two factors separately or together and estimated return periods for extreme water levels also at stations where only short records are available For seven long records for which estimations with other computing methods eg generalized extreme value GEV distribution and Gumbel can be attempted average estimations of extreme values appear slightly overestimated in relation to the actual maximum records by the uncorrected JPM +167 ± 72 cm and by the Gumbel method alone +103 ± 63 cm but appear closer to the reality with the GEV distribution −20 ± 53 cm and with the bestfitting correction to the JPM +29 ± 44 cm Because the GEV analysis can hardly be extended to short records it is proposed to apply at each station especially for short records the JPM and the sitedependent ad hoc technique of correction that is able to give the closest estimation to the maximum height actually recorded Extreme levels with estimated return times of 10 50 and 100 years respectively are finally proposed at all stations Because astronomical tide and surges have been computed or corrected in relation to the yearly mean sea level possible changes in the relative sea level of the past or foreseeable in the future can be considered separately and easily added to or deduced from the extremes obtained Changes in climate on the other hand may modify surge and tide distribution and hence return times of extreme sea levels and should be considered separatelyThis work was partly funded by the DISCOBOLE Project French Government Ministère de la Recherche and Ministère de l’Ecologie et du Développement durable We thank U Dornbusch for providing tidegauge data for British stations and B Simon for useful discussions The constructive criticism of four unnamed referees has been very useful to improve this paper The work for editing by Ms Jane Frankenfield was particularly appreciated

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