Abstract
Abstract. A good knowledge of extreme storm surges is necessary to ensure protection against flood. In this paper we introduce a methodology to determine time series of skew surges in France as well as a statistical approach for estimating extreme storm surges. With the aim to cope with the outlier issue in surge series, a regional frequency analysis has been carried out for the surges along the Atlantic coast and the Channel coast. This methodology is not the current approach used to estimate extreme surges in France. First results showed that the extreme events identified as outliers in at-site analyses do not appear to be outliers any more in the regional empirical distribution. Indeed the regional distribution presents a curve to the top with these extreme events that a mixed exponential distribution seems to recreate. Thus, the regional approach appears to be more reliable for some sites than at-site analyses. A fast comparison at a given site showed surge estimates with the regional approach and a mixed exponential distribution are higher than surge estimates with an at-site fitting. In the case of Brest, the 1000-yr return surge is 167 cm in height with the regional approach instead of 126 cm with an at-site analysis.
Highlights
The impact of astronomical tide and radiational tide on the sea is theoretically known
The integration or not of the surges at Saint-Jean-de-Luz to the regional data set does not compromise the homogeneity of the region, the impact being negligible on the estimates of the extreme surges
The region composed by the sites along the Atlantic coast and the Channel coast seems to be robust, with surge estimates that do not significantly depend on the sites chosen to be integrated to the regional data set
Summary
Combined with other extreme events (high-tide, high waves, intense precipitation), a surge can contribute to flooding or destruction of coastal facilities (Gerritsen, 2005; De Zolt et al, 2006). A skew surge at more or less an hour is the difference between the maximum of the hourly observed levels and the relative maximum of the hourly predicted levels These surges are not calculated by interpolation of the tide signals, which would make it possible to determine the real highest levels of tide as illustrated on Fig. 5. Non-significant data (tide gauge malfunction, bad transcription) have been eliminated for the highest surges (higher than 30 cm) owing to a visual analysis of the time series of surges and sea levels No replacement of these data has been done, such as with a linear interpolation from neigbouring data (Vilibicand Sepic, 2010), in order to only capture the “real” data. A qualification file validates the main functions and the results obtained with the program, in particular by comparing with the results of another statistical software (ASTEX) in collaboration with the Centre d’Etudes Techniques Maritimes et Fluviales (CETMEF)
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