The estimation of sea levels corresponding to high return periods is crucial for coastal planning and for the design of coastal defenses. This paper deals with the use of historical observations, that is, events that occurred before the beginning of the systematic tide gauge recordings, to improve the estimation of design sea levels. Most of the recent publications dealing with statistical analyses applied to sea levels suggest that astronomical high tide levels and skew surges should be analyzed and modeled separately. Historical samples generally consist of observed record sea levels. Some extreme historical skew surges can easily remain unnoticed if they occur at low or moderate astronomical high tides and do not generate extreme sea levels. The exhaustiveness of historical skew surge series, which is an essential criterion for an unbiased statistical inference, can therefore not be guaranteed. This study proposes a model combining, in a single Bayesian inference procedure, information of two different natures for the calibration of the statistical distribution of skew surges: measured skew surges for the systematic period and extreme sea levels for the historical period. A data‐based comparison of the proposed model with previously published approaches is presented based on a large number of Monte Carlo simulations. The proposed model is applied to four locations on the French Atlantic and Channel coasts. Results indicate that the proposed model is more reliable and accurate than previously proposed methods that aim at the integration of historical records in coastal sea level or surge statistical analyses.
Coastal planning implies the estimation of extreme sea levels. As the distribution of astronomical high tides can be predicted, most recent publications suggest focusing on the estimation of extreme skew surges. Historical information, record sea levels observed before the beginning of systematic tide gauge recordings, can improve estimations. The corresponding skew surges can be estimated but are not necessarily exhaustive. Indeed, some historical extreme skew surges can remain unnoticed if they are combined with low or moderate tides, or for a variety of reasons. To deal with this exhaustiveness issue, a previous publication proposed an unbiased method for combining systematic period skew surges with historical period extreme sea levels. This method appeared more reliable than previously proposed approaches. The present study aims at presenting a broader evaluation of this method, based on its application to nine sites located on the English Channel and North Sea coasts. The method is also improved to consider several historical periods and various types of historical information. Results confirm the method to be reliable, useful, and relevant. A number of recommendations is also formulated for the selection and use of historical information for sea level frequency analyses.
<p>The estimation of the sea levels with a high return period is crucial for coastal planning and the assessment of coastal flooding risk. Coastal facilities are designed to very low probabilities of failure and hence the design values are affected by significant uncertainties. Some recent coastal floods due to exceptional surges suggest that the design performed with the current statistical approaches may sometimes be significantly underestimated. This presentation is a contribution to the use of historical observations to improve the estimation of extreme sea levels. Historic records consist in observed major sea level values. The corresponding skew surges may be estimated but the exhaustiveness of historical skew surges, which is an essential criterion for an unbiased statistical inference cannot be guaranteed. . Indeed, Extreme skew surges can easily remain unnoticed if they occur at low or moderate high tide and do not generate extreme sea levels. This study proposes to combine, in a single Bayesian inference procedure, series of measured skew surges for the recent period and extreme sea levels for the historic period. The method is tested on four sites (tide gauges) located on the French Atlantic and Channel coasts. The proposed method appears to provide unbiased quantile estimates and to be more reliable than previously proposed approaches to include historic records in coastal sea level or surge statistical analyses.</p>
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