Nathalie Giloy, Alexa Latapy, Gaël André, Estimation of skew surge uncertainties when predicting tides in the past, Weather and Climate Extremes, Volume 41, 2023, 100597

The study of storm surges and more generally of extreme events is a subject of growing interest in the current context of coastal risk management in a changing climate. Recent studies show that the consideration of past events improves statistical models. In this context, important investigations have been carried out in the last decade to retrieve and quantify water levels and skew surges associated with historical events for the 18th or 19th century. The estimation of the historical surges depends on the calculated tidal prediction, and therefore directly on the tidal components and the mean sea level of the period under consideration. Still, as tidal constituents and mean sea level change over time, it is difficult to estimate these parameters for historical events when sea level data are missing or incomplete. Two cases are considered for calculating tidal predictions. Either short time series are available contemporary to the event of interest, allowing a harmonic analysis and subsequent calculation of tidal constituents and mean sea levels. Or there are no or insufficient contemporary data available for the event of interest and current tidal characteristics are used in combination with a mean sea level adjustment. The present study aims to answer the following questions: depending on the data available for a historical event, which tidal constituents and mean sea level should be used when considering events that occurred several decades or even centuries ago? To what extent does the length of the observation series used in the harmonic analysis influence the tidal predictions? How does a mean sea level correction affect tidal predictions made with current tidal components? Finally, what are the uncertainties associated with the estimation of historical extreme skew surges, when there are not enough contemporary observations to estimate tidal components? Two methods are considered to estimate the uncertainties associated with the calculation of tidal predictions. Either the use of contemporary tidal constituents and mean sea level or the use of current tidal constituents corrected for mean sea level. In this study, 14 sea level records of 100 years or more provided by the Global Extreme Sea Level Analysis (GESLA version 3) were analysed. First, the uncertainties of the astronomical tides are assessed. For this purpose, different lengths of observations are implemented in a harmonic analysis and the resulting tidal constituents and tidal predictions are compared. In a second step, current tidal constituents are used for past tidal predictions and a mean sea level trend is inferred to cope with sea level variations. Different time periods are used to estimate the mean sea level trend and their influence on the tidal predictions is analysed. Furthermore, these two methods are combined to study the effects on the estimated skew surges and to assess the associated uncertainties. This study shows that uncertainties of less than 10 cm can be obtained for skew surges by using contemporary tidal components calculated from at least two years of observations. Larger differences are obtained for microtidal or river-influenced sites. When using current tidal constituents and adjusting a mean sea level trend, linear trend correction gives reliable results when more than 60 years of data are used for the trend estimation. In this case, the extreme skew surges obtained are within a range of more or less 5 cm. Finally, on the basis of the results presented, some suggestions are made on the possible applications of these methods according to the availability of data, stressing the importance of site-specific studies.

Mis à jour le 15/02/2024

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