Michel Benoit

Application of regional frequency analysis to the estimation of extreme storm surges

Traditionally, extreme value theory is applied to single-site series of surge observations in order to estimate the probability of occurrence of extreme events at that particular site. However, single-site analyses give uncertain estimation of extreme quantiles, mainly because of the limited duration of observation periods. In order to reduce this uncertainty, regional frequency analysis (RFA) approaches suggest collecting information not only from a single-site series but also from all (statistically) similar available series of observation. The use of RFA is widely increasing in geosciences, but few applications have been attempted yet for surge estimation. The aim of this study is to examine the applicability of RFA to extreme storm surges. The surge data observed at 18 French harbors, located on the Atlantic coast from the Spanish to Belgian borders, were collected. The series span a period of 30 years, on average, with the longest series going back to the 19th century. Stationary and independent samples of extreme surges (peaks over a given threshold) are extracted and their (statistical) homogeneity has been tested via heterogeneity and discordancy measures based on L moments. Homogeneous regions have been identified and, in order to merge information on frequency of occurrence of surges from all the sites, a surge index pooling method is defined. Finally, a regional frequency distribution has been estimated. The hypothesis and the applicability of RFA application are discussed, with some ideas for future developments in the research direction.

Constitution et exploitation d'une base de données d'états de mer le long des côtes françaises par simulation numérique sur 23 ans : Base ANEMOC en Atlantique-Manche-mer du Nord

ABSTRACT A numerical wave data-base (called ANEMOC) has been built by hindcasting seastate conditions over a period of 23 years, from 1979 to 2002, with the TOMAWAC wave model. The wind fields used as input data come from the reanalysis ERA-40 of the European Center for Medium-Range Weather Forecasts (ECMWF). Some statistical processing of the results have been performed, so as to build synthetic charts of the medium and severe wave conditions over the European continental shelf. Finally two different methods are proposed to estimate extreme wave heights with return periods larger than the simulated period. Both methods for statistical extrapolation are applied to a selection of points and compared.

Construction of the Numerical Wave Databases Anemoc-2 on the Mediterranean Sea and the Atlantic Ocean Through Hindcast Simulations Over the Period 1979–2010

In the 2000s, CEREMA and EDF R&D have been collaborating to build two continuous wave databases through numerical hindcast simulations: one covers the Atlantic Ocean, the other the Mediterranean Sea. These databases are called ANEMOC. Over the last three years, new versions of the two numerical atlases have been created, in collaboration with Saint-Venant Laboratory. Several improvements have been made in the construction of ANEMOC-2: the Atlantic model covers a larger area, the temporal coverage of the atlases is larger (32 years from 1979 to 2010), direction and frequency discretization is finer, wind forcing is finer both in time (1 h resolution) and space (0.312) and the computation meshes are refined to reach 800–1000 m along the French coast. The simulations are performed with the numerical wave model TOMAWAC, a third generation spectral model, which is a module of the TELEMAC-MASCARET open source suite. The databases are calibrated with altimeters measurements, and validated in a second step against uncorrelated buoys measurements. The databases provide several wave parameters: significant wave height, mean, peak and energy period, mean direction, angular wave spreading, and wave power. More results regarding calibration and validation are presented for the Mediterranean wave model. Results of both Mediterranean and Atlantic databases are then presented. Their analyses by comparison with altimeter and buoy measurements provide an assessment of many of their characteristics. Finally, ANEMOC-2 ability to reproduce intense wave conditions is highlighted by the study of two storm events.

Efforts dus aux vagues sur un cylindre vertical en zone côtière : Etude expérimentale et méthodes de prédiction

ABSTRACT Wave-induced loadings acting on a monopile structure were investigated by experimental tests on a vertical cylinder in a wave flume with a sloping bottom, considering non-breaking, breaking and broken wave conditions. Measured forces were compared to numerical predictions obtained by applying Morison formula and by using different methods to compute the wave kinematics. This comparison highlights the need for accurate prediction methods for wave kinematics of shoaling and breaking waves. Indeed the linear wave theory often leads to significant underestimation of the measured forces. Using advanced stream function methods or the modified transfer function method, this under-prediction is significantly reduced, but additional work is needed to predict the impulsive force due to breaking on the structure better.

Methodologie de caracterisation et d’estimation des aleas climatiques extremes

Le dimensionnement des ouvrages de protection des sites de production necessite la determination d’aleas hydro-meteorologiques rares mais susceptibles d’intervenir au cours de leur duree d’exploitation.L’estimation des probabilites d’occurrence de valeurs extremes ou de niveaux associes a des probabilites tres rares est un sujet transverse a EDF et un effort de capitalisation et de mutualisation des pratiques des differents metiers a ete entrepris depuis quelques annees. La caracterisation est basee au maximum sur l’application de la theorie statistique des valeurs extremes, avec des extensions dans les cas ou elle n’est pas applicable directement. Un etat des lieux synthetique est ici propose.Cet etat des lieux eclairera la proposition d’EDF de considerer, dans le contexte post Fukushima, de nouveaux aleas dits «xa0hors-dimensionnementxa0». Ces aleas sont des aleas encore plus rares que les aleas de dimensionnement, dont la prise en compte vise a ameliorer la robustesse des sites vis-a-vis des agressions externes et notamment de l’inondation.

Prédiction de la cinématique de houle en zone de déferlement

ABSTRACT This paper deals with the prediction of wave kinematics in the nearshore and coastal zones, where bottom variations strongly affect wave propagation and their characteristics. Measurements of wave kinematics have been performed near the bottom for a large number of shoaling and breaking wave conditions, and also over the water column inside the breaking zone for three particular conditions. These experimental data serves to evaluate and compare several numerical methods, including Flat Bottom Theories (FBT), Sloping Bottom Methods (SBM) and Transfer Function Methods (TFM). It is shown that advanced TFM give the best predictions for wave kinematics, provided that the local free surface elevation signal is available. Particular SBM may also produce reliable prediction for the case of plane slope from a limited number of wave parameters, whereas FBT should not be used in the coastal zone.