Stéphane Abadie

A COUPLED NUMERICAL MODEL FOR TSUNAMIS GENERATED BY SUBAERIAL AND SUBMARINE MASS FAILURES

This paper presents a new numerical model simulating tsunamis generation by landslides. Water, air, and the slide (considered either as a viscous fluid or as a rigid bloc using the penalization method) are described by Navier-Stokes equations, expressed in a unified single fluid approach. The PLIC-VOF method is used to describe the motion of fluid interfaces. We present results for two test cases featuring solid blocks. The first one is that of a semi-elliptical body sliding over a plane slope, without surrounding water. We find, the slide motion is accurately reproduced by the model when the slide viscosity is around 10 Pa.s. For larger viscosity values, the conditioning of the matrix associated with the Navier-Stokes solver is degraded. In the second case, a falling rectangular block generates waves in a flume. We find, wave height and box velocity are in agreement with experiments by Monaghan and Kos (2000). The simulated flow close to the falling box is similar to that observed in experiment, except that the observed main plunging wave and associated air entrainment.do not occur. In future work, the model thus validated will be applied to simulate more realistic cases of tsunamis generated by subaerial landslides.

Using a Shore-Based Video System to Hourly Monitor Storm Water Plumes (Adour River, Bay of Biscay)

Abstract Significant river discharges are usually associated with episodic rainstorms, leading to turbid storm water plumes clearly visible in the vicinity of river mouths. The composition of the suspended particulate matter transported by these plumes can alter the quality of coastal waters. The survey of marine coastal zones has thus become a major issue for water managers. For several decades, satellite imagery has allowed the daily monitoring of river plumes behavior over large distances. Nevertheless, it fails to provide data on their hourly evolution, which is important to operationally manage coastal waters in tourist areas. This study investigates the possibility of using the Argus video system to continuously survey the evolution of a storm water plume impact region. The system was installed in Anglet, at the Adour River mouth (southwest of France), in February 2005. An image-processing technique, based on pixel intensity clustering, is proposed to delineate river plume fronts along beaches from images collected with the Adour video station. Application of this technique, tested on storm water runoff plume events occurring under different climate conditions, shows promising results.

3D NUMERICAL SIMULATIONS OF WAVES GENERATED BY SUBAERIAL MASS FAILURES: APPLICATION TO LA PALMA CASE

Three-dimensional (3D) waves generated by landslides are simulated using a three-fluid Navier-Stokes VOF model. With this approach, the interaction between slide and water is implicitly solved. The model capabilities are first tested for benchmark cases featuring rigid body motion. Results are good in two dimensions (2D) and encouraging in 3D. Wave generation by a potential collapse of the Cumbre Vieja Volcano, on La Palma island, is then studied. Stability analyses show that the Cumbre Vieja flank is currently highly stable and that potential slide volumes are likely to be closer to 100 km, rather than the 500 km predicted in earlier studies. Results of the Navier-Stokes model show that waves generated are highly dependent upon the details of slide mechanism and kinematics. In our worst 3D scenario (assuming an inviscid fluid), the initial wavelength is 20 km and the wave height decrease due to lateral spreading is high.

Video monitoring nearshore sandbar morphodynamics on a partially engineered embayed beach

ABSTRACT De Santiago, I., Morichon, D., Abadie, S., Castelle, B., Liria, P. and Epelde, I., 2013. Video observation of the morphodynamics of nearshore sandbars on a partially engineered embayed beach. Embayed beaches are prevalent environments along the northern Spanish coastline. The present work analyses the temporal and spatial variability of nearshore sandbars on the pocket beach of Zarautz using daily video observations over 2 years. This partially urbanized beach can be divided into two well defined distinct areas: a natural section backed by a sand dune system to the East and an engineered section with rigid seawalls to the West. The objective of the present work is to evaluate if the engineered and natural sections of the beach present different morphological behaviors. The video image analysis was performed in two steps: First, sand bars were characterized visually and classified following standard classification schemes. Then, the outer sand bar migration was measured using automatic bar detection. Results show that the nearshore sandbars evolution covers a wide range of temporal and spatial variability. A noteworthy exception is the persistent presence of headland rips. The beach is mostly double-barred, with both bars able to go through all the states within the intermediate classification. The most common double bar morphological configuration is an inner low tide terrace coupled to well-developed megacusps, with an outer subtidal crescentic bar. Various preferred locations of rip channel formation are identified along the beach, suggesting that the effects of the headlands can propagate far away from the headlands towards the center of the embayment. Interestingly, the western engineered and more sheltered section of the beach sometimes exhibits a different beach state to that of the eastern section.

THREE-DIMENSIONAL LARGE EDDY SIMULATION OF VORTICES INDUCED BY PLUNGING BREAKING WAVES

The scope of this paper is to show the results obtained for simulating 3D plunging breaking waves by solving the Navier-Stokes equations in air and water, coupled with a subgrid scale turbulence model. The breaking processes including overturning, splash-up and breaking induced vortex-like motion beneath the surface are presented and discussed. Plunging breaking waves have been simulated in periodic domains and compared with previous published works. We found a very good agreement with the general description we could find in the literature concerning the breaking processes.

A comparative study of models to predict storm impact on beaches

The storm impact scale of Sallenger (J Coast Res 890–895, 2000) was tested on a partially engineered beach. This scale is supposed to provide a convenient tool for coastal managers to categorize the storm impact at the shore. It is based on the relation between the elevation of storm wave runup and the elevation of a critical geomorphic or man-made structures in the present study. Two different approaches were tested to estimate the elevation of extreme storm wave runup: (1) a parametric model based on offshore wave conditions and local beach slope and (2) the XBeach process-based model that solves implicitly the runup. The study shows comparisons between impact regimes computed with the two methods and those derived from video observations acquired during 2 weeks while the site was battered by three consecutive storms. Storms scenario including wave conditions with higher return periods and different tidal range were also investigated. The advantages and disadvantages of the two methods used to compute extreme water level are then compared, and guidelines for the development of early warning system are drawn.

Nouvelle technique de mesure locale de l'évolution du fond en zone de surf

ABSTRACT The high frequency bathymetric surveys in the surf zone are difficult and hazardous because of the breaking waves. It is however in these intense energy zones that the morphological evolutions are typically useful to know. A local measurement technique of the bed level evolution that can be used in the surf zone has been developed. This is a resistivity measurement based method and we have here the preliminary results of a 30 days in situ measurement. The extended fields of application of this method are multiple. We present how they can be extended to water level measurement and under specific conditions, to give information on the variations of ground resistivity under wave action.

Analyse statistique du lien entre les variables d’aléa et les dommages par submersion

Résumé : Une base de données de niveau de dommages consécutif à la submersion couvrant les 60 dernières années et focalisée sur la Grande Plage de Biarritz a été récemment élaborée. Cette base repose sur l’analyse d’archives et de documents de presse locaux. Parallèlement, une base de données de variables d’aléas couvrant la même période a également été constituée. Elle utilise des simulations numériques pour les vagues et une analyse harmonique pour les niveaux d’eau, validées par les observations disponibles sur site. Dans le présent article, nous détaillons l’analyse du lien statistique existant entre les deux bases de données. L’objectif est de trouver quelles variables d’aléa ou combinaison de variables expliquent le mieux l’occurrence de la submersion de la Grande Plage et la survenue de dommage. Les résultats préliminaires montrent que le couple flux maximum d’énergie des vagues et niveau d’eau maximum durant l’événement expliquent très bien les dommages répertoriés sur cette plage spécifique. Mots-clés : Submersion, Biarritz, Indicateurs, Statistiques, Dommages, Données historiques.

Mesures in situ des impacts de vagues sur une digue composite : résultats préliminaires

1. UNIV PAU & PAYS ADOUR/ E2S UPPA, Laboratoire des Sciences de l’Ingénieur Appliquées à la Mécanique et au génie Électrique – IPRA, EA4581, 64600, Anglet, France. pa.poncet@univ-pau.fr 2. CNRS/UNIV PAU & PAYS ADOUR/E2S UPPA, Laboratoire de Mathématiques et de leurs Applications de Pau – Fédération*, UMR5142 64000, Pau, France. 3. MIO-USTV/AMU/CNRS/IRD, UM 110, Université du sud-Toulon-Var, La Garde, France.

OBSERVATIONS OF MOMENTARY LIQUEFACTION CAUSED BY BREAKING WAVES AROUND A COASTAL STRUCTURE

This paper aims at demonstrating the occurrence of momentary liquefaction in a sandy soil around a coastal structure subject to wave forcing. This was made in the field, using as coastal structure a bunker from the Second World War that is currently situated on the beach at Capbreton on the Atlantic coast of south-west France. By positioning pressure sensors at different levels inside the soil, time series of simultaneous pore pressure measurements were acquired while the position of the soil level was monitored. Analysis of the pressure measurements demonstrated that the sediment layer on top of the bed does not remain continuously in static equilibrium. An upward pressure gradient was sometimes observed, which could overcome the effective weight of the sediment. Pore pressure variations inside the soil were compared, using a Fourier analysis of the time series, with the theory put forward by Sakai et al. (1992) whereby the damping of pore pressure variations inside the soil can be quantified. A surprisingly good agreement is found when the gas content is adjusted in the model to 1%. This indicates the existence of a significant amount of gas inside the soil. This property is confirmed in the field by geo-endoscopic videos.