Vagues sur la côte aquitaine : régionalisation dynamique de 1958 à 2002

National audienceUn système de modélisation de vagues, forcé uniquement par des champs de vent a été mis en place avec le code WAVEWATCH IIITM sur une période de 44 ans (1958 à 2002) en vue d'étudier l'impact de la variabilité climatique passée sur les états de mer et sur l'érosion de la côte aquitaine. Les emboîtements hauturiers forcés par les champs de vent de la réanalyse ERA-40 ont été calibrés sur la période 1998-2002 sur 8 points de mesures. Les résultats sur 44 ans ont ensuite été validés sur 11 bouée

Longshore sediment flux hindcast and potential impact of future wave climate change along the Gironde/Landes Coast, SW France

In : International Coastal Symposium (ICS) : 2013 Proceedings (Plymouth, United Kingdom)International audienceThe West-facing Gironde/Landes Coast, SW France, comprises about 230 km of high-energy, nonengineered, sandy beaches. It extends from the Gironde estuary to the North, and to the Adour estuary to the South. Overall, the coast erodes at a rate of about 1 to 2 m/yr, with localized areas with stronger erosion or with a stable or slow accretive trend. The gradients in longshore sediment transport and the sediment budgets are presumably the most important factor controlling decadal scale evolution of the shoreline along the Gironde/Landes Coast. It is established that the net longshore drift, which is only disturbed at the tidal inlet system of the Arcachon Lagoon and shoreward of the Capbreton Canyon, is oriented southward (Michel, 1997). However, its intensity and longshore variability, as well as how it will evolve in the framework of climate change, are still a matter of debate. This important issue is addressed in this contribution. First, longshore sediment fluxes are hindcasted over the period 1958-2002, using an ERA-40 wind forced wave modelling and the most commonly used empirical longshore drift formulae (Charles et al, 2012). Results are (1) discussed with respect to the existing estimations based on smaller periods and using different datasets; (2) compared the shoreline evolution measured during this period and (3) further analyzed to address the respective contributions of the different representative wave classes (e.g. swell, storm waves, wind waves) to the total longshore drift. Wave projections, based on wind fields from one climate model (ARPEGE-Climat) for several emission scenarios (A2, A1B and B1), are carried out to estimate possible wave climate change in 2061-2100 (compared to a so-called REF scenario covering the 1961-2000 period). It shows that, for instance for the A2 scenario and the summer season, the modal significant wave height is likely to decrease together with a 5° northward rotation of incoming wave incidence. Based on the wave model outputs, longshore sediment fluxes along the Aquitanian coast are computed for the four scenarios. The analysis of the relative sediment flux evolution (A2, A1B and B1 scenarios with respect to present day REF scenario) shows an overall decrease of the southerly longshore drift magnitude. A thorough analysis of the wave components evolution responsible for this decreasing trend is done, as well as an analysis of the longshore drift gradients along the Aquitanian coast. The potential impact of longshore drift evolution on the shoreline evolution at the 2061-20100 is discussed

Dynamics of inner-shelf, multi-scale bedforms off the south Aquitaine coast over three decades (Southeast Bay of Biscay, France)

International audienceThis paper aims to investigate the seabed morphodynamics of the south Aquitaine inner shelf in the area known as “La Salie” (150 km2, Atlantic ocean, west coast of France), through a descriptive and comparative analysis (time lapse of 29 years) of geophysical and sedimentological datasets.At a water depth of 24–50 m, four orders of sedimentary body types were observed at different scales. The first order are large cross-shore “morphological ridges,” corresponding with the properties of very large sorted bedforms. The second order consisted in patchy sorted bedforms, composed of alternately medium to fine sand patches (0.5–2 m in thickness), cut by smaller, elongated coarse sediment depressions. In particular, the data from the sub-bottom profiler revealed that sand patches predominantly overlayed the coarse-grained blankets on the eastern (shoreward) extremities, while coarse-grained blanket wedges were found in front of the sand patches (southwestward) or locally overlying them on the southwestern extremities. The third order of bedforms involved groups of dune-like features (fine/medium sand), lying in wide areas of coarse-grained sediment. Finally, in the fouth order, the entire inner shelf was covered with wave-generated ripples, oriented N15°, that were larger where sediments were coarse (wavelengths of 2.2 m) than where sediments were fine (wavelengths of 0.3 m). Over the past 29 years, at a large scale of observation, patchy sorted bedforms have remained remarkably persistent, as has their overall appearance. However, at a smaller scale, weak but constant movements were observed. The coarse depressions have become elongated at their extremities (by a maximum of 300 m over 15 years), and certain coarse/fine sediment boundaries have moved toward the northeast and southeast (by a maximum of 75 m over 12 years). The general movement has been shoreward as has the migration of third-order submarine dune-like features. The persistence of sorted bedforms thus appears to be the consequence of sediment sorting feedback and recurrent storm events

Modelling the retreat of a coastal transverse dune under changing wind conditions

International audienceCoastal dunes can move in response to winds and cause serious hazard to human assets. Changes in wind patterns, potentially occurring as a consequence of climate change or variability, could affect rates of aeolian transport and migration velocity of coastal dunes. Understanding the response of coastal dune mobility to changes in transport conditions is a fundamental aspect of coastal zones management. However, most of previous modeling studies were conducted by assuming that aeolian bedforms are subject to a constant wind velocity. In the present work, we perform a modeling study of the mobility of a coastal transverse dune under varying wind velocities based on the example of the Dune du Pilat in Aquitaine. We apply well-established models and empirical formula for aeolian dune migration, including a model previously validated against measurements of real profiles of desert and coastal dunes. The average migration velocity of Pilat dune predicted by the models is about 3m/year, which is in good agreement with in-situ measurements of the dune retreat rates. To test the response of transverse dune mobility to changing winds and the impact of extreme events on dune migration rates, we generate virtual, but still realistic wind time series using a stochastic model. The approach consists in modifying a few parameters of the stochastic model, in order to generate virtual time series with different characteristics than today. The results suggest that more frequent storms have less impacts than more intense winds, but also that both lead to moderate changes in the average dune velocity i

White paper: Monitoring the evolution of coastal zones under various forcing factors using space-based observing systems

This paper is an outcome of the International Space Science Institute (ISSI) Forum on “Monitoring the evolution of coastal zones under various forcing factors using space-based observing systems” (http://www.issibern.ch/forum/costzoneevo/) held at ISSI, Bern, Switzerland on 11-12 October 2016 (convened by J. Benveniste, A. Cazenave, N. Champollion, G. Le Cozannet and P. Woodworth