Raphaël Certain

Physical modeling of intermediate cross‐shore beach morphology: Transients and equilibrium states

Laboratory experiments on cross-shore beach morphodynamics are presented. A lightweight sediment (density rho(s) = 1.19 g cm(-3)) model is used in order to fulfill a Shields number and Rouse number scaling. This choice aims at correctly reproducing bed load transport as well as suspension dynamics. Terraces and barred beach profiles obtained in the experiments also present close similarities with profiles observed in the field. In order to question the concept of equilibrium beach profile, wave forcings conforming to a JONSWAP spectrum were imposed over long periods (up to more than a hundred hours). An average bottom evolution velocity is defined and used to determine when the profile reaches equilibrium. Usually, beach profiles are characterized according to the Wright and Short (1984) classification based on the Dean number W. This well-known classification is investigated and refined in the intermediate range, that is, for 1 = Omega 5. For W close to 1, a typical reflective profile is obtained. Terraces are obtained for the Omega = 2.5 cases. For Omega approximate to 3.7, the profiles exhibit two parts: a mild dissipative offshore slope producing low reflection and a steeper beach face with slightly higher reflection. The wave dissipation, velocity skewness, and acceleration skewness are computed from the free surface elevation time series. The dissipation and wave nonlinearities patterns are similar for similar equilibrium beach profiles, that is, with the same Dean number. Dissipation peaks coincide with bottom slope transitions as higher energy dissipation occurs with milder bottom slope sections. Besides, the uniformity of volumetric wave energy dissipation seems to concern only a limited zone of beaches with a widely developed surf zone.

Integrated Onshore-Offshore Investigation of a Mediterranean Layered Coastal Aquifer

Most of the Mediterranean coastal porous aquifers are intensively exploited. Because of climatic and anthropogenic effects, understanding the physical and geological controls on groundwater distribution and flow dynamics in such aquifers is crucial. This study presents the results of a structural investigation of a system located along the coastline of the Gulf of Lions (NW Mediterranean). A key aspect of this study relies on an onshore-offshore integrated approach combining outcrops, seismic profiles, and borehole data analysis. This multidisciplinary approach provides constraints on pore-fluid salinity distribution and stratigraphic organization, which are crucial in assessing the modes of groundwater/seawater exchanges. Onshore, Lower Pliocene deposits dip gently seaward. They are unconformably overlain by Holocene clays in the lagoons. Offshore the Pliocene deposits either outcrop at the seabed or are buried below nonconsolidated sands infilling paleo-valleys. Beneath the lido, the groundwater salinity distribution consists of salty pore water, overlying fresher pore water. Active circulation of groundwater masses is inferred from the geophysical results. In particular, offshore outcrops and paleo-valleys may play an important role in salt water intrusion.

Wind-driven bottom currents and related sedimentary bodies in Lake Saint-Jean (Québec, Canada)

Lakes are major depositional systems for which the related depositional processes have long been considered relatively simple. Breaking this statement, this study presents a detailed analysis of deposits in Lake Saint-Jean, the third largest natural lake in Quebec. In addition to postglacial deltaic and coastal depositional systems fringing the lake, current-controlled features such as a large subaqueous prograding wedge and three sediment drifts have been identifi ed in its central portion based on two-dimensional (2-D) acoustic high-resolution subbottom profiles. The large subaqueous prograding wedge is a 4-km-long and up to 15-m-thick heterolithic shelf-like construction in the southeastern part of the lake. The three sediment drifts are 0.1–0.5-km-long and 2–5-mthick mud mounds distributed on the lake floor in the central portion of the lake. Diatom analyses and radiocarbon dating show that the development of these current-controlled features occurred during the lacustrine phase, after the disconnection with the postglacial marine Laflamme Gulf at 8.5 cal. k.y. B.P. Depositional facies show evidence of recurrent bottom-current activity. Related deposits alternate with pelagic sedimentation stages characterized by the settling of mud and biogenic accumulations. We investigated the origin of bottom currents using a numerical simulation (SYMPHONIE, an oceanographic model), with the aim of modeling wind-induced lake-scale water circulation. Simulations suggest that the subaqueous prograding wedge and the three sediment drifts result from wind-induced bottom currents generated by storm events having wind speed greater than 10 m s–1. Such strong winds are able to significantly affect sedimentation in the central portion of Lake Saint-Jean. The resulting wind-induced sedimentary features were integrated into a refi ned lacustrine depositional model that summarizes the evolution of a group of water bodies referred to as “wind-driven water bodies.” This study applies a new tool for lake strata characterization and highlights the potential diffi culty in differentiating them from marine deposits in the geological record.

Net offshore bar migration variability at a regional scale: Inter-site comparison (Languedoc-Roussillon, France)

ABSTRACT Aleman, N., Robin, N., Certain, R., Barusseau, J.-P. and Gervais, M., 2013. Net offshore bar migration variability at a regional scale: Inter-site comparison (Languedoc-Roussillon, France). The Languedoc-Roussillon coastline is a large unit stretching out over 200 km of sandy low coast in a wave dominated environment. The nearshore is characterized by a quasi continuous system of double sandbar that displays a wide range of typology. The interannual sandbar dynamic was investigated using 2D bathymetric profiles and 3D LiDAR imagery. This study has allowed determining the sandbar systems affected by the Net Offshore Migration (NOM). At a regional scale, conditions necessary to NOM development depend on the sandbar morphologies (crescentic or straight), the wave energy and the associated coastal orientation (low or high energy, sheltered area), the coastal structures (harbour and coastal defences) and the nearshore sedimentary budget. The areas where the NOM occurs show cycle dynamic differences. This regional inter-site comparison highlights that nearshore morphology and bar parameters seem to influence the nearshore bar behaviour. Interaction between the nearshore slope, width of the bar zone and the migration rate control the NOM duration. The sandbar volume and the regional wave climate influence also the migration rate of the system. On the Languedoc-Roussillon coast, the sediment grain size does not appear to influence the seaward bar migration.

Barrier shoreline evolution constrained by shoreface sediment reservoir and substrate control: The Miquelon-Langlade Barrier, NW Atlantic

ABSTRACT Billy, J., Robin, N., Certain, R., Hein, C. and Berné, S., 2013. Barrier shoreline evolution constrained by shoreface sediment reservoir and substrate control: the Miquelon-Langlade Barrier, NW Atlantic. The Saint-Pierre-et-Miquelon Archipelago (France) is located in the NW Atlantic Ocean, proximal to the Cabot Straight outlet of the Gulf of Saint-Lawrence, and 50 km south of Newfoundland (Canada). The Miquelon-Langlade Barrier is a 12-km-long, 100–2500-m-wide, north-south–oriented isthmus connecting two bedrock islands (Miquelon to the north; Langlade to the south). This study aims to improve our understanding of shoreface-shoreline sediment exchange processes by comparing medium-term (1949–2011) shoreline changes, determined from aerial photographs and differential GPS data, with total shoreface sediment reservoir volumes estimated using seismic along the west coast of the Miquelon-Langlade Barrier. Spatial variability between the northern and southern sectors of the study site are seen both in the volumes of shoreface sedimentary reservoirs and in multi-decadal shifts of the shoreline position. The northern region has the lowest shoreface sediment volume and the highest rate of shoreline retrogradation. By contrast, the center and southern regions contain the largest volume of sediment in the shoreface and have demonstrated either long-term stability or progradation. This study demonstrates the primary roles of geological control and the distribution of shoreface sediments in local shoreline change at multi-decadal time scales. The sedimentary reservoir, in conjunction with shoreline-monitoring studies and knowledge of transport patterns, may provide a good alternative proxy.

Wave-driven circulation over a double nearshore bar system during storm conditions

ABSTRACT Robin, N., Certain, R., Bouchette, F., Anthony, E.J., Meulé, S., and Aleman, N., 2014. Wave-driven circulation over a double nearshore bar system during storm conditions. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 084–089, ISSN 0749-0208. Current profiles and waves were recorded from a multi-instrumented transect over a double nearshore bar system in the Gulf of Lions, NW Mediterranean Sea (France) during storm conditions with shoreface significant wave heights of up to 3.2 m. The results constitute a preliminary analysis aimed at constraining the 3D nearshore circulation in a microtidal system. Significant time changes in the vertical distribution of nearshore velocities were observed, forced by the wind/wave conditions. Such vertical changes have been highlighted by theoretical velocity profiles in the literature, but our study demonstrates much larger variability than has hitherto been shown. Another result obtained was that the hydrodynamic pattern observed in the inner trough was distinct from that observed along the seaward flank of the inner bar. For a well-defined threshold in wave height, velocities in the trough increased abruptly and earlier, and remained strong over a longer time than those on the seaward flank. The trough thus behaves essentially as a drain for water piled against the shore. This behavior is altered by the width of the surf zone (and not only by the significant wave height), which modulates the mean current velocity. These results are a useful preliminary step in improving numerical modeling of the complex surf-zone circulation over bar-trough systems.