Florence Cayocca

Long-term morphological modeling of a tidal inlet: the Arcachon Basin, France

Abstract The Arcachon Lagoon on the French Atlantic coast is a triangular shaped lagoon of 20 km on a side connected to the ocean by a 3-km wide inlet between the mainland and an elongated sand spit. This tidal inlet exhibits a particularly active morphology due to locally strong tidal currents and rough wave conditions. During the past 300 years, minimum and maximum spatial extents of the Cap Ferret sand spit have varied by 8 km while one or two channels have alternately allowed circulation between the lagoon and the ocean. These impressive morphological changes have never prevented regular flushing of the lagoon, eventhough the spit came as close as 300 m from the coast during the 18th century. According to Bruuns concept of tidal inlet stability [Theory and Engineering (1978), 510 pp.], the balance between longshore littoral transport and the tidal prism ensures the perpetuity of the inlet. Process modeling was believed to give better insight into the respective roles of tides and waves in driving the long-term morphological changes of the inlet. A two-dimensional horizontal morphodynamic model was therefore developed, combining modules for hydrodynamics, waves, sediment transport and bathymetry updates. The use of process models at a scale of decades requires a schematization of the input conditions. We defined representative mean annual wave and tide conditions with respect to sediment transport, i.e. conditions that induce the same annual transport as measured in the field. Driven by these representative conditions, simulations run from the 1993 bathymetry show that the tide is responsible for the opening of a new channel at the extremity of the sand spit (where tidal currents are the strongest), while waves induce a littoral transport responsible for the longshore drift of sand bodies across the inlet. One particular simulation consisted in running the model from a hypothetical initial topography where the channels are filled with sand and the entire inlet is set to a constant depth (3 m). The results show the reproduction of a channel and bar system comparable to historical observations, which supports the idea that the lagoon is unlikely to be disconnected from the ocean, provided tide and wave conditions remain fairly constant in the following decades.

Evaluation of the risk of marine slope instability: A pseudo-3D approach for application to large areas

Abstract This article presents a methodology developed to evaluate the instability of submarine slopes that extend over a large area. Special attention was paid to (1) the complex geometry (bathymetry) and the expanse of the slope, (2) the heterogeneity of the sediment, and (3) the distribution of the pore pressure. The safety factor was considered as a spatially varying quantity. The General Formulation (GLE, Fredlund and Krahn 1977), which fully satisfies equilibrium conditions, was used for evaluating the stability of the marine slope. The submarine slope failure, which occurred on 16 October 1979 during the construction of the new Nice airport, was studied in order to test the developed model. Geotechnical parameters were taken from experimental tests carried out by IFREMER on 19 cores extracted at different depths (from 27 m to 1300 m) (Cochonat, Bourillet, and Savoye, 1993; Mulder et al., 1994). Many scenarios were proposed in order to explain the cause of the Nice slope failure (Habib, 1994). In th...

Analysis of submarine slumping in the Gabon continental slope

The Gabon continental slope is selected as a case study for slope-stability analysis because of evidence of previous slide activities. Different types of data were collected from the continental slope in the Gulf of Guinea off west Africa during Guiness and ZaiAngo surveys. The offshore investigation was carried out using swath bathymetry and associated imagery, deep-towed high-resolution subbottom profiles, side-scan sonar images, observation from remotely operated vehicle Victor, and Kullenberg cores. These data reveal different examples of seafloor instabilities commonly related to fluid-escape features. These slides occur on the continental slope at low declivities, showing that slope gradient has a secondary role on the marine slope instability with respect to external triggering mechanisms such as fluid flow, earthquake, shallow gas, and gas hydrates. One case of mass slide with small downslope displacement was studied on the Gabon slope.In this work, a pseudothree-dimensional slope-stability analysis (Sultan et al., 2001) was undertaken. Three scenarios of instability were tested to identify the possible trigger mechanism of the observed slide instability: (1) under static gravity loading, (2) under earthquakes, and (3) under upward fluid flow. Simulation results show that static stability of the area is satisfactory. However, the stability is very sensitive to fluid escape. These results agree with sonar images showing seepage features aligned along the upslope limit of the observed slide.

Chapter 29 Sedimentary processes in a shellfish farming environment, Mont Saint Michel Bay, France

Abstract Mont Saint Michel Bay is a 30 km wide bay located on the French side of the English Channel. The tidal range reaches 14 m during spring tides, which leads, along with the gentle slope of the bathymetry, to a tidal flat up to 11 km wide. The sedimentary coverage exhibits strong longshore and cross-shore gradients, with purely muddy environments to the west of the domain, and pure sands to the east, where the natural channels of three incoming rivers induce rapid morphological changes. The contributions of tides and waves to sedimentary processes are analysed by means of numerical modelling and results of field observations. Maximum tidal bottom shear stresses are shown to account for the sediment distribution throughout the bay, while the longshore gradient in wave intensity seems to drive the amount of suspended sediment concentration. The bay has been a priviledged ground for shellfish farming for over a century. Farming structures (oyster tables, mussel posts and wooden fences used as permanent fishing nets) have significantly hindered natural flow patterns, creating quiescent areas which significantly increase fine sediments deposits in farming areas. The paper focuses on introducing into a numerical model the effects of mussel farms on flow circulation and sediment dynamics.

3D application of the continuous modelling concept to mud slides in open seas

Most turbidity current models are vertically integrated, and use a parametric entrainment rate to simulate the growth and dilution of the dense layer. The continuous modelling concept integrates all physical processes related to high-concentrated suspensions (stratification-induced turbulence damping, hindered settling and molecular viscosity increase or even viscoplastic behaviour). It is applied in a three-dimensional frame to simulate turbidity currents on a slope. A sensitivity analysis has been carried out in a 2DV configuration. It shows the front celerity is dependent on the initial mass, not so much on the slope, whereas the velocity within the body of the turbidity current is controlled by the slope and vertical mixing processes. The resulting entrainment rate is in agreement with flume experiments. A vertical recirculation is observed at the head of the turbidity current. The contribution of bed erosion is pointed out: such an erosion is generated by the density current and then enhances the density gradient and thus the turbidity current. The application of the 3D model to the 1979 Nice slide and turbidity current is under process. Preliminary results are discussed.

Influence of bottom trawling on sediment resuspension in the ‘Grande-Vasière’ area (Bay of Biscay, France)

Sea trials were performed on two zones with different fishing efforts on the continental shelf of the Bay of Biscay (‘Grande-Vasière’ area of muddy sand) in order to assess particulate matter resuspension and seabed disturbances (i.e., penetration, reworking, grain size changes) induced by different types of trawls. Optical and acoustic measurements made in the water column indicate a significant trawling-induced resuspension mainly due to the scraping action of doors. It manifests as a highly dynamic turbid plume confined near the seabed, where suspended sediment concentrations can reach 200xa0mgxa0l−1. Concentration levels measured behind an “alternative” configuration (trawls with jumper doors instead of classical doors penetrating the sediment) are significantly lower (around 10–20xa0mgxa0l−1), which indicates a potential limiting impact regarding the seabed. Grain size analyses of the surficial sediment led to highlight a potential reworking influence of bottom trawling. On the intensively trawled zone, this reworking manifests as an upward coarsening trend in the first 5xa0cm of the cores. A significant decrease in mud content (30xa0%) has been also witnessed on this zone between 1967 and 2014, which suggests an influence on the seabed evolution. The geometric analysis of bottom tracks (4–5-cm depth, 20-cm width) observed with a benthic video sledge was used to compute an experimental trawling-induced erosion rate of 0.13xa0kgxa0m−2. This erosion rate was combined with fishing effort data, in order to estimate trawling-induced erosion fluxes which were then compared to natural erosion fluxes over the Grande-Vasière at monthly, seasonal and annual scales. Winter storms control the annual resuspended load and trawling contribution to annual resuspension is in the order of 1xa0%. However, results show that trawling resuspension can become dominant during the fishing high season (i.e., until several times the natural one in summer). In addition, the contribution of trawling-induced resuspension is shown to increase with water depth, because of the rapid decay of wave effects. Finally, the seasonal evolution of the respective contributions for erosion (mainly trawling and waves) could be mapped for the whole study area.

Characterization of Bottom Sediments in the Río de la Plata Estuary

ABSTRACT Moreira, D.; Simionato, C.G.; Dragani, W.; Cayocca, F., and Luz Clara Tejedor, M., 2016. Characterization of bottom sediments in the Río de la Plata estuary. Bottom sediments and surface water samples were collected in the intermediate and outer Río de la Plata (RDP) estuary, South America, during 2009–10, in six repeated cruises, with 26 stations each. Samples were processed for grain size and for water and organic matter content. The aim of this work is to analyze this new data set to provide a comprehensive and objective characterization of the bottom sediment distribution, to study the composition, and to construct a conceptual model of the involved physical mechanisms. Principal components analysis is applied to the bottom sediment size histograms to investigate the spatial patterns. Variations in grain size parameters contain information on possible sediment transport patterns, which were analyzed by means of trend vectors. Sediments show a gradational arrangement of textures: sand dominant at the head, silt in the intermediate estuary, and clayey silt and clay at its mouth. Textures become progressively more poorly sorted offshore, and the water and organic matter content increase and seem to be strongly related to the geometry and the hydrodynamics. Along the northern coast of the intermediate RDP, well-sorted medium and fine silt predominates, whereas along the southern coast, coarser and less sorted silt prevails from differences in tidal currents, in the tributaries water pathways, or both. Around Barra del Indio, clay prevails over silt and sand, and the water and organic matter content reach a maximum, probably because of flocculation and reduction of the currents. Immediately seaward of the bottom salinity front, net transport reverses its direction, and well-sorted, coarser sand from the adjacent shelf dominates. Relict sediment is observed between the Santa Lucía River and Montevideo, consisting of poorly sorted fine silt and clay. The inferred net transport suggests convergence at the bottom salinity front, where the limit of the estuarine deposits is observed.