Coastal erosion at Saint Louis in Senegal is affecting the local population that consists of primarily fishermen communities in their housing and their access to the sea. This paper aims at quantifying urban beach erosion at Saint Louis, Senegal, West Africa which is located on the northern end of the 13 km long Langue de Barbarie sand spit. The coastal evolution is examined quantitatively over a yearly period using Pleiades sub-metric satellite imagery that allows for stereogrammetry to derive Digital Elevation Models (DEMs). The comparison with ground truth data shows sub-metric differences to the satellite DEMs. Despite its interest in remote areas and developing countries that cannot count on regular surveys, the accuracy of the satellite-derived topography is in the same order as the coastal change itself, which emphasizes its current limitations. These 3D data are combined with decades-long regular Landsat and Sentinel-2 imagery derived shorelines. These observations reveal that the sand spit is stretching, narrowing at its Northern part while it is lengthening downdrift Southward, independently from climatological changes in the wave regime. A parametric model based on a stochastic cyclic sand spit behaviour allows for predicting the next northern opening of a breach and the urban erosion at Saint Louis.

The seafloor—or bathymetry—of the world’s coastal waters remains largely unknown despite its primary importance to human activities and ecosystems. Here we present S2Shores (Satellite to Shores), the first sub-kilometer global atlas of coastal bathymetry based on depth inversion from wave kinematics captured by the Sentinel-2 constellation. The methodology reveals coastal seafloors up to a hundred meters in depth which allows covering most continental shelves and represents 4.9 million km${}^2$ along the world coastline. Although the vertical accuracy (RMSE 6–9 m) is currently coarser than that of traditional surveying techniques, S2Shores is of particular interest to countries that do not have the means to carry out in situ surveys and to unexplored regions such as polar areas. S2Shores is a major step forward in mitigating the effects of global changes on coastal communities and ecosystems by providing scientists, engineers, and policy makers with new science-based decision tools.

This study describes the building of a common erosion law for fine sand and mud, mixed or not, in the case of a typical continental shelf environment, the Bay of Biscay shelf, characterized by slightly energetic conditions and a seabed mainly composed of fine sand and muddy sediments. A 3D realistic hydro-sedimentary model was used to assess the influence of the erosion law setting on sediment dynamics (turbidity, seabed evolution). A pure sand erosion law was applied when the mud fraction in the surficial sediment was lower than a first critical value, and a pure mud erosion law above a second critical value. Both sand and mud erosion laws are formulated similarly, with different parameters (erodibility parameter, critical shear stress and power of the excess shear stress). Several transition trends (linear or exponential) describing variations in these erosion-related parameters between the two critical mud fractions were tested. Suspended sediment concentrations obtained from simulations were compared to measurements taken on the Bay of Biscay shelf with an acoustic profiler over the entire water column. On the one hand, results show that defining an abrupt exponential transition improves model results regarding measurements. On the other hand, they underline the need to define a first critical mud fraction of 10 to 20%, corresponding to a critical clay content of 3–6%, below which pure sand erosion should be prescribed. Both conclusions agree with results of experimental studies reported in the literature mentioning a drastic change in erosion mode above a critical clay content of 2–10% in the mixture. Results also provide evidence for the importance of considering advection in this kind of validation with in situ observations, which is likely to considerably influence both water column and seabed sediment dynamics.

Hydro-sedimentary numerical models have been widely employed to derive suspended particulate matter (SPM) concentrations in coastal and estuarine waters. These hydro-sedimentary models are computationally and technically expensive in nature. Here we have used a computationally less-expensive, well-established methodology of self-organizing maps (SOMs) along with a hidden Markov model (HMM) to derive profiles of suspended particulate inorganic matter (SPIM). The concept of the proposed work is to benefit from all available data sets through the use of fusion methods and machine learning approaches that are able to process a growing amount of available data. This approach is applied to two different data sets entitled “Hidden” and “Observable”. The hidden data are composed of 15 months (27 September 2007 to 30 December 2008) of hourly SPIM profiles extracted from the Regional Ocean Modeling System (ROMS). The observable data include forcing parameter variables such as significant wave heights ( $Hs$ and $Hs50$ (50 days)) from the Wavewatch 3-HOMERE database and barotropic currents ( $Ubar$ and $Vbar$ ) from the Iberian–Biscay–Irish (IBI) reanalysis data. These observable data integrate hourly surface samples from 1 February 2002 to 31 December 2012. The time-series profiles of the SPIM have been derived from four different stations in the English Channel by considering 15 months of output hidden data from the ROMS as a statistical representation of the ocean for ≈11 years. The derived SPIM profiles clearly show seasonal and tidal fluctuations in accordance with the parent numerical model output. The surface SPIM concentrations of the derived model have been validated with satellite remote sensing data. The time series of the modeled SPIM and satellite-derived SPIM show similar seasonal fluctuations. The ranges of concentrations for the four stations are also in good agreement with the corresponding satellite data. The high accuracy of the estimated 25 h average surface SPIM concentrations (normalized root-mean-square error— $NRMSE$ of less than 16%) is the first step in demonstrating the robustness of the method.

Sénéchal, N., Sottolichio A., Bertrand, F., Goeldner-Giannella, L., Garlan, T., 2013. Analysis of currents and storm surge in a mixed-energy tidal inlet during storm conditions. Coastal morphodynamic processes around tidal inlets in mixed-energy environments are particularly complex due to severe tide and wave conditions. Collecting data in this area is generally very challenging. Here we present the first hydrodynamic data collected in the outer inlet of the Arcachon lagoon, a mixed-energy inlet situated on the southern part of the French Atlantic coast. Data consist in vertical current profiles and sea surface elevations collected on the offshore edge of the ebb delta under various tidal and wave conditions. In particular data were collected during the severe Joachim storm associated to wave heights up to 8 m. Preliminary results indicate that current profiles, intensities and direction are very sensitive to wave conditions. Under energetic conditions, the vertical profile of currents become un...

The goal of this work presented in a two-companion paper is to pave the way for reliably assessing the risks of damage to buildings on the shore, induced by the detonation of large-charge historical ordnance (i.e., countermining) in variable shallow water environments. Here, we focus on the impact of the marine environment, more specifically the unconsolidated sedimentary layer, on detonation-induced seismo-acoustic wave propagation. We rely on a multidisciplinary cross-study including real data obtained within the framework of a countermining campaign, and numerical simulations of the seismo-acoustic propagation using a spectral-element method. We first develop a strategy relying on physical insights into the different kind of waves that can propagate in a coastal environment, to provide clues for a computational cost reduction. The geological surveys and the hydroacoustic measurements provide input data for the 3-D axisymmetric modeling of wave propagation. The numerical simulations, obtained for one specific source–receiver path with a variable sedimentary facies, are compared with the real seismic data induced by the detonation of a charge either on the seabed, or in the water column, and recorded on the coast. Numerical analysis sheds light on the strong interaction between surface waves and the sedimentary facies. The short-scale and deep sedimentary basins favor a local wave-amplitude amplification and a frequency shift toward the low-frequency domain. However, the seismo-acoustic waves are globally attenuated during their propagation because of intrinsic attenuation and geometrical spreading, which generally prevents any large damage to nearby buildings on the shore.