Jules Fleury

A Jerba (Sud-Est tunisien), l’érosion marine pose un problème majeur qui menace les côtes d’une île aussi célèbre et les belles plages sableuses de cette destination touristique internationale. La frange littorale est aussi particulièrement riche en vestiges archéologiques présentant une grande vulnérabilité face à l’érosion des rivages. La superposition des traits de côte multi-dates des côtes meubles montre une tendance vers la régression qui se manifeste à travers leur rétrécissement et leur migration vers l’intérieur. Cette dynamique est due à la vulnérabilité du cadre naturel mais aussi à l’essor touristique et à la densification de l’occupation du littoral, à l’origine de différentes formes de dégradation et de menaces pour des aménagements récents ou pour des vestiges archéologiques.

Hyper-DRELIO (Hyperspectral DRone for Environmental and LIttoral Observations) is a custom, mini-UAV (unmanned aerial vehicle) platform (® and a field spectrometer, allowing performance of an initial calibration of the sensor in order to correct “vignetting effects” and a field standardization to convert digital numbers (DN) collected by the hyperspectral camera to reflectance, taking into account the time-varying illumination conditions. Radiometric corrections are applied to a subset of a dataset collected above mudflats colonized by pioneer mangroves in French Guiana. The efficiency of the radiometric corrections is assessed by comparing spectra from Hyper-DRELIO imagery to in situ spectrometer measurements above the intertidal benthic biofilm and mangroves. The shapes of the spectra were consistent, and the spectral angle mapper (SAM) distance was 0.039 above the benthic biofilm and 0.159 above the mangroves. These preliminary results provide new perspectives for quantifying and mapping the benthic biofilm and mangroves at the scale of the Guianese intertidal mudbanks system, given their importance in the coastal food webs, biogeochemical cycles, and the sediment stabilization.

The Senegal River delta in West Africa, one of the finest examples of “wave-influenced” deltas, is bounded by a spit periodically breached by waves, each breach then acting as a shifting mouth of the Senegal River. Using European Re-Analysis (ERA) hindcast wave data from 1984 to 2015 generated by the Wave Atmospheric Model (WAM) of the European Centre for Medium-Range Weather Forecasts (ECMWF), we calculated longshore sediment transport rates along the spit. We also analysed spit width, spit migration rates, and changes in the position and width of the river mouth from aerial photographs and satellite images between 1954 and 2015. In 2003, an artificial breach was cut through the spit to prevent river flooding of the historic city of St. Louis. Analysis of past spit growth rates and of the breaching length scale associated with maximum spit elongation, and a reported increase in the frequency of high flood water levels between 1994 and 2003, suggest, together, that an impending natural breach was likely to have occurred close to the time frame of the artificial 2003 breach. Following this breach, the new river mouth was widened rapidly by flood discharge evacuation, but stabilised to its usual hydraulic width of <2 km. In 2012, severe erosion of the residual spit downdrift of the mouth may have been due to a significant drop (~15%) in the longshore sand transport volume and to a lower sediment bypassing fraction across the river mouth. This wave erosion of the residual spit led to rapid exceptional widening of the mouth to ~5 km that has not been compensated by updrift spit elongation. This wider mouth may now be acting as a large depocentre for sand transported alongshore from updrift, and has contributed to an increase in the tidal influence affecting the lower delta. Wave erosion of the residual spit has led to the destruction of villages, tourist facilities and infrastructure. This erosion of the spit has also exposed part of the delta plain directly to waves, and reinforced the saline intrusion within the Senegal delta. Understanding the mechanisms and processes behind these changes is important in planning of future shoreline management and decision-making regarding the articulations between coastal protection offered by the wave-built spit and flooding of the lower delta plain of the Senegal River.

Photogrammetry is an old remote sensing technique that has undergone significant new developments related to the emergence of new computer vision algorithms, notably the workflow technique called Structure-from-Motion (SfM photogrammetry). These innovations facilitate the utilization of this technique by non-specialists. SfM photogrammetry now enables the production of high-resolution morphometric 3D models and derived products such as digital surface models (DSMs) and orthophotographs. We conducted three SfM-photogrammetric experiments, in French Guiana on an embayed beach in Cayenne and on a small mudflat sector of a coastal mud bank near the Sinnamary estuary, and finally in the French southern Alps on a section of the braided Buëch River near Sisteron. We developed a simple field protocol based on subvertical aerial photography from a microlight aircraft at low altitude (1000 feet). We generated DSMs and the other products with user-friendly Agisoft Photoscan Professional © software. These SfM implementations were aimed at highlighting the subtle morphological variations characterising these features and some of the processes involved in morphological change. The DSMs were produced at a resolution of 5 to 10 cm per pixel and had a mean vertical accuracy less than 10 cm compared to the Real-Time Kinematic Differential Global Positioning System (RTK-DGPS) control points, with a maximum of 20 cm in marginal sectors near high vegetation such as trees and shrubs and on wet surfaces close to water-filled channels or in the lower intertidal zone. Low-cost, user-friendly SfM photogrammetry offers interesting new perspectives in the fields of coastal and fluvial geomorphology requiring high-resolution topographic data. The technique combines the advantages of the reproducibility of GPS topographic surveys and the high density and accuracy of airborne LIDAR, but at very advantageous cost compared to the latter.