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International audience; Advances in image-based remote sensing using unmanned aerial vehicles (UAV) and structure-from-motion (SfM) photogrammetry continue to improve our ability to monitor complex landforms over representative spatial and temporal scales. As with other water-worked environments, coastal sediments respond to shaping processes through the formation of multi-scale topographic roughness. Although this topographic complexity can be an important marker of hydrodynamic forces and sediment transport, it is seldom characterized in typical beach surveys due to environmental and technical constraints. In this study, we explore the feasibility of using SfM photogrammetry augmented with an RTK quadcopter for monitoring the coastal topographic complexity at the beach-scale in a macrotidal environment. The method had to respond to resolution and time constraints for a realistic representation of the topo-morphological features from submeter dimensions and survey completion in two hours around low tide to fully cover the intertidal zone. Different tests were performed at two coastal field sites with varied dimensions and morphologies to assess the photogrammetric performance and eventual means for optimization. Our results show that, with precise image positioning, the addition of a single ground control point (GCP) enabled a global precision (RMSE) equivalent to that of traditional GCP-based photogrammetry using numerous and well-distributed GCPs. The optimal model quality that minimized vertical bias and random errors was achieved from 5 GCPs, with a two-fold reduction in RMSE. The image resolution for tie point detection was found to be an important control on the measurement quality, with the best results obtained using images at their original scale. Using these findings enabled designing an efficient and effective workflow for monitoring coastal topographic complexity at a large scale.

International audience; The Sillon de Talbert (Northern coast of Brittany) is a large 3.5 km-long swash-aligned gravel spit comprising a volume of sediment of 1.23x106 m3. Since 2002, a morphodynamic survey based on annual DEMs, and waves and water level measurements and/or modeling, has been carried out. The 17-year (2002-2019) monitoring program shows that cross-shore sediment transfers reaching 430,000 m3 are dominant, while the longshore sediment transfer -through cannibalization process- is about 52,000 m3. The maximum landward displacement of the spit due to rollover processes reaches –4 m.yr-1. Storm events control more than 95% of this retreat due to catastrophic overwash/inundation processes that led to the opening of a breach in March 2018. The morphological evolution of the Sillon de Talbert is driven by anthropogenic forcing (i.e., impact of coastal defence structures, cutting off of longshore sediment transport), and natural forcing such as the depletion of the supply of sediment from the platform.

International audience; The Sillon de Talbert is situated on the Northern coast of Brittany; it is a large 3.5 km-long swash-aligned gravel spit barrier comprising a volume of sediment of 1.23x106 m3. Since 2002 a morphodynamic survey was carried. It is based on annual DEMs and monthly beach profile measurements. Waves and water levels are also surveyed using modelling data and field measurements. The 15-years (2002-2017) sediment budget calculation shows that cross-shore transfers are dominant and represent a total volume of 370,000 m3, while the longshore sediment transfer is estimated at about 50,000 m3. However, considering the volume of sediment eroded from the beachface (as much as –411,000 m3), and the cross-shore and longshore sediment accumulation (reaching +420,000 m3), the global sediment budget is balanced for the whole survey period. The landward displacement of the spit due to rollover processes has increased almost doubled during the last 15 years with rates of retreat of 2 m.yr-1 between 2002 and 2017, vs 1.2 m.yr-1 between 1930 and 2010. Storm events control more than 95% of this retreat due to catastrophic overwash/inundation processes. The spit retreat has led to the opening of a breach in the proximal section at the beginning of March 2018, which has experienced a rapid enlargement (35 vs 15 m) and deepening (3.4 vs 1.25 m) over the last seven months. The morphological evolution of the Sillon de Talbert is driven by anthropogenic forcing (i.e., impact of coastal defence structures, cutting off of longshore sediment transport), and natural forcing such as the depletion of the supply of sediment from the platform, and the increase of extreme storm events over the last few years.; Le Sillon de Talbert est situé sur la côte nord de Bretagne ; c’est une flèche littorale de galets faisant face à la houle, qui s’étend sur 3,5 km et dont le volume de sédiment atteint 1,23 106 m3. Depuis 2002 un suivi morphodynamique a été entrepris.Il est basé sur des levés annuels, permettant de calculer des MNT, et des mesures mensuelles de profils de plage. Le calcul du bilan sédimentaire sur les 15 ans (2002-2017) montre que les transits transversaux sont dominants et représentent un volume total de 370 000 m3, alors que le transit longitudinal est estimé à environ 50 000 m3. Toutefois, considérant le volume de plus de –411 000 m3 de sédiment érodé à la plage avant, et le volume transversal et longitudinal accumulé atteignant +420 000 m3, le bilan sédimentaire global sur l’ensemble de la période de suivi est équilibré. Le déplacement vers la terre lié au phénomène de rollover a doublé durant les quinze dernières années avec un taux de recul de 2 m/an entre 2002 et 2017, contre 1,2 m/an entre 1930 et 2010. Les évènements tempétueux contrôlent plus de 95 % de ce recul lors des processus catastrophiques de submersion/inondation. Le recul de la flèche a conduit, au début du mois de mars 2018, à l’ouverture d’une brèche dans la partie proximale qui s’est rapidement élargie et approfondie, passant respectivement de 15 à 35 m, et de 1,25 à 3,4 m, au cours des 7 derniers mois. L’évolution morphologique du sillon de Talbert est commandée par des forçages anthropiques (i.e., l’impact des ouvrages de défense côtière qui bloquent le transit sédimentaire longitudinal), et des forçages naturels, comme la pénurie de sédiments accumulés sur la plateforme pour l’alimentation de la flèche, ou l’augmentation des évènements extrêmes durant les dernières années.

The paper analyses the conservation policy devised for the Bas-Champs coast of Picardy. The policy proposes a coastal defence strategy that raises a large number of controversial issues, calling into question the overall management of this coast., La politique de conservation du trait de côte des Bas- Champs de Cayeux (Somme, Picardie) est analysée dans cet article. Cette stratégie de défense contre la mer pose un nombre important de questions et de controverses qui remettent en cause la politique globale de gestion de cette région., Dolique Franck. Le littoral des Bas-Champs de Cayeux (Somme) : conflits et controverses pour une stratégie de défense contre la mer / The Bas-Champs de Cayeux coast (Picardy) : conflicts and controversy over a sea defence strategy. In: Revue de géographie de Lyon, vol. 74, n°1, 1999. Géographie des littoraux : la nature et les hommes. pp. 59-64.