On the influence of reflection over a rhythmic swash zone on surf zone dynamics

The reflection of incident gravity waves over an irregular swash zone morphology and the resulting influence on surf zone dynamics remains mostly unexplored. The wave-phase resolving SWASH model is applied to investigate this feedback using realistic low-tide terraced beach morphology with well-developed beach cusps. The rhythmic reflection generates a standing wave that mimics a subharmonic edge wave, from the superimposition of incident and two-dimensional reflected waves. This mechanism is enhanced by shore-normal, narrow-banded waves in both direction and frequency. Our study suggests that wave reflection over steep beaches could be a mechanism for the development of rhythmic morphological features such as beach cusps and rip currents

High-resolution marine flood modelling coupling overflow and overtopping processes: framing the hazard based on historical and statistical approaches

A modelling chain was implemented in order to propose a realistic appraisal of the risk in coastal areas affected by overflowing as well as overtopping processes. Simulations are performed through a nested downscaling strategy from regional to local scale at high spatial resolution with explicit buildings, urban structures such as sea front walls and hydraulic structures liable to affect the propagation of water in urban areas. Validation of the model performance is based on hard and soft available data analysis and conversion of qualitative to quantitative information to reconstruct the area affected by flooding and the succession of events during two recent storms. Two joint probability approaches (joint exceedance contour and environmental contour) are used to define 100-year offshore conditions scenarios and to investigate the flood response to each scenario in terms of (1) maximum spatial extent of flooded areas, (2) volumes of water propagation inland and (3) water level in flooded areas. Scenarios of sea level rise are also considered in order to evaluate the potential hazard evolution. Our simulations show that for a maximising 100-year hazard scenario, for the municipality as a whole, 38 % of the affected zones are prone to overflow flooding and 62 % to flooding by propagation of overtopping water volume along the seafront. Results also reveal that for the two kinds of statistic scenarios a difference of about 5 % in the forcing conditions (water level, wave height and period) can produce significant differences in terms of flooding like +13.5 % of water volumes propagating inland or +11.3 % of affected surfaces. In some areas, flood response appears to be very sensitive to the chosen scenario with differences of 0.3 to 0.5 m in water level. The developed approach enables one to frame the 100-year hazard and to characterize spatially the robustness or the uncertainty over the results. Considering a 100-year scenario with mean sea level rise (0.6 m), hazard characteristics are dramatically changed with an evolution of the overtopping ∕ overflowing process ratio and an increase of a factor 4.84 in volumes of water propagating inland and 3.47 in flooded surfaces

Natural remobilization and historical evolution of a modern coastal transgressive dunefield

International audienceThe vast majority of coastal dunes in Europe have been stabilized by increasing vegetation cover since the mid‐20th century. However, some systems may experience a remobilization phase, generally occurring locally and further propagating alongshore, the drivers of which remain poorly documented. This study investigates the evolutionary paths (stabilization/destabilization/remobilization) from 1945 to 2020 of a 2 km‐long modern coastal transgressive dunefield located in southwest France with a holistic approach (GPR profiles, aerial photographs and LiDAR topographic data). Results show a landward migration of the transgressive dune by approximately 233 ± 7.5 m, through two distinct stages of rapid landward migration from 10 to 23 m/yr (Stage I: 1949–1959 and Stage III: 2000–2021) separated by an approximately 40‐year stage of slow to no migration, but with substantial windward slope deflation (Stage II). The onset of Stage II is due to the fixation of vegetation by human action between 1950 and 1959. The onset of Stage III is hypothesized to be driven by long and sustained upper backshore/dune toe erosion beginning in 1968 due to a massive shoal welding that locally disturbed the longshore drift. It induced a destabilization of the dune and erosion of the vegetation cover over some decades. A non‐synchronization is therefore observed between the start of the perturbation (1968), then the migration (2000), in line with the hysteresis concept of Tsoar (2005). This study shows that almost all of the sedimentary volume of the 1945 dune has been remobilized by translation to shape the dune system in its current form. The 2.2 km dunefield has grown by approximately 673 000 ± 190 000 m 3 during the 2005–2020 period. Among this volume, there is a new foredune that was built from 2005 between the upper beach and the transgressive dune (volume in 2020 of about 394 000 ± 68 000 m 3 )

Correction: Almar et al. Sea State from Single Optical Images: A Methodology to Derive Wind-Generated Ocean Waves from Cameras, Drones and Satellites. Remote Sens. 2021, 13, 679

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Coastal flood: a composite method for past events characterisation providing insights in past, present and future hazards—joining historical, statistical and modelling approaches

International audienc