Naval Hydrographic and Oceanographic Service

Coastal boulder deposits (CBD) provide relevant information on coast-erosion wave events, -defined here as extreme events combining energetic storm wave and high spring tide level-, and are crucial for understanding giant storm wave impacts on coastal geomorphology. Banneg Island, located off the coast of Brittany (France), faces Atlantic storm waves that are not attenuated in this highly exposed area. This island is characterized by CBD resulting from the erosion of western cliffs, that have accumulated at the top of the island as a result of storm wave/inudation processes. These accumulations have been subjected to topo-morphological survey for the last two decades (using GNSS measurements from 2005 to 2012, and using UAV-based measurements since 2012). Hydrodynamic conditions (wave and water levels) were also analyzed. We used these data to analyse the frequency of extreme events over the last 20 years. The results show a good relationship between boulder transport (measured as volume of boulder displacement) and the frequency of extreme events inundating the island. Over the survey period, we found significant interannual variability in the occurrence of these extreme events, depending on winter weather conditions. This storminess variability is significantly correlated with the WEPA climatic index, calculated on the 3 winter months (December to February). Finally, the results from Banneg Island were compared with long-term monitoring carried out in other sectors along the coast of Brittany, to build a comprehensive picture of storminess and its impact on coastal change in Brittany over the last 20 years.

Coral reefs serve as a highly effective natural barrier against incoming ocean waves. However, climate change and human-induced degradations are threatening reefs, reducing their capacity to protect coastal populations by diminishing the frictional processes that dissipate wave energy. The most common approach to represent wave energy dissipation through bottom friction relies on the estimation of a representative roughness length, whose definition on coral environments remains to be elucidated. As a consequence, a generic parametrization for friction processes on coral reef systems is still lacking. In this study, high-resolution hydrodynamical and topographical data collected at the South-West barrier reef of Mayotte, Indian Ocean, were used to perform an estimation of wave friction. The hydraulic roughness estimated across the reef varies spatially, and attempts are made to connect this information with the diverse bed morphologies observed in the field.

Natural estuarine morphology exerts strong control over tidal propagation. Human activities, such as dredging and land reclamation, modify the natural geometry, altering tidal dynamics and the ecosystems linked to them. Here we analyse changes in tidal dynamics, specifically the amplitude and propagation of tides, over decadal to centennial timescales, using archival maps, hydrographic surveys, tide gauge records and modern records from 25 estuaries worldwide, spanning the coast to their landward boundaries. Over the past two centuries, local interventions have typically amplified tidal ranges, accelerated tidal propagation and shifted tidal duration asymmetry. The most pronounced changes occurred far inland, often more than 100 km from the coast. Land reclamation and channel deepening are the most widespread and impactful interventions, affecting nearly all systems studied. The magnitude and inland location of maximum changes point to local human activities as the dominant drivers, exceeding the influence of long-term processes such as sea-level rise and natural subsidence and demonstrating that anthropogenic modifications have historically had the larger influence on estuarine water levels. Recognizing this human footprint opens opportunities for targeted local management strategies to reverse past changes, reduce flood risk and build resilience to climate change. Human activities such as dredging and land reclamation have pervasively altered the ways that tides propagate through large estuaries with negative ecological impacts, according to an analysis of recent and historical records.

Coastal flooding is increasing due to rising sea levels and changes in extreme weather events such as heightened wave energy, winds, and surges. This poses a significant threat to coastal cities. The study focuses on assessing the risk of coastal flooding in Taghazout and Imourane Bay (Moroccan Atlantic), which includes sandy beaches and a valuable tourist and commercial district. The methodology involves estimating extreme sea levels using the NWWIII model, the SWAN wave propagation model, statistical analysis of extremes (GEV model), and sea level rise scenarios. Results are mapped onto a high-resolution Digital Terrain Model based on recent topo-bathymetric data, supplemented by GEBCO data. Currently, Imourane is more exposed to the risk, especially in its southern part, while Taghazout is less exposed, particularly in its northern part due to its bay shape and protection from Anchor Point and Cape Ghir. Future projections for 2100 suggest the entire bay could be submerged, threatening seaside infrastructure. This underscores the need for coastal managers to develop a protective strategy against this risk.

We present a large-scale laboratory model of the Strait of Gibraltar that reproduces realistic topography, tidal forcing, stratification, and rotation, enabling controlled investigation of key exchange processes linking the Mediterranean Sea and Atlantic Ocean. Velocity and density measurements confirm dynamic similarity with ocean observations. Analysis of the flow near Camarinal Sill shows that bottom boundary layers are the primary source of turbulent kinetic energy, exceeding contributions from shear at the interface between Atlantic and Mediterranean waters. The enhanced role of bottom-generated turbulence is linked to separation of the Mediterranean gravity current induced by an adverse pressure gradient during outflow, providing a new explanation for the well-documented detachment of the Mediterranean plume west of the sill. This detachment intensifies during spring tides, driving diluted waters farther into the Atlantic, while during neap tides bottom-generated and interfacial turbulence coincide, offering a consistent explanation for the high dissipation rates reported in field measurements. Overall, tidal forcing promotes full-depth mixing, with up to 20 % density reduction west of CS and oscillatory 20 % variations east, consistent with field data, and simultaneously introducing an important shift between vertical gradients of along-strait velocity and density, with implications for parameterizing turbulent exchange and definition of the composite internal Froude number for reliable diagnostics of hydraulic control. During spring tides, hydraulic control is intermittently lost during inflow (for about 50 % of the tidal cycle) and this loss propagates eastward, while additional control points arise west of the sill. Neap tides exhibit signatures of control which persist much longer during a tidal cycle (up to 88 %) as compared to spring tides, but does not propagate to the east when the tide reverses, offering an explanation of the different release of internal solitary waves observed in the literature (Roustan et al., 2024a). Transport and energy budgets reveal strong longitudinal and transverse variability, highlighting the need for fully three-dimensional diagnostics. Volume and buoyancy transports, dominated by transverse topographic variability, exceed tidal and turbulent transports by up to two orders of magnitude, confirming net Atlantic inflow. These results demonstrate that high-fidelity laboratory modeling can capture the essential three-dimensional dynamics of energetic straits and provides a powerful complement to observational and numerical approaches.