Ángel David Gutiérrez Barceló

Ocean wave extremes drive major coastal hazards, yet whether they evolve consistently with mean conditions under climate change remains unclear. Here, we present a global assessment of projected changes in wave height extremes based on a large CMIP6 multi-model ensemble, combining complementary extreme-value and exceedance-based approaches within a structured uncertainty framework. We find consistent increases in extreme wave heights across the mid-latitudes of the North Pacific, North Atlantic and Southern Ocean. In the North Atlantic and North Pacific, however, such increases occur despite projected decreases in mean conditions, demonstrating that changes in extremes cannot be reliably inferred from the changes in the means. Coastal indicators reveal sensitivity to projected wave climate changes along more than 60% of the global coastline. Together, our results show that wave extremes exhibit coherent large-scale changes that can diverge from changes in mean conditions, highlighting the need for dedicated assessments of future coastal hazards based on extremes.

Coastal regions are increasingly at risk of flooding and erosion from sea-level rise, extreme storm events, and anthropogenic activities. Wave runup, which can contribute over 50% to extreme total water levels at the shoreline, is a critical driver of both flooding and morphological change. Consequently, accurate prediction of wave runup is essential for assessing coastal risk and enhancing the resilience of coastal infrastructure, especially during high tides and storm events. However, directly measuring wave runup under storm conditions is challenging due to its highly dynamic nature. To address this, shore-based video systems provide a practical, non-intrusive solution for continuous, wide-view monitoring of shoreline movement.This research focuses on storm-induced wave runup along the dissipative, gently sloping beach of Villers-sur-Mer in northwest France, integrating a phase-resolving numerical model (SWASH) with high-resolution shore-based video observations. The study assesses the contributions of infragravity waves and sea-swell components to shoreline excursions and total water levels during energetic conditions, providing critical insights for flood risk assessments and coastal resilience strategies. Villers-sur-Mer is characterized by a macrotidal, highly dissipative system, notable for its complex nearshore bathymetry, strong wave-tide interactions, a gentle intertidal slope (~1%), and a substantial tidal range (approximately 3–10 m). Since 2019, a video monitoring system has been collecting 10-minute timestacks at a frequency of 2 Hz, capturing shoreline position and runup variability, providing a robust dataset for model validation under macrotidal conditions. The SWASH model was configured in two-dimensional, non-hydrostatic mode at high spatial resolution, using an October 2019 LiDAR-derived topo-bathymetric surface, forced with conditions from Storm Ciara (February 2020), one of the most energetic storms to impact the French coastline.Model results show spatial variability in water levels, with relatively weak offshore gradients that intensify toward the inner surf zone where bathymetric slopes and curvature are greater. Along the representative transect, the wave energy spectra reveal a persistent sea–swell peak that diminishes shoreward, reflecting the strong dissipation characteristics of the surf zone. The video timestacks show quasi-periodic shoreline excursions, indicative of low-frequency modulation of runup during energetic conditions; runup maxima align with the arrival of distinct swash fronts. More gently sloping, highly dissipative sections display predominantly infragravity-driven shoreline motion, with smaller excursion amplitudes under comparable offshore forcing.Overall, the integrated framework provides process-based insights into storm-driven runup on macrotidal, dissipative coasts, supporting improved site-specific hazard mapping, flood risk assessment, and early-warning applications. By resolving the joint roles of infragravity and sea–swell motions in controlling runup and shoreline excursions during severe storms, the study advances process-informed coastal resilience planning and design for dissipative beach environments.

Climate wave data, derived from significant wave height (SWH) altimetry, provide accurate information towards nearshore and coastal areas. Their use is crucial to enhance our capabilities of observing, understanding, and forecasting storm waves, even in complex coastal basins. In this study, SWOT nadir data were combined with nine existing altimeters for assessing waves and monitoring their evolution during storms in the English Channel, near UK–French coasts. Validation against wave buoys and numerical models shows high accuracy, with correlations around 95%, decreasing to 85% when buoy track offsets > 50 km, producing the largest errors. The multi-source approach enables depth-resolved monitoring, with SWH mapping revealing ~20–25% modulation in the Channel and ~36% dissipation near the Seine Bay during storms. Spectral analysis of multi-source altimeter-derived merged observations improve time-sampling, resolving high-frequency variability from monthly to daily scales and capturing ~75% of storms. Most storm wave features along altimetry tracks are resolved, with CFOSAT mapping nearshore areas and SWOT capturing coastal zones, both achieving ~80% variance. This temporal and spatial monitoring would be further enhanced with SWOT’s 2D wide swath. This finding provides a complementary, comprehensive understanding of coastal waves and offers valuable input for data assimilation, to improve storm wave estimates in coastal basins.

Nowadays, most satellite altimeters have proven effective in providing datasets on sea surface changes in oceans towards nearshore zones. The need to extend satellite sea level and wave records closer to the coast has been a focus of several studies investigating the accuracy of observations in shallow water areas and coastal zones, with the aim of monitoring spatial and temporal changes in the hydrodynamics. The recently launched Surface Water and Ocean Topography (SWOT), is the first wide-swath interferometric altimetry mission. Here, we integrate SWOT observations with 9 existing altimeters for assessing the Significant Wave Height (SWH) at regional and local scales with a special focus on high energetic periods. The monitoring of storms using multisource altimeters offer the possibility to enhance their evolution with full coverages. The combination of SWOT with conventional altimeters improves the time-sampling to capture the variability modes from inter-monthly to inter-daily scales, including storms; and the spatial sampling that would be expanded thanks to the SWOT 2D consistent view. The frequency analysis of SWH along tracks highlights the wavelength of wave features and their changes during storms. This challenging issue benefits from ongoing works of new altimeters, including SWOT, to offer reliable estimates of coastal waves.

Storms are some of the most powerful natural phenomena on Earth. From a climate perspective, they are driven by atmospheric perturbations that induce high waves and extreme storm surges. Coastal storms can have significant effects on coastal environments, including erosion of beaches, coastal infrastructure damage, or flooding, among other impacts. These impacts will likely increase with climate change and the rising human pressure due to the overexpansion of urbanization and infrastructure that takes place in these areas. Among all these risks, flooding in coastal and low-lying beach environments, is considered one of the major hazards that humanity must face, especially in recent decades. Investigating coastal flooding and its impact on coastal systems requires the consideration of the different natural drivers induced by marine forces with their specific return period. In this context, and in the framework of the Normandy coastal risk management, an integrative approach of numerical modelling has been developed to generate waves and sea-level hydrodynamics from the Atlantic Sea and the North Sea to the English Channel. In this context, our work aims to examine the dynamics of coastal storms and their impacts close to Normandy coasts with gravel and composite beaches. Employing the outputs of the downscaling, extreme waves impacting on the coast have also been investigated at a local scale, simulating real storms on different profiles on the beach of Hautot-sur-Mer, France. This beach is an interesting location because of its morphology in the intertidal part of the profile. It is formed by a layer of sand with a mild slope covered by a layer of pebbles forming a steep slope. We have used the numerical model SWASH to obtain overwash and overtopping data in response to high energetic intervals of time for different sections of the beach. This allowed us to define the protective effect of the pebbles. The characterization of this effect has highlighted the importance of employing groins to slow down coastal transit in order to create natural morphologies that serve as barriers against extreme events produced by the action of the sea. In particular, the central part of this beach, sustained by groins, presented a much more dissipative response to high waves than the sides, where this kind of structures are no longer maintained.

This research was carried out in the framework of the Surface Water and Ocean Topography (SWOT) program of the French National Centre of Space Studies (CNES). In the context of global climate change, increases in frequency and intensity of extreme events are considered potential threats. Hence, many efforts have been devoted to acquiring a better understanding of the natural processes driving the hydrodynamics’ multiscale variability in order to produce a more accurate estimation of their fluctuations. In this situation, and in the framework of Normandy’s coastal risk management, the dynamics of extreme events were investigated from regional scales (the English Channel basin) to local scales (the Normandy beaches in Etretat and Hautot-sur-Mer). Offshore wave data were obtained and hourly wave data were simulated for two years using the SWAN (Simulating WAves till Nearshore) model, including ten highly energetic stormy events selected by their different physical characteristics. Spectral analysis was conducted to compare the transformation in frequencies of the wave spectra from the Channel to shallow waters. A special focus was given to the hydrodynamics close to Normandy during these extreme events depending on their duration, their strength, and the persistence of the maximum wave height.