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.

Swell propagation and transformation across variable bathymetry and coastal obstacles fundamentally control nearshore wave energy redistribution, sediment transport, and shoreline evolution. Conventional satellite altimetry cannot adequately resolve these dynamics, particularly refraction and diffraction as swell transition into shallow waters. Leveraging high‐rate (HR) observations from the Surface Water and Ocean Topography (SWOT) mission, we present the first spaceborne measurements of nearshore‐coastal wave transformation at sub‐kilometric scales. SWOT HR observations over the English Channel during Storm Mathis reveal wavelength shortening, directional shift and distinctive fan‐shaped diffraction patterns, including a 27.5° island‐induced diffraction signal undetected by operational models. Comparisons with in situ measurements and the Atlantic‐European Northwest Shelf (NWShelf) Reanalysis, a regional configuration of the WAVEWATCH III spectral wave model, validate the robustness of the SWOT‐derived wave parameters. This finding establishes SWOT HR data as a new observational capability for quantifying wave‐bathymetry interactions and the resultant redistribution of wave energy that conventional altimetry fails to resolve.

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.

Storm waves undergo rapid transformation as they propagate into shallow, complex coastalwaters, yet these processes are difficult to observe at the scales at which they occur. We assess the capability of Surface Water and Ocean Topography (SWOT) High-Rate (HR) altimetry to map storm-wave wavelength, propagation direction and period across the English Channel. Four storms (Mathis, Ciarán, Debi and Nelson, 2023–2024) were analysed over HR scenes on 5 km boxes, using two independent spectral estimators (2D Welch and Radon–Fourier). The 180° ambiguity was resolved from the SSH–𝜎 0 cross-spectrum, period was derived through finite- depth dispersion including a depth-weighted current Doppler correction, and every retrieval carries bootstrap uncertainty and an observability grade. Propagation direction is the most robust product, it agrees with a WAVEWATCH III forecast within about 11° RMSE, and the two estimators agree within 10° in 96% of common boxes. Wavelength is systematically longer than modelled (+32 to +45 m). Decomposition of the retrieved wavevector attributes this bias to across-track compression near nadir and along-track compression consistent with known KaRIn imaging distortions, which the derived period inherits. Relative changes along transects are nevertheless robust. For Storm Ciarán, SWOT HR resolves 19% depth-controlled wavelength shortening consistent with linear dispersion, refraction reproduced in sign and magnitude by ray tracing, and a 36–49° directional rotation with spectral broadening in the lee of Sark, characteristic of diffraction. SWOT HR thus provides a new spatial observation of coastal storm- wave transformation, while inversion of the imaging response is required to retrieve absolute wavelength, period and energy.

Coastal water levels during storms result from interacting tides, storm surges, waves, currents, bathymetry, and coastal geometry, producing spatially heterogeneous responses that remain difficult to characterize from point observations alone. The Surface Water and Ocean Topography (SWOT) mission provides a new opportunity to investigate this complexity through wide-swath High-Resolution (HR) observations of sea surface height (SSH). Here, SWOT HR water levels are first evaluated against five tide gauges in the English Channel and subsequently analyzed at three contrasting sites, Cherbourg, Villers-sur-Mer, and Étretat, using complementary spatial and scale-resolved approaches. SWOT HR shows strong agreement with tide gauges at the best-performing sites, with RMSE values of approximately 0.11 m and correlations exceeding 0.99, while accuracy remains site dependent. Beyond point-based validation, the two-dimensional HR fields reveal pronounced spatial heterogeneity, including localized and elongated SSH anomalies with distinct geometries and orientations. EOF decomposition shows that more than half of the variance remains beyond the first six modes at all three sites, highlighting the complexity of the HR spatial signal. These residual structures are spatially organized rather than random, while their point-by-point relationship with radar backscatter (σ⁰) remains negligible. Two-dimensional Morlet wavelet analysis identifies a recurrent energetic range of approximately 0.5–1.0 km across the three domains, showing strongly contrasting intensity, localization, geometry, and persistence across neighboring scales. Comparison with independent wave, surface-current, and bathymetric fields further shows that this variability occurs within markedly different hydrodynamic and morphological environments, consistent with the combined influence of storm forcing and local coastal configuration rather than a single controlling process. The complementarity of these analyses demonstrates the added value of SWOT HR for resolving the two-dimensional, multiscale complexity of storm-time coastal SSH, providing new spatial information for understanding coastal dynamics and for high-resolution model validation and data assimilation.