Integrated Numerical Modeling and Video Observations of Storm-Driven Wave Runup on a Dissipative Macrotidal Beach

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.