Dynamics of Nearshore Waves during Storms: Case of the English Channel and the Normandy Coasts

Abstract

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.

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“…According to Lopez et al [29], dominant waves originate in the Atlantic Ocean, characterized by Gaussian behavior (Figure 2a) with a bimodal distribution, indicating the presence of sea and swell waves (Figure 2b). This behavior is consistent from deep water to shallow areas, with a reduction in average wave height (H s ) of around 50% and 25% of energy dissipation, respectively, from the Atlantic to the English Channel due to changes in water depth [29]. During storms, changes in wave direction occur, with an average clockwise variation of 18 • at the peak of the storm, shifting from the sectors W-WNW to sectors WNW-NW.…”

“…During storms, changes in wave direction occur, with an average clockwise variation of 18° at the peak of the storm, shifting from the sectors W-WNW to sectors WNW-NW. These changes are predominantly influenced by bathymetry variation and bottom friction, leading to wave refraction and energy dissipation processes [29]. According to Lopez et al [29], dominant waves originate in the Atlantic Ocean, characterized by Gaussian behavior (Figure 2a) with a bimodal distribution, indicating the presence of sea and swell waves (Figure 2b).…”

“…These changes are predominantly influenced by bathymetry variation and bottom friction, leading to wave refraction and energy dissipation processes [29]. According to Lopez et al [29], dominant waves originate in the Atlantic Ocean, characterized by Gaussian behavior (Figure 2a) with a bimodal distribution, indicating the presence of sea and swell waves (Figure 2b). This behavior is consistent from deep water to shallow areas, with a reduction in average wave height (H s ) of around 50% and 25% of energy dissipation, respectively, from the Atlantic to the English Channel due to changes in water depth [29].…”

“…During storms, changes in wave direction occur, with an average clockwise variation of 18 • at the peak of the storm, shifting from the sectors W-WNW to sectors WNW-NW. These changes are predominantly influenced by bathymetry variation and bottom friction, leading to wave refraction and energy dissipation processes [29].…”

“…The wave data were initially extracted from a wave model [29] as there is no active buoy close to the beaches of Normandy. Waves were then propagated to shore using the SWAN model [33].…”