Wave Dissipation and Mean Circulation on a Shore Platform Under Storm Wave Conditions

Abstract

While wave processes on shore platforms have been recently advanced by a number of field‐based studies, few attention has been paid to the role of bed roughness on wave dissipation and wave setup dynamics in these environments. This study reports on a new field experiment conducted under storm wave conditions on a gently sloping shore platform which was instrumented from 10 m water depth up to the shoreline. Data analyses are complemented with numerical simulations performed with a 3D fully coupled modeling system using a vortex force formalism to represent the effects of short waves on the mean circulation. An accurate representation of wave dissipation by both depth‐induced breaking and bottom friction is found essential to reproduce the transformation of short waves across the platform and the resulting wave setup. Wave energy dissipation by bottom friction is dominant in the subtidal part of the platform and contributes to about 40% of the total wave energy dissipation. The enhanced wave bottom friction on the platform decreases the wave height before breaking, which reduces the contribution of wave forces to the wave setup compared to a smooth bottom (mechanism 1). Conversely, an idealized analysis of the cross‐shore momentum balance reveals that the wave‐induced circulation increases the wave setup, this process being enhanced on a rough bottom (mechanism 2). The contribution of mechanism 2 increases with the bottom slope, accounting for up to 26% of the wave setup for a 1:20 sloping shore platform, and overcoming mechanism 1.

Wave-induced mean currents and setup over barred and steep sandy beaches

“…The wave setup predictions in the 2D-VF configuration are mostly explained by the η wafo relative contribution, with an additional contribution coming from the bottom shear stress. At the shoreline, where the beach is the steepest (1:14), the depth-varying circulation explains 18-20% of the computed wave setup, which is consistent with the results obtained over planar beaches by Guérin et al (2018) and Lavaud et al (2022). Among the depth-varying circulation terms, the vertical mixing term is dominant and accounts for 10-15% of the wave setup across the entire surf zone (Fig.…”

“…1 2 34 5 67 this is then extremely relevant for nearshore applications of this model at regional and national scales (Guérin et al, 2018;Pezerat et al, 2021Pezerat et al, , 2022Lavaud et al, 2020Lavaud et al, , 2022.…”

“…In the surf zone, the wave forces associated with depth-induced breaking processes are the dominant forcing term for the wave setup and its cross-shore evolution (e.g., see Guérin et al, 2018;Lavaud et al, 2022). It is thus essential to accurately reproduce the cross-shore evolution of wave heights in order to reduce as much as possible the bias in wave setup predictions owing to the wave forcing.…”

“…The accuracy of the wave predictions gives us great confidence for analysing the wave setup dynamics and the importance of accounting for the depth-varying surf zone circulation. The various contribution to the simulated wave setup can be analysed via a steady-state momentum balance in the cross-shore direction (Buckley et al, 2016;Guérin et al, 2018;Lavaud et al, 2022):…”

“…The vortex-force (VF) formalism extends this approach to the vertical, and allows for the reproduction of depth-varying wave-induced circulation such as Langmuir cells in deep water (e.g. Leibovich, 1980) and nearshore currents (e.g., Newberger and Allen, 2007a;Uchiyama et al, 2010;Kumar et al, 2012;Lavaud et al, 2022;Pezerat et al, 2022). Using the approximated Generalized Lagrangian Mean (GLM) equations derived by Ardhuin et al (2008), Bennis et al (2011) proposed a set of equations for the depth-varying effects of short waves on currents which, when integrated over depth, are closely equivalent to those derived by Smith (2006).…”