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Tridimensional nonhydrostatic transient rip currents in a wave-resolving model

Abstract : Flash rips and surfzone eddies are transient horizontal structures of order 10-100 m scale, that can be generated in the surfzone in the absence of bathymetric irregularities. They are traditionally assessed within a depth-averaged framework that involves intrinsic horizontal shear instabilities or/and direct short-crested wave vorticity generation. In this paper, we revisit surf eddy generation processes with a new tridimentional, wave-resolving model (CROCO). We first present a quick overview of a free-surface, compressible approach adapted to nearshore dynamics. Its ability to simulate surface gravity wave propagation and nearshore breaking is validated against laboratory experiments. Next, we present a full 3D, wave-resolving real-case simulation of nearshore currents in Grand Popo Beach, Benin, forced by waves with frequency and directional spreading. The generation of surfzone eddies by the 3D model is shown to differ considerably from that produced by depth-averaged models, owing to the vertical structure of currents produced by shallow breaking that prevents a strong inverse energy cascade. Processes of both horizontal shear instability and short-crested wave breaking are restricted. Instead, the vertical shear flow is prone to forced variability by wave groups and to a Kelvin-Helmholtz-type instability. The latter shows primary and secondary instabilities, that generate spanwise and streamwise vorticity connecting breaking-induced rolls to larger horizontal surfzone structures. Streamwise filaments, appearing as ribs or mini rips, can extend beyond the surfzone but with lower energy than depth-averaged model rips 1 (offshore mean EKE is reduced by a factor 2). Energy spectra and visual inspections of tracer and sediment concentration shows much better fit between the 3D model patterns and in-situ and aerial observations in Grand Popo Beach. Our results thus suggest that nearshore vortical motions are driven by nonhydrostatic dynamics, at least for moderate wave forcing.
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Preprints, Working Papers, ...
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https://hal.inria.fr/hal-02883697
Contributor : Laurent Debreu <>
Submitted on : Monday, June 29, 2020 - 12:20:06 PM
Last modification on : Friday, July 17, 2020 - 2:40:05 PM

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  • HAL Id : hal-02883697, version 1

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Patrick Marchesiello, Francis Auclair, Laurent Debreu, James Mcwilliams, Rafael Almar, et al.. Tridimensional nonhydrostatic transient rip currents in a wave-resolving model. 2020. ⟨hal-02883697⟩

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