How do Wave Properties Determine the Surfzone Horizontal Eddy Diffusivity?
Abstract
Two-dimensional horizontal surfzone motions are responsible for surfzone eddy fluxes and therefore contribute to the cross-shore exchange of material between the surfzone and waters offshore. Like other turbulent flows, surfzone eddy fluxes are commonly quantified and parameterized with an eddy diffusivity. Based upon 120 wave and eddy resolving surfzone flow simulations, we demonstrate how various properties of surfzone horizontal eddy diffusivities, such as the maximum diffusivity K, are related to the incident wave conditions: significant wave height (Hsig), wave directional spread (σθ), and beach slope (β). The maximum cross-shore diffusivity (Kx) is related to the rotational cross-shore velocity u and a suitably scaled surfzone width Lx so that Kx = uLx. Both u and Lx can be anticipated from Hsig, σθ, and β. In general, Kx increases with Hsig and σθ, but decreases with β. The maximum alongshore diffusivity (Ky) is related to the rotational alongshore velocity v and the alongshore eddy length-scale Ly so that Ky = vLy. Like u and Lx, v and Ly can be anticipated from Hsig, σθ, and β. However, unlike Lx which is the geometrical extent of the surfzone turbulence, and thus NOT related to an eddy length-scale, Ly is a proper eddy length-scale found from the alongshore velocity wave-number spectra. We find that σθ affects Kx and Ky differently. Both u and v increase similarly with σθ, but Lx and Ly are effected very differently: Lx is nearly independent of σθ whereas Ly decreases with σθ. Hence, the dependence of σθ on Ky is more subtle than the dependence of σθ on Kx. Implications for material spreading and transport in non-eddy resolving models are addressed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMOS23B2023S
- Keywords:
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- 4544 Internal and inertial waves;
- OCEANOGRAPHY: PHYSICALDE: 4546 Nearshore processes;
- OCEANOGRAPHY: PHYSICALDE: 4558 Sediment transport;
- OCEANOGRAPHY: PHYSICALDE: 4560 Surface waves and tides;
- OCEANOGRAPHY: PHYSICAL