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Shore protection by oblique seabed bars

Published online by Cambridge University Press:  21 February 2017

Louis-Alexandre Couston Open the ORCID record for Louis-Alexandre Couston [Opens in a new window] ,
Mir Abbas Jalali  and
Mohammad-Reza Alam Open the ORCID record for Mohammad-Reza Alam [Opens in a new window]
Louis-Alexandre Couston
Affiliation:
Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA
Mir Abbas Jalali
Affiliation:
Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA Department of Astronomy, University of California, Berkeley, CA 94720, USA
Mohammad-Reza Alam*
Affiliation:
Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA
*
†Email address for correspondence: reza.alam@berkeley.edu
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Abstract

Shore protection by small seabed bars was once considered possible because seafloor undulations strongly reflect surface waves of twice the wavelength by the so-called Bragg resonance mechanism. The idea, however, proved ‘unreliable’ when it was realized that a patch of longshore seabed bars adjacent to a reflective shore could result in larger waves at the shoreline than for the case of a flat seabed. Here we propose to revamp the Bragg resonance mechanism as a means of coastal protection by considering oblique seabed bars that divert, rather than reflect, shore-normal incident waves to the shore-parallel direction. We show, via multiple-scale analysis supported by direct numerical simulations, that the creation of a large protected wake near the shoreline requires a bi-chromatic patch to deflect the incident waves to the shore-parallel direction. With two superposed sets of oblique seabed bars, the incident wave energy becomes efficiently deflected far to the sides, leaving a wake of decreased wave activity downstream of the patch. We demonstrate that the shore protection efficiency provided by this novel arrangement is not affected by reflection of leaked waves at the shoreline, and that it is relatively robust against small frequency detuning.

JFM classification

Geophysical and Geological Flows: Coastal engineering Waves/Free-surface Flows: Surface gravity waves Geophysical and Geological Flows: Topographic effects

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Papers
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Journal of Fluid Mechanics , Volume 815 , 25 March 2017 , pp. 481 - 510
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© 2017 Cambridge University Press 

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