Shore protection by oblique seabed bars

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 Yu & Mei (J. Fluid Mech., vol. 404, 2000, pp. 251-268) showed that a patch of longshore seabed bars adjacent to a reflective shore could result in larger waves at the shoreline than in the open ocean. 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. The incident wave energy is therefore fully deflected to the sides, leaving a wake of decreased wave activity downstream of the patch. We show, via multiple-scale analysis supported by direct numerical simulations, that the creation of a large protected wake requires a bi-chromatic patch to deflect the incident waves to the shore-parallel direction. We demonstrate that the shore protection efficiency provided by this novel arrangement is not affected by reflection of leaked waves at the shoreline, nor by small frequency detuning.