Experimental and numerical investigations on wave motions over platform reef-flat

Abstract

Previous studies have shown that high-frequency (HF) waves, low-frequency (LF) waves and wave set-up coexist on shallow coral reef-flat and jointly contribute to potential floods and subsequent damages of infrastructures and islands on it. To better understand the reef-flat wave dynamics with incident waves and still water level, a wave-flume experiment was performed based on an idealized platform reef composed of a steep reef-face (1:4), a relatively mild reef-rim (1:14) and a horizontal reef-flat. Also, the non-hydrostatic phase-resolving model SWASH was validated against the experiment and then applied to further numerically investigate the effects of reef-rim topographic features on the reef-flat wave motions. The results show that incident waves of a larger wave height and a longer wave period can generate larger LF waves and wave set-up, thereby inducing greater HF waves on the reef-flat. Higher still water level can lead to larger HF waves but result in smaller wave set-up. In contrast to HF waves and wave set-up, LF waves are minimally affected by the still water level. A rim of milder slope and larger edge depth will induce smaller HF and LF waves and set-up on the reef-flat, and thus provide better protection for the reef-flat region. Furthermore, on the reef-flat, the ratio of HF significant wave height to water depth ${H_{s\_h}}/\left( {{h_r} + \overline \eta } \right)$ is approximately constant; the dimensionless LF significant wave height H_{s_l}/H_s0 and the dimensionless wave set-up $\overline \eta /\left[ {{T_p}{{\left( {g{H_{s0}}} \right)}^{1/2}}} \right]$ can be related to the inverse wave steepness parameter gT ²_p /H_s0 and the relative reef-flat submergence $\left( {{h_r} + \overline \eta } \right)/{H_{s0}}$ respectively.