Laboratory study of wave processes over fringing reefs with a reef-flat excavation pit

Abstract

Dredging and excavation activities on fringing reefs have potential impacts such as the coastal inundation during extreme wave events, and no published laboratory experiments have addressed this issue. To verify the previous findings from both field observations and numerical simulations, and focus on evaluating the alteration of wave processes over fringing reefs by an excavation pit, a new set of laboratory experiments are carried out in a wave flume based on an idealized fringing reef model with and without a reef-flat pit. Experimental results are reported for a series of spectral wave conditions and pit widths. It is found that the sea and swell (SS) waves near the shoreline can be increased or decreased, depending both on the incident wave condition and the pit width. Whereas the infragravity (IG) waves reaching the shoreline are consistently reduced by an average of 10.5% when the pit exists, and such reduction is enhanced with increasing pit width. An analysis of the involved hydrodynamic processes indicates that the variation of shoreline SS wave energy depends on the relative importance of such processes as increased wave reflection around the pit, decreased dissipation with wave breaking, decreased energy transfer with nonlinear wave-wave interaction and wave resonance excited in the pit. However, the reduction of shoreline IG wave energy is due to the combined effects of a disruption of resonant standing wave pattern on the reef flat and increased wave reflection around the pit.