Reef flat wave processes and excavation pits: Observations and implications for Majuro Atoll, Marshall Islands

摘要

An experimental deployment of pressure sensors was undertaken to assess the impact of reef flat excavation pits on wave processes at Majuro Atoll, Marshall Islands. Experiments were undertaken on two sections of an 80-m-wide fringing reef flat, one modified by the excavation of a 17-m wide, 4-to 5-m deep pit and the other an unmodified reef flat of comparable width, topography, and incident wave energy. A wave-driven inundation event during the experiment led to minor amounts of debris overwashing the road surface. The event was associated with southerly swell that was normally incident to the study site at high tide, resulting in enhanced shoreline energy in both the sea and swell and the infragravity frequency bands. The shoreline with the excavation pit received slightly smaller wave heights (～8%) for all wave conditions, including the overwash event, compared to the unmodified shoreline. The statistically significant difference is largely a result of a decrease in infragravity wave energy over and shoreward of the pit that compensates for a weak increase in sea and swell energy. The influence on infragravity energy levels is likely to depend on the pit geometry and position, as well as the wave forcing; hence, not all pits are likely to lead to net wave energy dissipation. Implications of the findings are discussed with respect to the impacts of reef flat excavation on a highly developed, urbanised atoll.