Abstract:

To study the effect of varying the location of an excavation pit on wave transformation over the coral reef coast, we tested a series of irregular wave conditions with different pit locations using physical model experiments. We then compared them with the case in the absence of excavation pit. Results show that as the location of the excavation pit moves towards the shoreline until the case with no pit, the short-wave height near the shoreline decreases. The existence of excavation pit also reduces the low-frequency wave height near the shoreline. When the excavation pit is located near the shoreline, the low-frequency wave height is further reduced due to the increase of local water depth. When the excavation pit moves from the reef edge to the shoreline until the case without pit, wave setup near the shoreline increases, and this trend is more evident when the water level of the reef flat is higher. Analysis of the coherence function shows that the low-frequency waves on the reef flat are generated by the break-point shift when the grouped short waves break, and the impacts of varying pit locations on the low-frequency wave generation are insignificant. Transfer function analysis indicates that the first-order resonance amplification exists, associated with the low-frequency wave motions on the reef flat. The effect of resonance amplification is reduced when the pit is present, and such effect is more significant when the pit is located on the central reef flat or near the shoreline.

Key words: coral reef, excavation pit, low-frequency wave, reef-flat resonance, transformation