Experimental Study on Complex Hydrodynamics of Tsunami-Like Waves over Fringing Reefs with Uneven Flat Topography

Coral reefs, widely distributed in tropical and subtropical waters, generally have uneven flat topography. Sudden changes in reef flat topography significantly impact the propagation and evolution characteristics of tsunami-like waves; however, related studies are rare. This study systematically investigates the propagation and evolution of tsunami-like waves over uneven islands and reefs through wave flume physical experiments, analyzing the influences of incident wave height and reef flat water depth on these characteristics. The experimental results show that an uneven reef flat blocks the propagation of solitary waves and generates reflected waves. The slope, reef margin, and first reef flat in front of the reef are deep underwater, causing solitary waves to undergo shallow deformation without breaking. The wave height increases to a maximum at the step of the reef flat at shallower water depths and then breaks on the second reef flat. After breaking, the solitary wave propagates over the second reef flat as a swell, with wave height slowly attenuated due to energy dissipation from breaking in the limited water depth. As the incident wave height increases, the local wave height at each measuring point rises, with the increase in wave surface further amplified by the shallowing effect at the reef flat step. An increase in incident wave height causes the uneven reef flat to dissipate more wave energy and enhances wave attenuation. Consequently, the spatial distribution of local wave height Hi/H0 decreases with an increase in incident wave height. Increasing the water depth of the reef flat gradually weakens the shallowing effect, resulting in a decrease in the range and peak value of the maximum wave height near the reef flat step. When the water depth of the reef flat exceeds a certain limit, such as hr = 0.05 m in this study, the influence of the uneven reef flat on local wave height diminishes. An increase in the water depth leads to a reduction in wave energy consumed by the uneven reef flat, while the maximum wave height on the second reef flat increases monotonically with deeper water. During the propagation of solitary waves over uneven islands and reefs, two reflections from reef margins and flat steps occur. The reflection coefficient CR1 decreased with increasing incident wave height and reef flat water depth, while the reflection coefficient CR2 decreases significantly with increasing water depth. When the reef flat water depth is low, sudden changes in reef flat terrain greatly enhance the reflection intensity of incident waves. In particular, when hr = 0 m, the reflection coefficient CR2 reaches approximately 0.3.