Experimental study of transmission of waves over coastal reef systems

Abstract

The wave transformation and breaking play an important role in shaping the physical structure of the coral reefs. Through the exchange of sediments and nutrients, they also maintain the health of the coral reefs and shoreline. This final year project is about a comprehensive study of the wave dynamics over fringing reefs using the former results of ridge width 0.5 m from Yao (2012) and the present results of ridge widths 0.25 m and 0.125 m obtained from laboratory experiments, numerical modelling and theoretical analyses on monochromatic waves. Laboratory experiments were first conducted in the wave flume to measure the main aspects of wave-breaking characteristics over submerged idealized fringing reefs. Results were obtained for different types of monochromatic waves, reef-flat submergence, water depths and ridges. The experiments have shown that the relative submergence on a reef flat is the dominating factor affecting the breaking-wave characteristics. An additional laboratory study of wave-induced setup over an idealized fringing reef in the presence of a steep ridge was conducted. Experimental results were reported for a range of reef-crest submergence under monochromatic waves. The behaviours of the wave transformation and wave setup in the 0.5 m ridge were compared to the ridges of 0.25 m and 0.125 m. An analytical model of Gourlay and Colleter (2005) was used to calculate the wave-induced setup in the water above the reef flat. This is to compare the accuracy of the present results to the analytical model with two constant empirical parameters, the reef profile (Kp) and breaker depth index (γ). Iteration of a series of empirical parameters was completed for the results obtained in the laboratory experiments. Analyses of Yao‘s (2012) results for the 0.5 m ridge with the empirical parameters and the comparison of the present results were done. The analyses show that the model of Gourlay and Colleter (2005) correlates well with the experimental results and predicts significantly accurate results. The values of Kp and γ that best fitted the results were both 0.4 and would be further elaborated in the report. The findings in this study are useful for comparing the wave setup on different ridges and improving the analytical model for cross-shore wave transformation, wave-induced setup and wave-driven flow over submerged fringing reefs.