Wave dissipation on a complex coral reef

Abstract

Atolls and coral islands in and around the Pacific and Atlantic Ocean largely rely on coral reefs for their coastal protection. On these reefs a large roughness is created by different coral species. The dissipative character of these species reduces the incident wave height, thereby reducing the amount of run-up on the coast. In our current society we are confronted with large issues due to climate change such as global sea level rise and temperature rise. Because of these issues, coral reefs are more prone to bleaching and degradation. Research has also shown that the wave climate will intensify due to sea level rise. As the sea level will rise globally, the water level above the rough coral elements will increase leading to less wave dissipation and more extreme wave conditions at the coast. It is therefore of vast importance to gather more knowledge about the dissipative character of corals to quantify the risks coral reef islands and atolls will face in the coming decades due to climate change.

For this master thesis, an experimental study was performed in the wave flume of the Fluid Mechanics Laboratory of the Technical University of Delft, investigating dissipation rates due to complex corals. 384 corals were reproduced and exposed to 21 different wave conditions. It was found that the presence of corals significantly enhanced dissipation rates. Shallower water depths, higher wave frequencies, and higher wave heights positively influenced the dissipation rates due to the corals. By increasing the amount of corals in flumewise direction and over the cross section, rates also increased.

Lowe et al. (2005b) and Lowe et al. (2007) developed a canopy model that is able to estimate in-canopy flow velocities and dissipation rates due to coral canopies. In the final part of this thesis, this canopy model was assessed and its output was compared with the experimental results. In general, the model predictions were in good agreement with the experimental results.

The research presented in this thesis is a contribution to the understanding of hydrodynamic processes around coral reefs. The good performance of the analytical model of Lowe et al. (2005b) and Lowe et al. (2007) can be of value for quantifying risks atolls and coral reef islands will face in the coming decades. The model can also be used to estimate the impact of coral restoration projects, when coastal protection is of the essence.