Laboratory Investigation of Energy Dissipation and Turbulent Mixing During Internal Solitary Wave Breaking on the Rough Shelf Slope

When internal solitary waves (ISW) propagate to the continental shelf, they typically runup and break on the forereef. This process might lead to periodic temperature drops at the reef slope, which has potential to protect coral reefs from bleaching threats. A series of laboratory experiments were conducted to investigate turbulence characteristics and energy dissipation during the ISW breaking events on the shelf slope with different bottom roughness to mimic the forereef environment. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) were coupled to obtain simultaneous high-resolution velocity and density fields, based on which we then calculate the energy budget and turbulent dissipation rate. We found that the ISW breaking can be divided into two turbulent processes, the production of shear instability and coherent structures. Ratio of energy loss is weakened with the increase of bottom roughness and the collapse height, while the rough structure substantially enhanced the turbulent dissipation rate. The mixing efficiency is between 0.1 and 0.3, close to its maximal rate at 0.25. Our results reveal that the bottom roughness has impact on the mixing efficiency and turbulent process during the ISW breaking events. We found that rough structures at the shelf slope inhibit the mixing efficiency, especially in cases with small amplitude incline ISW. This finding may help us to better understand the reef damage and coral reef decline and support the management of coral protection strategy. Coral reefs are one of the most valuable ecosystems on earth, but they are now facing great risks of bleaching and degrading in response of numerous natural and anthropogenic drivers, including the global warming. Previous researches suggested that the internal solitary waves have ability to transport cold and nutrient-rich deep water shoreward, which might benefit corals during thermal stress. To simulate this process, we conducted a series of laboratory experiments to simulate internal solitary wave (ISW) breaking on a shelf slope with different structure of bottom roughness. ISW with different amplitudes on various rough structures on the shelf slope are examined. We found that the rough surface of coral reefs can enhance dissipation and inhibit the buoyancy mixing process. This may explain that the roughness of coral reefs can protect itself from strong turbulence during the ISW breaking events, leading more rich-nutrient sediment to settle down in the coral region, and hence has potential to promote the growth of coral reefs. Turbulent kinetic energy budget and mixing efficiency were calculated using the velocity and density data measured from PIV and PLIF Two kinds of instabilities (shear instability and convective instability) were observed during ISW breaking events on the shelf slope Rough structure on the shelf slope substantially enhanced the turbulent dissipation rate and decrease the mixing efficiency