||In this study, Acoustic Doppler Velocimeter (ADV), current profiler and wave gauge were used to measure the turbulent kinetic energy dissipation rate (ε) and shear stress (τ) in Hobi shallow coral reef and Howan coastal waters. Data pre-processing including despike, data quality check, coordinate transformation and filtering of wave signal were applied.|
The inertial dissipation (ID) method is commonly used to estimate the turbulence due to its broad distribution in the frequency domain. The spectral slope of -5/3 and its frequency range within the inertial sub-range can be clearly seen with the increasing flow speed. At the Hobi coral reef which is situated shoreward of the surf zone characterized by small currents and large waves, the observed values of ε are O(〖10〗^(-5)-〖10〗^(-3))W/kg and vary in phase with the tide and wave height. On the other hand, the Howan experiment is dominated by unidirectional flows and wind waves, turbulence is generated at the bed by shear flows, and the observed values of ε are O(〖10〗^(-5)-〖10〗^(-4))W/kg. The average turbulent kinetic energy dissipation rate ε_BL, estimated directly from the observed ε with a bottom boundary layer scaling, indicates that a close correlation exists between the wave energy dissipation rate D_wave and turbulence dissipation.
Three methods were used to estimate the bed shear stress and their results are inter-compared. When the Reynolds number is smaller and wave-current interaction is important, correction of the friction velocity u_* by the ID method is required. As a result, an increase of τ by approximately 60 percent shows better correlation with the wave height. However, the applicability and reliability of the ID method become worse as the Reynolds number decrease and turbulence is not fully developed. The EC method will not produce reasonable values of bed shear stress when the Reynolds number is not sufficiently high and the turbulence is not fully developed. Finally, estimating τ by the turbulent kinetic energy (TKE) method with velocity fluctuations is considered be the best and most common method for this study. The drag coefficient Cd is found to be close correlated with the mean flow, but it is not suited for the wave-dominated shallow reefs.