In situ measurements of turbulent dissipation rate over an algal reef

In situ measurements of wind, waves, current, and turbulence over an algal reef off the northwest coast of Taiwan are presented. Surface waves and flow velocities were measured by pressure sensors and wave gauges and by acoustic Doppler velocimetries (ADVs), respectively. Turbulence dissipation rate ε was estimated by fitting turbulence spectra in the inertial subrange. Under a minimum wave breaking condition, the observed ε over the algal reefs ranged from 10^-6to 10^-3(m²s^-3), which is much higher than that reported in the literature for sandy beaches, and is higher than that for unidirectional flows over the fringing coral reefs at Red Sea. The values are comparable to the recent observations over the shallow coral-reef lagoon around Lady Elliot Island (LEI) [Huang et al., 2012]. In the present study site, both the wave-induced flow and the tidal current contribute to the turbulent dissipation rate in contrast wave dominating process in LEI. The classic wall-layer scaling for the observed friction velocity and the turbulence dissipation rate is found to be inapplicable to the bottom boundary layer within the shallow waters over the algal reef in the presence of tidal currents and waves.