| Turbulent motions along the seabed generated by the oscillatory motion of surface waves are of great interest in coastal waters because wave boundary layers are the site of sediment entrainment, suspension and transport from the seabed as well as wave energy dissipation. Parameterizations of bed roughness have been addressed for homogenous beds (i.e. ripples), but little attention has focused on more complex roughness. For coral reef, roughness is highly inhomogeneous and individual elements can often be of the same order of magnitude as the wave orbital amplitude.;Field observations were carried out at the Kilo Nalu Nearshore Reef Observatory on the south shore of Oahu, Hawai'i, beginning in 2004. A first set of observations focuses on a 3-meter long stretch of coral reef. A novel technique is investigated to measure the velocity field in the vertical plane of the wave motion: remotely controlled acoustic instruments (ADCP and ADV) are mounted downward-looking on an automated horizontal profiler to obtain a two-dimensional view of the wave-boundary layer over a wave orbital excursion in a phase-averaged sense. Observations of the phase-averaged velocity field as a function of varying wave orbital diameter reveal high spatial variability in the near-bed flow, consistent with vortex shedding and flow separation around roughness elements. Two different regimes are identified for near-bed dynamics for small and large wave orbital diameters.;A second set of observations was carried out with high resolution instruments to obtain vertical profiles of the near-bed region at a fixed location along the bed. A downward-looking ADV, mounted on a vertical profiler, measures velocity between 2 to 30 cm above bed, within the vertical plane of the ADCP observations. The vorticity field is consistent with vortex shedding due to flow separation around the crests of two nearby coral heads, with different regimes associated with varying wave orbital diameters.;Finally, vertical profiles of dissipation are estimated from the observed ADV velocity power spectra. A new method of wave-turbulence decomposition is investigated to obtain estimates of Reynolds stresses in a wave-dominated environment over this inhomogeneous bathymetry. |
