Presenter: Kurt Rosenberger (U.S. Geological Survey)

Description:
Modeling of currents and waves on reefs will become increasingly important to understanding coastal flooding of reef-lined coasts as sea level rises. Critical to these predictions is an accurate assessment of turbulent kinetic energy (TKE) dissipation on reefs which create a high bottom roughness compared to typical coastal environments. Recent advances in 5-beam acoustic doppler current profilers (ADCPs) could allow for more accurate TKE estimates over such complex bathymetry with an independent estimate of vertical velocity from the fifth beam; however, a robust evaluation of these methods is still needed. A 5-day deployment of a downward-facing 5-beam ADCP on a patch of reef with relatively high roughness (10’s of cms) on the south coast of Molokai, Hawaii, yielded high quality data on currents in the bottom 80 cm of the water column. The instrument collected half-hour long bursts every hour of pulse-coherent profiles of beam-coordinate currents on all 5 beams at 4 Hz in 2-cm vertical bins. The high roughness on the reef resulted in profiles from individual beams within the coral canopy that differed in length (range to bottom) by up to 25%, making accurate determination of earth-coordinate velocities and TKE by the variance technique difficult. Nevertheless, it was possible to make estimates of Reynolds stress, TKE and dissipation rates from several widely used methods including the spectral method, structure function, and the variance technique (in the upper water column). Here we present a comparison of the results of these methods as other researchers have in estuarine environments.