Boundary layer turbulence and flow structure over a fringing coral reef

Measurements of velocity and rates of turbulence were made across a fringing coral reef in the Gulf of Aqaba, Red Sea, to determine the effect that the rough topography has on boundary layer mixing and flow dynamics. Observations were made at two fore-reef sites and a nearby sandy slope. The friction velocity, u*, and drag coefficient, C_D, were determined directly from turbulent Reynolds stresses measured using acoustic Doppler velocimeters. Values of C_D for the coral substrates ranged from 0.009 to 0.015, three to five times greater than over the sandy bottom site. The turbulence dissipation rate, ε, was determined by fitting spectra of vertical velocity to the theoretical "5/3" law expected for the inertial subrange of turbulence. There was a local balance between production and dissipation of turbulent kinetic energy, signifying that we could estimate u* from either the mean velocity profile, turbulence, or dissipation rate of turbulent kinetic energy. Estimates from all three measures agreed well with mean u*/U_o ranging from 0.10 ± 0.03 to 0.12 ± 0.03, indicating that existing turbulent boundary layer flow theory can be applied to flows over the rough topography of coral reefs. The bottom topography, by enhancing both reef scale and local drag and mixing levels, allows reef biota to more effectively exchange dissolved and particulate matter with oceanic waters.