Hydrodynamics of a Tidally-driven Alongshore Flow over a Fringing Coral Reef

Andre Miguel Amador, University of Puerto Rico‑Mayagüez, Department of Mechanical Engineering, Mayagüez, United States, Geno R Pawlak, University of California San Diego, Mechanical and Aerospace Engineering, La Jolla, CA, United States, Isabella Beatriz Arzeno-Soltero, University of California at Los Angeles, Civil and Environmental Engineering, Irvine, United States, Sarah N Giddings, University of Washington, Seattle, WA, United States and Mark Merrifield, University of California San Diego, Integrative Oceanography Division, La Jolla, CA, United States
Abstract:
A tidally driven alongshore flow over a fringing coral reef on the leeward coast of O'ahu, Hawai'i is examined using autonomous underwater vehicle (AUV)-based spatial velocity measurements and time series observations of the alongshore pressure gradient. Depth-averaged tidal velocities are reconstructed as a function of cross-shore distance using velocity observations from multiple AUV surveys assuming a sinusoidal tidal periodicity. Ensemble averages of the alongshore velocities and the associated pressure gradient reveal characteristics akin to an oscillatory boundary layer, with the nearshore flow leading the offshore flow and with a corresponding attenuation of the velocity magnitude in shallower water depths. Analysis of the depth-averaged alongshore momentum balance indicates that the cross-shore structure and evolution of the tidal boundary layer is described adequately by a balance between the local acceleration, the barotropic pressure gradient, and the bottom drag. This primary balance allows the estimation of the drag coefficient as a function of cross-shore distance. Results indicate that drag coefficients range from 0.004 to 0.010 over a 600 m section of the forereef with depths spanning from 25 to 5 m. These estimates are in good agreement with analysis of time series data at the 12 m isobath, and compare favorably with drag coefficients estimated from AUV-based roughness mapping. Roughness data suggest that larger scales, with wavelengths comparable to the local depth, play a more significant role than smaller meter-scale roughness in determining drag.
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