Cross‐Shore Structure of Tidally Driven Alongshore Flow Over Rough Bathymetry

A tidally driven alongshore flow on the western coast of O'ahu Hawai'i is examined using velocity measurements from an autonomous underwater vehicle (AUV) along with time series observations of the alongshore pressure gradient. Depth‐averaged velocities over the forereef shelf are reconstructed from AUV‐based velocity observations as a function of cross‐shore distance assuming a sinusoidal tidal periodicity. Ensemble phase averages of the alongshore pressure gradient and velocities from multiple AUV surveys reveal characteristics akin to an oscillatory boundary layer, with the nearshore flow leading the offshore flow in phase and with a corresponding velocity attenuation at shallower depths. Analysis of the depth‐averaged alongshore momentum equation indicates that the cross‐shore structure and evolution of the tidal boundary layer is well described by a balance between the local acceleration, the barotropic pressure gradient, and bottom drag. This primary balance allows estimation of the drag coefficient as a function of cross‐shore distance over depths spanning from 24 to 6 m. Results indicate that drag coefficients range from 0.004 to 0.010 [±0.002] over a 600 m cross‐shore forereef section. These estimates are in good agreement with previous results obtained at the 12 m isobath using fixed observations and compare favorably with roughness estimates from LIDAR and AUV‐based mapping. Roughness data suggest that larger scales, with wavelengths of O (10 m), play a more significant role than smaller meter‐scale roughness in determining the drag on the tidal flow.