Dynamics of Tidally-Driven Flows in Coral Reef Shelves: Observations from Autonomous and Fixed Instruments

The present work examines the hydrodynamics of the inner-shelf region, focusing on tidally-driven alongshore flows over coral reef shelves. This study draws on field data collected in O’ahu, Hawai’i using fixed and mobile assets to develop new modes of observational research.First, a theoretical model is developed to describe how autonomous underwater vehicle (AUV)-based water velocity measurements are influenced by a surface wave field. The model quantifies a quasi-Lagrangian, wave-induced velocity bias as a function of the local wave conditions, and the vehicle’s depth and velocity using a first-order correction to the linear wave solution. The theoretical bias is verified via field experiments over a range of wave and current conditions. The analysis considers velocity measurements made using a REMUS-100 AUV, but the findings apply to any small AUV (vehicle size ≪ wavelength) immersed in a wave field. The observed wave-induced biases [O(1–5) cm/s] can be significant, and can be comparable to steady flow velocities for inner-shelf regions.Second, a new approach to estimate lateral turbulent Reynolds stresses (u′v′) in wavy coastal environments using acoustic Doppler current profilers (ADCPs) is described. The performance of the proposed method is evaluated via comparisons with independent acoustic Doppler velocimeter (ADV)-based stress estimates at two sites, and the vertical structure of the tidally-averaged turbulent Reynolds stresses is examined for an unstratified, tidally-driven flow over a rough coral reef seabed in weak waves. Observations and analysis indicate that lateral stresses are sustained by the cross-shore gradient of the mean alongshore flow, and driven by bottom-generated turbulence. Scaling considerations suggest that cross-shore transport by lateral turbulent mixing could be relevant to coral reef shelves with steep cross-reef slopes and rough bottoms.Finally, a tidally-driven alongshore flow over a forereef shelf is examined using AUV-based spatial velocity measurements along with time series data of the alongshore pressure gradient. Ensemble phase averages of AUV-based velocities reveal characteristics akin to an oscillatory boundary layer, with the nearshore flow leading the offshore flow in phase and with a corresponding velocity magnitude attenuation near the shallower regions of the reef. Analysis of the depth-averaged alongshore momentum equation indicates that the cross-shore structure and evolution of the alongshore flow is well described by a balance between local acceleration, barotropic pressure gradient, and bottom drag. This primary balance allows the estimation of a spatially-averaged drag coefficient as a function of cross-shore distance over depths spanning from 24 to 6 m. Seabed roughness data suggest that larger scales, with wavelengths of O(10 m), are more relevant than smaller meter-scale roughness for drag