ADCP-based estimates of lateral turbulent Reynolds stresses in wavy coastal environments

We assess the use of a four-beam, Janus-type ADCP for the measurement of lateral Reynolds stress ((u'v') over bar) in wavy coastal environments. The calculation of (u'v') over bar derives either from the fluctuating beam velocity equations or directly from the fluctuating part of the Cartesian velocities (u', v'), with each requiring different assumptions. We adapt existing wave-turbulence decomposition strategies to isolate the lateral turbulent motions at frequencies below those of surface gravity waves. The performance of the proposed method is evaluated via comparisons with independent ADV-based stress estimates at two sites. Comparisons show good quantitative agreement over the tidal cycle, and indicate that ADCPs can effectively resolve lateral turbulent fluxes via ensemble-averaging. Assessment of ensemble-averaged turbulence cospectra indicates that the proposed approach is effective in isolating the low-frequency (below the waveband) turbulent stresses from wave-induced errors. Furthermore, the vertical structure of the turbulent Reynolds stresses is examined as a function of tidal phase in an unstratified, tidally-driven flow over a rough coral reef seabed in weak swell conditions. Observations and analysis indicate that lateral fluxes are tied to the cross-shore (lateral) gradient of the mean alongshore flow, though vertical and lateral stresses are primarily driven by bottom-generated turbulence. Scaling considerations indicate that cross-shore transport by lateral turbulent mixing could be relevant to coral reef shelves with steep cross-reef slopes and rough bottoms.