Observations of Near-Surface Mixing Behind a Headland

Field observations were collected near the mouth of the Bagaduce River, Maine, in order to understand how complex features affect the intratidal and lateral variability of turbulence and vertical mixing. The Bagaduce River is a low-inflow, macrotidal estuary that features tidal islands, tidal flats and sharp channel bends. Profiles of salinity, temperature, and turbulent kinetic energy dissipation (epsilon) were collected for a tidal cycle across the estuary with a microstructure profiler. Lateral distributions of current velocities were obtained with an acoustic doppler current profiler. Results showed intratidal asymmetries in bottom-generated vertical eddy diffusivity and viscosity, with larger values occurring on ebb (K-z: 10(-2) m(2); A(z): 10(-2) m(2)/s) compared to flood (K-z: 10(-5) m(2)/s; A(z): 10(-4) m(2)/s). Bottom-generated mixing was moderated by the intrusion of stratified water on flood, which suppressed mixing. Elevated mixing (K-z: 10(-3) m(2); A(z): 10(-2.5) m(2)/s) occurred in the upper water column in the lee of a small island and was decoupled from the bottom layer. The near-surface mixing was a product of an eddy formed downstream of a headland, which tended to reinforce vertical shear by laterally straining streamwise velocities. These results are the first to show near-surface mixing caused by vertical vorticity induced by an eddy, rather than previously reported streamwise vorticity associated with lateral circulation.