Turbulence Observations in a Macrotidal Estuary with Five Beam Acoustic Doppler Current Profilers

Turbulent mixing is a primary control on circulation and transport of dissolved and particulate matter in estuaries. The nonlinear dynamics of turbulence are notoriously difficult to model at environmental scales. Field observations of turbulence are essential to critically test numerical models and to improve our understanding of estuarine dynamics, leading to more accurate predictive tools for coastal and estuarine management.

Acoustic Doppler current profilers (ADCP) are commonly used for measuring currents and turbulence in estuaries and the ocean. However, they usually cannot provide direct measurements of anisotropy in spite of its importance in estuarine flows. We present here new measurements of turbulence using a five-beam ADCP. The recent development of five beam systems enables the direct estimation of five components (out of six) of the Reynolds stress tensor, and subsequently of the turbulent kinetic energy and anisotropy ratio. Shear production and turbulence dissipation rate are also calculated, thus providing more complete turbulence information than previously feasible.

Our study reports on summer and winter campaigns, which were part of a study to measure sediment fluxes, and during which two Nortek Signature 1000 ADCPs were deployed in the Blackwater Estuary (SE England). This estuary is macrotidal and varies from well-mixed to partially mixed regimes. The Reynolds stress tensor, turbulent kinetic energy, anisotropy ratio are calculated for most of the water column from beam variances accounting for tilt corrections. The turbulence dissipation rate is calculated using structure functions. We will investigate turbulence variability and its driving mechanisms by comparing results obtained for a well-mixed period and for a period with periodic stratification. Preliminary results indicate that turbulence anisotropy varies significantly at tidal timescales with extremely anisotropic turbulence corresponding to peaks in turbulent kinetic energy themselves likely due to mean advective processes.