The impact of tidal straining on the cycle of turbulence in a partially stratified estuary

Fast sampling ADCPs (Acoustic Doppler Current Profilers) have been used to observe the cycle of turbulent properties in a partially stratified estuary over a springs-neaps cycle in parallel with measurements of water column T / S from an anchored vessel. Using the variance method, estimates of the Reynolds stress τ , the rate of turbulent kinetic energy production P and the eddy viscosity N z are derived from measurements of the along beam velocities with a vertical resolution of 0.5 m. Combined with vertical profile measurements of temperature and salinity, the results reveal the important influence of tidal straining in controlling the structure and intensity of turbulent stresses and TKE production on both ebb and flood phases of the tide. he M2 cycle at neaps, τ and P both showed a significant response to water column structure with higher values restricted to the lower part of the water column at times when the water column was stratified by tidal straining. Conversely at the end of the flood, when stratification was minimal due to the de-stabilising effect of straining in supplementing tidal mixing, significant stresses and relatively high production levels extended upwards to near the surface. At springs, strain-induced stratification is relatively weaker but the influence of straining is still apparent in the development of some stability at the end of the ebb and in relatively high values of N z on the flood due to the action of tidal straining in inducing a negative buoyancy flux. This asymmetry between mixing conditions on the ebb and the flood is apparent in plots of N z versus the local gradient Richardson number Ri . N z generally increases rapidly for Ri <1 but there is considerable scatter and only limited conformity to functional relations like that of Munk and Anderson.