Residual Currents and Flux Estimates in a Partially-mixed Estuary

An intensive sampling programme of physical parameters has been carried out at three representative areas to examine the variable circulation and mixing conditions in Southampton Water and the Test Estuary (Hampshire, southern England). Field data of current velocity and salinity were collected under neap and spring tidal cycles, and under winter and summer circulations, to account for the fortnight and seasonal effects in the estuary. Both were collected to a very high vertical resolution using ADCP and CTD instrumentation. The instantaneous and residual fields of the above measured variables were examined, together with a thorough consideration of the mechanisms responsible for residual transport. Variation of the instantaneous field during different stages of the tidal cycle is shown, corresponding mainly to changes in the intensity of turbulence introduced in the water column. Tidal effects appear to be much more important than river flow discharge and wind shear stress for the longitudinal–vertical distribution of physical variables. Analysis of residual longitudinal and lateral currents and fluxes of water and salt, however, reveals the significance of non-tidal effects in the mean tidal transport. The Stokes drift mechanism appears mostly positive in direction, thus pushing water and salt upstream and increasing its magnitude under spring tidal conditions. Lateral Eulerian transports are of the same order of magnitude as the longitudinal ones, but with smaller values, especially under spring tidal amplitudes, when the flow coincides better with the longitudinal axis of the estuary. Vertical eddy diffusivity and viscosity coefficients were examined showing near zero values at the surface and bottom layers and maximum values at the mid-depth region, where most intense mixing occurs