RESIDUAL CURRENTS AND FLUX ESTIMATES IN A PARTIALLY-MIXED ESTUARY

An intensive sampling programme of physical parameters has been carrie d out at three representative areas to examine the variable circulatio n and mixing conditions in Southampton Water and the Test Estuary (Ham pshire, southern England). Field data of current velocity and salinity were collected under neap and spring tidal cycles, and under winter a nd summer circulations, to account for the fortnight and seasonal effe cts in the estuary. Both were collected to a very high vertical resolu tion using ADCP and CTD instrumentation. The instantaneous and residua l fields of the above measured variables were examined, together with a thorough consideration of the mechanisms responsible for residual tr ansport. Variation of the instantaneous field during different stages of the tidal cycle is shown, corresponding mainly to changes in the in tensity of turbulence introduced in the water column. Tidal effects ap pear to be much more important than river flow discharge and wind shea r stress for the longitudinal-vertical distribution of physical variab les. Analysis of residual longitudinal and lateral currents and fluxes of water and salt, however, reveals the significance of non-tidal eff ects in the mean tidal transport. The Stokes drift mechanism appears m ostly positive in direction, thus pushing water and salt upstream and increasing its magnitude under spring tidal conditions. Lateral Euleri an transports are of the same order of magnitude as the longitudinal o nes, but with smaller values, especially under spring tidal amplitudes , when the flow coincides better with the longitudinal axis of the est uary. Vertical eddy diffusivity and viscosity coefficients were examin ed showing near zero values at the surface and bottom layers and maxim um values at the mid-depth region, where most intense mixing occurs. ( C) 1998 Academic Press Limited.