Residual fluxes of water and salt at two stations in the Severn Estuary

Abstract

Observations of currents and salinity are presented for two stations in the Severn Estuary; the observations extend over 50-h periods at both stations, and the data are analysed to provide estimates of the residual fluxes of water and salt. Relative to the depth averaged values, the axial residual currents show a seaward flow in the upper layer, and a landward flow in the bottom layer for both stations; the associated salinity profiles show only small differences in salinity between the sea-bed and the sea-surface (~0·1‰). The tidally averaged velocity and salinity profiles are used to estimate the effective vertical eddy viscosity and diffusivity which are applicable to residual variables, estimated maximum values being of the order of 50 and 100 cm 2 s −1 respectively. The computation of residual fluxes shows that the Stokes drift is an important feature of the physical regime of the Severn Estuary. The residual transport of water is the dominant mechanism for the residual transport of salt, of which the transport effected by tidal pumping is of the same order of magnitude as the salt transport due to freshwater discharges into the estuary; the vertical shear appears to be of secondary importance in the salt balance of the estuary.