Circulation, Salinity and River Discharge in the Mersey Estuary

Observations of currents and salinity in the Narrows of the Mersey estuary have been analysed in terms of the mixing processes and the dynamics of the flow. At each of three stations in a section across the estuary there was a downstream flow in the upper layer and an upstream flow near the bottom, relative to the depth-mean current. Computed values of the coefficient of vertical eddy diffusion, Kz, reached a maximum of 28 cm2/s at mid-depth at the central station. From observations of salinity in two sections at opposite ends of the Narrows, repeated at approximately monthly intervals throughout a year, a high correlation was found between the properties of the salinity distribution and the river discharge into the estuary. While the salinity at the two sections decreased with increasing discharge, the horizontal and vertical gradients of salinity both increased. The probable change in the circulation, associated with these salinity changes, is discussed. Variations in the tidal range had a comparatively small effect on the salinity distribution.