Tidal Mixing in the Narrows of the Mersey Estuary

Summary Observations of salinity in the Narrows of the River Mersey have shown that vertical mixing due to tidally induced turbulence may be complete at mid-tide. As tidal currents abate horizontal differential velocities, which constitute a seaward flow in the upper layer and an upriver flow near the bottom, become apparent and produce vertical salinity gradients. A second series of observations showed that vertical homogeneity may not be reached at any stage of the tide if the riverflow is high. Applying the theory for a vertically mixed estuary, coefficients of longitudinal eddy diffusion were computed for a low riverflow of 26 m3/s and gave a mean value of 160 m2/s. During a period of greater fresh water influx (103 m3/s) when conditions were less clearly defined, the average coefficient was considerably higher at 360 m2/s.