Tidal Variation of Salt Flux and Dispersion in a Polluted Estuary

Velocity and salinity data collected during the summer of 1983 in the Tyne estuary are used to investigate the dominant mechanisms for the flux of salt and dispersion. The instantaneous salt flux arises mainly from the combination of velocity fluctuation with the tidal mean salinity, and the instantaneous dispersion results almost entirely from the combination of vertical oscillatory velocity and steady salinity. Of the net downstream salt flux, 92.4% is accounted for by the river discharge. The non-tidal drift appears to be of most importance in the net upstream salt flux. The estuary has a strong vertical salinity gradient and due to the strong secondary flow, has a uniform transverse gradient; thus, the dominant factor in the longitudinal dispersion arises from the vertical shear in the oscillatory tidal current. The dispersion coefficient is highly time-dependent within the tidal cycle as well as a function of distance. The importance of time-varying longitudinal dispersion coefficients are demonstrated in a water quality model by using the sensitivity analysis.