Modelling circulation and vertical mixing in estuaries

A laterally averaged two-dimensional model of estuarine circulation has been developed and applied to the Danshuei River estuarine system in northern Taiwan to study the changes in estuarine circulation and salinity distribution when certain parameters of the flow are varied systematically. The numerical experiments were conducted with the real geometry of the Danshuei River system. The boundary conditions include a simple harmonic tidal oscillation and salinity forced at the Danshuei River mouth and a constant river discharge applied at the head of the three tributaries. Two internal parameters, the coefficients of vertical turbulent viscosity and diffusivity, A_z and K_z, respectively, were taken: (a) as time varying and (b) as constant. It was found that the use of variable coefficients, A_z and K_z, instead of constant values, has significant effects on the vertical profiles of current and salinity distributions during a tidal period. Experimental runs with different values of the tidal amplitude, the river discharge and salinity showed the effects of interaction between the tidal-driven and density-driven components. The simulation results reveal that turbulent mixing terms had substantial impact on estuarine circulation and salinity distribution. Comparisons of field data and modelling results with constant and non-constant A_z and K_z were also investigated.