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Water quality modelling in estuaries

University of British Columbia

A tidally varying and a tidally averaged mass transport model are applied to the Fraser River Estuary to investigate the significance of tidal effects on the concentrations resulting from assumed effluent discharges. The tidally averaged model is due to Thomann [1963]. The tidally varying model is developed from first principles. A hydrodynamic model was used to determine the tidally induced temporal variation in the longitudinal velocity and cross-sectional area along the estuary. All models are "mathematical" and one-dimensional. Finite difference techniques are used to solve the underlying partial differential equations of all three models. The problems of stability and numerical dispersion are examined. Numerical dispersion is seen to result from the solution of the mass transport equation over a fixed space grid rather than along the advective characteristics. Advantages of solving the equation along the characteristics are: no numerical dispersion; the advective and dispersive transport processes are usefully separated; lateral dispersion can be partially assessed with a one-dimensional model; and time dependent behaviour in coefficient of longitudinal dispersion can be taken into account. The tidally varying flows along the estuary are seen to cause a variation in the initial dilution of a discharged effluent. This, together with the effects of tidal flow reversal produces spikes in the concentration profile along the estuary. The concentration of these spikes is then reduced by the dispersion process, the peak concentration during the first two tidal cycles being sensitive to the form and magnitude of the coefficient of longitudinal dispersion. Time dependent variations in this coefficient are considered. The effect of the lateral dispersion process on the estimated concentrations is also considered and secondary flows are tentatively explained in terms of the generation and advection of vorticity. The predicted peak tidally varying concentration was found to be significantly greater than the tidally averaged value.

Text

2010-01-28

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http://hdl.handle.net/2429/19218

Civil Engineering

University of British Columbia

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