UBC Theses and Dissertations
Salinity intrusion in the Fraser River, British Columbia Hodgins, Donald Ormond
The dynamics of salt water intrusion in the tidal Fraser estuary was studied by both a programme of field measurements and the use of numerical solutions of the equations of motion. Time series conductivity measurements spanning several tidal cycles indicated significant penetrations exceeding an estimated 15 kilometers above Steveston for tides of large diurnal inequality. Large ebb tides washed salt water out of the river despite low winter discharges averaging 1100 m³/sec. Mixing sufficient to disperse the salt water throughout the water column was not observed although surface currents typically ebb between 2 and 3 meters/second, and the salt wedge appeared to flood and ebb in a fairly well-defined layer. Longitudinal salinity gradients were detectable in each layer, indicating that two-way mixing took place during flood and ebb periods. Both conductivity and velocity data revealed that maximum intrusion lagged high water by 60 to 80 minutes near the river mouth. A numerical two-layer model predicted the salt water thickness within 10 per cent of the total depth and a phase agreement of ± 40 minutes at maximum intrusion. Velocities were comparable to measurements within 15 cm/sec. The model neglected mixing across the interface but included the Reynold's stress formulated as KipU|U|Fi where Ki=0.0075, U is the relative layer velocity and Fi is an interfacial Froude number. The bottom stress was included as Kbp¹u¹|u¹| where Kb=0.0055, and both stresses were found to be significant in the dissipation of energy in the flows.
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