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Variations of the Fraser River plume : observations and computer simulations

University of British Columbia

Temporal and spatial variations of the Fraser River plume, in the central Strait of Georgia (British Columbia, Canada), are monitored by continuous salinity sampling of the engine cooling water on two B.C. ferries. Travelling along two different routes between Vancouver Island and the mainland the ferries provide eight crossings per day both north and south of the river outflow. From each crossing, characteristic measures of the plume are extracted, such as the average salinity and the maximum salinity gradient. These parameters are then formulated as time series and used to compute cross-correlations and cross-spectra with the probable driving forces of wind and river discharge. The effect of the tides is examined using harmonic analysis. Periods of high river discharge lead to decreases in the average salinity for each section, and peaks in the magnitude of the maximum salinity gradient. The correlation of the plume characteristics (average salinity, maximum salinity gradient) on the southern section with the along-strait component of the wind is consistent with advection by the wind. Weak correlation is found between the plume characteristics on the northern section and the wind. Linear combination of the wind and the discharge variations reproduce the general trend of the average salinities but cannot explain the level of variability. A shift to a nonlinear combination of the wind and discharge improves this comparison. The phases of parameter fluctuations at tidal frequencies, on the southern section, agree with the expected effects of tidal currents and the modulation of the river discharge. The agreement is not as apparent for the northern section. The level of the discharge is seen to affect the tidal amplitudes of the salinity fluctuations on the southern section. A numerical model, previously developed to examine the effect of tidal forcing on the plume, is modified to input the hourly wind and daily discharge data record. Equivalent average salinities along the ferry section are outputed and compared to the observed ferry data. Good agreement is reached after manipulating the entrainment velocity and the momentum transfer from the wind to the plume. The tides are seen to add a tidal modulation to the general salinity pattern resulting from the combined effect of the wind and the discharge. Horizontal distributions from the model and from CTD cruise results agree fairly well with each another.

Text

2010-05-02

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http://hdl.handle.net/2429/24352

Oceanography

University of British Columbia

Graduate