Proceedings - Teaching and Learning Centre - Simon Fraser ...

Fraser River Action Plan 3rd Research Workshop
Transport and Sedimentation of Fine Grain Sediments
M. Church, M. Hassan and H. Wetherley
Department of Geography
University of British Columbia
In this project, the Water Survey of Canada (WSC) historical archive of suspended sediment observations along
Fraser River was analyzed to determine the regime of fine sediment transport in the river. This is by far the most
extensive collection of sediment transport observations available in the basin, but it has limitations which must
be understood. Most significantly, it consists of observations of clastic sediment coarser than about one micron.
Very fine sediment is not included, and organic particles are not separated, identified or analyzed. Grain size
distribution data are available for most higher flows, so that sediment ratings are accessible for selected subranges.
We constructed the data set of all primary observations from WSC field notes using different (and statistically
more rigorous) methods than were originally used by WSC. Our results were not systematically different than
those originally reported, however. We constructed sediment rating surveys by size range and by year for all
stations. In our current work, we are seeking to determine whether the individual ratings are distinct, or reflect a
common underlying relation of sediment concentration in the water column to flow variates. The outcome of this
exercise will be the key result of our analysis: if the relations are distinct from year to year, there is no
alternative to continued monitoring if natural suspended sediment concentrations are required to be known.
The principal explanatory flow variates are current discharge and discharge 10 days prior. The latter variate
reflects synoptic effects on sediment pickup processes along the channel margins. The rating curves are not
simple. Commonly, they exhibit anticlockwise hysteresis (higher sediment concentration on the rising limb),
which is common in rivers with supply-limited transport.
Stations available for analysis over many years between 1966 and the present include Hansard, upstream from
Prince George; Marguerite Ferry, south of Quesnel; Hope, at the south end of Fraser Canyon; and Agassiz and
Mission City within the Lower Mainland. These stations conveniently divide reaches with important contaminant
sources and ones with potentially significant impact reaches, but they do not provide a high resolution within the
Fraser and Thompson rivers.
Conclusions
Monitoring data of the type represented by WSC records can be incorporated into a model for the prediction of
water quality along the river by setting a suspended sediment value at some known site (e.g., Hansard) and
propagating suspended sediment downstream in a hydraulic model that includes sediment pickup and deposition.
Two sorts of corrections are possible. At downstream stations, a new value can be introduced. Otherwise,
knowledge of the location of major sources and sinks of fine sediment along the river can guide estimates of
sediment recruitment rate between known stations. Knowledge of significant bank sources is available and
sediment exchange with the bed can be computed (See Krishnappan, Engel and Stephens).
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