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Hydraulic geometry of secondary channels of lower Fraser River, British Columbia, from acoustic Doppler velocimetry Ellis, Erica R.

Abstract

The hydraulics and morphology of secondary channels within the lower Fraser River gravel reach have been examined using data collected during the 2002 freshet utilising an acoustic Doppler current profiler with integrated GPS. A range of sub-reach types was sampled, corresponding to position within an individual secondary channel. Surface and sub-surface grain size distribution data were collected as well. Sub-reach averaged water surface width, mean hydraulic depth and mean velocity data generally conformed well (R2 > 0.9) to the form of classical at-a-station hydraulic geometry relations, at higher flow. Spatial and frequency distributions of near-bottom velocity and channel depth were examined. In general, sub-reach types stratified along gradients of width, depth, velocity and sedimentology, although there were exceptions. Additional data collected at high flow were used to generate bankfull scaling relations for secondary channels in the gravel reach. Again, the data conform well to a simple power law, up to and including data points from the main channel. The water surface width to discharge relation agrees with work by Bray (1973) in Alberta gravel bed rivers and with the more general trend of a one-half power relation. However, the mean hydraulic depth to discharge relation (and by continuity, the mean velocity to discharge relation) deviate from previous results. Data collection in the field was somewhat facilitated by the use of the acoustic Doppler current profiler but post-processing time requirements were high and, ultimately, the operating frequency led to certain sampling problems. Issues surrounding the use of these instruments in river channels are discussed.

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