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Bedforms and Bars
Bedforms in the Pitt River, British Columbia
The Pitt River is a meandering river channel linking the Fraser River estuary and Pitt Lake, which acts as a temporary reservoir for tidally diverted Fraser River flow. Large-scale bedforms can be divided into dunes and sand waves on the basis of three dimensional geometry. Sand waves have linear crests and height/spacing ratio of < 1:20; dunes (2-3 m spacing) have sinuous crests and height/spacing ration of > 1:10 and are found mainly on the stoss side of sand waves. Sand waves do not occur in a continuous range of sizes but rather form discreet height/spacing groups: .8 m/10-15 m; 1.5 m/25-30 m; 3 m/50-60 m. Most (70%) are of the intermediate size. These groups fall on a straight line on a log height-log spacing plot implying a common hydraulic control. Repeated depth soundings revealed reorganization of bedforms from one scale range to another, by halving or doubling. The size and morphology of bedforms appear to be independent of depth. Small forms are characteristic of areas with flat channel topography whereas large forms predominate in reaches of shallowing depths (slope ≥ 1°). Bedforms are flood-oriented, reflecting dominant flow conditions and direction of net sediment transport, and appear to be in quasi-equilibrium with tidal and seasonal flow variations. Both bedforms and channel geometry appear to be scaled solely to the winter flood peak discharge (2400 m3. sec−1) which is thus considered the effective discharge, Qe.
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