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Channel adjustment of a gravel-bed stream under episodic sediment supply regimes Elgueta, Maria Alejandra

Abstract

Sediment supply is a key control on sediment transport rates and bed evolution in a stream. This study examined the adjustment of a gravel-bed stream under episodic sediment supply regimes by conducting a flume experiment in the Mountain Channel Hydraulic Experimental Laboratory at the University of British Columbia. The experiment consisted of a sequence of runs with no feed, constant feed and episodic supply regimes; but constant water discharge and feed texture. The observations indicated that sediment transport rates, the texture of bedload and the bed surface, sediment storage, bed slope, and bed topography adjusted to changes in sediment supply. The relative mobility of sediment instead did not change significantly. Under constant feed, transport rates showed a slow and small increase. The texture of the surface was fluctuating, the same as of the bedload. Sediment storage was relatively large, and the bed slope presented small changes. If the same amount of sediment entered in one or few pulses, transport rates and the texture of the surface exhibited pronounced changes just after the pulse, and returned to conditions similar to previous the pulse after some time. The size of the pulses influenced the results, and larger pulses caused larger increases of transport rates and finer textures on the bed surface. Cumulative storage, bed slope and bed morphology adjusted to episodic supply; but did not return to the conditions before the pulse, revealing that the effects of sediment supply over the bed were cumulative and persisted under periods of no feed. During these periods, transport rates decreased, the bed texture coarsened, and there was little change in the bed slope. After few hours of no feed, transport rates were relatively low and changes in elevation were small. Our results suggested that episodic supply produced interesting patterns of channel adjustment (different from constant feed regimes) that depend on the size and frequency of the supply.

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