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Temporal adjustments of a streambed following an episodic sediment supply regime von Flotow, Claudia
The objective of this research is to evaluate temporal adjustments of a streambed surface due to changes in the sediment supply regime. To achieve the goals, laboratory experiments were conducted in an 18 m long, 1 m wide flume with a gradient of 0.0218. The bed was brought to an armored state by running water with no sediment feed before releasing a sequence of sediment pulses. Water discharge and grain size distribution of the sediment feed were held constant over the set of experimental runs, while supply input rates and magnitudes varied. Sediment flux at the outlet of the flume was continuously measured. At frequent time intervals, bed surface texture, 1 mm vertical resolution bed elevation, and microtopographical cluster features were measured over a two meter section of the flume bed. Instantaneous three-dimensional observations of flow velocities were made using an Acoustic Doppler Velocimeter. Surface texture adjustments in response to the sediment supply regime followed similar patterns between all runs involving pulses of sediment in that (1) the D₁₆ – the diameter at which 16% of the particles are smaller than – was the most variable characteristic grain size value and (2) medium-gravel patch areas were least variable, likely due to higher relative mobility of fine material. Fine-gravel patch areas developed following sediment pulses and persisted over time even once the overall characterization of the bed had returned to an armored state. Increases in bed roughness, quantified by the standard deviation of bed elevations (σz), were consistently paralleled with increases in the number of identified clusters and their combined surface area. Cluster formation was seemingly random, but expansion occured only once surface texture became relatively coarse and bed roughness increased. These results inform about the degree of bed surface evolution complexity under conditions of variable sediment supply and can be linked to observations and predictions of sediment transport in field settings.
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