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Effects of episodic sediment supply on channel adjustment of an experimental gravel bed

University of British Columbia

A flume experiment was conducted to study channel adjustment of gravel beds to episodic sediment supply. The bed and sediment feed included grains 0.5–64 mm with geometric mean size 5.7 mm. Flow discharge was constant and every 40 h, 300 kg of sediment was supplied through different feed regimes. Sediment transport and storage, bed slope, and bed surface texture responded to sediment supply regime. The preferential storage of grains > 8 mm caused a cumulative increase in bed slope, which probably increased transport efficiency. Within a run, sediment transport rate qb and bed-surface texture were controlled by the magnitude and frequency of sediment feed and not the total mass. Constant feed promoted gradual increases in qb and small changes in bed surface texture, whereas large infrequent sediment pulses caused pronounced increases in qb and strong surface fining, followed by monotonic decreases in qb as surface re-coarsened. Pronounced trends caused stronger memory in bedload time series for runs with episodic feed than in those for runs with constant feed, although within each run, the structure of memory varied. Long memory was observed for periods when bedload rate was nearly stable, which indicates that it could result from local changes in storage. Patterns of grain-size dependence were not affected by sediment feed and the limit for full-mobility was stable around 8 mm. Scaling statistics for total bedload were similar to those for fine gravel, which was fully-mobile and dominated bedload. A decrease in the frequency of movement with size for gravel fractions caused a reduction in the memory strength of fractional bedload signals. Size-selective transport promoted the storage of coarse grains upstream and downstream fining on the bed surface. Although fully-mobile, more than 60% of the sand fed got stored in the bed, probably because of its high potential to infiltrate and get caught within larger grains. Memory was weaker for sand bedload rates than for fine gravel, which indicates that sand mobility was more influenced by short-term stochastic dynamics (e.g., clustering) and less affected by long-term processes like the evolution of large bedforms and sediment pulses.

Text

2018-03-28

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/64980

Geography

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/