THE RESPONSE OF SINUOUS CHANNELS TO CHANGES IN SEDIMENT SUPPLY

Channel patterns are strongly related to bar morphology and sediment supply, yet little is known about how supply change might affect bar morphology. To address this question, we conducted an experiment in the 17 m X 6.1 m flume at UC Berkeley’s Richmond Field Station. Roughened walls were placed in the flume approximating the channel boundary from previous alfalfa experiments, with a sinuosity of 1.5. The fixed walls simplify the channel response by precluding channel migration and cutoffs in response to supply changes. The channel width was 0.45 m, and the bed sediment and feed had a median diameter of 0.85 mm. The channel was run to equilibrium at a feed rate of 6 kg/hr and then the sediment feed rate was doubled to 12/kg/hr. Once equilibrium was reached for the higher sediment supply the feed was then cut off to investigate the channel response to decreased feed.

The transient response to the sediment feed increase lasted approximately 50 hours while the channel slope adjusted. Pool relief decreased through deposition of coarser sediment while the channel adjusted to the increased supply, but the degree of filling varied. The grain size distribution of the sediment flux out the bottom of the flume remained constant during the transition phase, and there were only minor changes to the areal extent of sediment facies. During this period, the water surface slope increased 30% from 0.0048 to 0.0062. The sediment flux out the bottom of the flume equilibrated shortly after the water surface slope stabilized. Once the water surface stabilized, the bar-pool morphology returned to a condition very similar to the equilibrium condition at the lower slope. Cutting off the feed caused the slope to decrease and the channel to narrow as incision occurred in pools and left bars behind as terraces. Alluvial meandering channels differ from these experiments in that they can respond to changes in sediment supply through width increase, channel migration, and slope increase through cutoffs. We are currently conducting similar experiments in freely migrating channels to examine differences in response between alluvial and fixed bank channels.