HR: 1340h
AN: H53D-0519    [Abstracts]
TI: The influence of variable sediment supply and bed roughness on the spatial distribution of incision in a laboratory bedrock channel
AU: * Demeter, G I
EM: gez4_dem@hotmail.com
AF: San Francisco State University, Department of Geosciences, San Francisco, CA 94132 United States
AU: Sklar, L S
H53D-0519 AF: San Francisco State University, Department of Geosciences, San Francisco, CA 94132 United States
AU: Davis, J R
H53D-0519 AF: San Francisco State University, Department of Geosciences, San Francisco, CA 94132 United States
AB: Numerical models of river incision into bedrock typically predict incision rates averaged across the channel width and over a substantial reach length. However, at-a-point rates of bedrock wear are not likely to be spatially uniform but rather should depend strongly on local conditions, including the degree of partial bed alluviation, patterns of bedrock bed roughness, and variations in local shear stress. We report results of laboratory experiments in which we measured the spatial distribution of erosion under varying sediment supply, bedrock roughness and average boundary shear stress. The experiments utilized a tilting flume 8 m long and 0.3 m wide with an erodible bedrock bed made of sand and cement. We varied bedrock roughness, starting initially with a planar bed, which was roughened by chiseling and then allowed to evolve by repeated and sustained erosion. For a second set of runs we created an extreme initial bedrock roughness bed using egg-carton style mold, which then evolved as erosion significantly modified the bed surface. Digital elevation maps of the bedrock beds were made with a laser microtopography scanning device that has a resolution of 0.2 mm vertical and 5 mm horizontal. We calculated the local rate of bedrock wear by differencing the pre- and post-run local elevations. For each degree of bedrock roughness we systematically varied the rate of supply of uniform 5 mm gravel and measured sediment flux out the downstream end of the flume with a pair of submerged baskets suspended from load cells. Transient alluvial patches, if any, were repeatedly mapped by hand using a 5 cm grid transparent overlay. We also used the laser scanner to map the alluvial deposits left at the end of several runs. We found that high sediment feed rates tend to cover low points and erode the high points preferentially. In contrast, low sediment feed rates favor a positive feedback in which eroded low points capture bedload and erode more rapidly than neighboring high points, cutting slots in the bedrock. Erosion also tends to focus on the upstream faces of high points, and vibrating and rotating grains trapped in the low points erode in an incipient pothole-style pattern. We conclude that bedrock roughness interacts with partial alluvial cover to modulate where and how fast bedrock incision occurs. Over longer time scales, fluctuations in sediment supply and shear stress, and resulting variations in the location and extent of alluvial cover, may result in all portions of the bed lowering at a similar, long-term average rate.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
DE: 8175 Tectonics and landscape evolution
SC: Hydrology [H]
MN: Fall Meeting 2005