Paper No. 11
Presentation Time: 8:00 AM-6:00 PM
LATERAL BEDROCK EROSION IN AN EXPERIMENTAL CHANNEL: THE INFLUENCE OF BED ROUGHNESS on WEAR BY BEDLOAD IMPACTS
FULLER, Theodore K., Department of Geology and Geophysics, University of Minnesota, Saint Anthony Falls Laboratory, 2 Third Avenue SE, Minneapolis, MN 55414, SKLAR, Leonard S., Department of Geosciences, San Francisco State University, San Francisco, CA 94132 and GRAN, Karen B., Geological Sciences, University of Minnesota Duluth, 1114 Kirby Dr, Duluth, MN 55812, fulle155@umn.edu
Lateral bedrock erosion is potentially important in setting boundary conditions for landscape evolution, yet little is known about the controls and mechanisms of this process. We conducted a series of flume experiments with erodible ‘bedrock’ walls to investigate the influence of bed roughness on lateral erosion in bedrock channels. Bed roughness was varied along the length of the channel by changing the size of particles embedded in a non-erodible bed material. Single experiments consisted of 3 or 4 bed roughness sections, each spanning 2 m in length with a distinct embedded particle size. Embedded particle size ranged from 1.2 mm to 16.0 mm over the series of experiments and varied by a factor of 2.5 to 6.5 within a single experiment. In addition to varying the size of embedded particles, we varied the downstream trend in bed roughness (increasing, decreasing or alternating). Experiments consisted of an initial time period of clear water flow followed by multiple periods in which 4mm gravel was introduced at a constant rate. At the end of each time period, detailed elevation data of the experimental channel were collected using a laser scanning system.
Lateral erosion from clear water flow was negligible compared to erosion during periods of bedload supply. Erosion was focused near the base of the channel walls resulting in banks that were undercut by as much as 25% of initial channel width. Reach-average erosion rates in ‘rough’ sections (2.4 mm to 16 mm embedded particles) were 3 to 5 times greater than those in control sections (no embedded particles). Within a single experiment, erosion rates in ‘rough’ sections of different embedded particle size are similar. This similarity suggests that rates of lateral erosion are insensitive to increases in bed roughness beyond a threshold value (embedded particle size : transport particle size ≈ 1). Longitudinal variability in cross-section-scale erosion increases continuously with increases in embedded particle size, likely reflecting the larger gap between individual embedded particles. Finally, in the ‘rough’ bed sections, we observe a three-fold decrease in erosion rates between the first and second periods of bedload supply. Our results indicate: 1) bedload impacts are a viable mechanism for lateral bedrock wear and 2) bed roughness exerts a control on rates of lateral erosion.
