Lateral erosion in an experimental bedrock channel: The influence of bed roughness on erosion by bed load impacts

Physical experiments were conducted to evaluate the efficacy of bed load particle impacts as a mechanism of lateral bedrock erosion. In addition, we explored how changes in channel bed roughness, as would occur during development of an alluvial cover, influence rates of lateral erosion. Experimental channels were constructed to have erodible walls and a nonerodible bed using different mixtures of sand and cement. Bed roughness was varied along the length of the channel by embedding sediment particles of different size in the channel bed mixture. Lateral wall erosion from clear-water flow was negligible. Lateral erosion during periods in which bed load was supplied to the channel removed as much as 3% of the initial wetted cross-sectional area. The vertical distribution of erosion was limited to the base of the channel wall, producing channels with undercut banks. The addition of roughness elements to an otherwise smooth bed caused rates of lateral erosion to increase by as much as a factor of 7 during periods of bed load supply. However, a minimum roughness element diameter of approximately half the median bed load particle diameter was required before a substantial increase in erosion was observed. Beyond this minimum threshold size, further increases in the relative size of roughness elements did not substantially change the rate of wall erosion despite changes in total boundary shear stress. The deflection of saltating bed load particles into the channel wall by fixed roughness elements is hypothesized to be the driver of the observed increase in lateral erosion rates.