Relationships between block quarrying, bed shear stress, and stream power: A physical model of block quarrying of a jointed bedrock channel

Abstract

We examine interactions among substrate characteristics, hydraulic forces, and erosive processes acting on a jointed, resistant substrate in a flume experiencing supercritical flow. We conducted six experimental runs with no sediment in transport, constant slope, varying discharge, varying channel width, and varying geometry of concrete blocks forming the channel bed. The initial downstream boundary was a vertical knickpoint lip perpendicular to flow. Observed changes in bed morphology occurred primarily by episodic horizontal knickpoint retreat. The knickpoint lip can develop a curve that reflects interlocking of the irregular boundaries between otherwise physically disjointed blocks during rotation of the blocks. Sometimes a block acts like a keystone in an arch and mobilization of the key block facilitates mobilization of the linked blocks on either side, resulting in a large magnitude event. In contrast to previous studies where blocks were vertically plucked, blocks were removed by sliding from the knickpoint. Building on previous work, we develop a theoretical force threshold for sliding block mobilization, which suggests that increased block side length parallel to flow increases block stability, whereas increased side length perpendicular to flow decreases block stability. Stream power was also linked to observed erosion rates as a function of block length, consistent with the idea that block loosening rates play a key role in controlling average erosion rates. Inferred amplification of bed shear stress and pressure at the knickpoint lip, stream power, and joint spacing correlate with the rate of block erosion. The results, although limited in scope and scale, suggest a linear relationship between bed shear stress and stream power with the rate of block erosion. Block sliding, especially at near-vertical knickpoints, can play a key or even dominant role in the morphologic dynamics of the river channel.