RIVER EROSION BY PLUCKING ILLUMINATED IN LAB AND FIELD STUDIES

Plucking regulates the style and rate of erosion in many bedrock rivers, but the physical forces governing the hydraulic conditions that initiate plucking need further study. Wilkinson et al. (2018) proposed three potential mechanisms that explain how fractured bedrock can be plucked from a smooth surface in the presence of non-uniform or changing water surface topography. Building on former flume studies of plucking, we used a 2.44 m X 14.3 cm X 33 cm acrylic flume with 3D-printed plaster blocks with a 0.5-mm-aperture crack network to further investigate Wilkinson’s hypotheses and learn more about the role of hyporheic flow and associated pressure differentials during plucking in non-uniform flow. We created hydraulic jumps at an upstream step and controlled its position and geometry by changing the flume slope and the flume outlet using an adjustable gate. Dye injected under the simulated bedrock blocks illustrates the dynamics of pulsed, upstream flow in the hyporheic zone and pressure sensors record pressure below the blocks along the flume centerline. With these tools, we seek to illustrate pressure pulses and flow below the blocks in response to the dynamics of flow over the blocks and in the hydraulic jump. To correlate these lab data with field conditions, we continued the development of an instrumented block removed from fractured, bedrock bed below a step in the Cowpasture River, Virginia. A sensor capsule designed to measure motion and pressure during high-flow events. The capsule emplaced in a 4.5 cm hole drilled through the block consists of two pressure sensors and three-axis gyroscopes and accelerometers. The pressure sensors are placed at the top and bottom of the rock within the capsule to collect and record pressure data to identify neutral buoyancy. The gyroscopes and accelerometers measure angular and linear inertial data respectively to track vibrations and lift. The inertial and pressure data are synchronously measured and recorded to the SD card. Analysis of the recorded data lends itself to a more complete understanding of plucking through synthesizing experimental results from the lab and field.