Plucking in Mixed Alluvial-Bedrock Rivers: The Incipient Motion Problem

Abstract:

Bedrock river channel erosion is an important factor in the evolution of landscapes, driving the relief of mountainous drainage basins and setting the lowest erosional positions of terrestrial landscapes. The mechanics behind erosional processes (predominantly plucking and abrasion) in these rivers are only recently being explored in depth. Plucking, the fracture and extraction of jointed blocks, is observationally an order of magnitude more efficient than abrasion, but if a river cannot provide the force necessary to move the plucked block, erosion by plucking cannot proceed. Therefore, incipient motion of blocks starting at rest on a solid surface is an important factor in erosion by plucking. Calculations of forces necessary for incipient motion require values of drag coefficients, which do not exist for bedrock contact geometry. We discovered from experiments on a flume that drag coefficients (CD) are inversely proportional to aspect ratios (RA), defined as the frontal block height to width. We used the relationship with field data from plucked blocks at a stream at Montgomery Bell State Park in Burns, TN, a mixed-alluvial bedrock channel with an actively incising knick zone, to support our theory and experimental data. Sizes of plucked blocks were compared to the velocities needed to move them, and then calculations done for bankfull velocities at the stream at Montgomery Bell to determine if it could attain these velocities. It was discovered that this stream has a bankfull depth-averaged velocity of 1.26 m s^-1 and is capable of moving a large range of plucked block sizes. Therefore, erosion of this particular stream is plucking-limited, not transport-limited.