Toward Entrainment Thresholds in Fluvial Plucking

Plucking of blocks is an important, but understudied component of erosion in steep bedrock rivers. With the goal of developing realistic entrainment thresholds for use in bedrock channel evolution models, we re‐formulate the block entrainment problem. We derive the force and torque balances representing sliding and toppling of blocks occupying backward‐facing steps in the channel floor. We employ a computational fluid dynamics package to calculate the pressures and shear stresses on all faces of such a block, therefore explicitly representing the horizontally directed drag force and vertically directed lift force. We find that the pressure difference between the upstream and downstream block faces can significantly impact the force and torque balances. At typical flows, downstream pressure becomes ∼1%–2% lower than total upstream pressure, reflecting the reduction in fluid pressures in the recirculation zone. This pressure difference scales with the block Reynolds number squared and dramatically increases if the block protrudes above the plane of the upstream ledge. The resulting net downstream‐directed force both adds to the forces promoting sliding and provides a torque about the lower downstream corner of the block. Armed with the full set of forces on a block, we explore the roles of block geometry, flow velocity and depth, toppling pivot point location, and bed slope in facilitating sliding and toppling. Incorporating the small pressure difference between upstream and downstream faces greatly lowers the threshold for plucking, and suggests that blocks on river channel floors should move at lower and hence more frequent discharges than previously thought.