Toward Entrainment Thresholds in Fluvial Plucking

Abstract:

Rivers erode bedrock channel floors by abrasion and by plucking of blocks. Whereas abrasion has been extensively studied, quarrying has seen less attention despite its demonstrable importance in many settings. We revisit the block entrainment problem to develop entrainment thresholds for bedrock evolution models. Blocks that occupy backward-facing steps in the channel floor can be extracted from their niches by either sliding or toppling. We develop force and torque balances that represent these modes of extraction. Ours differs from past formulations of the problem by reformulating the fluid drag and lift on blocks to explicitly acknowledge the role of a pressure difference between upstream and downstream faces of blocks. We exploit a computational fluid dynamics (CFD) package to calculate the detailed velocities and pressure fields around a block defining a negative step on the bed. We find that the pressure difference between the upstream face of a block and the downstream face that represents the bed step scales with the Froude number squared. At typical flows seen in rivers, the downstream pressure is typically ~1-5% lower than the upstream. The low pressure on the downstream face reflects the lowered fluid pressures in the recirculation zone. The resulting net downstream-directed force adds to the forces promoting sliding and provides a significant torque about the lower downstream corner of the block. Incorporation of the pressure difference between upstream and downstream faces greatly promotes plucking and suggests that blocks on the floors of river channels should move at lower discharges than previously thought.