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.

中文翻译：

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在河流采摘中达到诱捕阈值

在陡峭的基岩河流中，采掘块体是一个重要但未被充分研究的侵蚀组成部分。为了开发用于基岩通道演化模型的现实夹带阈值，我们重新制定了块夹带问题。我们推导出了代表在通道地板中占据朝后台阶的块的滑动和倾倒的力和扭矩平衡。我们采用计算流体动力学软件包来计算此类块体所有面上的压力和切应力，因此明确表示了水平方向的曳力和垂直方向的举升力。我们发现上游和下游块面之间的压力差会显着影响力和扭矩的平衡。在典型流量下，下游压力比上游总压力低约 1%–2%，反映了再循环区流体压力的降低。此压力差与块雷诺数成正比，如果块突出到上游壁架平面上方，压力差会急剧增加。由此产生的净下游导向力既增加了促进滑动的力，又提供了围绕块体下部下游拐角的扭矩。借助块体上的全套力，我们探索了块体几何形状、流速和深度、倾覆枢轴点位置和河床坡度在促进滑动和倾覆方面的作用。考虑到上游和下游面之间的小压力差，大大降低了采挖的门槛，并表明河道底部的块应该以比以前认为的更低的速度移动，因此更频繁地排放。