Author(s): A. Z. Luo; N. S. Cheng; Y. S. Lu; M. X. Wei
Linked Author(s):
Keywords: Bedload transport; Local scour; Shear scour; Vortex scour; Plane wall jet
Abstract: Classical bedload transport models, which predominantly assume a state of simple shear turbulence in uniform open channel flow, fall short in capturing the intricacies of sediment movement during local scour, a complex non-equilibrium sediment transport process under non-uniform and transient flow conditions. For this reason, our study investigates a two-dimensional local scour induced by a plane wall jet. This simplified scenario facilitates simultaneous measurements of the evolving scour profile and flow field. We discern that the local scour process bifurcates into two distinct stages: an initial "shear scour" phase, governed by bed shear forces, followed by a "vortex scour" phase, where large-scale vortices prevail. During the shear scour stage, the increasing scour volume per unit width is linear with time, yielding a constant bedload transport rate. Conversely, during the vortex scour stage, the scour volume per unit width aligns logarithmically with time, and the bedload transport rate inversely correlates with scouring duration. In the shear scour phase, flow energy predominantly concentrates near the bed, whereas in the vortex scour phase, jet flow detaches from the bed and extensively mixes with the ambient fluid, resulting in a decrease in flow energy near the bed. Utilizing the sheet-flow bedload transport model, we establish that the dimensionless bedload transport rate scales with the cube of the densimetric Froude number during the shear scour stage. Furthermore, we derive a formula to compute bed shear stress within the scour hole based on the phenomenological theory of turbulence. With the so-obtained bed shear stress, we note that bed shear stress-based model, typically employed to predict the bedload transport rate in uniform flow conditions, is also applicable to the scouring process. This finding suggests that the bed shear stress is the “first principle” force driving bedload transport.
Year: 2024