Migration of Backward‐Facing Step

Abstract

A backward‐facing step sometimes forms in an alluvial river. The step deforms and migrates when the inflow sediment and flow conditions change. A series of experiments were conducted to investigate the migration behavior of a backward‐facing step composed of uniform sediment. According to the experimental observations, the phenomena observed were that the bed material picked up from the upstream reach was deposited in the separation zone at the step and caused the step to migrate downstream. Due to contraction of the flow cross section in the vicinity of the knickpoint, the bed‐shear stress increased locally, which caused scouring. The scouring propagated either upstream or downstream depending on step height, flow conditions, and sediment characteristics. A linear‐wave model was proposed to simulate the process. The model was calibrated and verified with experimental data. Due to the nonuniformity of the flow conditions in the vicinity of the step, the model can only provide qualitative information in this region. Fortunately, the nonuniform‐flow conditions last at most 60 s, and hence the overall simulation results are quite satisfactory.