MIGRATION OF BACKWARD-FACING STEP

A backward-facing step sometimes forms in an alluvial river. The step deforms and migrates when the inflow sediment and flow conditions chan ge. A series of experiments were conducted to investigate the migratio n behavior of a backward-facing step composed of uniform sediment. Acc ording to the experimental observations, the phenomena observed were t hat the bed material picked up from the upstream reach was deposited i n the separation zone at the step and caused the step to migrate downs tream. 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 depend ing on step height, flow conditions, and sediment characteristics. A l inear-wave model was proposed to simulate the process. The model was c alibrated and verified with experimental data. Due to the nonuniformit y of the flow conditions in the vicinity of the step, the model can on ly provide qualitative information in this region. Fortunately, the no nuniform-flow conditions last at most 60 s, and hence the overall simu lation results arc quite satisfactory.