Studies of Bed Elevation Variations in Sand Bed Rivers Caused by Two Successive Knick Points

Knick points are generated, naturally or artificially, across the river beds in the form of sequential steps. In the present work, the experimental and numerical migration of two successive Knick points in a sandy bed of a rectangular channel is studied for different discharges. At the start of flow, the two Knick points became unstable in a short period of time. Erosion caused the migration of the first Knick point backward upstream and deposition, down the second Knick point, moved it forward to the downstream channel. At the same time a side bar was formed down the first Knick point and extended upward. Downstream of this bar and before the second Knick point the canal was meandered. Between the two Knick points, the channel developed a steep slope bed. Increase in discharge augmented the rate of movement of the two Knick points. Consequently, migration not only changed the channel elevation but also destabilized its alignment in plan. The flow and sediment transport equations were integrated by HEC-RAS (V4.1.0) for different sediment transport functions. In comparison with the available experimental data, Larsen and Ackers-White equations proved a better estimation for the prediction of bed level variations.