Performance of Riprap Structures in the Stabilization of Knick Points in a Rectangular Canal, an Experimental Study

Knick points are normally shaped in riverbeds in the form of successive drops. Backward migration of Knick points in a canal result in instability, erosion, bed subsidence and sediment transport and may cause serious damages to the canal banks and upstream structures such as bridges. In the present research, two successive Knick points, with 10% slope and 1 meter distance are constructed at the sandy bed of a rectangular open channel with a 0.003 longitudinal slope. The bed erosion, Knick point migration and its development were studied under different discharges. The performance of four grade control structures as riprap sill, riprap sloping sill, Newbury rock riffle and cross-vane riffle were studied experimentally for the stabilization of each Knick point. All the structures were successful in controlling the bed erosion. However, the riffles operated much better than sills. Specifically, Newbury rock riffles that concentrated the flow in the middle of the channel, prevented Knick point migration and stabilized its position, completely. The experiments demonstrated that the construction of a control structure is not only effective in the stabilization of a Knick point but retarded the development of its nearby Knick point too.