Experimental study of retrogressive erosion efficiency evaluation in Xiaolangdi Reservoir model

Retrogressive erosion is a special phenomenon of fluvial processes, where the channel scour develops from downstream to upstream. Retrogressive erosion occurs due to the rapid reduction in water level, which is closely related to the engineering controlling conditions, water and sediment conditions, scouring time and duration, and initial reservoir conditions. Various impacts of destruction degree of a sandbar in a tributary on the retrogressive erosion in the main stream have been investigated. By use of a physical model for the Xiaolangdi Reservoir, which was constructed based on the similarity laws of sediment-laden physical reservoir model, four tests were conducted to investigate the effect of retrogressive erosion due to lower the pond water level quickly. On the constrain of different sandbars evolved in the tributary of Zhenshui, different flow processes and quantities from the interior tributary caused different sediment processes and quantities of the main stem channel below the branch. Restoration efficiencies of reservoir storage capacity for test 1 to test 4 was 11.6%, 6.8%, 12.2% and 6.6%, respectively. More water quantity would be released from the tributary for a severer sandbar destruction degree, which led to a higher restoration efficiency of reservoir storage capacity due to a larger volume of channel scour. In addition, a lower base level of erosion could produce the similar effects. When the volume of reservoir deposition reached 4.2×10⁹m³, the scour effect of retrogressive erosion by lowering the pool level would be better than the case with the volume of reservoir deposition of 3.2×10⁹m³. The results show that the calculated results are in close agreement with the experimental data. The formula for calculating the daily volume of retrogressive erosion has been improved, which was originally proposed by Tsinghua University and Northwest Institute of Hydraulic Science of China. The resulted from the improved formula indicate that a larger water volume released from the tributary could cause a higher amount of channel scour in the main-stem channel below the branch, and the calculations would be in close agreement with the measurements. These results can be used in comparing various schemes of water and sediment regulation of the reservoir in a sediment-laden river. Those enrich the subject content.