Simulation analysis of river channel evolution after small dam removal: a case study of Xihe dam

As the serving time of low-head dams prolonged,security,economy and ecology issues of these dams have become increasingly prominent. For small dams with little repair values,forced retirement or demotion would be the best management measure. After removal of the dam,large amounts of sediment in the reservoir will be released,which leads to the change of river morphology hence affecting aquatic habitats. To predict the evolution characteristics of river channel in different time scales,Xihe dam in Wujiang drainage system was chosen as an example. By establishing two-dimensional sedimentation model,riverbed evolution in different time scales after dam removal was simulated and studied. The results showed that:in the short term after dam removal,the upstream of the dam experienced a strong scour,and sedimentation appeared in the front part of the reservoir delta;in contrast,the riverbed changes in the downstream of the dam was not obvious,only a small amount of sediment deposition occurred near the estuary and downstream of the dam;as for the long-term evolution after dam removal,both the upper and lower reach experienced different degrees of scour. After 2 years of dam removal,the morphology of downstream riverbed tended to be stable,while the evolution of the upstream transformed from scour to alternating deposition and erosion,and the riverbed morphology was under constantly adjusting. Results of our study can provide important basis for further river channel managements,aquatic habitat restorations after removal of sick dams and small hydropowers.