Responses of channel morphology to flow-sediment variations after dam construction: a case study of the Shashi Reach, middle Yangtze River

Upstream damming has profoundly impacted downstream channel morphology by altering inflowing water and sediment conditions, which can mostly be ascribed to variations in the flow hydrograph and sediment supply regime. In this paper, channel erosion and deposition during different flow-sediment processes are quantified using a 2D hydro-morphodynamic model. Our results revealed that the net erosion mainly occurred during the flood season when the flow discharges were above 15,000 m(3)/s. Together, larger peak discharges and less sediment supply could produce greater net erosion. Flow hydrograph variations could alter the inundation extent, thus creating a more widespread redistribution of channel deposition and erosion and possibly causing a shift in the active channel adjustment area, where more channel scouring and siltation occurred. The channel adjustments caused by the sediment supply regime variations underwent a gradual downward propagation process, and most of the riverbed thalweg profile variations could first be observed at a very short distance from the studied reach entrance. A larger cross-sectional area and channel depth as well as a lower width-depth ratio could result from larger floods and less incoming upstream sediment load. We found that a comprehensive flow-sediment combination coefficient with a value of m ranging from 2 to 4 most appropriately reflected the post-dam flow-sediment imbalance regime at the studied reach, which implied the leading role of flow hydrograph variations in shaping channel morphology. In summary, the combined results presented herein for the Shashi Reach of the Yangtze River can provide a better understanding of the downstream morphological impacts of different flow-sediment processes caused by dam operation.