Causes of local gravel deposition in gravel-sand transition of the Middle Yangtze River under clear water scour

After impoundment of the Three Gorges Reservoir (TGR), channel scour is ongoing in the reaches downstream the dam and gravel erosion, transport, and deposition have occurred in some areas of the gravel-sand transition (GST) of the Yangtze River. To investigate the causes of these phenomena, field data of flow, sediment and channel topography was collected and computational analysis was performed using a 2-D hydrodynamic numerical model. Especially, the spatial distribution characteristics of the incipient diameter under various discharges in the GST were simulated and examined. Impacts of the variations in river flow and the changes in water level downstream the reach on channel adjustment were also analyzed. The following results are obtained:① The large grain size gravel (D>30 mm) can be continuously transported along the entire GST when the incoming discharge is higher than 45 000 m³/s. On the contrary, the large grain size gravel is only locally transported in some shoal sections when the discharge lower than 15 000 m³/s. The flow dynamics are relatively weak for the transport of the same size gravel when the discharge ranges from 15 000 m³/s to 45 000 m³/s. ② The number of days with floods reduced and that with low discharge increased under the effects of the TGR operation. This trend is adverse to the long-distance transport of gravel. The water level lowering downstream the GST caused erosion in areas that previously remained stable. ③ The phenomenon of the local siltation was caused by the gravel transport in the GST driven by the increasing flow dynamics in the dry season. The main cause of this phenomenon is relevant to the special morphologic structures and sedimentary environment of the GST. This type of local bed adjustment may exist in the several locations within the GST for a long time and should receive considerable attention.