清水冲刷条件下长江中游沙卵石河段局部卵石淤积成因

为探究三峡水库持续下泄清水条件下长江中游沙卵过渡段内卵石局部冲刷、搬运和淤积现象的成因,采用近期水文、泥沙和地形观测资料,结合河道平面二维水动力模型,计算了研究河段内各级流量下的泥沙起动粒径分布,分析了上游来流、下游水位变化的影响,辨析了河床冲淤的成因。结果表明:①45 000 m³/s以上流量时,粒径大于30 mm的卵石可沿洪水主流带连续搬运;流量低于15 000 m³/s时,大粒径卵石只能沿枯期主流在浅滩河段局部搬运;流量介于两者之间时,水流对大粒径卵石的输移动力相对较弱。②三峡建库后,洪水量级削减而枯水天数增多,不利于卵石长距离下移,而水位下降不断溯源传递,促使枯水流路上原本稳定的区域开始冲刷。③局部淤积现象由枯期水动力增强所导致,与长江中游沙卵过渡段特殊的地貌和沉积环境有关。卵石局部冲淤调整可能在河段内多个位置长期存在,需引起关注。

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.