Channel adjustments in response to the operation of large dams: The upper reach of the lower Yellow River

摘要

The Yellow River in China carries an extremely large sediment load. River channel-form and lateral shifting in a dynamic, partly meandering and partly braided reach of the lower Yellow River, have been significantly influenced by construction of Sanmenxia Dam in 1960, Liujiaxia Dam in 1968, Longyangxia Dam in 1985 and Xiaolangdi Dam in 1997. Using observations from Huayuankou Station, 128 km downstream of Xiaolangdi Dam, this study examines changes in the river before and after construction of the dams. The temporal changes in the mean annual flow discharge and mean annual suspended sediment concentration have been strongly influenced by operation of theses dams. Observations of sediment transport coefficient (ratio of sediment concentration to flow discharge), at-a-station hydraulic geometry and bankfull channel form observed from 1951 to 2006 have shown that, although variations in flow and sediment load correspond to different periods of dam operation, changes in channel form are not entirely synchronous with these. The channel has been subject to substantial deposition due to the flushing of sediment from Sanmenxia Dam, resulting in a marked reduction in bankfull cross-sectional area. Flows below bankfull had a greater impact on channel form than higher flows because of very high sediment load. At-a-station hydraulic geometry shows that the variation of channel cross-sectional area below bankfull in this wide and relatively shallow system largely depends on changes in width. Such at-a-station changes are significantly influenced by (1) events below bankfull and (2) overbank floods. Bankfull depth is the main component of channel adjustment in that depth adjusts synchronously with channel area. The channel adjusts its size by relatively uniform changes in depth and width since 1981. Channel morphology is not the product of single channel-forming flow frequency. It is determined by the combination of relatively low flows that play an important role in fine sediment transport and bed configuration as with relatively high flows that are effective at modifying the channel's morphology. The sediment transport coefficient is a useful index for efficiently guiding the operation of the dams in a way that would minimize channel changes downstream. Sedimentation over the nearly 60 years of study period caused the lower Yellow River to aggrade progressively, the only significant exception being the two years following completion of Sanmenxia Dam.