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

Abstract

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 channelforming 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.

Progradation of the Yellow (Huanghe) River delta in response to the implementation of a basin-scale water regulation program

“…1a). Although the Sanmenxia Reservoir initially had a volume of 36 billion m 3 , heavy sedimentation after its construction has reduced its available volume to less than 1 billion m 3 (Chen et al, 2002;Ma et al, 2012). Despite such a substantial loss, it still has some capacity for adjusting runoff and sediment, and the joint operation of these three large dams has successfully ended more than two decades of no-flows in the lower Yellow River and the YRD (e.g.…”

“…Wang et al, 2010;Chen et al, 2012;Chu, 2014). Due to the heavy deposition of sediment, the channel bed of the lower reach has become elevated above the adjacent valley floor by some 8 to 12 m over the past 40 years, so this water-sediment modulated scheme helps to reduce the discrepancy in channel and ground elevation (Ma et al, 2012;Chu, 2014).…”

“…The average annual runoff and sediment supply are the highest in the first period (1950)(1951)(1952)(1953)(1954)(1955)(1956)(1957)(1958)(1959)(1960)(1961)(1962)(1963)(1964)(1965) when the river adopted the Shenxiangou course, 61.5% and 64.1% higher than the corresponding averages for the entire 64 years (30.5 billion m 3 yr − 1 and 0.71 billion t yr −1 ), respectively. In the period [1966][1967][1968][1969][1970][1971][1972][1973][1974][1975] patterns of runoff and sediment load are closely linked to the operation of the four large reservoirs in the upper and middle reaches of the Yellow River, as detailed by Ma et al (2012) and Chu (2014).…”

“…Consequently, the amount of water and sediment delivered to the YRD has reduced and the rate of sedimentation in the estuary slowed greatly, causing significant coastal erosion in many parts of the delta (e.g. Chu et al, 2006;Saito et al, 2007;Wang et al, 2010;Ma et al, 2012;Yu et al, 2013a,b;Chu, 2014;Liu et al, 2014;Zhou et al, 2014). The delta today supports a human population of more than two million who obtain their livelihood from farming, fishing and oilexploiting, so changes to its water and sediment supply and land area can have profound impacts.…”

Controls of channel morphology and sediment concentration on flow resistance in a large sand-bed river: A case study of the lower Yellow River

“…The river channel bed is composed of fine sand (mean grain size b 0.1 mm; Table 2). The high suspended sediment load in the lower Yellow River exerts great influence on channel morphology change (Qian and Zhou, 1965;Wang et al, 2009b;Ma et al, 2012). Although some investigations have studied flow resistance in the lower Yellow River (Qian et al, 1959;Qian and Zhou, 1965;Qin et al, 1995;Cheng et al, 1997;Sun et al, 2005), a clear recognition of the major controls on flow resistance is still lacking.…”