Dramatic sediment load changes and sedimentation characteristics upstream of the Three Gorges Dam due to the large reservoirs construction
Jie LIU1,2, Wenwu ZHANG1,3, Ying SHEN1,3, Xin WANG1,4()
. National Inland Waterway Regulation Engineering Research Center, Chongqing Jiaotong University, Chongqing 400074, China . State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin 300072, China . School of River & Ocean Engineering, Chongqing Jiaotong University, Chongqing 400074, China . Tianjin Research Institute for Water Transport Engineering, Tianjin 300456, China
After the construction of cascade reservoirs in the upper reaches of the Three Gorges Reservoir (TGR), the sediment load outflow of the upper Yangtze River Basin (YRB) has been significantly altered, decreasing from 491.8 Mt/yr (1956–2002) to 36.1 Mt/yr (2003–2017) at Yichang station. This has widely affected river hydrology, suspended sediment grain size distribution, and channel morphology. This study analyzed hydrological variations in water discharge and sediment load of the upper YRB over the past 62 years (1956–2017) by employing a double mass curve. The variations in the source areas of sediment yielding for the upper YRB were quantified, and field measurement data of the cross-channel profile were collected to investigate the sedimentation process in the TGR from 2003 to 2017. More than 90% of the sediment load reduction in the upper YRB may be explained by human activities. The Jinshajiang River was no longer the largest sediment source area for the Zhutuo station (accounting for 5.23%) in the 2013–2017 time span, and the sediment rating rates for the inflow and outflow of the TGR shifted to negatively correlated. A longitudinal fining trend was revealed in the suspended sediment size. Still, the mean median grain size of suspended sediment in the TGR had an increasing trend in the 2013–2017 period. This result may be closely related to sediment regulation in reservoirs and incoming sediment load reduction. Sedimentation in the TGR decreased sharply from 299.8 Mt/yr in 2003–2012 to 47.2 Mt/yr in 2013–2017, but the sedimentation rate of the TGR remained at > 80% annually. Moreover, some cross sections in the fluctuating backwater zone experienced scouring.
Online First Date: 03 July 2024Issue Date: 29 September 2024
Cite this article:
Jie LIU,Wenwu ZHANG,Ying SHEN, et al. Dramatic sediment load changes and sedimentation characteristics upstream of the Three Gorges Dam due to the large reservoirs construction[J]. Front. Earth Sci.,
2024, 18(3): 565-578.
Fig.1 Map of the study area. (a) Yangtze River Basin (YRB) with main hydrometric stations and dams of the upper YRB. (b) The variation in water level in front of the TGD, based on Wusong elevation, (1) cofferdam stage: 135–139 m; (2) initial impoundment stage: 144–156 m; (3) pilot impoundment and the normal operation stage: 145–175 m.
Fig.2 Temporal variations of annual water and sediment discharge at four hydrological control stations in 1956–2017. The water and sediment discharge dates at Zhutuo from 1967 to 1971 were not measured. Bold vertical lines label the operation time of large reservoirs in tributaries and the main stem of the upper YR in recent years.
Fig.3 Variation of the median grain size and grain-size composition of suspended sediment along the upper YR and its tributaries in 1987–2002, 2003–2012, and 2013–2017.
Fig.4 Double mass curves of the annual sediment load and water discharge at four key hydrological stations in the upper YRB.
Fig.5 Contributions of climate change and human activities to sediment load in the upper YRB.
Rivers
Sediment load/(Mt·yr?1)
Water discharge/km3
1956?1997
1998?2012
2013?2017
1956?1997
1998?2012
2013?2017
Xiangjiaba (Jinshajiang River)
250.45
194.57
1.98
143.22
151.37
131.92
Gaochang (Minjiang River)
50.39
30.12
12.5
86.43
79.84
77.7
Fushun (Tuojiang River)
12.47
10.53
8.39
10.61
19.7
12.08
Hengjiang (Hengjiang River)
14.24
8.38
7.06
8.34
6.71
8.54
Zhutuo
311.25
216.89
37.86
264.81
265.17
254.24
Tab.1 Mean annual sediment load and water discharge at five hydrologic stations in 1956–1997, 1998–2012, and 2013–2017. The corresponding rivers for the control hydrological stations are indicated in parentheses
Fig.6 The proportion of water and sediment source areas for Zhutuo stations in 1956–1997, 1998–2012, and 2013–2017. (a) Water discharge; (b) sediment load.
Fig.7 The rating curves of annual water discharge versus sediment load in 1956–1989, 1990–2002, 2003–2012, and 2013–2017. The annual water and sediment discharge that flows into the TGR is the sum of the values at Zhutuo, Beibei, and Wulong. The dates at Zhutuo from 1967 to 1971 were excluded from the calculation for those years that were not measured. The data for 1998 and 2006 were not calculated for their respective catastrophic floods and extreme droughts. (a) Inflow of three gorges; (b) outflow of three gorges.
Fig.8 The annual sediment inflow variation and sedimentation in the TGR.
Fig.9 The monthly sedimentation rate of the TGR.
Fig.10 Change in the longitudinal profile of the backwater zone after the operation of the TGR.
Fig.11 Cross-channel scouring and sedimentation profiles of typical cross sections in the TGR.
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