Response of the gravel–sand transition in the Yangtze River to hydrological and sediment regime changes after upstream damming

Abstract

In the Yangtze River, the Three Gorges Dam (TGD) has altered the flow-sediment regimes, resulting in channel degradation and bed surface coarsening, which have been reported in recent years. However, there has been no systematic study on the impacts of the TGD on the downstream gravel-sand transition (GST). Based on detailed field observations and numerical modelling, this study fills this gap and reveals significant adjustments in the GST and the leading causes. The results show that after operation of the TGD began, the GST migrated 49.5 km downstream during 2003-2010 due to a reduction in the suspended-load supply (grain size < 0.5 mm) and then remained stable from 2010 to 2015 under the control of a braided river morphology. In addition, as channel degradation continued, the steepening bed profile upstream of Chenjiawan and the lowering of the downstream water level caused some gravel within the GST to become more frequently mobile during low-flow periods (discharge <15,000 m 3 /s). Combined with the effects of the TGD-induced prolonged low-flow season and reduced frequency of large floods (discharge ≥ 40,000 m 3 /s), the GST exhibited a poorly understood phenomenon in which gravel was mainly transported downstream during low-flow periods. However, these processes affected only localized sediment movement, contributing little to overall GST migration in the Yangtze River. The results also show that when a low sediment supply persists long term, additional GST migration after transient downstream migration depends primarily on the river morphology. This article has implications for the channel regulation and evolutionary processes of GSTs below dams.

Gravel‐Sand Transition of the Yangtze River: Human Disturbances, Migration Processes, and Controlling Factors

“…Due to the difference in bedforms, channel morphologies and human‐made structures (i.e., the factors related to form drag), the $$\alpha $$ value of GSTs varied in individual large rivers. The $$\alpha $$ values in the middle of the Yangtze River have been reported previously (He, Sun, Li, et al., 2022), with values ranging from 0.3 to 0.33. Here, we consider the adjustments in the $$\alpha $$ value over time to be negligible for simplicity and assumed $$\alpha =0.32$$ in 1975, 2002, and 2016.…”

“…Such distributions indicated that the end of the GST seemed to be located upstream of KM 129. To further pinpoint the GST position, we use an inverse method based on the fact that the GST position in 2002 was located in KM 0-61 (He, Sun, Li, et al, 2022), with the downstream surface material characterized by sand (Figures 5c and 5d). Dai et al (2005) reported that, due to the GZB operation in 1982, the YXR had undergone erosion within KM 0-61 until the bed returned to equilibrium in 1987.…”

“…Thus, we set 1975 as the dividing point between our first two periods instead of 1972, when the JMC was completed. During the baseline period (1965–1975), the river channel was in quasi‐equilibrium, with stable average annual water and sediment supplies of approximately 4.38 × 10 11 m 3 and 5.28 × 10 8 t, respectively. The sediment supply was characterized by a weakly bimodal grain size distribution, with fine sediment (grain size <0.5 mm) accounting for 98% of the total upstream supply (He, Sun, Li, et al., 2022). Water discharge and sediment outputs from the two outlets also had limited adjustments, accounting for 14.2% and 14.0%, respectively, of the upstream supply. During the second period with limited human interference (1976–2002), the JMC caused the outlet base level at KM 217 to rapidly decrease by 0.6–0.8 m during flood flows in 1973 and then to slowly decrease by 0.2–0.4 m in the following years (Figure S1 in Supporting Information S1).…”

Mechanisms Controlling Water‐Level Variations in the Middle Yangtze River Following the Operation of the Three Gorges Dam

“…The jacking force is an interactive force in that the lakes create a backwater effect on the Yangtze River and in turn, the Yangtze River has a block effect on the outlet of lakes. The MYR is a predominately meandering channel with many floodplains, and a riverbed composition that tends toward finer grain sizes downstream (He et al., 2021; Lyu et al., 2020).…”

Impact of upstream reservoirs on geomorphic evolution in the middle and lower reaches of the Yangtze River

“…However, the extent and frequency of riverbank collapses have decreased in recent times due to the implementation of riverbank protection works (Huang et al, 2016). Previous studies observed abrupt changes in the sandy and pebbly reaches downstream of the TGD, mostly due to the adjustments of riverbed morphology, often accompanied by declining scouring intensities (He et al, 2022;Zhang et al, 2017). Some studies also determined that scouring prevailed in the low-water channels (Dai et al, 2018;Han et al, 2014Han et al, , 2017Xu et al, 2011Xu et al, , 2013Yang et al, 2022), and there was a lag in the response of channel morphological adjustments resulting from changing water-sediment conditions (Lyu et al, 2020).…”