Research papers
Impact of water-sediment diversion and afflux on erosion-deposition in the Luoshan-Hankou reach, middle Yangtze River, China

https://doi.org/10.1016/j.jhydrol.2022.128110Get rights and content

Highlights

  • New empirical formulae for estimating erosion-deposition in Luoshan-Hankou reach.

  • A functional chart differentiating erosion-deposition types of Luoshan-Hankou reach.

  • Diversion-afflux states for maximum and equilibrium erosion-deposition in the reach.

Abstract

It is not yet fully understood how water-sediment diversion and afflux along a mainstream reach of a river affect erosion-deposition in downstream reaches. This study focuses on the Luoshan-Hankou mainstream reach of the middle Yangtze River, China. The Luoshan-Hankou reach is vitally important for flood control, being located downstream of three diversion mouths and an afflux outlet along the Jingjiang reach. We establish empirical formulae for sediment transport rates at boundary cross-sections, and hence estimate the amount and proportion of erosion-deposition and its relative increase (termed erosion-deposition promotion) in the Luoshan-Hankou reach. We then propose critical net water supplies from Dongting Lake to Luoshan-Hankou reach based on maxima and equilibria of erosion-deposition and its promotion. It is found that net water supply partly drives erosion-deposition in the Luoshan-Hankou reach where maximal proportions of deposition and deposition-promotion may be approximated by 0.01c-37.67 and 0.01c-37.67 + c-1, in which c is a dimensionless parameter representing the erosion-deposition condition in Luoshan-Hankou reach for no water-sediment exchange. At Zhicheng hydrological station, the critical ratio of net water supply to overall water discharge is 0.418c-33.33-1, and critical net water supply ratios for equilibria of erosion-deposition and its promotion are −1 (or c-33.33-1) and 0 (or (0.06 + 3.257c54.61)-1-1). A chart based on net water supply and c is devised representing four types of erosion–deposition and its promotion for the Luoshan-Hankou reach. Historical data over the past 65 years demonstrate that erosion-deposition and its promotion in the reach are respectively governed by c and net water supply; there is a remarkable shift from alternate erosion-deposition to monotonic erosion whilst the erosion-deposition effect remains consistent. The foregoing are in agreement with observed data, and comparable with data for the Jingjiang reach (affected by the three water-sediment diversion mouths). Satisfactory flood-control conditions in the convergence zone between the Yangtze mainstream and Dongting Lake accompanied by increasing erosion in the Luoshan-Hankou reach are predicted for the future.

Introduction

Water and sediment diversions from a river into distributaries, and affluxes from tributaries into a river affect net erosion–deposition in mainstream reaches. This can have engineering consequences, such as altered flood risk, navigation obstruction and land-resource loss. In China, water diversion for agricultural irrigation along the Kubuqi (Hobq) Desert reach of the Yellow River as it passes through Inner Mongolia has caused the mainstream flow velocity to decrease, enhancing sediment deposition and raising the mainstream flood discharge level (Pan et al., 2015). In the US, large-scale sediment diversion along the lower Mississippi River, through distributaries or small canals to restore sub-deltas, has diminished the deposition rate in mainstream reaches, lowering flood flow lines and effectively reducing navigation-related dredging volumes (Kemp et al., 2014). In South America, the decreasing trend in sediment afflux from the Madeira River (a major tributary of the Amazon River) has partly triggered muddy coast degradation and increased the risk of wetland recession in the Amazon Estuary (Li et al., 2020). Several previous studies have quantitatively dissected the influence mechanism of water–sediment diversion and afflux on erosion–deposition in a river mainstream (Lindner, 1953, Kerssens and Urk, 1986, Wang and Yin, 1989). To data, the focus has been on cases where either diversion or afflux solely occurs. However, there is a need to understand how the river mainstream evolves in cases where diversion and afflux occur concurrently along a given river. This is especially pertinent at the present time as many large rivers worldwide are experiencing intense human interference from dam construction, water-soil conservation works, sand excavation, etc.

The Yangtze River, China, has received widespread attention regarding erosion–deposition of its riverbed. The Luoshan-Hankou reach is located in the middle Yangtze River, immediately downstream of a water–sediment diversion and afflux system between Jingjiang reach and Dongting Lake, and occupies ~90% of the length of the Chenglingji-Hankou reach whose net erosion–deposition is of major concern owing to its critical impact on local flood control, especially near Chenglingji at the afflux outlet of Dongting Lake (Zhou, 2005, Han, 2006, Han et al., 2017). A major debate has taken place as to whether net sediment erosion or deposition would occur in the Chenglingji-Hankou reach after the impoundment of the Three Gorges Dam (TGD). One view (Zhou, 2005) was that riverbed evolution of the Chenglingji-Hankou reach was primarily determined by the amount of coarse sediment (of median diameter d > 0.1 mm) entering the reach. Given that the Jingjiang reach had experienced substantial sediment erosion after the impoundment of the TGD and thence supplied abundant coarse sediment to the Chenglingji-Hankou reach, it was most likely that persistent (>100 years) sediment deposition would occur in the Chenglingji-Hankou reach. Another study (Han, 2006) argued that the Chenglingji-Hankou reach had experienced both sediment erosion and deposition after impoundment of the TGD, with erosion resulting from the sediment trapping effect of the TGD, and deposition from the settling out of eroded material from the Jingjiang reach and a reduction in sediment diversion from the Jingjiang reach into Dongting Lake. This latter argument placed emphasis on the bed-forming effects of both fine (d < 0.1 mm) and coarse (d > 0.1 mm) sediment present in the river. Later researches demonstrate that the Chenglingji-Hankou reach has indeed experienced erosion after the impoundment of the TGD (Yuan et al., 2012, Han et al., 2017, Guo et al., 2019).

The Luoshan-Hankou reach exhibits similar riverbed evolution characteristics to those of the Chenglingji-Hankou reach and also acts as a key river segment for flood control, motivating many studies of its net erosion–deposition (Fang et al., 2012, Dai and Liu, 2013, Han et al., 2017, Lai et al., 2017, Dai et al., 2018b, Yang et al., 2018, Guo et al., 2019). The foregoing researches agree that the Luoshan-Hankou reach has changed from a sediment-sink before the impoundment of the TGD to a sediment-source after, during which time the TGD-induced sharp reduction in sediment delivery downstream has played the dominant role, expedited by water-soil conservation and sand extraction activities (Dai and Liu, 2013, Dai et al., 2018a). Meanwhile, the TGD has smoothed downstream hydrologic processes, thus altering the geomorphological evolution of bed features, such as mid-channel bars, in this reach (Mei et al., 2015, Lou et al., 2018).

Although the TGD is the determining factor behind erosion–deposition in the Luoshan-Hankou reach, it has also been observed that water–sediment diversion and afflux between the Jingjiang reach and the Dongting Lake also contribute (Han, 2006, Mei et al., 2015, Dai et al., 2018b). Specifically, Dongting Lake is not only receiving decreased water–sediment through diversion from the Jingjiang reach while facilitating the entry of additional mainstream water and sediment into the Luoshan-Hankou reach (Han, 2006), but has also weakened the flattening effect of the TGD on hydrological behavior in this reach (Mei et al., 2015). Moreover, the lake has also changed from a sediment-sink to a sediment-source (Dai et al., 2018) since TGD impoundment. The foregoing necessarily modulates water and sediment budgets in the Luoshan-Hankou reach and affect its riverbed erosion–deposition. Nevertheless, previous studies have mainly focused on variations in water–sediment diversion and afflux and corresponding erosion–deposition changes in the Dongting Lake area (Chang et al., 2010, Ou et al., 2014, Yang et al., 2014, Zhang et al., 2015, Zhu et al., 2015, Li et al., 2016, Wang et al., 2017, Yu et al., 2018). To date, little attention has been paid to the impact of water–sediment diversion and afflux on riverbed erosion–deposition in the Luoshan-Hankou reach.

Hu et al. (2016) carried out a relevant study of the Jingjiang reach which overlaps the three diversion mouths and is located upstream of the afflux outlet of Dongting Lake at Chenglingji. Specifically, Hu et al. (2016) discovered that the water–sediment diversions inherently promote deposition in adjacent mainstream reaches. However, this deposition-promotion has attenuated in recent decades due to the decreasing discharge trend in the water–sediment diversions, with an average deposition-promotion ratio of ~20% achieved during 1957–2010. (The deposition-promotion ratio is defined as the proportion of increased sediment deposition caused by the water–sediment diversions, accounted in the sediment flux entering the Jingjiang reach.) The Luoshan-Hankou reach receives water and sediment from the upstream Jingjiang reach (after water–sediment diversion and afflux have occurred) and the Hanjiang River (a tributary of the Yangtze River) (Zhou, 2005, Han, 2006, Han et al., 2017, Yang et al., 2018), and so undoubtedly undergoes a different erosion–deposition response to changes in systemic water–sediment exchanges between the Jingjiang reach and the Dongting Lake.

The present study explores the net erosion–deposition response of the Luoshan-Hankou reach to changes in water–sediment diversion and afflux along the Jingjiang reach, based on integrated data on daily water–sediment discharges at selected hydrological stations and multi-year average net erosion–deposition in the Luoshan-Hankou reach over the past 65 years (1955–2019), afforced by sediment-budget computations at prescribed cross-sections. Critical water–sediment exchange conditions are deduced for erosion–deposition and its relative increase (termed erosion–deposition promotion) in the Luoshan-Hankou reach, and an assessment chart devised that delineates four types of erosion–deposition condition. Historical variations of erosion–deposition and its promotion are calculated, and key influence factors identified. Erosion-deposition promotion effects of water–sediment diversion and afflux on the Luoshan-Hankou and Jingjiang reaches are compared, erosion–deposition types to be avoided in the Luoshan-Hankou reach are proposed, and an assessment made of the future flood situation in the reach. This paper quantifies the reaction of riverbed erosion–deposition processes to distributary diversion and tributary afflux along part of the middle Yangtze mainstream, and deepens our knowledge of the interaction between the Yangtze mainstream and Dongting Lake. Our research findings are instructive for water–sediment regulation and regional flood-control in the convergence zone between the Yangtze River and Dongting Lake, and might be applicable to other river–lake connection systems that are also undergoing complex variations in water–sediment exchanges.

Section snippets

Geographical setting

The Luoshan-Hankou reach is situated in the middle Yangtze River, stretching 251 km from Luoshan hydrological station to Hankou hydrological station (Fig. 1a and 1b), and connects to the Hanjiang River, a tributary of the Yangtze River (Fig. 1a and 1b)). The Jingjiang reach is located 31 km upstream of the Luoshan-Hankou reach and extends 347 km from Zhicheng hydrological station to the outlet of Dongting Lake at Chenglingji (Fig. 1b). Three water–sediment diversion mouths at Songzikou,

Identification of impact of water–sediment diversion and afflux on net erosion–deposition in Luoshan-Hankou reach

Fig. 2 is a schematic diagram of the Yangtze mainstream side of the river–lake connection system from Zhicheng to Hankou. To identify the impact of the three water–sediment diversions and single afflux on net erosion–deposition in the Luoshan-Hankou reach, sediment transport rates at the two boundaries of the reach are estimated for two cases: Case I, the actual reach with water–sediment diversion and afflux; and Case II, the idealized reach without any water–sediment diversion and afflux. In

Erosion-deposition response of the actual Luoshan-Hankou reach to water diversion and afflux along the Jingjiang reach

The actual Luoshan-Hankou reach experiences an overall trend from sediment erosion to deposition as ηTDM increases (Fig. 3a) and from sediment deposition to erosion as ηQilishan and ηQilishan-ηTDM increase (Fig. 3b and 3c). In other words, higher ηTDM corresponds to decreased Yangtze mainstream discharge entering the actual Luoshan-Hankou reach, whereas higher ηQilishan and ηQilishan-ηTDM indicate an increased net water supply from Dongting Lake, respectively resulting in lower and higher

Erosion-deposition promotion in different mainstream reaches

The Luoshan-Hankou reach is located downstream of three diversion mouths and an afflux outlet, which are overlapped by the Jingjiang reach (Fig. 1b and 2). Discrepancies therefore occur in the erosion–deposition promotions produced by the mainstream-lake water and sediment exchanges in the two Yangtze mainstream reaches. Erosion-deposition promotion in the Luoshan-Hankou reach is determined by the net water supply from the Dongting Lake. Water afflux at Chenglingji is generally larger than the

Conclusion

We present empirical formulae for evaluating the amount and proportion of erosion–deposition and its promotion caused by river–lake water–sediment exchanges in the Luoshan-Hankou reach of the middle Yangtze River, and deduce critical ηQilishan-ηTDM values from Dongting Lake to the reach for the maxima and equilibria of erosion–deposition and its promotion. An erosion–deposition assessment chart, consisting of six subareas, has been devised based on two parameters, c and ηQilishan-ηTDM. The

CRediT authorship contribution statement

Boyuan Zhu: Conceptualization, Data collection, Formal analysis, Investigation, Methodology, Writing - Original draft preparation, Writing - Reviewing and Editing. Jianhao Qin: Conceptualization, Data collection, Formal analysis, Investigation, Writing - Original draft preparation, Writing - Reviewing and Editing. Yitian Li: Conceptualization, Formal analysis, Writing - Original draft preparation. Gexuanzi Luo: Data collection, Visualization. Qi Xu: Visualization. Lingfeng Liu: Visualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded by the Natural Science Foundation of Hunan Province (Grant No. 2021JJ40607), the Scientific Research Foundation of Hunan Provincial Education Department (Grant No. 20B021) and the National Natural Science Foundation of China (Grants No. U2240224, 52071031, 51879198 and 52171245). The authors thank the Changjiang Water Resources Commission for providing hydrologic and terrain data. We are also grateful to the three anonymous reviewers for their insightful suggestions.

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