Elsevier

CATENA

Volume 188, May 2020, 104438
CATENA

Decadal changes in sediment budget and morphology in the tidal reach of the Yangtze River

https://doi.org/10.1016/j.catena.2019.104438Get rights and content

Highlights

  • Erosion occurred in the reach during pre- and post-TGD periods 1992–2003 and 2003–2008.

  • Contribution of erosion in the reach to output sediment flux increased.

  • Erosion and deposition were characterized by a length scale of 4–32 km.

  • Horizontal location of the thalweg had limited changes during 1992–2008.

Abstract

In the last few decades, riverine sediment flux to the coastal zones has been decreasing globally. The sediment flux from rivers to seas is usually estimated based on the lowermost gauging stations free of tides in the rivers, for example the Yangtze River. Knowledge of decadal sediment budget and the morphological evolution in the tidal reach is still limited. Here, using historical bathymetry of the tidal reach of the Yangtze, the changes in the sediment budget and local morphology in this reach for three periods between 1970 and 2008 were investigated. In terms of input sediment flux, Period I 1970–1992 is a relatively steady period, Period II 1992–2003 is a decline period with a significant decrease in the sediment flux, and Period III 2003–2008 is a decline period after the closure of the Three Gorges Dam (TGD). The reach was nearly in equilibrium during Period I. Net erosion occurred in the reach during both the pre- and post-TGD decline periods 1992–2003 and 2003–2008, with annual erosion of approximately 65 and 37 Mt/yr, respectively. The contribution of the annual erosion in the reach to the annual sediment flux at the downstream end of the reach increased from around 16.9% during 1992–2003 to 19.1% during 2003–2008. These changes in the sediment budget are closely related to the decrease in the input sediment flux, coarsening of the sediment grain size after the closure of the TGD, and changes in the riverine water flux and tidal conditions. Characterized by a length scale of 4–32 km, local deposition/erosion patterns were quite complex, which relates to the complexity of the local geometry, bathymetry and flow condition. The horizontal location of the thalweg of the reach had limited changes during 1992–2008, likely owing to the fixed riverbank due to human intervention.

Introduction

Since the last century, generally a decrease in the riverine sediment flux into seas is observed, which is mainly due to the constructions of dams on rivers globally. This holds especially for the rivers in Africa and Asia (Vorosmarty et al., 2003, Syvitski et al., 2005, and references therein), e.g., the Nile and Orange in Africa, and the Indus and Yangtze in Asia. The decrease in the sediment supply to the seas not only affects the morphology of estuaries, deltas, coastlines and coastal wetlands (Bird, 2008, Syvitski et al., 2009, Kirwan and Megonigal, 2013, and references therein), but also has impacts on the aquatic environment in the coastal zones (Jickells, 1998).

To understand the impact of the decrease in the riverine sediment flux on the coastal zones during the last few decades and in the future, it is of great importance to quantify the amount of the sediment supply to the seas. Most of the studies (Milliman and Farnsworth, 2011) estimated the sediment input from the rivers to the seas based on the measurement at the lowermost gauging stations free of tides in the rivers. However, many lowermost gauging stations of large rivers, e.g., the Amazon, Mississippi, Brahmaputra and Yangtze, are actually located hundreds of kilometers from the coast (Meade, 1996). Recently, several studies (Ralston and Geyer, 2017, Wang and Xu, 2018, Zheng et al., 2018a, Zheng et al., 2018b) have been conducted to investigate the decadal sediment budget in the river reaches downstream of the lowermost gauging stations. In the first two studies, they showed that a significant amount of riverine sediment was trapped in the tide-influenced reach of the Hudson during 2004–2015 (40%) and the lowermost Mississippi (70%) during 1992–2013, respectively. In contrast, Zheng et al., 2018a, Zheng et al., 2018b found erosion in the tidal reach of the Yangtze during 1998–2013. Despite these recent progresses, for many rivers, how much sediment is trapped or removed downstream of the lowermost gauging stations during the last few decades still needs investigation.

The effect of the decreasing sediment supply on the river morphology has been studied extensively (e.g., Williams and Wolman, 1984, Petts and Gurnell, 2005, ICOLD, 2009, and references therein). Based on Lane’s principle (Lane, 1955) for stable/equilibrium states, in general net erosion of the channel would be expected if the sediment supply decreases. However, for each reach the detailed transitional response in time and space could vary significantly depending on the local conditions such as hydrology and geology. For instance, overall the lowermost 500 km Mississippi experienced a deposition of 0.4 m during 1992–2004 and an erosion of 0.3 m during 2004–2013, and spatial and temporal variations in the riverbed changes were observed (Wang and Xu, 2018). Researches on the transient state on the spatial scale of the reach over a timescale of decades (management timescale) are recommended (Petts and Gurnell, 2005). Moreover, the tidal river or tidal reach, i.e., the reach between the tidal limit and the limit of salinity intrusion (Dyer, 1997), has received little attention despite its vital role in the river-sea system (Hoitink and Jay, 2016, and references therein). The tidal limit is the farthest point upstream of the river where the tidal fluctuations are negligible, which could be hundreds of kilometers upstream of the estuary mouth, e.g., the Yangtze, Mekong and Hudson. The long distance of tidal propagation into the river lies in the fact that tides are long waves which transfer energy instead of mass.

The Yangtze River is a good prototype for studying the sediment flux in the river-sea system under anthropogenic influences, for its large water and sediment fluxes and intensive human intervention. Fig. 1 shows the location of the lowermost gauging station Datong in the Yangtze, which is approximately 600 km from the river mouth. The Yangtze estuary is influenced by both the fluvial and tidal forcings. In the Yangtze the tidal limit is located near Datong in the dry season (Zhang et al., 2018), and the limit of salinity intrusion lies near Xuliujing (Shen et al., 2003).

Since the 1950s, due to the construction of more than 50,000 dams (especially the Three Gorges Dam, TGD) and soil conservation in the Yangtze River basin, the annual sediment flux at Datong has decreased by almost 80% from 1956–1968 to 2013–2015 (Yang et al., 2018). The decrease in the sediment supply has resulted in erosion of the channel downstream of the TGD and the subaqueous delta of the Yangtze (see e.g. Yang et al., 2011, Yang et al., 2014, Dai and Liu, 2013, Guo et al., 2018, Lyu et al., 2019). In these studies the focus was either on the reach upstream of Datong or the estuary downstream of Xuliujing. Exceptions are the studies of Zheng et al., 2018a, Zheng et al., 2018b (study area between Datong and Xuliujing/Wusongkou) mentioned above and Wang et al. (2009). In Wang et al. (2009), the sedimentation and erosion in the Jiangsu reach (between Maanshan and Xuliujing, see Fig. 1) during 1963–2003 was investigated. A switch from sedimentation to erosion in the 1980s in the reach was observed, which was considered to be due to the decrease in the riverine sediment flux. However, what changes occurred in the reach after the closure of the TGD in 2003 is unclear and the local morphological changes were not presented. Comparison of the sediment budget in the tidal reach before and after the closure of the TGD was not available in Zheng et al., 2018a, Zheng et al., 2018b either, as only bathymetry data in 1998 and 2013 were used.

Apart from limited understanding of the sediment budget in the river reach downstream of the tidal limit Datong during the last few decades, the characteristics of the local morphological changes within this region have not been thoroughly investigated. Existing studies either focused on a relatively small segment of the tidal reach or the time span was relatively short. In Dai and Liu (2013), it was found that in the tidal reach near Nanjing (see Fig. 1), the thalweg change was in the range of −1 m during 1998–2006. Zheng et al. (2018b) showed that during 1998–2013 the average bed level in the entire 565-km lowermost Yangtze River decreased by 1.2 m, with the largest degradation occurring downstream of the tidal reach.

The aims of this study are to quantify the changes in the sediment budget and the morphology of the tidal reach of the Yangtze both before and after the operation of the TGD during the last few decades, and to explore the possible causes for the changes. To fulfill these aims, data sets of the bathymetry of the tidal reach of the Yangtze between 1970 and 2008 were collected. Digital elevation models (DEMs) built from historic bathymetry data, were used to estimate the sediment budget and to explore the local morphological changes in the reach.

Section snippets

Study area

The Yangtze River is the third longest river (6300 km) in the world, and is the fourth and fifth largest river in terms of average annual sediment flux (~470 Mt/yr in the 1960s) and water flux (~900 km3/yr), respectively (Milliman and Farnsworth, 2011). Fig. 2 shows the annual suspended sediment flux and water flux at the tidal limit Datong (see Fig. 1) from the 1950s (data source: the Changjiang Water Resources Commission, the Ministry of Water Resources of China). The total sediment load at

Material

The digitized river relief maps of the tidal reach of the Yangtze were obtained from Yangtze River Delta Science Data Center, National Earth System Science Data Sharing Infrastructure, National Science & Technology Infrastructure of China (http://www.geodata.cn). Table 1 lists the information of the relief maps of the tidal reach (between Datong and Jiangyin) of the Yangtze River. These maps were published in the years of 1972, 1992, 2003 and 2008, which contain the information of the depth of

Sediment budget

Fig. 3 shows the difference of the DEMs for the tidal reach of the Yangtze (between Datong and Jiangyin) in 1970, 1992, 2003 and 2008. A zoom-in of an area in the reach was also presented to show in more detail the bed level change. It is seen that alternate sedimentation and erosion occurred in the whole reach in the steady period of sediment supply 1970–1992 (Period I), and in the decline periods of sediment supply 1992–2003 (Period II) and 2003–2008 (Period III) as well. Based on the

Possible causes for the morphological changes in the tidal river of the Yangtze

The results in Section 4.1 show that during the period of steady input sediment flux 1970–1992 (Period I), erosion and deposition was nearly balanced in the tidal reach between Datong and Jiangyin of the Yangtze. Afterwards, during the decline periods of input sediment flux 1992–2003 (Period II) and 2003–2008 (Period III), the reach experienced substantial erosion. Although the annual erosion in the reach decreased by nearly 50% from Period II to III (65±5.0 to 37±2.9 Mt/yr), the contribution

Conclusions

Globally intensive human intervention such as damming has brought about significant changes in the riverine sediment flux to the seas and morphology of the rivers, estuaries and coasts over the last few decades. Historical bathymetry data of the tidal reach of the Yangtze (between Datong and Jiangyin) were used to investigate the changes in the sediment budget and the local morphology in the reach. Main findings are as follows.

From 1970 to 2008, the reach experienced overall erosion. In the

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.

Acknowledgments

The project is funded by the National Key R&D Program of China (2016YFE0133700 and 2016YFA0600901) and the National Natural Science Foundation of China (51779121and 51679125). Yangtze River Delta Science Data Center, National Earth System Science Data Sharing Infrastructure, National Science & Technology Infrastructure of China (http://www.geodata.cn) are acknowledged for the bathymetry data support.

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