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Journal of Geographical Sciences >

2017 , Vol. 27 >Issue 10: 1268 - 1278

DOI: https://doi.org/10.1007/s11442-017-1434-0

Orginal Article

Interacting effects of multiple factors on the morphological evolution of the meandering reaches downstream the Three Gorges Dam

  • HAN Jianqiao , 1, 2, * ,
  • ZHANG Wei , 2 ,
  • FAN Yongyang 3 ,
  • YU Mengqing 2
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  • 1. Institute of Soil and Water Conservation, Northwest A&F University, Yangling 712100, Shaanxi, China
  • 2. State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China
  • 3. Changjiang Institute of Survey, Planning, Design and Research, Wuhan 430010, China

Author: Han Jianqiao, PhD, specialized in river geomorphology and numerical simulation of fluvial processes. E-mail:

*Corresponding author: Zhang Wei, PhD, E-mail:

Received date: 2017-03-22

  Accepted date: 2017-05-02

  Online published: 2017-09-06

Supported by

National Natural Science Foundation of China, No.51479146

Doctoral Foundation of Northwest A&F University No.2452015337

National Key Research and Development Plan, No.2016YFC0402303, No.2016YFC0402101

Copyright

Journal of Geographical Sciences, All Rights Reserved
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Abstract

Elucidating the influence of dams on fluvial processes can inform river protection and basin management. However, relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches. Using hydrological and topographical data, we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam (TGD). Our conclusions are as follows. (1) The meandering reaches can be classified into two types based on their evolution during the pre-dam period: G1 reaches, characterized by convex point bar erosion and concave channel deposition (CECD), and G2 reaches, characterized by convex point bar deposition and concave channel erosion (CDCE). Both reach types exhibited CECD features during the post-dam period. (2) Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars. However, changes in the river regime, river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches. (3) Flood discharges ranging from 20,000 to 25,000 m³/s result in greater erosion of convex point bars. The point bars become scoured if the durations of these flows, which are close to bankfull discharge, exceed 20 days. In addition, the reduction in bedload causes the decreasing of point bar siltation in the water-falling period. (4) During the post-dam period, flood abatement, the increased duration of discharges ranging from 20,000 to 25,000 m³/s, and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features. Our results are relevant to other meandering reaches, where they can inform estimates of riverbed change, river management strategies and river protection.

Key words： reaches downstream dams; meandering river; morphological evolution; Three Gorges Dam

Cite this article

HAN Jianqiao , ZHANG Wei , FAN Yongyang , YU Mengqing . Interacting effects of multiple factors on the morphological evolution of the meandering reaches downstream the Three Gorges Dam[J]. Journal of Geographical Sciences, 2017 , 27(10) : 1268 -1278 . DOI: 10.1007/s11442-017-1434-0

1 Introduction

As one of the most common river patterns, the meandering river is closely associated with channel flood control and navigation (Edwards et al., 2016; Czapiga et al., 2015). The riverbed evolution of meandering rivers exhibits complex characteristics and is affected by multiple factors, including the upstream flow regime, channel boundaries, hydrological regime and downstream water stages (Bradley and Smith, 1984; Yao et al., 2010). Following dam impoundment, it is necessary to adapt to the changes in hydrological conditions and other factors in the downstream meandering reaches (Frascati and Lanzoni, 2009; Ibisate et al., 2013). Investigation of these changes and the mechanisms by which meandering rivers respond to multiple factors will help facilitate river protection and management.
The phenomenon of bend bypassing and shoal cutting following dam construction has been documented in the Mississippi River, the Bollin River and the Han River (Harmar and Clifford, 2006; Luchi et al., 2010; Qin et al., 2009). During the 1980s, morphological regulation via bend bypassing and shoal cutting occurred in 11 of the 13 river bends in the Huangzhuang to Zekou reaches downstream the Danjiangkou Reservoir (Tan et al., 1996). The morphological regulation and its mechanisms have been given attention by many researchers (Li et al., 2013; Camporeale et al., 2008; Grenfell et al., 2012; Zhang et al., 2007). Different modes of shoal cutting are associated with various evolutionary processes. The flow conditions resulting from dam construction can alter riverbed processes, with the river making these adjustments to achieve optimal bend morphology. The changes in river boundaries are the main determining factor for bend bypassing, riverbed around convex bank is sourced while the development of concave bank is prevented by bank protection works, and greater channel curvature can cause erosion of point bars located on the convex side (Li et al., 2013; Hasegawa, 1989).
The lower Jingjiang River is a reach of the middle Yangtze River and has a predominant meandering pattern. The river bore the brunt of downstream erosion following the operation of the Three Gorges Dam (TGD) in 2003, as demonstrated by multiple examples of bend bypassing and shoal cutting in the meandering reaches downstream the dam (Li, Zeng and Wu, 2013; Yao et al., 2011). These changes are difficult to explain because bank protection works was implemented during the 1990s and the river boundary was stable in the pre- and post-dam periods. Some researchers have suggested that a significant change in sediment load could cause bend bypassing in meandering reaches. Flow with a low sediment concentration will scour a convex point bar when sediment becomes trapped in the reservoir (He et al., 2011). More water is directed from the concave bank to the convex bank due to deposition in the concave channel, which is clogged by bed load during the dry season, thus eroding the convex point bar (Han and Yang, 2000). Other studies have shown that bend bypassing is caused by changes in flow dynamics. For example, point bars can be eroded when the axis of flow shifts to the convex side (Hey et al., 2005). Sediment cannot be effectively transported from the concave bank to the convex bank by secondary circulation, resulting in a change in sediment transport mode (Albers and Steffler, 2007).
Previous studies have mainly focused on reductions in sediment concentration and changes in flow dynamics. However, these factors cannot explain the changes in the meandering reaches downstream the TGD, and little information is available on the interactions between multiple factors. Further study is needed to investigate the morphological evolution of the meandering reaches and the combined effects of multiple factors, including changes to the channel boundary, river regime, flow processes, and sediment transport. The objectives of this study are as follows: (1) to explore the morphological regulation of the meandering reaches and (2) to identify the effects of multiple factors on the morphological evolution of the meandering reaches downstream the TGD. These results will provide insight into how multiple interacting factors affect the morphology of the meandering reaches. The results will also inform estimates of riverbed change and strategies for river management and river protection in river reaches downstream of dams.

2 Study area

The Yangtze River is the longest river in China, with a catchment area of 1.8×106 km2. It is divided into upper, middle and lower reaches at Yichang, Hukou and Datong, respectively (Dai et al., 2016). The TGD, located 40 km upstream from Yichang, was constructed in 1992 and began operation in 2003. This project plays an important role in the management of the Yangtze River. The total storage capacity of the reservoir is 39.3×109 m3 and its flood control capacity is 22.15×109 m3 (Xu and Yan, 2010). The annual sediment load at Yichang decreased from 492×106 t (1950-2002) to 43×106 t (2003-2013).
Most of the meandering reaches occur on the Lower Jingjiang River (LJR), which extends from Ouchikou to Chenglingji. The total length, linear distance and bending rate of the LJR are 175.7 km, 84.5 km and 2.08, respectively. The incoming flow and sediment within the LJR mainly originate from upstream of the TGD. A portion of the flow and sediment in the LJR is diverted into Dongting Lake through Ouchikou, Dongting Lake’s outflow and the Jingjiang River outflow converge near Chenglingji. The bankfull discharge of the LJR is 22,000 m3/s.
The LJR consists of eight meandering reaches, including the Tiaoguan, Laijiapu, Jianli, Fanzui, Xiongjiazhou, Qigongling, Qizhou and Shazui reaches (Figure 1). Main branch is regarded as meandering pattern in Jianli and Xiongjiazhou reach. The LJR is a severely eroded reach downstream the TGD and is generally representative of the meandering reaches in the middle Yangtze River.
Figure 1 Study area, showing the meandering reaches downstream the Three Gorges Dam

3 Data sources and methods

3.1 Data sources

Daily discharge and sediment concentration data of Shashi and Jianli hydrological stations on the LJR were collected from the Changjiang Water Resources Commission (CWRC), China (www.cjh.com.cn). The sectional velocity distribution of the meandering reaches was obtained from the Changjiang Waterway Planning Design and Research Institute (CWPDRI), China. The topography data consist of sectional profiles measured in October 1996, October 2003 and October 2016.
Table 1 Sources of measurements
Data type Station Measured time Sources
Hydrologic data Shashi
Jianli		 2003-2013
1996-2015		 CWRC
Sectional velocity Six typical sections August 2014, January 2015 CWPDRI
Sectional profiles Laijiapu reach 2003-2016 CWPDRI
Eight typical sections 1996, 2003, October 2016 CWRC

3.2 Classification of meandering reaches

Based on evolutionary patterns during the pre-dam period, these meandering reaches can be classified into two types: type G1, characterized by convex point bar erosion and concave channel deposit (CECD), and type G2, characterized by convex point bar deposition and concave channel erosion (CDCE). Figure 2 shows the morphological patterns of the G1 and G2 reaches. The G1 type includes the Qigongling, Qizhou and Shazui reaches and the G2 type contains the Tiaoguan, Laijiapu, Fanzui, Xiongjiazhou and Jianli reaches.
Figure 2 Morphological patterns of the G1 and G2 reaches
To analyze point bars and channel deformation, the channels were classified as low-flow, basic and bankfull channels. Low-flow channels had riverbed elevations below the low water stage. Basic channel corresponds to the water stage parallel with the point bar. Bankfull channels correspond to the bankfull stage. The discharges of the Jianli that correspond to each channel type are 5500 m3/s, 9000 m3/s and 22,000 m3/s, respectively. Point bars were classified as low beach, high beach and floodplain (Figure 3).
Figure 3 Schematic diagram of the channel divisions. The water stages representing low-flow channel, basic channel, and bankfull channel are indicated by 1, 2, and 3, respectively.

3.3 Characterizing multiple factors

Sectional profiles of eight typical cross-sections were compared to describe the riverbed evolution. The planform of thalweg was analyzed to investigate the influence of the river regime on riverbed processes. Similarly, the bending radius of the channel was calculated to assess the geomorphic effects of the river boundary during different periods.
The flow dynamic axis and sediment transport capacity were used to describe changes in flow. Flow dynamic axis was calculated as follows (Qin et al., 2009):
(1)
where R0 is the bending radius of the flow dynamic axis, R is the bending radius of the channel, Q is the discharge, and A is the flow area. The sediment transport capacity was estimated using a simplifying indicator based on the following formula proposed by Zhang (Yuan et al., 2012):
(2)
where S is the sediment transport capacity, u is the velocity, h is the water depth, and g is the acceleration due to gravity (9.8 m/s2). The values of the coefficient k, the exponent m and the settling velocity of the sediment w can be considered constants. The simplifying indicator u3/h was used to represent the sediment transport capacity.
The bedload (D50>0.125 mm) runoff was estimated to explore the effects of changes in sediment on riverbed morphology.

4 Results and discussion

4.1 Morphological evolution of meandering reaches

Changes in the typical cross-sectional profiles of the G1 reaches are shown in Figure 4, showing the features caused by convex point bar erosion and concave channel deposition during the pre-dam (1996-2003) and post-dam periods (2003-2016). The floodplain, high beach and low beach experienced continuous erosion, and the thalweg migrated from a concave bank to a convex bank. For instance, in the Qizhou reach, the maximum scour depth of the convex bars exceeded 15 m, the maximum silting thickness of the channel in the concave banks was greater than 10 m, and the thalweg point shifted to a new, deeper channel. These phenomena also occurred in the Qigongling and Shazui reaches.
Figure 4 Typical cross-sectional profiles of G1 reaches during the pre- and post-dam periods
The G2 reaches showed different flow patterns from the G1 reaches during the pre-dam period: the convex point bar deposition and concave channel erosion occurred from 1996 to 2003. During the post-dam period, the G2 reaches evolved in similar ways to the G1 reaches: point bars in convex banks were scoured and deposition occurred in channels in the concave banks (Figure 5). For example, during the pre-dam period in the Tiaoguan reach, the sedimentation thickness of the low beach exceeded 7 m, and the erosion depth of the concave channel was greater than 15 m. By contrast, from 2003 to 2016, the erosion depth of the low beach reached 10 m. Moreover, changes to the high beach were minor, the maximum depth of deposition in the channel exceeded 20 m, and one mid-channel bar was generated in the original channel.
Figure 5 Typical cross-sectional profiles of the G2 reaches during the pre- and post-dam periods

4.2 Effects of flow regime

As shown in Figure 6, the flow regime of the upstream reach has a strong effect on the evolution of the G1 reaches downstream. The location of the thalweg in the river bends shifted to a left-right-left pattern after the flow regime changed in the upstream Qigongling, Qizhou and Shazui reaches.
For the G2 reaches, the thalweg location shifted to a right-left-right pattern in the Laijiapu reach, although the upstream flow regime was stable. However, this shift was consistent with flow regime changes in the upstream Fanzui reach, the location of the thalweg in the Fanzui reach moved to the left in the pre-dam period and then to the right in the post-dam period.
These results demonstrate that changes in the flow regime resulted in morphological changes to the meandering river reaches. However, both the G1 and G2 reaches showed CECD features regardless of changes in the upstream flow regime. Thus, the flow regime was not the key determining factor for the morphological evolution of the meandering reaches after TGD operation.
Figure 6 Changes in thalweg position in the meandering bends of the Lower Jingjiang River

4.3 Effects of river boundaries

In natural geomorphic processes, the river boundary plays an important role in the evolution of the meandering reaches. A small bending radius should result in convex point bar erosion and concave channel deposition. Lateral deformation is limited by bank protection projects, which alter the evolutionary regulation of riverbed morphology. Since 1998, many bank revetments have been installed along the LJR. Both the bank and bending radius were stable before and after the TGD. However, the morphology of the G2 reaches changed significantly after TGD impoundment, even though the river boundaries remained stable. Therefore, river boundaries do not control convex point bar erosion and concave channel deposition.

4.4 Effects of flow processes

Differences in the sediment transport capacity indicator (u3/h) for various discharges on the meandering reaches were used to compare the flow dynamics of convex and concave bank sides and thus the effect of flow processes on riverbed evolution (Figure 7). When the discharge increased from 7000 m³/s to 19,500 m³/s, the u3/h values of the convex bank sides increased significantly and the peak area of sediment transport capacity moved to the concave bank side. Due to the sudden change of overbank characteristics of flow dynamics, the flow dynamics should experience a dramatic decline around the bankfull discharge (22,000 m³/s). This relationship also affected the amount of daily erosion on the Jingjaing reach, the erosion intensity dropped to 5000 m³/s when the flow reached 25,000 m3/s (Figure 8). These results demonstrate that the critical discharges that result in convex point bar erosion range from 20,000 to 25,000 m3/s, close to bankfull discharge.
Figure 7 Changes in u3/h along the cross sections during different discharges in the Lower Jingjiang River
Figure 8 Erosion intensity caused by different discharges in the sandy bed of the Jingjiang River (Han et al., 2014)
Relationship between the convex point bar morphology and flow processes is exemplified in the Laijiapu reach. The flow regime and river boundaries did not change significantly and thus did not alter riverbed processes in the reach. The width of the high beach remained stable when a significant reduction in the flood discharge; the average duration of flood discharge exceeding 35,000 m3/s was reduced to 1 day during the post-dam period (Figure 9 and Table 2). In addition, the width of the low beach decreased during the post-dam period. However, during years in which discharges of 20,000-25,000 m³/s occurred less frequently (such as 2007, 2012 and 2016), the low beach experienced deposition. These results indicate that a discharge of 20,000-25,000 m³/s affects point bar evolution in meandering reaches. When the duration of discharge at this level is less than 20 days, siltation of the convex point bars tends to occur. Otherwise, for discharge durations greater than 20 days the point bars tend to become scoured.
Figure 9 Changes in width of the high beach and low beach in the Laijiapu reach
Table 2 Durations of different discharge ranges at the Jianli station after impoundment of the TGR (days)
Discharge (m³/s) Average 2003 2004 2005 2006 2007 2008
Bankfull discharge (20,000-25,000) 32 43 36 40 5 27 33
Flood discharge (>35,000) 1 0 5 1 0 6 0
Discharge (m³/s) 2009 2010 2011 2012 2013 2014 2015
Bankfull discharge (20,000-25,000) 41 46 17 29 38 67 15
Flood discharge (>35,000) 0 0 0 3 0 2 0
During the post-dam period, most years experienced almost all discharges of 20,000- 25,000 m³/s for more than 20 days. These altered flow processes, including flood abatement and the increased number of days with a discharge of 20,000-25,000 m³/s, are the main factors controlling the morphological evolution of the meandering reaches.

4.5 Effects of sediment transport

With the recovery of sediment concentrations along the river, the coarse sediment load during the post-dam period resembled that of the pre-dam period at the Jianli station (Han et al., 2014). Both the G1 and G2 reaches exhibited the CECD features regardless of whether the sediment concentration had fully recovered. The reduction in sediment transport is not the determining factor on the morphological evolution of the meandering reaches. However, the decrease in bed load should increase the sediment transport dynamics of the unsaturated flow, causing serious erosion of the bankfull channel and decreasing of point bar deposition in water-falling period. Convex point bars will continue to scour if the bed sediment transport capacity decreases in the future.

4.6 Effect of the jacking of Dongting Lake

The flow dynamics of the G1 reaches located near Chenglingji are influenced by discharge from Dongting Lake, thus affecting the riverbed processes of meandering reaches. Existing research shows that no significant changes have occurred in the export channel of the Dongting Lake into the Yangtze River. The ratio of Dongting Lake discharge to river flow has not changed and the downstream water stage has been relatively stable (Feng et al., 2013). Thus, the jacking intensity of Dongting Lake has not changed significantly since impoundment of the TGD. Therefore, the jacking of Dongting Lake is not the critical factor for morphological evolution of meandering reaches.

5 Conclusions

In summary, to understand the effects of dams on riverbed processes in meandering reaches, we investigated the effects of multiple factors on the morphological evolution of the meandering reaches downstream the TGD. During the pre-dam period, the G1 and G2 reaches were characterized by CECD and CDCE features, respectively. Both the G1 and G2 reaches showed the CECD behavior during the post-dam period. The bankfull channels in the meandering reaches downstream the TGD experienced scouring, and erosion was concentrated in low beaches located near convex point bars. The results of this study show that the flow regime, river boundaries and jacking of Dongting Lake are not determining factors for the morphological evolution of the meandering reaches. Rather, the riverbed processes were driven by flow processes and sediment transport. Erosion intensity was the greatest at discharges of 20,000-25,000 m³/s, and when the flow durations were less than 20 days, siltation occurred at convex point bars; for durations greater than 20 days, the point bars were scoured. During the post-dam period, almost all years experienced discharges of 20,000-25,000 m³/s lasting more than 20 days. In addition, the reduction in bedload caused the decreasing of point bar siltation in water-falling period. Therefore, we attribute the change in the evolution of the meandering reaches to a significant decline in sediment load, a reduction in flood flows, and the increased duration of discharges ranging from 20,000 m3/s to 25,000 m3/s. In the future, the meandering reaches should exhibit CECD features if the current management strategy of the TGD continues.

The authors have declared that no competing interests exist.

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[9]
Grenfell M, Aalto R, Nicholas A, 2012. Chute channel dynamics in large, sand-bed meandering rivers.Earth Surface Processes and Landforms, 37(3): 315-331.Meander bends of many large, sand-bed meandering rivers are partitioned by chute channels that convey permanent flow, and co-exist with the mainstem for decades. As a first step toward understanding the dynamics and morphodynamic implications of these ‘bifurcate meander bends’, this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics. Regression models developed for the Strickland River, Papua New Guinea, the lower Paraguay River, Paraguay/Argentina, and the Beni River, Bolivia, revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate (the rate at which a bend elongates in a direction perpendicular to the valley axis trend). Image analyses of all rivers and field observations from the Strickland suggest that the majority of chute channels form during scroll–slough development. Rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank, resulting in widely-spaced scrolls with intervening sloughs that are positively aligned with primary over-bar flow. The rivers plot in order of increasing chute activity on an empirical meandering-braided pattern continuum defined by potential specific stream power (ωpv) and bedload calibre (D50). Increasing stream power is considered to result in higher bend extension rates, with implications for chute initiation. In addition, chute stability is shown to depend on river sediment load relative to flow discharge (Qs/Q), such that while the Beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power, the formation of stable chute channels is suppressed by the high sediment load. This tendency is consistent with previous experimental studies, and results in a planform that is transitional between single-thread meandering and braided. Copyright 08 2011 John Wiley & Sons, Ltd.

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[10]
Han J, Sun Z, Huang Yet al., 2014. Features and causes of sediment deposition and erosion in Jingjiang reach after impoundment of the Three Gorges Project.Journal of Hydraulic Engineering, 45(3): 277-285. (in Chinese)Using hydrologic and morphological observations from the Jingjiang reach of the Yangtze River, the differences of bed deformation among different channel types are examined and the response of channel form adjustment to regulated flow and sediment process has been investigated. It is revealed that the magnitude of channel deformation is uneven in the whole reach,although the channel bed is totally covered with erosive sand. The erosion amounts mainly concentrate in the part of channel bed below the low flow stage, which causing the section geometry of channel becoming narrow and deep. Meanwhile,the erosion is more intensive in shallow and wide reaches than deep and narrow reaches,which tends to be uniform for the whole channel. Obviously,the relation between channel geometry and process of flow and sediment discharge has changed after the Three Georges Dam closure. In the pre-dam period,although erosion or deposition took place in the channel under different discharges,channel geometry adapted to the process of flow and sediment discharge and the sediment transport process had shown equilibrium in long term. In the post-dam period,the dramatic reduction of incoming sediment load,the elimination of large flood and the prolongation of medium flow duration are inducing an uneven erosion magnitude and unequal channel forming frequency in different part of the channel.

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[11]
Han Q, Yang K, 2000. The evolution trend of Lower Jingjiang River after the impoundment of Three Gorges Projects.Journal of Sediment Researches, (3): 1-11. (in Chinese)

[12]
Harmar O P, Clifford N J, 2006. Planform dynamics of the Lower Mississippi River.Earth Surface Processes and Landforms, 31(7): 825-843.This paper presents an analysis of the planform behaviour of the Lower Mississippi River (LMR) using a series of maps and hydrographic surveys covering the period 1765-1975. Data allow analysis at various time and space scales, using fixed and statistically defined reaches, both before and after extensive channel modification. Previous research has interpreted planform change in relation to geomorphological or engineering regime-type analyses of channel length and width for the LMR as a single system. The analysis here is broadly consistent with these approaches, but highlights the importance of meander geometry, in the form of the radius of curvature:width ratio. This neglected factor helps resolve paradoxes relating to observed changes in sediment transport and channel stability. When viewed over smaller time and space scales, analysis of dynamics using fixed reach boundaries reveals a downstream trend in the pattern of planform behaviour, which is closely related to the distribution of valley floor deposits, and which also reflects neotectonic influences. Analysis of changes using statistically determined reach boundaries shows that, over shorter time scales, meander trains are continually formed and modified over a period of approximately 120 years. Zones of more-or-less dynamic behaviour thus move through the LMR. The research also provides a context for 20th century engineering interventions to the river. These have constrained the magnitude of planform adjustment, but also altered the kind of response that is now possible in relation to changes in discharge and sediment load, and as a consequence of internal feedbacks within the LMR system. Copyright 2006 John Wiley & Sons, Ltd.

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[13]
Hasegawa K, 1989. Universal bank erosion coefficient for meandering rivers.Journal of Hydraulic Engineering, 115(6): 744-765.

[14]
He G, Yao S, Jin Z, 2011. Study of atypical erosion of convex bank of river bend in Jingjiang Reach of Yangtze River.Yangtze River, 42(17): 1-3. (in Chinese)According to the fact that the annual variation of convex bank of river bend in Jinjiang Reach was in continuous erosion in recent years,the evolution characteristics of the convex bank in different stages was analyzed so as to find out the main causes.The results showed that the main causes included obvious decrease of incoming sediment of Jingjiang Reach after impoundment of TGP,the mainstream line of the river bed swinging to the convex bank,the eroding of the waves in medium and dry period,among which,the decrease of incoming sediment was dominated.It was suggested that proper bank protection works should be conducted at the convex bank in order to solve the problems of flood control and navigation caused by the atypical erosion.

[15]
Hey R D, Catmur S E, Sellin R H Jet al., 2005. Behavior of meandering overbank channels with graded sand beds. Journal of Hydraulic Engineering, 131(8): 665-681.Measurements of velocity distributions, depth variation, and sediment transport have been made under bankfull and overbank flow conditions in meandering channels with a graded sand bed, using the large-scale U.K. Flood Channel Facility. The overbank conditions depend upon the relative strength of opposing secondary circulation cells generated by shear at the channel crossover and centrifugal forces around the meander bend. Generally the shear-generated secondary flow either reversed or weakened the centrifugal circulation around the next downstream bend. This led to considerable modification of the main channel bed morphology, which, in turn, altered flow distributions. Measurements of the lateral distribution of bed load were made using a14-scale Helley mith sampler. This demonstrated that the bed load was generally concentrated within a limited width of the channel and tended to take the shortest route through the meanders. Comparisons of observed and calculated bed material load gives an indication of how secondary circulation around meanders, under both bankfull and overbank conditions, affects the predictive performance of formulas derived for predominantly one-dimensional flow.

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[16]
Ibisate A, Díaz E, Ollero Aet al., 2013. Channel response to multiple damming in a meandering river, middle and lower Aragón River (Spain).Hydrobiologia, 712(1): 5-23.Small hydropower plants (SHP) affect river flow and sediment transport and thus impact river morphology. Eight hydropower schemes were studied along the meandering middle and lower reaches of Arag n River (Spain) to assess their effects on channel morphology and sediment dynamics from 1927 to 2010. GIS tools were used to measure changes in fluvial surfaces, channel planform and lateral and vertical dynamics. Three periods (early, middle and late twentieth century) were analysed to discern the effects of the main pressures, such as changes in land use, large reservoirs upstream and SHPs. Results were combined with field and topographical measurements and hydrological analysis. Active channel width and channel migration suffered a clear reduction in the whole period. They started as a consequence of land cover changes in the drainage basin, but their speed increased after a large reservoir was built upstream. More recent changes occurred since most of the SHPs were put into operation in the 1990s, especially in their short-circuited reaches and in the four more downstream ones. These changes are interpreted as a consequence of reduced discharge, transitory sediment trapping and reactivation of sediment transport after weirs became filled as well as by the impact of flood hydrology.

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[17]
Li N, Zeng Y, Wu Z, 2013. Preliminary study of causes of mainstream bending of Qigongling Bend in Jingjiang reach of Yangtze River.Yangtze River, (1): 22-25. (in Chinese)After the impoundment of Three Gorges Reservoir,Jingjiang reach is scoured generally,and the mainstream bending occurred at lower Jingjiang reach.The historical evolution and formation process,recent variation of shoreline,talweg and erosion and siltation of Qigongling Bend are analyzed.The result are compared with that of bend bypassing of lower reach of Danjiangkou Dam after its impoundment,so as to analyze the cause of mainstream bending of Qigongling Bend.The results show that the mainstream bending of Qigongling Bend is related to the factors such as process and amount of incoming flow and sediment,river channel configuration,riverbed geology and relationship between Dongting Lake and Yangtze River.

[18]
Li Z, Wang Z, Xu Met al., 2013. Patterns and mechanisms of neck cutoffs on meandering rivers. Journal of Tsinghua University (Science and Technology), 53(5): 618-624. (in Chinese)Neck cutoff is a sudden event occurring in meandering rivers which reduces the sinuosity and complexity of the morphology.The patterns and mechanisms for neck cutoffs were analyzed in remote sensing images of neck cutoffs to divide the cutoffs into bank collapse,cutting and erosion patterns to theoretically investigate the cutoff conditions,processes and mechanisms.The bank collapse pattern occurs on extremely narrow necks where the left and right banks simultaneously collapse or a single bank collapse which forces the flow through the neck.The cutting pattern is the most common neck cutoff mechanism when overflow surface scouring forms a new channel and intensively scours the new channel until creating a stable channel.The erosion pattern is the first flood scouring that forms a ditch,with subsequent floods along the ditch scouring with headward erosion until a new channel is completely formed.

[19]
Luchi R, Hooke J M, Zolezzi Get al., 2010. Width variations and mid-channel bar inception in meanders: River Bollin (UK).Geomorphology, 119(1/2): 1-8.The extent and pattern of width variations along a meandering channel and its association with variation in bed topography and development of mid-channel bars have been examined through field survey evidence for a reach of the River Bollin in NW England. The wet width has been quantified along the reach by applying a hydraulic model to surveyed cross sections under a range of discharges between low flow and defined bankfull conditions. This approach allows an objective, modelling-based method to compute channel width. The high spatial resolution of the topographical survey allows capture of significant variations in the cross-sectional morphology at the meander wavelength scale. Results disclose some features of longitudinal and stage-dependent width variability in meanders. Width variation is shown to be highly correlated with curvature: at bankfull conditions width peaks in bend apex sections exceed those at inflection sections and can be up to twice greater. The width-curvature behavior is correlated with the pattern of bed and banks morphology, which is different in bend apices and in meander inflections. The survey shows that the bedform morphology can be characterized by a mid-channel bar pattern that is initiated at the inflection section and that the bedform dynamics can be closely associated with channel width variations.

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[20]
Qin L, Yu M, Tan Get al., 2009. Discussion on the relation between the change of the flow dynamic axis and the bend bypassing and shoal cutting in the bend river course.Journal of Hydrodynamics, 24(1): 29-35.Abstract Based on field data of the river bend in middle and lower Hanjiang River and in Jingjiang River, the relation between the variation of bend radius of flow dynamic axis and bend bypassing and shoal cutting is expounded. A great number of field data show that if income flow and bend conformation change, the river regime and the direction of flow dynamic axis will change as well, then bend bypassing and shoal cutting will occur. A mathematical expression about the radius of flow dynamic axis was obtained by multiple linear regression, the contrast relation which is used to judge the occurrence probability of bend bypassing and shoal cutting between the radius of flow dynamic axis and the radius of bend was proposed. The research may be helpful in layout of river regulation and flood control.

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[21]
Tan G, Ning L, Li F, 1996. The mainstream movement and sandbars erosion in Hanjiang River from Huangzhuang to Zekou.Journal of Sediment Research, (2): 113-117. (in Chinese)

[22]
Xu J, Yan Y, 2010. Effect of reservoir construction on suspended sediment load in a large river system: Thresholds and complex response.Earth Surface Processes and Landforms, 35(14): 1666-1673.Dam construction greatly alters the channel boundary of rivers, making the dammed river system a human-controlled system. Based on hydrometric data in the upper Changjiang River basin, the change in behaviour of sediment transport of some dammed rivers was studied. As a result, some phenomena of threshold and complex response were found. When the coefficient (Cr,a) of actual runoff regulation by reservoirs, defined as the ratio of total capacity of reservoirs to annual runoff input, is smaller than 10%, suspended sediment load at Yichang station, the control station of the Changjiang River, shows a mild decreasing trend. When this coefficient becomes larger than 10%, suspended sediment load decreases sharply. The coefficient of 10% can be regarded as a threshold. The Cr,a of 10% is also a threshold, when the variation of suspended sediment concentration (SSC) with Cr,a at Yichang station is considered. The impacts of reservoir construction can be divided into several stages, including road construction, dam building and closure, water storage and sediment trapping. During these stages, some complex response was identified. At the station below the dam, SSC increases and reaches a maximum, and then declines sharply. This phenomenon was found on the main-stem and several major tributaries of the upper Changjiang River. In the Minjiang River, where a series of dams were built successively, the response of SSC is more complicated. Copyright 2010 John Wiley & Sons, Ltd.

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[23]
Yao S, Huang L, Lu J, 2011. Review of research on river pattern in downstream of Three Gorges and Danjiangkou reservoirs.Yangtze River, 42(5): 5-10. (in Chinese)Based on the existing research results of river pattern in the downstream of Three Gorges and Danjiangkou reservoirs,the formations of different river patterns are summarized and analyzed,including incoming flow and sediment condition,boundary condition of river channels and gradient ratio.It is concluded that after completion of Danjiangkou Reservoir,the river channel of middle Hanjiang River changes into a stable braided or single channel and the bend bypassing and shore cutting occur in the lower reach;after operation of Three Gorges Reservoir,the changes of downstream river channel are smaller than lower reach of Hanjiang River.Furthermore,we point out the complexity of river pattern changes in downstream of Three Gorges Reservoir and the problems need to be studied.The research results provide reference for studying the mechanism of river pattern change in downstream of large-scale hydro project.

[24]
Yao W, Zheng Y, Zhang M, 2010. Discussion on the mechanism of river meandering.Advances in Water Science, 21(4): 533-540. (in Chinese)The mechanism of river meandering is investigated experimentally based upon the principle of energy conservation. Our study reveals that meandering is a natural property of rivers. The curvatures of a river bend are largely controlled by the energy of river flow,i. e.,the flow rate and the channel slope. River meandering is a natural fluvial process to compensate the energy loss so that the river could keep itself moving forwards. The meandering channel process can result in a rise in the upstream water level,consequently increasing the potential energy in the channel reach and leading to a rising in the downstream channel slope as well as the kinetic energy increases. The latter,in turn,results in a relative equilibrium state between the flow and the sediment transport in the channel reach. The mechanism of river meandering may thus be rephrased as a river automatic regulating process. The degree of curvature in a channel reach is subject to the gained kinetic energy,i. e.,the difference in kinetic energy between upper-and downstream cross-sections determines the degree of river meandering. The automatic kinetic energy compensation in river reaches could be used as the mechanism to explaining river meandering. Such a mechanism is justified by the indoor experiments and field observation.

[25]
Yuan W, Yin D, Finlayson Bet al., 2012. Assessing the potential for change in the middle Yangtze River channel following impoundment of the Three Gorges Dam.Geomorphology, 147/148: 27-34.The geomorphic impacts of dams on downstream river channels are complex, not readily predictable for specific cases, but widely reported in the literature. For the Three Gorges Dam on the Yangtze (Changjiang) River in China, no studies of the impact of the changed flow and sediment conditions below the dam on the behaviour of the channel were included in the pre-dam feasibility report. We have assembled a database of flow and sediment data for the middle Yangtze River from Yichang to Hankou and used this to analyse changes following the closure of the dam. While total flow is little affected, the operating strategy for the dam that provides for storage of part of the summer high flows to maintain hydroelectric power generation in winter (the low flow season) is reflected in changes to the seasonal distribution of flow below the dam. We calculated potential sediment carrying capacity and compared it with measured sediment concentrations for both pre- and post-dam conditions. While channel sedimentation is indicated along the middle Yangtze for pre-dam conditions, scour is indicated for post-dam conditions, highest at Yichang immediately below the dam and decreasing downstream.

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[26]
Zhang J, Chen L, Liu Let al., 2007. Perfect morphology of meandering river for middle and lower reaches of Hanjiang River.Engineering Journal of Wuhan University, 40(1): 37-41. (in Chinese)Based on field data and former research,the perfect morphology of meandering river is studied.The perfect morphology of meandering river was influenced not only by flow conditions including flow discharge and slope of river,but also influenced by the constitute and median diameter of river bed.The expression of curvature semidiameter is formed.Based on the field data of the middle and lower reaches of the Hanjing River,an empirical formula is developed.This empirical formula may be helpful both in river engineering and waterway improvement.

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