Evolution of the mid-channel bars in the middle and lower

Vol.:(0123456789)1 3

Environmental Earth Sciences (2018) 77:394 https://doi.org/10.1007/s12665-018-7576-2

ORIGINAL ARTICLE

Evolution of the mid-channel bars in the middle and lower reaches of the Changjiang (Yangtze) River from 1989 to 2014 based on the Landsat satellite images: impact of the Three Gorges Dam

Yaying Lou1 · Xuefei Mei1 · Zhijun Dai1,2 · Jie Wang1 · Wen Wei1

Received: 11 August 2017 / Accepted: 16 May 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018

AbstractThe mid-channel bars have long been identified as essential landforms in the large rivers of the world, and the significance of connectivity between morphology and flow-sediment dynamics has been intensively emphasized. In this study, remote sensing images and associated hydrological data from 1989 to 2014 were used to explore mid-channel bars evolution in the middle and lower reach of the Changjiang and their responses to the Three Gorges Dam (TGD), the world’s largest hydrologi-cal engineering. The results indicated that mid-channel bars, respectively, exhibited deposition and erosion in the flood and dry season in pre-TGD period, while mild deposition in flood season and deposition in dry season were found in post-TGD period. As a consequence, mid-channel bars area was characterized by ‘remarkable seasonal differences in pre-TGD period, mild seasonal pattern in post-TGD period’. The obvious shift in seasonal features could be attributed to the TGD operation in 2003. Specifically, flood duration decrease and sediment load reduction following TGD regulation suppressed the bars growth in flood season. TGD-induced variations in differences between sediment carry capacity and suspended sediment concentration resulted in the bars transformation in dry season. Meanwhile, the change trends of downstream mid-channel bars became weak as their locations’ distance to TGD increases because of the river adjustment and tributaries supplement. Moreover, mid-channel bars in different river patterns presented various change trends with the most remarkable variation being detected in goose-head-shaped river pattern. The results of this paper provide a theoretical basis for the river channel improvement in the middle and lower reaches of the Changjiang River.

Keywords Mid-channel bars · Morphodynamic process · Sediment · Changjiang (Yangtze) River · Three Gorges Dam (TGD)

Introduction

Mid-channel bars, i.e., a free bar in the middle of a chan-nel, are typical accumulation landforms that are distributed worldwide over river channels (Bristow and Best 1993). Generally seen in diamond or lozenge-shaped, mid-channel

bars can be vegetated, with flow discharge passing through both sides (Hooke and Yorke 2011). As unique geomorphic cells and components are present in braided river and estuary area, mid-channel bars are of vital importance to maintain the river and estuarine channel stability and protect adja-cent wetlands (Hooke 1986; Bristow and Best 1993; Sanford 2007). Thus, morphodynamic evolution of the mid-channel bars along fluvial rivers has received attention in the litera-ture (Leopold and Wolman 1957; Hooke 1986; Lippmann and Holman 1990).

The earlier studies about river bars leaned to analyze their formation mechanism and natural evolution and aggradation (Knighton 1972; Church and Jones 1982; Ash-worth 1996). Knighton (1972) recognized that a number of mid-channel bars on the Bollin River were arisen where the bank was rapidly eroded and the channel became wid-ened. More and more researches have focused on the

* Zhijun Dai [email protected]

Yaying Lou [email protected]

1 State Key Lab of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China

2 Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266100, China

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contribution of detached bars to the meander form devel-opment and variations. Church and Jones (1982) identi-fied three major causes of bars’ formation: reduce in shear stress, widening of the channel and tributary entrances (all of which lead to flow divergence). Thereafter, Ash-worth (1996) demonstrated the bars’ development process and linked the process to changes in channel geometry and local flow strength and direction. In addition, a series of recent research works further conducted the specific bars’ formation and evolution mechanisms globally, and showed that their evolutions were related to the hydrologi-cal regime (Ashworth et al. 2000; Lunt and Bridge 2004; Burge 2006; Szupiany et al. 2009; Baubinienė et al. 2015).

With intensive human intervention and occupation, the natural river regimes were gradually replaced by the regu-lated flow, which dramatically affected the mid-channel bars’ evolution and gained international concerns (Sanford 2007; Skalak et al. 2013; Kiss and Balogh 2015; Raška et al. 2016). For example, Sanford (2007) showed that in the Missouri River, dam regulated rate of flow resulted in the decrease in the sand bars’ area, less centroid migra-tion and more easy bars aggregation. Kiss and Balogh (2015) showed the point-bars developed quickly upstream and laterally in the Dráva River, because of the coarse sediment supply and the decreasing stream energy by the dam effect. Raška et al. (2016) discovered most islands have disappeared because of the construction of dams and lock chambers in Elbe River. Meanwhile, a series of water conservancy projects have been constructed along the Changjiang River, especially, the Three Gorges Dam (TGD), currently the world’s largest water conservancy project, which influences the hydrological regime (Chen et al. 2017) and may affect dozens of sand bars in the downstream reaches (Xu 2013; Zhu et al. 2015; Li 2011). For instance, Zhu et al. (2015) analyzed the evolution characteristics and trend of the braided channels in the middle Changjiang reach, and found that the mid-channel bars in lower Jingjiang reach experienced the most severe erosion after the TGD impoundment. However, the previ-ous understanding on bars change have been mainly con-ducted along Jingjiang reach and ignored variations along the further downstream reach (Shao et al. 2005; Jiang et al. 2010). Besides, according to classification of river chan-nel by Rust (1978), Chenglingji–Datong reach belongs to a braided river with different sinuosities, which could also affect the bars’ development which is why it needs an urgent research.

Therefore, the objectives of this paper are to (1) explore the evolution of braided bars along the Chenglingji–Datong reach of Changjiang River, (2) identify the change character-istics of the mid-channel bars in braided river with different sinuosities, (3) discuss associated variation mechanism of mid-channel bars evolution.

Study area

Changjiang River is the longest river (about 6300 km) in Asia with a drainage area of about 1.8 × 107 km2 (Wang et al. 2009; Dai and Liu 2013; Dai et al. 2014; Mei et al. 2016). The reach between Chenglingji and Datong (CD) belongs to the middle and lower reach of the Changji-ang River, which is one of the densely populated area of Changjiang River (Dai and Liu 2013). Chenglingji–Datong (CD) reach is about 753.4 km in length with the Han River and Poyang Lake mingling in at Hankou and Hukou, respectively (Fig. 1). Accordingly, this reach can be further divided into three reaches: CH reach from Chenglingji to Hankou, HJ reach from Hankou to Jiuji-ang and JD reach from Jiujiang to Datong. The hydrologic regimes in these reaches are monitored by five hydrologi-cal stations, namely, Luoshan, Hankou, Huangshi, Jiujiang and Datong. The annual runoff (from 1951 to 2002) in Luoshan, Hankou and Datong were about 6.5 × 1012 m3, 7.1 × 1012 m3 and 9 × 1012 m3, respectively, while the con-temporary sediment load were 4.15 × 109 t, 3.98 × 109 t and 4.28 × 109 t. Due to the effect of monsoon, the runoff and sediment in Chenglingji–Datong reach are mainly con-centrated in the flood season (May to October), when the runoff, respectively, accounted for 74.2, 73.3 and 71.1% of the total runoff in the three reaches from upstream to downstream, while the sediment took up 85.5, 87.6, 87.7% of the total sediment load (Pan 2011).

As a braided river with sandy beds, CD reach has devel-oped a number of mid-channel bars (Fig. 1) (Yu 2005). To facilitate statistic, stable and relatively large bars that can be observed in both flood and dry seasons were selected in this study. Besides, based on the channel classification theory of Rust (1978) and Chien (1987), it was further pro-posed that braided river with different sinuosities can be divided into straight braided river, bending braided river and goose-head-shaped braided river. These three river types are described in Fig. 2, while the related specific mid-channel bars number in each river type are shown in Table 1.

Materials and methods

Traditional mid-channel bars’ research methods included model simulation (Leopold and Wolman 1957), numeri-cal method (Davoren and Mosley 1986; Ashworth et al. 2000), historical maps and aerial photos analysis (Hooke 1986; Sanford 2007; Raška et al. 2016). The methods of model simulation and numerical method are prone on the theoretical analysis, which, however, cannot document the


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real situation properly. Besides, although historical maps and aerial photos are more intuitive in exploring the bars’ development, there are no lasting yearly observed maps or photos in this study. In recent decades, with technological advancement, remote sensing images have been widely used in bars’ observation and study because of high resolu-tion and easy accessibility (Moretto et al. 2012; Yang et al. 2015; Deng et al. 2017), which, therefore, was applied to this study as well.

Water depth has an inevitable effect on bars’ exposed area, which, accordingly, was used to select the Landsat satellite images in this study. To improve the results preci-sion and reduce the comparison errors, the selected inter-annual images should correspond to the constant water level (Wang et al. 2013a, b). Besides, the threshold segmentation was adopted to distinguish the mid-channel bars from water. The bars area thereafter could be extracted to compare the changing tendencies between pre- and post-TGD periods. Furthermore, the independent samples T test method was

used to examine the significance of area changing between two periods (Makwana et al. 2016).

Satellite imagery selection and data source

According to the Worldwide Reference System, three TM scenes (path 123, row 39; path 122, row 39; path 121, row 39) can absolutely cover the study area. TM scene selection was controlled by the water level. Only those correspond-ing to the same or similar water level could be used in this study (Tables 2, 3). Considering the water and sediment differences between flood and dry season, two remote sens-ing images with high and low water levels were selected within 1 year. According to the hydrological regime along the Changjiang River, the dry season ranges from Novem-ber to April, while the flood season covers May to October. Occasional remote sensing images were missing over the study period because of the weather limits. Detailed remote

Fig. 1 Study area of Chenglingji to Datong reaches

Fig. 2 Three kinds of braided river diagram; a straight braided river; b bending braided river; c goose-head-shaped braided river


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sensing images and their water level information were shown in Tables 2 and 3.

Three hydrological stations (Hankou, Huangshi, Datong) with daily water level information during 1989–2014 were

chosen to control the TM scenes selection. All these data were obtained from Changjiang Water Resources Commis-sion, China (http://www.Cjh.com.cn). In addition, monthly discharge and sediment load data at Yichang, Luoshan,

Table 1 The information of channel bars and corresponding unified water levels

Bar Name River pattern Long axis slope Short axis slope Unified water level (flood season, m)

Unified water level (dry season, m)

Nanyang bar Straight 1/60 1/23 22.5 15.0Nanmen and Xinyu bar Straight 1/75 1/40 22.5 15.0Zhong bar Goose-head-shaped 2/5 2/5 22.5 15.0Xin bar Goose-head-shaped 1/60 1/20 22.5 15.0Fuxing bar Straight 1/208 1/39 22.5 15.0Tuan bar Goose-head-shaped 1/56 1/100 22.5 15.0Tianxing bar Bending 1/160 1/10 22.5 15.0Dongcao bar Goose-head-shaped 1/80 1/50 19.0 11.5Daijia bar Bending 1/40 1/20 19.0 11.5Xin bar (Longpin) Goose-head-shaped 1/30 1/30 19.0 11.5Zhangjia bar Bending 1/100 1/30 15.5 10.0Xiasanhao bar Straight 1/60 1/30 15.5 10.0Mianchuan bar Bending 1/100 1/30 15.5 10.0Yudai bar Straight 1/90 1/20 15.5 10.0Xin bar (Guanzhou) Bending 1/130 1/40 15.5 10.0Meimao bar Bending 1/335 1/140 15.5 10.0Jiangxin bar Bending 1/18 1/14 15.5 10.0Tieban bar Goose-head-shaped 1/20 1/60 15.5 10.0Fenghuang bar Bending 1/160 1/10 15.5 10.0Chongwen bar Bending 1/96 1/40 15.5 10.0

Table 2 List of satellite images used in the present work (dry season)

Remote sensing image number (123/39)



Acquisition date Hankou water level (m)

Acquisition date Huangshi water level (m)

Acquisition date Jiujiang water level (m)

1989/2/11 14.20 1989/2/4 11.07 1989/2/13 9.721994/3/5 15.00 1994/1/25 10.95 1994/1/2 9.971995/12/5 15.78 1995/12/30 11.15 1995/12/7 9.921996/12/23 15.13 1997/3/6 12.1 1997/2/11 10.351999/12/24 15.47 1999/12/25 11.98 1999/12/18 10.112000/2/26 15.17 2000/3/6 11.98 2000/1/27 9.842001/12/29 15.13 2001/12/22 11.93 2002/2/1 8.512003/1/17 15.95 2003/2/19 12.13 2003/1/27 9.922003/12/27 14.85 2004/3/9 12.35 2004/1/30 8.152004/12/13 15.92 2005/1/7 11.58 2005/1/16 8.782007/4/10 15.38 2007/4/19 11.54 2007/2/23 9.082008/12/24 14.75 2008/12/17 12.66 2009/2/12 8.312009/11/25 14.49 2010/2/22 11.31 2010/1/14 8.532011/3/4 14.92 2011/1/8 12.23 2011/2/2 9.842013/11/20 15.77 2014/3/5 12.34 2013/11/6 8.89

http://www.Cjh.com.cn


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Hankou and Datong were also acquired from Changjiang Water Resources Commission. The Landsat data from a Thematic Mapper (TM) and an Enhanced Thematic Mapper (ETM+) were provided by the Earth Resources Observation and Science Center (http://glovi s.usgs.gov/).

Methods

Waterlines extraction

In this paper, threshold segmentation method was utilized for extracting the waterlines to separate the land and water through ENVI and ArcGIS. Specifically, the near infrared band that has high resolution for water and land was chosen as the research object, first. Then, gray histogram of each band could be obtained using ENVI software. Thereafter, the gray value was set as the bottom between the two peaks of the histogram, which was used as the threshold to separate the land from water. For the selection of the bottom value, the minimum threshold value method was utilized (Abutaleb 1989; Guo and Pandit 1998; Cuevas et al. 2010):

where h(z) represents the histogram.Then, the image was loaded into the ArcGIS to carry

out the image binary based on the threshold. Eventually, mid-channel bars’ exposed area data could be obtained in the ArcGIS.

Considering the resolution of remote sensing images is 30 m, if mid-channel bars with small areas will be used, the situation would lead to comparatively larger relative errors.

(1)𝜕h(z)

𝜕z= 0

𝜕2h(z)

𝜕z2> 0,

To reduce these data errors, some larger bars were selected for analysis. Because, it is difficult to require the bars eleva-tion change information from the remote sensing images, the exposed bar areas were extracted from different remote sensing images to analyze the bar dynamics.

Exposed area calculation

In spite of careful selection of the remote sensing images to avoid the effect of water level, differences still existed in the inter-annual water level among the images (Tables 2, 3). To decrease such uncertainties, the generalized geometric model was utilized to simplify the shape of mid-channel bars (Fig. 3) for a better unifying of water levels. Then, the horizontal distances of bars’ isobaths between 0 and 2 m could be measured by the Changjiang river channel graph (http://www.cjien c.com/), thus the slopes of sand bars can be calculated.

Table 3 List of satellite images used in the present work (flood season)




Acquisition date Hankou water level (m)

Acquisition date Huangshi water level (m)

Acquisition date Jiujiang water level (m)

1991/9/21 22 1991/6/26 19.5 1991/10/9 15.031993/10/12 22.1 1993/7/17 19.67 1993/6/8 15.551994/7/27 22.56 1994/10/24 18.7 1994/8/30 15.681995/8/31 23.46 1995/6/5 19.54 1995/9/18 15.271997/6/17 22.1 1997/8/29 18.21 1997/9/7 15.372000/9/13 23.75 2000/9/22 19.68 2000/9/23 16.712002/8/2 24.5 2002/8/3 21.3 2002/6/17 16.332003/9/22 23.43 2003/5/26 20.34 2003/8/23 15.922005/6/23 22.52 2005/9/20 19.92 2005/9/29 15.492007/6/29 22.81 2007/8/25 19.66 2007/10/5 15.272009/8/21 23.73 2009/7/13 19.45 2009/6/4 15.752013/6/13 23.11 2013/7/8 20.41 2013/8/18 14.7

Fig. 3 The mid-channel bars generalization

http://glovis.usgs.gov/

http://www.cjienc.com/


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The bars outcrop area changes with the length and width of quadrilateral, which could be adjusted through the horizontal and vertical slopes. The specific formulas are as follows:

where S is the bars area under the actual water level (m2), a is the length of channel bar, b is the bar’s width (m), k is a coefficient representing the ratio between the actual bars area and the parallelogram area, which is set as a constant value for the same bar. And, i represents the bar’s horizontal slope (m), j represents the bar’s vertical slope, hf is the actual water level (m), hu is the unitive water level (m), subscript ‘1’ represents the value under the unitive water level. Fur-therly, each TM scene contains a water level control station, and corresponding unitive water level can be seen in Table 1.

Independent samples T test

Comparison of mid-channel bars area variation rates between pre- and post-TGD periods was carried out through independent samples T test. Independent samples T test pro-cedure compares the means of two or more groups of cases (Larjani et al. 2014; Makwana et al. 2016). In this study, the entire data set was classified into two groups by a specify-ing cutoff point. Here, 2002, 2003 and 2005 was selected as cutoff points, respectively. Then, the area variation rates for the two sub-groups were computed and compared.

By default, a 95% confidence interval for the difference in means is displayed (Larjani et al. 2014; Makwana et al. 2016). If p ≤ 0.05, there is a significant difference between the two groups, otherwise, there is no significant difference.

Sediment carrying capacity formula

Flow hydraulics, sediment transport and mid-channel bars evolution in braided rivers were closely related (Bridge 1993; Ashworth 1996; Ashworth et al. 2000). In this paper, sediment carrying capacity was used to quantitative explore of the variation mechanism of mid-channel bars before and after the TGD operation.

According to the gravitation theory, the sediment-carry-ing capacity could be estimated by (Chien and Wan 1983):

where Svm is the suspended sediment carrying capacity; U is the flow velocity (m/s); g is the gravitational acceleration

(2)k = S∕(ab),

(3)a1 = a + (hf − hu)∕i × 2,

(4)b1 = b + (hf − hu)∕j × 2,

(5)S1 = ka1b1,

(6)Svm = k

(

U3

gh�

)m

,

(9.8 m/s2); h is the water depth (m); ω is the sediment set-tling velocity; k and m are empirical coefficients. Here k is equal to 0.07 and m is 1.14 (Zhang 1998; Gao et al. 2009). Then, U is determined from the fitting curve between dis-charge and flow velocity (Wang et al. 2009). h is obtained through the same way.

ω can be calculated by the following formulas (Rich-ardson and Zaki 1954):

where ω0 is the settling velocity of a sphere of diameter D; D is the grain diameter; γs is the sediment density (2300 kg/m3); γ is the density of water (1000 kg/m3); Sv is the sus-pended sediment concentration (kg/m3) (Yuan et al. 2012).

Results

Seasonal changes in channel bar area

According to the T test results in dry season, the best cut-off point for the entire 1989–2014 series is 2003 (Tables 4, 5, 6). The bars area variations have significant difference in the two sub-periods of 1989–2002 and 2003–2014 (Sig = 0.16, Sig2-tailed = 0.011 < 0.05) (Table 4). A clear downward trend was detected in the dry season with a mean decrease rate of 0.89 km2/year before 2003 (Fig. 4a). However, during the post-TGD period, the bars area had observable increase trend with a rate around 0.815 km2/year.

During the flood season, the whole area of mid-channel bars between CD reach presented upward trend with an increased rate of 1.07 km2/year between 1991 and 2014 (Fig. 4b). The T test shows that the bars’ variation rates have a statistical significant variance between the two sub-stages (Sig = 0.16, Sig2-tailed = 0.021 < 0.05) (Table 4). It means that compared to those in natural situation with-out TGD, the mid-channel bars still kept growing in flood season during 2003–2014, but the increase tendency was relatively insignificant (Fig. 4b).

Thus, mid-channel bars’ evolution was entitled as ‘deposition in the flood season, erosion in the dry season’ during the pre-TGD stage, indicating remarkable seasonal changes. Nevertheless, in post-TGD stage, mid-channel bars’ variation was summarized as ‘mild deposition in the flood season, deposition in the dry season’. Taken alto-gether, mid-channel bars area showed a notable difference following the construction of TGD in 2003.

(7)�0 = 1.72

√

�s − �

�gD,

(8)� = (1 − Sv)m�0,


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Tabl

e 4

Inde

pend

ent s

ampl

es T

test

for m

id-c

hann

el b

ars v

aria

tion

rate

s bet

wee

n C

heng

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i-Dat

ong

reac

hes

Sig

leve

l of s

igni

fican

ce

Leve

ne’s

test

for

equa

lity

of v

ari-

ance

s

T te

st fo

r equ

ality

of m

eans

FSi

gt

dfSi

g (2

-taile

d)M

ean

diffe

renc

eSt

d. e

rror

dif-

fere

nce

95%

con

fiden

ce

inte

rval

of t

he d

if-fe

renc

e

Low

erU

pper

Are

a va

riatio

n (d

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)G

roup

1: 1

989–

2000

Gro

up2:

200

2–20

14

Equa

l var

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ssum

ed0.

400.

540.

4212

0.68

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290.

68−

1.20

1.77

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419.

50.

690

0.29

0.70

− 1.

291.

87

Are

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roup

1: 1

989–

2002

Gro

up2:

200

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14

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240.

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120.

011

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550.

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2.73

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Are

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400.

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888.

90.

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1.89

0.66

0.40

3.38


1 3

394 Page 8 of 18

Tabl

e 5

Inde

pend

ent s

ampl

es T

test

for m

id-c

hann

el b

ars v

aria

tion

rate

s bet

wee

n C

heng

lingj

i-Han

kou,

Han

kou-

Jiujia

ng a

nd Ji

ujia

ng-D

aton

g re

ache

s

Leve

ne’s

test

for

equa

lity

of v

ari-

ance

s

T te

st fo

r equ

ality

of m

eans

FSi

gt

dfSi

g (2

-taile

d)M

ean

diffe

renc

eSt

d. e

rror

di

ffere

nce

95%

con

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inte

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of t

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Low

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Are

a va

riatio

n (d

ry se

ason

CJ)

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up1:

198

9–20

02G

roup

2: 2

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2014

Equa

l var

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es a

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660.

13−

3.83

12.0

0.00

2−

0.66

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0.28

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Page 9 of 18 394

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1 3

394 Page 10 of 18

Changes in the mid‑channel bars along the river course

There are obvious spatial differences of the mid-channel bars along the course of the Changjiang River (Fig. 5).

Specifically, in the dry season before TGD operation, the total area of the mid-channel bars was about 55 km2 in CH, 95 km2 in HJ, and 325 km2 in JD reach, respectively. In flood season, the mid-channel bars areas in the three reaches were approximately 37, 75 and 280 km2, respectively. Similar

Fig. 4 The overall change trend of mid-channel bars from Chenglingji to Datong: a in flood season from 1991 to 2014. b in dry season from 1989 to 2014

Fig. 5 The mid-channel bars area statistic in different reaches in dry/flood season; a Chenglingji–Hankou (CH) reach in dry season. b Hankou–Jiujiang (HJ) reach in dry season. c Jiujiang–Datong (JD)

reach in dry season; d CH reach in flood season. e HJ reach in flood season. f JD reach in flood season


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Page 11 of 18 394

seasonal variety can be found in flood/dry seasons during post-TGD period (Fig. 5).

Meanwhile, in dry season, the mid-channel bars area in CH and HJ decreased significantly before the impound-ment of TGD, with the inter-annual reduction rate reach-ing 0.67, and 0.21 km2/year, respectively, (Fig. 5a–c). Bars area in JD reach presented significant increase with a rate of 0.79 km2/year during 1994–2002. It was noted that the year of 1989 was excluded from the analysis because of the abnormal broken of Mianchuan bar and Xiasanhao bar, which can affect the accuracy of the result. Nevertheless, after the TGD operation, mid-channel bars in CH and HJ reach turned from erosion to deposition with an increas-ing rate of 0.3 and 0.49 km2/year, respectively. On the contrary, mid-channel bars in JD reach experienced ero-sion, indicating an average yearly declining area of 0.3 km2 (Fig. 5a–c). Moreover, all reaches had significant difference between pre- and post-TGD periods according to T test (CH reach: Sig = 0.13, Sig2-tailed = 0.002 < 0.05; HJ reach: Sig = 0.22, Sig2-tailed = 0.006 < 0.05; JD reach: Sig = 0.24, Sig2-tailed = 0.010 < 0.05) (Table 5).

Nevertheless, in flood season, the growth area of mid-channel bars mainly concentrates on the JD reach with an increasing rate of 0.86 km2/year during 1991–2014. The other two reaches showed slight deposition or even ero-sion with an increasing rate of 0.068 and − 0.039 km2/year, respectively. After TGD construction, the mid-channel bars

variations in the three reaches basically maintains the previ-ous forms, but with smaller variation rates (Fig. 5d–f). By the independent samples of T test, there were insignificant differences in bars variations between pre- and post-TGD stages along the Changjiang River, except the CH reach (CH reach: Sig = 0.36, Sig2-tailed = 0.045 < 0.05; HJ reach: Sig = 0.74, Sig2-tailed = 0.07 > 0.05; JD reach: Sig = 0.5, Sig2-tailed = 0.07 > 0.05) (Table 5).

Changes of mid‑channel bars located in different river patterns

In dry season, mid-channel bars area in straight and bend-ing braided river manifested a decreased trend between 1989 and 2002 with a decreasing rate of 0.62 and 0.42 km2/year, respectively (Fig. 6a, b). However, goose-head-shaped braided river presented slight deposition at a rate of 0.15 km2/year in bars area (Fig. 6c). Along with the TGD operation in 2003, change tendencies of the three river types had all changed. There were obvious devia-tion of bars variation rates between pre- and post-TGD stages, except for straight braided river (straight braided: Sig = 0.03, Sig2-tailed = 0.64 > 0.05; bending braided: Sig = 0.01, Sig2-tailed = 0.04 < 0.05; goose-head-shaped braided: Sig = 0.78, Sig2-tailed = 0.007 < 0.05) (Table 6). Specifically, for the mid-channel bars in the straight braided river, while remaining erosion, their average inter-annual

Fig. 6 The mid-channel bars area statistic under three river patterns in dry/flood season; a Straight braided river in dry season. b Bending braided river in dry season. c Goose-head-shaped braided river in dry

season. d Straight braided river in flood season. e Bending braided river in flood season. f Goose-head-shaped braided river in flood sea-son


1 3

394 Page 12 of 18

area reduction rate was significantly lower (the decreasing rate was 0.131 km2/year). Mid-channel bars in the bending braided river turned from erosion to deposition (the depo-sition rate could reach 0.3 km2/year). In the goose-head-shaped braided river, mid-channel bars area kept the growth trend with the yearly average deposition area increasing from 0.15 km2 during 1989–2002 to 0.65 km2 during 2003–2014 (Fig. 6a–c).

However, during 1991–2014, mid-channel bars area in the three river patterns all showed increasing trends in flood sea-son, with an annual increase rate of 0.27, 0.74 and 0.04 km2/year, respectively (Fig. 6d–f). Unlike the bars’ variations in dry season, only goose-head-shaped braided river showed significant difference following the operation of TGD (T test, Sig = 0.12, Sig2-tailed = 0.009 < 0.05) (Table 6), with bars area decreasing by 0.17 km2/year (Fig. 6f). Meanwhile, straight and bending braided river experienced insignificant difference in bars area between pre- and post-TGD peri-ods (T test, Sig = 0.15, Sig2-tailed = 0.08 > 0.05; Sig = 0.51, Sig2-tailed = 0.07 > 0.05) (Table 6). Mid-channel bars in above two river types still kept growth trends in post-TGD period, but the increasing rates reduced to 0.32 and 0.66 km2/year (Fig. 6d, e), respectively.

In summary, goose-head-shaped braided river indicated the most remarkable variation in these three river patterns, which deposited in both flood and dry season during pre-TGD stage, but turned into erosion in flood season and more deposition in dry season after 2003.

Discussion

Variations in water and sediment pre‑ and post‑dam periods

The natural hydrological regime and sediment transport pro-cesses along the Changjiang River have been altered by the TGD operation (Yang et al. 2006; Mei et al. 2015; Dai et al. 2016), which could be likely responsible for the changes in mid-channel bars. Runoff and sediment in Changjiang River were mainly concentrated in the flood season (Table 7). Mid-channel bars in CD reach generally indicated deposi-tion in flood season before TGD operation due to rich sedi-ment materials and decreasing flow velocity when the high water flows over the bars surface (Yu 2005; Li 2011; Pan 2011). While the annual discharge revealed slight change during 1991–2014, the seasonal features indicated signifi-cant variations. For instance, the regulation of TGD not only decreased the runoff amount in flood season (Fig. 7), but also cut down the magnitude and frequency of high flow (Mei et al. 2015). Specifically, the days with great discharge at Luoshan (> 30,000 m3/s), Hankou (> 35,000 m3/s) and Datong (> 40,000 m3/s) fall from 91 in pre-TGD stage to 66 in post-TGD stage, from 74 to 49 and from 90 to 66, in turn (Fig. 8). Moreover, since TGD began to operate in 2003, the sediment load has reduced drastically (Yang et al. 2006, 2007a; Dai and Liu 2013; Dai and Lu 2014; Dai et al. 2016), especially in flood season (Fig. 9). For example, the sedi-ment load at Yichang, Luoshan, Hankou, Datong stations decreased by 89, 73, 68, 61% following the construction of TGD in comparison with the natural situation (Table 7; Fig. 9). Due to shorter duration of high discharge and sharp reduction in fluvial sediment load, the bars’ growth trend in the flood season weakened, relatively (Fig. 4b). On the other hand, if sediment release is coupled with relatively large

Table 7 The flood and dry season data about flow and sediment in four main stations

Runoff volume dry season (109 m3)

Runoff volume flood season (109 m3)

Sediment load dry season (104 t)

Sediment load flood season (104 t)

Yichang Pre-dam 957 3377 976 39,523 Post-dam 988 3005 56 4301

Luoshan Pre-dam 1798 4822 4933 29,261 Post-dam 1702 4220 1957 7630

Hankou Pre-dam 2034 5273 3839 28,987 Post-dam 1850 4342 1943 8993

Datong Pre-dam 2848 6656 4293 29,329 Post-dam 2630 5779 2799 11,214


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Page 13 of 18 394

Fig. 7 The monthly average flow of four hydrologic stations in pre- and post-TGD periods (pre-TGD: 1989–2002; post-TGD: 2003–2014)

Fig. 8 The duration of high discharge in three stations in pre-and post-TGD periods

Fig. 9 The monthly average sediment load at four hydrologic stations in pre- and post-TGD periods (pre-TGD: 1989–2002; post-TGD: 2003–2014)


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394 Page 14 of 18

floods, their deposition occurs on a relatively elevated area of the bars, which increase the bars elevations instead of the bars area (Asaeda and Rashid 2012). Under this condition, the lateral increasing tendencies of sandbar area could be unobservable (Fig. 5d–f, 6d–f).

Besides, TGD’s influence on flow and sediment exhib-ited spatial differences along the Changjiang River because of downstream tributary input and riverbed compensation (Yang et al. 2007a, b) (Table 7), which made the bars in lower reach more developed. Specifically, Yichang station, 37 km immediate to the TGD, showed the most remark-able hydrological variations along with TGD construc-tion, where the annual sediment load reduced to 4 × 108 t in the post-TGD period, only about one-tenth of its original value. At Jingjiang reach, directly below TGD, occurred the greatest erosion after 2003 (Yang et al. 2015; Zhang et al. 2017). Meanwhile, annual sediment load in Luoshan, Hankou and Datong reduced by 71, 66, 58% respectively

(Table 7). These hydrological differences were consistent with the mid-channel bars area spatial variations, namely, the further away from the dam, a weaker effect is reflected. In addition, mid-channel bars area in JD reach had different change trend from the upstream owing to the supplement of flow and sediment from the Poyang Lake.

Variation of sediment carrying capacity along the river course

River sediment concentration and sediment carrying capacity directly dominated scour and deposition in the river channel (Petts 1979; Curtis et al. 2010), which, accordingly, could quantitatively explain the mid-channel bars’ area change. Based on the linear simulation curves of discharge with suspended sediment concentration (SSC), water depth and average flow velocity (Fig. 10), the sedi-ment carry capacities in Luoshan, Hankou and Datong

Fig. 10 Relationships between discharge and SSC, and flow velocity (U), and water depth at three hydrologic stations in pre-and post-TGD peri-ods


1 3

Page 15 of 18 394

could be obtained under different hydrological condi-tions (Fig. 11). Compared with pre-TGD period, the sedi-ment carrying capacity decreased obviously in post-TGD period, as sediment grain in middle and lower reaches of the Changjiang River have increased gradually (Fig. 11).

In dry season, the SSC is generally smaller than the sediment carry capacity before TGD operation, suggest-ing that the sediment in water was starving (Fig. 11). In such case, mid-channel bars would be eroded to keep the balance between flow and sediment, which satisfies the concepts of the graded river (Hoover Mackin 1948) and dynamic equilibrium of Hack (1960). However, after TGD operation, sediment carrying capacity of CH, HJ and JD reach dropped to 23, 31, 76% of the initial values in turn, while their corresponding SSC just reduced to 38, 42, 55%, respectively. These variations were in agreement with the bars changes along the course of the Changji-ang River in dry season. Furthermore, sediment carrying capacity of CH reach reduced to 14% of the pre-TGD level in flood season, while that in HJ and JD reduced by 87 and 51%, respectively. In the meantime, the suspended sedi-ment concentrations of these three reaches declined to 36, 38, 61% of the pre-TGD value.

Besides, the clear water released from TGD would scour downstream channels and as a consequence, take the coarse riverbed particles to the downstream, especially the coarse sand (D > 0.125 mm) (Zhang et al. 2017). Sediment coars-ening along the downstream reach coupled with sediment carrying capacity decrease in post-TGD period were likely to promote the sediment deposition in the mid-channel bars (Yuan and Li 2016; Yuan et al. 2012). To make things worse, the shallow water in dry season mainly covered coarser par-ticles (Rust 1972; Ashworth 1996), which further contrib-uted to, mid-channel bars deposition.

Impact of geomorphic conditions

As for the alluvial rivers, the lateral changes of river bed not only depended on the sediment and flow from upstream, but also were affected by the original environment conditions, such as the river patterns and river bed forms (Friedman et al. 1998).

The channel nodes are typical topographic conditions in the middle and lower reaches of the Changjiang river, which can control the river regimes and affect the channel evolution (Chien 1987). Due to the effect of deflecting flow of channel nodes under different discharges, various sediment scour-ing and depositing degrees could occur (Yu 1987). Under low flow conditions, due to the constraint of channel nodes, discharges are concentrated in the river channel and corre-spondingly water velocity increases, which together led to the bars erosion. The restraint effect of channel nodes turns out to be relatively weak under the high discharge conditions (Xia and Yan 2000; Zhu et al. 2015). With the TGD opera-tion, the natural flow regime was replaced by the regulated one (Curtis et al. 2010; Mei et al. 2018). The effect of the channel nodes, as a result, also has been adjusted, which further caused the variation of mid-channel bars.

Fig. 11 Sediment carrying capacity and actual suspended sediment concentration (measured SSC) at three hydrologic stations in pre- and post-TGD periods

Table 8 The geometrical morphology of different river patterns

River pattern Widening rate Tortuous rate Branch-ing numbers

Straight braided 2.17 1.08 2.3Bending braided 4.21 1.27 2.3Goose-head-shaped

braided6.72 2.04 3.4


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394 Page 16 of 18

Expect the influence of channel nodes, river patterns, including the widening rate (the ratio of full branches’ widths and import straight channel width), tortuous rate (the ratio of the branch channel arc length and its straight length) and branching numbers (Yu 2005; Li 2011), also affected the bars development. Geometrical morphology of straight, bending and goose-head-shaped braided river patterns in the middle and lower reaches of Changjiang river are summa-rized in Table 8 (Chien 1987).

Generally, the route of discharge tends to be straight in flood season and bent in dry season (Chien 1987). Accord-ingly, in flood season, mid-channel bars in straight braided river with the least tortuous rate (1.08) could obtain a sig-nificant amount of sediment from flood. Besides, it generally developed the narrow and low mid-channel bars, and had poor stability. Therefore, straight braided river presented larger growth rate than the other two river patterns in flood season. In dry season, due to greater tortuous rate (2.04) and the existence of the natural rocky nodes, the import flow could be restricted, which facilitated the settling of sedi-ment in goose-head-shaped braided river and make the bars develop (Ma and Gao 2001).

In addition, comparing with the other two river patterns, goose-head-shaped braided river had greater widening rate, tortuous rate and more branch channels (Table 8), which could result in a shorter cycle of mutual transformation between the main branch and other branches, as well as more unstable mid-channel bars evolution (Brice 1982). There-fore, goose-head-shaped braided river was most sensitive to the water and sediment variation, where the mid-channel bars had the greatest variation since the TGD construction in 2003 (Ma and Gao 2001).

Conclusion

As important geomorphic features and components are present in middle and lower reaches of Changjiang River, mid-channel bars development has important influence on the river channel stability. However, under the influence of the TGD, the downstream hydrologic regime have dramati-cally changed, which also affected the mid-channel bars. Therefore, this paper explored the variation of mid-channel bars and potential driving factors of these changes using the remote sensing method. The main conclusions were sum-marized below:

1. The mid-channel bars in Chenglingji–Datong reach experienced erosion in dry season (− 0.89 km2/year) during 1989–2003 and turned from erosion to deposition with a growth rate of 0.82 km2/year after 2003. In flood season, bars kept silting up state with an increasing rate of 1.05 km2/year. However, the growth rate reduced fol-

lowing the construction of TGD. Therefore, mid-channel bars in Chenglingji–Datong reach is characterized by ‘remarkable seasonal difference in pre-TGD period, and mild seasonal pattern in post-TGD period’.

2. Owing to the effect of tributary input, mid-channel bars in CH, HJ, JD reaches showed different degrees of variation in response to the regulation of TGD. Farther downstream from TGD, the bars variation is weaker. Besides, mid-channel bars variation in JD reach in dry season was contrary to the upstream as a result of the Poyang Lake influence, indicating the self-adjustment of the river system.

3. Mid-channel bars in different river patterns had different responses to TGD-induced water and sediment varia-tion. Among the three river types, goose-head-shaped river pattern indicated the most remarkable alteration in mid-channel bars. Specifically, the bars’ deposition rate increased from 0.155 km2/year during 1989–2002 to 0.53 km2/year in dry season following the construc-tion of TGD. In flood season, however, the bars showed erosion with a rate of 0.17 km2/year after 2003.

Above all, this study shows the changing trend of mid-channel bars in the middle and lower reaches of the Changji-ang River in response to TGD regulation. Such knowledge is essential for the sustainable management of channel bars and can provide scientific support to other mega rivers under a similar context.

Acknowledgements This study was supported by the National Science Foundation of China (NSFC) (41706093). The Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Tech-nology (MGQNLM201706) and the Key Laboratory of Coastal Science and Engineering, Beibu Gulf, Guangxi (2016KYB01).

Author contributions ZD jointly conceived the study. JW collected the field data and processed the remote sensing images. WW and YL undertook the sediment carry capacity computation. XM provided valuable suggestions on the mid-channel bars evolution analysis. All co-authors contributed to the discussion. YL drafted the main manu-script, which was then commented and edited by XM and ZD.

Compliance with ethical standards

Conflict of interest The authors declare no conflict of interest.

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