Introduction
Morphological evolution of river systems is important to river management and regulation, and has become a growing issue over the past decades (
Li et al., 2014;
Zhu et al., 2017;
Schletterer et al., 2019). Braided reaches are common place in rivers, with alternate development and shrinkage occurring between the main stem and secondary branches; such reaches often extend from the head source to the estuary (
Jain and Sinha, 2004;
Latrubesse, 2008;
Jansen and Nanson, 2010;
Chen et al., 2016;
Li et al., 2016;
Zhu et al., 2017 and
2019;
Han et al., 2018). It has been established that differences in evolutional processes between bifurcated branches are primarily caused by changes in lateral flow dynamics, driven by variations in flow discharge (i.e., upstream runoff discharge or downstream tidal discharge) (
Chen et al., 2016;
Zhu et al., 2017 and
2019;
Han et al., 2018), local human activities (e.g., channel improvement works and sand excavation) (
Kuang et al., 2014;
Zheng et al., 2018;
Dai and Ding, 2019), and Coriolis-induced circulation in estuarine areas (
Li et al., 2011 and
2014;
Wang et al., 2013). Dams modulate runoff discharge in rivers worldwide and can drive the morphological evolution of braided reaches, as demonstrated by many inland rivers (
Petts and Gurnell, 2005;
Graf, 2006;
Han et al., 2018;
Alcayaga et al., 2019;
Mendoza et al., 2019;
Zhu et al., 2019) and estuarine areas (
Warne et al., 2002;
Sloff et al., 2013;
Zhu et al., 2017;
Liu et al., 2018;
Zhou et al., 2018), noting that tidal discharges are relatively stable at the yearly time scale (
Horrevoets et al., 2004;
Jiang et al., 2012a;
Zhu et al., 2017). For fluvial braided reaches, shrinking or developing trends of branching channels are often aggravated or interchanged after dam impoundment, caused by changes in unidirectional flow dynamics driven by the altered runoff discharge (
Han et al., 2018;
Zhu et al., 2019). However, the morphological evolution in branches of tidal-affected braided reaches (including bifurcated estuaries) are more intricate, mainly due to the complexity of bifurcating systems and the jacking effect of tidal currents (
Zhang et al., 2015;
Zhu et al., 2017 and
2018). Marine dynamic factors, such as waves, longshore currents, and storm surges, further complicate morphological changes in estuarine areas (
Kaliraj et al., 2014;
Rangoonwala et al., 2016;
Shen et al., 2019).
As the largest river on the Eurasian continent and the third longest in the world, the Yangtze has accommodated the construction of more than 50,000 dams since the 1950s (
Yang et al., 2011,
2015). Moreover, the Yangtze River hosts 49 major braided reaches in its middle and lower region (including the Yangtze Estuary). These reaches are classified into three main types: 1) straight braided reaches; 2) slightly bending braided reaches; and 3) goose-head braided reaches (
Yu, 2013). Under the impact of the dams, the intra-annual distribution of runoff discharge has flattened, whereas the change in total annual runoff flux has been minimal (
Zhao et al., 2018;
Zhu et al., 2017,
2018, and
2019). Consequently, changes to the natural evolutional trends of the branching channels have been generally identified in the braided reaches along the Yangtze River (
Zhu et al., 2017 and
2019; Han et al., 2018;). Nevertheless, the braided near-estuary reach, which extends from the tidal current limit of the Yangtze River to the upper boundary-node of the Yangtze Estuary (Yu and Lu, 2005; Yang et al., 2012 and 2016), has not yet been comprehensively investigated. Researchers have chiefly analyzed the evolutional courses of thalwegs, cross-sections, and flow hydrodynamics (e.g., net discharge ratios and flow velocities) in the branching channels of the near-estuary reach (
Chen et al., 2012 and
2016;
Jiang et al., 2012b;
Fan et al., 2017;
Zhang and Xu, 2017), but have not considered overall variations in channel morphology and systemic relationships between erosion-deposition patterns and variations in the flow hydrodynamics. This implies that predictions of the future evolution of branching channels in this reach (
Jiang et al., 2012b;
Chen et al., 2016) could be unreliable. Moreover, the law of depo-center movement in the branching channels of the near-estuary reach has not been explored. (The depo-center is defined as the location where the sediment deposition rate is at maximum.) Depo-center movement has not been considered adequately for other braided reaches or river systems as well, and deserves in-depth analysis given its indicative role regarding erosion-deposition distributions in the branching channels of a braided river.
In the present study, the overall morphological evolution and the law of depo-center migration in branching channels of the near-estuary reach of the Yangtze River are investigated, based on terrain and hydrodynamic data from 1950 to 2014. The findings may be transferable to other braided reaches worldwide, and could be useful in guiding future engineering projects that are planned for the near-estuary reach of the Yangtze River.
Study area
The near-estuary reach of the Yangtze River is located at the distal section of the Yangtze River (Fig. 1(a)), extending from Jiangyin (the tidal current limit) to Xuliujing (the upper boundary-node of the Yangtze Estuary) (Yu and Lu, 2005; Yang et al., 2012 and 2016). The reach measures ~90 km in length (Fig. 1(b)). Its main braided waterways comprise the Fujiangsha, Rugaosha, Tongzhousha, and Langshansha Waterways (Fig. 1(b)). The middle branch of the Rugaosha Waterway connects with the north branch of the Fujiangsha Waterway, while the Liuhaisha branch of the Rugaosha Waterway connects to the middle and south branches of the Fujiangsha Waterway (Fig. 1(b)). Similarly, the east and west branches of the Langshansha Waterway connect with the corresponding branches of the Tongzhousha Waterway (Fig. 1(b)). This braided reach is influenced by tides, with multi-year (1950–2014) average tidal ranges of 1.68 and 2.04 m at Jiangyin and Xuliujing (Fig. 1(b)) (
Zhu et al., 2018). Due to the relative stability of tidal forcing at the yearly time scale (
Horrevoets et al., 2004;
Jiang et al., 2012a;
Zhu et al., 2017), the mean annual tidal level at Xuliujing (the lower boundary of this reach, Fig. 1(b)) is almost constant (
Zhu et al., 2018). By comparison, runoff discharge from the Datong hydrological station (Fig. 1(a)), representing the most downstream reach (
Zhao et al., 2018;
Zhu et al., 2017,
2018, and
2019), experiences significant intra-annual variability, exhibited by a reduced flood discharge occurrence frequency and an increased middle-low discharge occurrence frequency (
Zhu et al., 2017,
2018, and
2019). However, the annual runoff discharge is almost constant (
Zhu et al., 2017,
2018 and
2019;
Zhao et al., 2018), with a multi-year (1950–2014) average value of 28300 m
3/s (
CWRC, 2016). To achieve the national goal of a Golden Waterway, extensive channel improvements have been implemented along the near-estuary reach, including an upstream extension of the Deepwater Channel Project (Fig. 1(b);
Chen et al., 2012;
Wu et al., 2013;
Yang and Lin, 2013;
Ni et al., 2014;
Xu et al., 2014).
Materials and methods
Data information
Observed daily runoff discharge time series at Datong Station from 1950 to 2014, and hourly ebb tidal discharges in the branching channels and hourly ebb tidal levels at temporary tide gauges in the vicinity of the waterways from 30th August to 10th September 2004 and from 17th January to 12th February 2005 were obtained from the Changjiang Water Resources Commission (China). Bed-elevation point data digitized from surveyed navigational charts in 2005 and 2007 were provided by the Shanghai Estuarine & Coastal Science Research Center (China); those in 2011 and 2014 were obtained from the Changjiang Waterway Bureau (China). Channel volumes below -5 m and -10 m isobaths in the two branches of the Tongzhousha Waterway were acquired from the Changjiang Waterway Bureau (China). The following data on hydrodynamics and morphology were gathered from the open literature: 1) yearly wet-season average ebb partition ratios for branching channels in 1977, 1983, 1993, 1998, 2006, and 2011; 2) minimum widths of -8 m and -10 m isobaths in the north branch of the Fujiangsha Waterway from 2005 to 2012; 3) cross-sectional profiles at the entrance of the south branch of the Fujiangsha Waterway in 1977, 1983, 1993, 1998, 2006, and 2011; and 4) cross-sectional areas of the two branches of the Rugaosha Waterway under bankfull discharge in 1977, 1983, 1993, 1998, 2006, and 2011. Table 1 summarizes the data sources.
Processing of bed-elevation point data
Bed-elevation point data from 2005, 2007, 2011, and 2014 were projected onto Beijing 54 coordinates using ArcGIS 10.2 during digitization, with reference to 1985 national elevation benchmarks. Bed elevations and point locations had previously been determined from measurements using dual-frequency echo sounders and GPS positioning. The measurement errors for bed-elevation of ±0.1 m and location of ±1 m were taken to be acceptable, noting the huge scale of bed-elevation changes that can occur annually (
Luan et al., 2016). The proportional scales for all the four sets of terrain data are 1:10000, with sample density of 10–122 pts/km
2 (i.e., spacing of 50–500 m between two neighboring points), thus a grid resolution of 25 m × 250 m was adopted when calculating morphological changes using Kriging interpolation.
Interpretation of depo-centers
A depo-center in a branching channel is defined as the location where the maximum depositional rate of sediment occurs. Upstream and downstream depo-center movements in branching channels are identified by interpreting changes in river-bed elevation caused by erosion and deposition in upper and lower sub-reaches of roughly the same length. Increases in depositional rate or decreases in erosional rate of the upper or lower sub-reaches indicate that the depo-centers in the corresponding channels are moving toward the sub-reaches, whereas decreases in depositional rate or increases in erosional rate of the sub-reaches indicate depo-center movements away from the sub-reaches. Figure 2 illustrates the divisions of upper and lower sub-reaches in the branching channels of the four main braided waterways.
Results
Hydrodynamic variations
Runoff discharge
Figure 3 shows minimal change in annual runoff discharge, whereas the intra-annual distribution of runoff discharge flattened significantly, from the pre-TGD period (in which the GZD and the DJKD impounded water, Fig. 1) to the post-TGD period (in which the TGD, the XJD, and the XLDD impounded water, Fig. 1). It can be seen that the multi-year average duration days with discharges<10,000 m3/s and>50,000 m3/s decreased significantly, while those of 10000–20000 m3/s increased substantially.
Ebb partition ratio
Yearly trends
Figure 4 shows that the annual wet-season average ebb partition ratios in the north region (including the north branch, middle branch, and Shuangjian shoal, Fig. 1(b)) of Fujiangsha Waterway, the Liuhaisha branch of Rugaosha Waterway, the west branch of Tongzhousha Waterway, and the west branch of Langshansha Waterway exhibited decreasing trends from 1977 to 2011, whereas the ebb partition ratios for the other branches of the four waterways presented increasing trends.
Changes under different runoff conditions
Figure 5 presents the variations in ebb partition ratio with tidal range obtained for the branching channels from 30th August to 10th September 2004 and from 17th January to 12th February 2005, for runoff discharges of 36000 m3/s and 11000 m3/s. The tidal data were obtained using temporary tide gauges (see Fig. 1(b) for locations). For all tidal range values considered, the ebb partition ratio at a runoff discharge of 36000 m3/s was invariably larger than that at 11000 m3/s in the north region of Fujiangsha Waterway, the Liuhaisha branch of Rugaosha Waterway, the west branch of Tongzhousha Waterway, and the west branch of Langshansha Waterway. This implies that the higher runoff discharge caused flow to divert into these branches, with the opposite occurring in the other waterway branches.
Morphological variations
Whole channel
Yearly trends
Both the annual time series of minimum widths of the -8 m and -10 m isobaths in the north branch of the Fujiangsha Waterway present decreasing temporal trends (Fig. 6(a)), indicating that the north branch has been progressively shrinking. Given that the north branch is the main channel at the north of Fujiangsha Island (Fig. 1(b)), this shrinkage implies that the northern region (including the north branch, middle branch, and Shuangjian shoal) of the Fujiangsha Waterway has been experiencing morphological decline. By contrast, the deep channel of the cross-section at the entrance of the south branch of Fujiangsha Waterway underwent significant erosion from 1977 to 2011 (Fig. 6(b)), suggesting the morphology of the south branch was undergoing rapid development.
The cross-sectional areas of the middle branch and the Liuhaisha branch of Rugaosha Waterway presented increasing and decreasing trends under bankfull discharge (Figs. 6(c)-6(d)), implying the middle branch and Liuhaisha branch were experiencing developing and declining morphological trends, respectively.
Figures 6(e)–6(f) show that the channel volume below the -10 m isobath in the east branch of the Tongzhousha Waterway has been presenting an increasing trend, whereas that below the -5 m and -10 m isobaths in the west branch of the Tongzhousha Waterway has a decreasing trend, indicating development and decline of the east and west branches, respectively.
It should be noted that field observations by
Chen et al. (2016) have shown that the east and west branches of the Langshansha Waterway have exhibited respective developing and declining trends.
Changes under different runoff conditions
Table 2 lists erosion-deposition rates in the branching channels and the corresponding duration days of relevant runoff discharges during 2005-2007, 2007-2011, and 2011-2014. Figure 7 displays plan distributions of erosion-deposition rates for the whole near-estuary reach in 2005, 2007, 2011, and 2014.
Of these periods, 2005–2007 was the driest, with the least duration days of flood discharges (>50000 m
3/s and>60000 m
3/s) and the most duration days of low and middle-low discharges (<10000 m
3/s and 10000–20000 m
3/s) (Table 2). This is due to the extremely dry year event that affected the Yangtze Basin in 2006 (
Zhu et al., 2018) when no discharge exceeded 40000 m
3/s, and the duration days of low and middle-low discharges were at 7 days and 185 days. The 2007–2011 period was wettest, with the largest number of duration days for flood discharges (especially>60000 m
3/s) and lower number of duration days for low and middle-low discharges (Table 2). The 2007–2011 period included the flood year of 2010 (
Zhu et al., 2018), the only year from 2005 to 2014 during which the discharge exceeded 60000 m
3/s, lasting 36 days. The runoff intensity in 2011–2014 was between that in the foregoing two periods (Table 2).
The entire northern region of Fujiangsha Waterway (including the north branch, middle branch, and Shuangjian shoal) experienced deposition during 2005–2007, severe erosion during 2007–2011, and slight erosion during 2011–2014 (Table 2), indicating that low and high values of runoff intensity promoted deposition and erosion, respectively. This erosion-deposition behavior in the north region is also confirmed by changes in the deep channel area (see Fig. 7), which shrank during the period of 2005 to 2007 (Figs. 7(a)-7(b)) before experiencing significant growth from 2007 to 2011 (Figs. 7(b)-7(c)) and from 2011 to 2014 (Figs. 7(c)-7(d)). Erosion occurred in the south branch during all three periods, with a much larger erosional rate during 2005–2007 than 2011–2014 (Table 2). Even though severe erosion occurred in the south branch during 2007–2011, when the largest number of flood discharge duration days were experienced, the rate of erosion was smaller than in the north region (Table 2; Figs. 7(b)-7(c)). This implies that the north region and south branch underwent roughly the reverse erosion-deposition behavior under runoff changes.
In accordance with changes in runoff intensity, the Liuhaisha branch of Rugaosha Waterway experienced deposition during 2005–2007, significant erosion during 2007–2011, and less significant erosion during 2011–2014 (Table 2). This erosion-deposition behavior was linked to changes in the deep channel area (Fig. 7) which witnessed obvious shrinkage from 2005 to 2007 (Figs. 7(a)
-7(b)) and significant growth from 2007 to 2011 (Figs. 7(b)
-7(c)) and 2011 to 2014 (Figs. 7(c)
-7(d)). The middle branch of Rugaosha Waterway exhibited a similar erosion-deposition pattern to that of the Liuhaisha branch (Table 2), influenced by the flood-tide-driven sediment supply from the lower two braided waterways during the dry period of 2005-2007 (
Zhu et al., 2018) and engineering projects that were implemented in the vicinity (Fig. 1(b);
Chen et al., 2012;
Wu et al., 2013).
The two branches of Tongzhousha Waterway did not exhibit opposite erosion-deposition patterns under runoff change (Table 2). This was perhaps because the gradual decline of the west branch in recent years (
Ni et al., 2014) caused the Tongzhousha Waterway to become a single river channel dominated by the east branch. In this case, the discharge, regardless of runoff intensity, passed mainly through the east branch, leading to erosion or deposition depending on the discharge within the branch (Table 2). Meanwhile, regulation projects implemented along the Tongzhousha Waterway also impacted the erosion-deposition pattern (
Ni et al., 2014). Even so, the low runoff intensity during 2005–2007 promoted shrinkage of the west branch and shortened the deep channel of the west branch (Figs. 7(a)
-7(b)), whereas the high runoff intensities during 2007–2011 and 2011-2014 facilitated development of the west branch, lengthening its deep channel (Figs. 7(b)
-7(c) and Figs. 7(c)
-7(d)). The upper and lower deep channels became connected within the west branch from 2011 to 2014 (Figs. 7(c)
-7(d)).
Depositional rates in the east branch of Langshansha Waterway were smallest during 2005–2007 and largest during 2007-2011 (Table 2), indicating that low and high runoff intensities, respectively, facilitated the development and decline of the east branch. As shown in Fig. 7, the deep channel area in the east branch experienced obvious growth from 2005 to 2007, and altered from a bifurcating to a single channel pattern as its width increased (Figs. 7(a)-7(b)). However, from 2007 to 2011 and 2011 to 2014, the deep channel area re-established a bifurcated pattern, with decreased width (Figs. 7(b)-7(c) and Figs.7(c)-7(d)). The west branch underwent an almost opposite erosion-deposition pattern, with deposition during 2005–2007 and 2011–2014, and erosion during 2007–2011 (Table 2). This implied that low runoff intensity promoted shrinkage of the west branch, whereas high runoff intensity promoted growth. Meanwhile, the deep channel of the west branch shortened during 2005-2007 (Figs. 7(a)-7(b)) and lengthened during 2007-2011 (Figs. 7(b)-7(c)) and 2011-2014 (Figs. 7(c)-7(d)).
In summary, low runoff intensity generally promoted development of the south branch of Fujiangsha Waterway, the middle branch of Rugaosha Waterway, the east branch of Tongzhousha Waterway, and the east branch of Langshansha Waterway, while facilitating morphodynamic decline of the other branches of the braided waterways. High runoff intensity produced essentially the opposite effect.
Depo-center movement
The data listed in Table 3 indicate that depo-centers in the north region of Fujiangsha Waterway, the Liuhaisha branch of Rugaosha Waterway, the west branch of Tongzhousha Waterway, and the west branch of Langshansha Waterway moved upstream after the runoff intensity declined, and moved downstream after the runoff intensity increased. However, the observed effects were quite different in the other waterway branches as detailed below.
In the north region of the Fujiangsha Waterway, the depositional rate in the upper sub-reach was higher than in the lower sub-reach during 2005
-2007 (with low runoff intensity) (Table 3), indicating that the depo-center of this region was located in the upper sub-reach. However, both sub-reaches experienced erosion during 2007
-2011 and 2011
-2014 (with higher runoff intensities) (Table 3), implying that the depo-center moved into the channel downstream from this region. Moreover, due to the higher runoff intensity during 2007
-2011 than during 2011–2014, the erosion rates of the two sub-reaches were greater, respectively. In addition, the erosion rate in the upper sub-reach was greater than in the lower sub-reach during 2007
-2011 (Table 3). This suggests that the depo-center moved further downstream during 2007
-2011 than 2011
-2014. Due to the likely impacts from the regulation projects in the Fujiangsha Waterway (Fig. 1(b);
Xu et al., 2014), the depo-center in the south branch did not exhibit the reverse behavior (Table 3).
The depositional rate in the lower sub-reach of the middle branch of Rugaosha Waterway was higher than in its upper sub-reach during 2005-2007 (Table 3), indicating that the depo-center was located in the lower sub-reach. During 2007-2011, both sub-reaches underwent erosion (Table 3), implying that the depo-center was located in the channel downstream from the middle branch. However, the erosional rate in the upper sub-reach was less than in the lower sub-reach during 2007-2011 (Table 3). This suggests that the downstream movement of the depo-center was eased by an upstream transport of sediment (eroded from the lower sub-reach during flood-tide) into the upper sub-reach during this flood period. The runoff intensity levels from 2011 to 2014 were between those observed during 2005-2007 and 2007-2011, thus the position of the depo-center (reflected by the erosion-deposition rates of the two sub-reaches, Table 3) occupied an intermediate location. The depo-center in the Liuhaisha branch exhibited almost the opposite behavior. Both sub-reaches of the Liuhaisha branch experienced deposition during 2005-2007 and erosion during 2007-2011 and 2011-2014 (Table 3), suggesting that the depo-center was located in the Liuhaisha branch during the former period but in the channel downstream of the Liuhaisha branch during the latter two periods. In short, the depo-center migrated downstream from 2005 to 2014. Meanwhile, the decrease in erosional rate of the upper sub-reach was greater than that of the lower sub-reach from the periods of 2007–2011 to 2011–2014 as runoff intensity fell (Table 3), indicating upstream migration of the depo-center.
Both sub-reaches of the east branch of the Tongzhousha Waterway experienced erosion during 2007–2011 (Table 3), corresponding to the depo-center located in the channel downstream of the east branch. However, the erosional rate in the upper sub-reach was less than in the lower sub-reach (Table 3). This meant that erosion in the upper sub-reach was relieved by upstream transport of sediment (eroded from the lower sub-reach by the flood-tide) into the upper sub-reach. During 2005–2007 and 2011–2014, the upper and lower sub-reaches underwent erosion and deposition (Table 3), implying that the depo-center was located in the lower sub-reach. In the west branch, the upper and lower sub-reaches, respectively, experienced erosion and deposition during 2005–2007 (Table 3), indicating that the depo-center was located in the lower sub-reach. However, both sub-reaches experienced erosion during 2011–2014, with the erosional rate of the upper sub-reach increasing significantly (Table 3), as the depo-center moved into the channel downstream of the west branch.
During 2005-2007, the upper and lower sub-reaches of the east branch of the Langshansha Waterway experienced erosion and deposition, respectively (Table 3), with the depo-center located in the lower sub-reach, accordingly. During 2007-2011 and 2011-2014, the upper sub-reach accreted sediment, while the lower sub-reach underwent deposition from 2007 to 2011, followed by erosion from 2011 to 2014 (Table 3), meaning that the depo-center moved upstream, even entering the upper sub-reach. The upper and lower sub-reaches of the west branch experienced deposition and erosion, respectively, during 2005-2007 and 2011-2014 (Table 3) when the depo-center was located in the upper sub-reach. However, the upper and lower sub-reaches underwent erosion and deposition, respectively, from 2007 to 2011 (Table 3), as the depo-center migrated downstream into the lower sub-reach during this flood period.
Discussion
Linkage-mode between channel erosion-deposition and depo-center movement
Through the foregoing analysis, a linkage-mode can be identified between the erosion-deposition patterns of branching channels and their depo-center movements. That is, as a channel experiences erosion/deposition, its depo-center tends to move downstream/upstream. In the northern section of Fujiangsha Waterway, the Liuhaisha branch of Rugaosha Waterway, the west branch of Tongzhousha Waterway, and the west branch of Langshansha Waterway, erosion and concomitant downstream depo-center migration occur as runoff intensity increases, whereas deposition and accompanying upstream depo-center migration occur as runoff intensity falls (Table 2; Fig. 7; Table 3). In other branches of the waterways, the two cases of erosion-deposition behavior and concomitant depo-center migration occur as runoff intensity falls and rises, respectively (Table 2; Fig. 7; Table 3).
Mechanism behind the linkage-mode
Figure 5 indicates that there is a robust relationship between ebb partition ratio and runoff discharge for a branching channel in the near-estuary reach, given the morphological changes in the river bed are small, owing to the short timespan from the wet period (30th August to 10th September, 2004) to the dry period (17th January to 12th February, 2005), and due to the weak runoff intensity during this water-recession timespan.
The relationships in Fig. 5 are driven by the geographic features of the near-estuary reach. Several raised nodes (formed by mountains) exist along the south bank at the entrance of Fujiangsha Waterway (
Chen et al., 1988). These nodes tend to drive the ebb tidal current into the north region of Fujiangsha Waterway, with this effect strengthening as runoff intensity rises (
Chen et al., 1988). Hence, a high runoff discharge corresponds to a high value of ebb partition ratio in the north region and a low value of ebb partition ratio in the south branch, with the reverse occurring for a low runoff discharge (Figs. 5(a)
-5(b)). The Liuhaisha branch of Rugaosha Waterway is much wider than the middle branch of Rugaosha Waterway and connects with the north region of Fujiangsha Waterway (Fig. 1(b)). Hence, a high runoff discharge also facilitates diversion of the ebb tidal current into the Liuhaisha branch while restraining diversion of the ebb tidal current into the middle branch (Figs. 5(c)
-5(d);
Chen et al., 2012). Given that the cross-section and water depth of the east branch of Tongzhousha Waterway are much greater than those of the west branch (Fig. 1(b)), the ebb tidal current tends to flow into the east branch when the runoff discharge is low, which increases the ebb partition ratio in the east branch and decreases the ebb partition ratio in the west branch (Figs. 5(e)
-5(f)). Conversely, the tidal level rises as runoff discharge increases, causing part of the ebb tidal current to divert into the west branch (
Chen et al., 2012), leading to the ebb partition ratio exhibiting opposite behavior in the two branches (Figs. 5(e)
-5(f)). Given that the two branches of Langshansha Waterway connect directly with those of Tongzhousha Waterway (Fig. 1(b)), the relationships between ebb partition ratio and runoff discharge of the branches are similar to those for the Tongzhousha Waterway (Figs. 5(g)
-5(h)).
In the near-estuary reach, the ebb tidal flow consists of runoff discharge and the flood tidal current, both of which are relatively stable at the yearly time scale (
Zhu et al., 2017 and
2018). Consequently, the yearly ebb tidal flow is also stable, implying that ebb partition ratios in the branching channels determine the allocation of ebb tidal amplitudes among these channels. Existing theory has established that the ebb tidal force dominates channel evolution in tide-affected reaches (
Dou, 1964). Hence, the ebb partition ratio is responsible for morphological change in a branching channel. During a dry period with low runoff intensity (e.g., 2005–2007), the values of ebb partition ratio (i.e., ebb tidal force) for the south branch of Fujiangsha Waterway, the middle branch of Rugaosha Waterway, the east branch of Tongzhousha Waterway, and the east branch of Langshansha Waterway were large (Figs. 5(b), 5(c), 5(e), 5(g)). Hence, downstream transport of sediment tended to occur in these channels, resulting in erosion or reduced deposition in the channels (Table 2); meanwhile, the channel depo-centers were pushed downstream by the strong ebb tidal current (Table 3). Conversely, the values of ebb partition ratio for other waterway branches were small (Figs. 5(a), 5(d), 5(f), 5(h)), which promoted the relative strength of the flood tide in these channels, driving upstream sediment transport from downstream reaches into the channels, leading to deposition or reduced erosion (Table 2). The channel depo-centers were pushed upstream simultaneously, by the strong flood tidal current (Table 3). During flood periods of high runoff intensity (e.g., 2007
-2011 and 2011–2014), the opposite occurred (Fig. 5; Table 2; Table 3).
Trends in channel erosion-deposition and depo-center movement
The presence of dams caused decreases in duration days of discharges exceeding 50000 m
3/s and 60000 m
3/s and increases in duration days of discharges in the range of 10000–20000 m
3/s (Fig. 3). This resulted in decreasing trends in ebb partition ratios for the north region of Fujiangsha Waterway, the Liuhaisha branch of Rugaosha Waterway, the west branch of Tongzhousha Waterway, and the west branch of Langshansha Waterway, and increasing trends for the other waterway branches (Fig. 4). Accordingly, a branching channel with decreasing ebb partition ratio presented a declining trend, and vice versa (Fig. 6;
Chen et al., 2016). Meanwhile, depo-centers in declining branches tended to migrate upstream into the upper sub-reaches, whereas those in developing branches tended to move downstream into the lower sub-reaches, as supported by the regulating schemes of recent channel-regulation projects (
Wu et al., 2013;
Yang and Lin, 2013;
Ni et al., 2014).
Construction is currently underway for a cascade of large dams along the upper Yangtze, which will continue to flatten the intra-annual distribution of runoff discharge (
Duan et al., 2016), coupled with future effects of climate change (
Cao et al., 2011;
Sun et al., 2013;
Zeng et al., 2013;
Chai et al., 2019). Consequently, recent trends in ebb partition ratios, patterns of channel erosion-deposition, and depo-center movements in the near-estuary reach of the Yangtze are likely to persist well into the future.
Conclusions
The north region of Fujiangsha Waterway, the Liuhaisha branch of Rugaosha Waterway, the west branch of Tongzhousha Waterway, and the west branch of Langshansha Waterway in the near-estuary reach of the Yangtze River tend to experience increased deposition or reduced erosion in periods of low runoff intensity, and vice versa. The depo-centers in these channels have been found to move upstream and downstream under low and high runoff intensity scenarios, respectively. Meanwhile, the other waterway branches in the near-estuary reach experience opposite trends in erosion-deposition patterns, and depo-center movements with varying runoff intensity.
The mechanism behind the foregoing morphological changes relates to variations in ebb partition ratio in the branching channels as the flow hydrodynamics alters, owing partly to geographic features (raised nodes and connections among the branches) of the near-estuary reach. As runoff discharge increased, the ebb partition ratios in the north region of Fujiangsha Waterway, the Liuhaisha branch of Rugaosha Waterway, the west branch of Tongzhousha Waterway, and the west branch of Langshansha Waterway increased. Thus, sediment in these branching channels tended to be transported into downstream reaches by the ebb tidal current, resulting in erosion or reduced deposition, with the depo-centers pushed downstream. Ebb partition ratios in the other waterway branches decreased, with sediment in downstream reaches transported into the branches by the relatively stronger flood tidal current, leading to deposition or less erosion in the branches, and causing the depo-centers to migrate upstream. As runoff discharge fell, the opposite occurred.
The runoff-flattening effect of dams in the Yangtze Basin has greatly decreased the duration of flood discharges exceeding 50000 m3/s and 60000 m3/s, and increased that of the middle-low discharge between 10000 and 20000 m3/s. In turn, the values of ebb partition ratio in the north region of Fujiangsha Waterway, the Liuhaisha branch of Rugaosha Waterway, the west branch of Tongzhousha Waterway, and the west branch of Langshansha Waterway were significantly reduced. Therefore, these branching channels presented a decline in morphological trends, with their depo-centers tending to move upstream, thus locating in the upper sub-reaches. Dam-induced runoff flattening has enhanced ebb partition ratios in the other waterway branches, promoting morphological development and downstream migration of depo-centers into the lower sub-reaches of the branches. As a cascade of large dams continues to be constructed along the upper Yangtze and climate change is ongoing, current overall trends in the evolution of branching channels and migration of depo-centers are likely to continue into the future.
Although the current study has primarily focused on a local tide-affected braided reach of the Yangtze River, it may be instructive for other braided rivers experiencing similar hydrodynamic processes, given its investigation of the morphological evolution in an intermediate zone between fluvial and estuarine areas. A numerical model giving full consideration of water, sediment, and engineering projects, will be created in the near future to quantify the morphological evolution of this reach.
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