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Variations in downstream grain-sizes to interpret sediment transport in the middle-lower Yangtze River, China: A pre-study of Three-Gorges Dam
Geomorphology 113 (2009) 217–229
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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g e o m o r p h
Variations in downstream grain-sizes to interpret sediment transport in the middle-lower Yangtze River, China: A pre-study of Three-Gorges Dam Zhanqiao Wang a, Zhongyuan Chen b,⁎, Maotian Li b, Jing Chen a, Yiwen Zhao c a b c
Department of Geography, East China Normal University, Shanghai 200062, China State Key laboratory for Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China Department of Geography, University of Leeds, Leeds LS2 9JT, UK
a r t i c l e
i n f o
Article history: Accepted 30 January 2009 Available online 27 March 2009 Keywords: Grain size Hydrological parameter Erosive and accumulative riverbeds Damming effect 3-Gorges Dam Yangtze River
a b s t r a c t In 2000 and 2003 before the closure of 3-Gorges Dam, numerous sediment samples were taken from the middlelower Yangtze River channel to examine sediment transport processes and associated hydromorphological nature of the river. Analytical results show that the riverbeds consist mostly of medium to coarse sands and gravelly sands, and fine sand occurs locally, especially near the river coast. The results further indicate a downstream fining trend in riverbed sediment from Yichang to the river mouth, totaling about 1900 km long with 12 sediment zones (I-XII). These were identified as alternate coarse- and fine-grained sediment on the riverbed, although the zonation of I–III below Three-Gorges Dam site is weaker. The mode of sediment transport in the river is dominated by saltation (20–80%), followed by bed-load transport with 3–15%; transport by suspension is quite low. Grainsizes associated with hydrological parameters have greater values in the Jingjiang Reaches (from Yichang to Chenglingji; unit stream power: 5–18 N m− 1s− 1, boundary shear stress: 14 Nm− 2 and mean flow velocity: 2– 3.2 ms− 1), whereas the values obtained from Chenglingji downstream are considerably low (b 5 N m− 1s− 1, 1– 4 Nm− 2 and b 0.7–1.5 ms− 1). These values, when compared with on-site measured velocity of the ADP flow column, revealed the erosive riverbed sediment transport in the Jingjiang Reaches, and the accumulative riverbed transport downstream, from Wuhan to the river coast. Hydrological parameters together with distribution of grain-sizes indicate a coarsening riverbed in the Jingjiang river, largely because damming peaked since the last half-century. This corroborates the weakening sediment zonation in the Jingjiang Reaches, which is expected to extend further downstream towards the river coast in response to the potential impact of 3-Gorges Dam in the coming decades. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Study of riverbed sediment is a useful tool to verify the processform and environmental feedback from hierarchical natural and anthropogenic controls. Sediment formulates the riverbeds, and the grain sizes that characterize different river patterns, such as braided, meandering and anabranching, etc., result from various physical controls (Reading, 1978; Gordon et al., 2004; Harmar and Clifford, 2007). Grain sizes bridge regional hydrology and geomorphology to reflect sediment dynamical processes (Vanoni, 1975; Allen 1982; Qian and Wan, 2003). Downstream variation in riverbed grain sizes would serve as a fingerprint for a better understanding of the morphological evolution of the river channel relating to sediment transport, mitigation of the river channel, hazardous floods, and human impact, such as damming, dyking and sand-mining (William and Wolman, 1984; Dade, 2000; Surian, 2002; Eaton and Millar, 2004; Harmar and Clifford, 2007; Wang et al., 2007).
⁎ Corresponding author. E-mail address: [email protected] (Z. Wang). 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.03.009
The parameters of grain sizes, including Medium (Mz) and Sorting (SD), are traditionally useful to help examine the river-channel morphdynamical processes and geoengineerings, but the simulated hydro-parameters derived from grain sizes, such as shear stress, mean flow velocity, unit stream power, etc. have been considered more of practical significance (Allen, 1985; Qian and Wan, 2003; Gordon et al., 2004). Of note, this was confirmed by several other relevant studies in small-intermediate-scale river basins, where closer correlation exits among the simulated hydro-parameters and river morphology (cf. Schumm, 1977; Qian, 1987; Dade, 2000; Gordon et al., 2004; Harmar and Clifford, 2007). The applicability of these studies is not much relevant to the large-scale river system, like the mega-rivers of Asia, largely because of unconfined river boundaries (cf. Qian, 1987). Grain sizes can also denote the river channel morphodynamical equilibrium — the eternal topic while addressing the changes of extrinsic and intrinsic conditions in response to geomorphological and climatic fluctuations (Reading, 1978; Sam, 1987; Gupta, 2002). In most rivers, the presence of the riverbed sediment has proved as the longerterm (multiyearly) hydrodynamical facies, chiefly resulting from annual high-floods. For example, the monsoonal precipitation of Asia often peaks to 30,000–70,000 m3s− 1 in many large river
218 Z. Wang et al. / Geomorphology 113 (2009) 217–229 Fig. 1. Study area showing the locations of riverbed sampling transects (171) in the middle-lower Yangtze River, each with 3–5 samples taken from the North, Central (thalweg) and South river channels. Hydrological gauging stations used in the present study are marked. Also shown are A) the water surface gradient and riverbed topography (modified after Chen et al., 2007b); B) meandering river pattern of the middle Yangtze and C) anabranching river pattern of the lower Yangtze. I–XII denotes sediment zonation of the study area (discussed in text).
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catchments (Liebault and Piegay, 2001; Chen et al., 2007a). In this context, the riverbed distribution of grain sizes has to adapt itself to the corresponding river geometry and associated flow dynamics (Gupta, 2002). Process-form relates to hierarchical physical controls, including nature of rock, discharge, water surface and riverbed gradient. Using mono-control to interpret the process-form has proved problematic (Harmar and Clifford, 2007). The water surface gradient of basin scale determines the distribution trend in the downstream fineness in the riverbed sediment, and the riverbed topographic gradient of sub-basin scale determines the variations in the distribution of grain sizes in the along-river profile. This trend is also influenced by the local river morphology, e.g. coarser grains usually appear in a braided river channel, and finer grains occur in the meandering pattern. The recent impoundment, however, has inevitably modified the river hydrological regime. During the past few decades, artificial intervention has significantly altered the river hydomorphology no matter whether it is riverbed sediment or the sediment of the flood levees (O'connor and Grant, 2003; Gordon et al., 2004). The objectives of the present study aim to examine the downstream variation in riverbed sediment in relation to the uniqueness of river hydomorphology in the middle-lower Yangtze River channel (Fig. 1). In addition, this study attempts to test for the fitness of how the parameters derived from grain sizes are accommodated in sediment transport and associated process-form of middle-lower Yangtze River channel. This is particularly vital since the present study serves as a pre-survey of 3-Gorges Dam, being completed in 2009. A comparative study between pre- and post-dam conditions will reveal human intervention in the evolution of the river channel and the management of the integrated river basin. 1.1. Description of river channel morphology The Yangtze River originates in the Tibetan plateau and flows across its vast catchment of N106 km2 in the upper region, and enters the middle and lower reaches by Yichang (Fig. 1). The total length of the middle-lower Yangtze River channel is about 1900 km, primarily characterized by meandering and anabranching river patterns with a gentler topographic slope (Fig. 1A). The meandering river starts from the upstream Yichang (the exit of Three-Gorges Dam) to Hukou of the outlet of Poyang lake (about 955 km long; Fig. 1), and the anabranching extends from this geographic location to the mouth of the river (about 938 km long) (Chinese Academy of Science Beijing Institute of Geography, 1985). Most of the meandering river morphology in the middle Yangtze River occurs from Yichang to Dongting Lake; this course is known as ‘Jingjiang Reaches’ as indicated in the early Chinese documents (Lin, 1965, 1978) (Fig. 1). The river channel, especially the lower Jingjiang Reach (Fig. 1), remained remarkably unstable in the early of 20th century and its lateral migration extends from the northern to southern flood plain up to 20–30 km (Yang and Tang, 1998). Many meandering cut-offs are still left as polders and lakes that are still seen today (Fig. 1B). Geoengineering countermeasures, such as straightening the river course and diking along the river bank, were applied to some critical river sections to stabilize the river channel (Yang and Tang, 1998; Yangtze Water Conservancy Committee, 1999). Jingjiang Reaches are characterized by varying sizes of sandy bars that range from 1–10 km in length (Fig. 1). In the areas where these sandy bars exist, the channel becomes deeper and curvier, and in the absence of them the channel takes a straight course. The river channel of the middle Yangtze is about 1–2 km wide, and the water depth ranges generally from 20–40 m. Extensive flat topography occurs in the meandering fluvial pattern as represented by an overall water surface slope of 0.000033 (Chen et al., 2007a,b) (Fig. 1). The lower Yangtze River is termed an anabranching course because it contains a series of goose-head-like sandy islands (Fig. 1C).
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Wherever these appear, the river channel tends to be wider to some extent. In contrast, narrower river sections exist because of many individual rock exposures on the river sides, serving as ‘river-knot’ (Shi et al., 2007). Recently, to prevent river bank collapses from flood erosion, artificial river knots in the form of dykes were constructed along the curvy river belt (Tang, 1999). In the lower Yangtze River, the channel widens to 2–4 km and the water depth is about 30–40 m. At certain sections of the river where the rock knots exist the water depth can reach 60–90 m. The overall water surface slope of the lower Yangtze reach is 0.000008. 2. Method and materials In the April, 2000, a boat survey sampled riverbed sediment from the river mouth to Wuhan of the middle Yangtze basin, totaling about 1000 km (Fig. 1). About one hundred transects were selected with 10 km intervals to collect three samples, each from the North (left), Central (thalweg) and South (right) of the river channel in downstream direction. The total samples collected were 300. Sampling sites were positioned by GPS and the water depth was recorded by Echo Sounder mounted on the boat. Simultaneously sediment descriptions were made in the field. A similar survey was conducted in the May, 2003 from Wuhan to Yichang (800 km) which is located immediately below 3-Gorges Dam site. Two-hundreds and twenty (220) samples were collected from 71 transects with 10-km interval in the North, Central and South river channel. Three to five samples on average were collected from each transect. Two sets of samples were placed into plastic bags with consecutive numbering from 001 to 171 from Yichang extending to the river mouth area. Samples were tested to study the nature of grain sizes. The steps include: 1) finer-grain samples (b1 mm) were examined directly by the Laser Particle Size Analyzer (Ls-13320); 2) coarser-grain samples were sieved with 1-mm sieve, and then the sieve method applied to the coarser materials. In the latter case, the final results were integrated from both methods. Parameters of grain sizes (Mz, SD, SK, and KG) are chosen for the present study. The sediment component of sand (N63 µm), silt (4–63 µm) and clay (b4 µm) were calculated as percentage. In the present study, ordered sample cluster analysis, using Mz, SD, Sk, and KG without disordering sample number sequence, was used to identify each individual cluster, in relation to the nature of riverbed sediments and the zones of riverbed sediments in the different river patterns. Besides the percentage of sand, silt and clay, each sample is plotted against the North, Central and South along-river sampling profile. Cumulative and frequency distribution mode of grain sizes for the different zones of riverbed sediments are well marked along with the fluvial topography of the middle-lower Yangtze basin (Yangtze Water Conservancy Committee, 1999). From these modes of sediment transport patterns, namely bed-load, saltation and suspension, were defined. Another hydromorphological survey was conducted during 2002 from Yichang to the river mouth area by using ADP (Acoustic Doppler Profilers SONTEK-500). Data on flow velocity and the water depth were collected during the survey when flow discharge remained 25,000–40,000 m3s− 1. The water surface gradient of the study area was extracted from the previous river channel navigation map (1:25,000–1:80,000; unpublished data, archived in our project database, see inset in Fig. 1A). The above parameters were incorporated into the present grain sizes associated-hydrodynamical simulation, including unit stream power, boundary shear stress and flow velocity. On the basis of mean velocities of flow collected from numerous on-site measurements recorded at the 14 hydrological gauging stations of the middle Yangtze River, the mean velocities of flow were simulated for different discharges of 10,000–60,000 m3s− 1 (Yangtze Water Conservancy Committee, 1950–1980). These data enabled assessment of the sediment transport property in the river
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course immediately below 3-Gorges and Gezhou Dams under the high-flow season (Fig. 1). 2.1. Observation Analyses of five-hundred-twenty (520) samples reveal that a large extent of the medium to fine sands (Mz: 100–400 µm) constitute in large part the middle-lower Yangtze riverbed (Yichang to the river mouth area). Coarse sand (N500 µm) occurs locally and silty sand to fine sand spreads in the river mouth area. Occasionally, the riverbed is comprised of silty mud (b63 µm). Sand proportion (N63 µm) of most samples reaches as high as 70–90%, silt (63–4 µm) ranges from 4–50%, and clay (b4 µm) is less than 2–7% (Fig. 2). The distribution of
standard deviation (SD) reveals a general increase in value, ranging from 0.5 to 1.5 towards downstream of Yichang (Fig. 2). SD in the upper part of the river mouth area reduces before reaching a higher value (2–2.5) of the lower river mouth area. Skewness (SK) in alongriver distribution tends to be similar with that of SD, ranging from 0– 0.6. Increase in value is more prominent from Yichang to Wuhan than that from Wuhan downstream. Kurtosis (KG) in the along-river distribution shows a slightly lower value (~ 1–3) from Yichang to Wuhan, and remains relatively stable (2–3) from Wuhan downstream, before meeting the river mouth area where the value decreases (to 0.9). Parameter of grain sizes (Mz, SD, SK, KG) in cluster analysis helps define 12 (I–XII) sediment zones in the middle-lower Yangtze River
Fig. 2. Downstream distribution of riverbed grain-size parameters of the middle-lower Yangtze riverbed (Mz, S.D., SK, KG). The parameters are shown as the three (North, Central and South) along-river profiles.
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channel (Fig. 3). The overall spatial distribution of sediment zones highlights two major riverbeds, i.e. from Yichang to Chenglingji (~220 km upstream of Wuhan; zone I–III; Fig. 1) and from Chenglingji to the river mouth (zone IV–XII). The analysis indicates that the
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former is coarser than the latter, and shows a weaker ‘coarse-fine’ alternated riverbed; especially in the Central (thalweg) river channel (Fig. 3). This is in contrast with the riverbed sediment downstream, where zones of coarse-fine-grained riverbed sediments occur. Sand,
Fig. 3. Dendrogram of ordered cluster plots based on the parameters of grain-sizes(Mz, S.D., SK, KG). On this base, twelve (I–XII) sediment zones were defined. Sediment components (sand — N63 µm, silt — 63–4 µm and clay — b 4 µm) of the three along-river samples were denoted.
222 Z. Wang et al. / Geomorphology 113 (2009) 217–229 Fig. 4. Triangular plot of the three along-river profiles (North, Central and South) showing distribution of the components of the riverbed sediments. Based on different river morphology, each profile is divided into 3 sections, i.e. from Yichang–Wuhan (meandering river pattern — zone I–V), from Wuhan–Datong (anabranching river pattern — zone VI–VIII), and from Datong — the river mouth area (slightly anabranching river pattern — zone IX–XII).
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Fig. 5. Patterns of cumulative and frequency grain size curves (coarser and finer sediment grouped, respectively). Morphological feature of the middle-lower Yangtze River is shown. Inserted is the mode of sediment transport proportional of bedload, saltation and suspension.
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Fig. 6. Indicative hydro-parameters along the middle-lower Yangtze River channel (thalweg).
silt and clay proportions for 12 zones (I–XII) also demonstrate these riverbed characteristics (Fig. 3). The triangular plot of sand, silt and clay proportions of 520 samples from the middle-lower Yangtze River has revealed that all the samples change proportions along a linear trend from sand-end to 70%-siltlean-end (Fig. 4). This can be further differentiated from various sections of the river morphologies, i.e. meandering river pattern in the Jingjiang Reaches of the middle Yangtze basin (above Dongting Lake, Fig. 1), anabranching river pattern (Wuhan to Datong), and slightly anabranching to the river mouth area (below Datong). In the Jingjiang Reaches the coarsest sediments occur in the Central along-river profile, where sand reaches N80%, and silty sand remains lower. From Wuhan to Datong, sand lessens to 60%, and further decreases downstream to the river mouth area as silt increases to 70%. The proportion of sediment components of North and South along-river profiles is rather scattered, especially from Datong to the river mouth area, but all the samples retain the proportional change along the sand-end to 70%-silt-lean end. The mode of sediment transport in the middle-lower Yangtze River can be identified by the cumulative and frequency patterns of grain sizes (Fig. 5). In the Upper and Lower Jingjiang river channel bedload accounts for 10–15%, while saltation takes about 70–80% and suspension is quite lower. Bedload transport decreases to 5–10% downstream, i.e. below Wuhan, and 2–5% near the river mouth area (Fig. 5). This is in contrast with saltation and suspension transport, which increases to 40–70% and about 20–50% downstream, respectively. Sediment in the form of silt occurs occasionally in the riverbeds above Datong of the lower Yangtze River and also frequently from Datong to the river mouth area (Fig. 5). Unit stream power of the study area reveals that the riverbeds in the Jingjiang reaches have the highest value (5–18 N m− 1s− 1) (Fig. 6A). The value below this geographic location lowered to b5 N m− 1s− 1 downstream, in addition to the zone VI–VII (Fig. 1), where rock knots exist along the river bank (Inset C in Fig. 1). A similar trend was
observed for the boundary shear stress distribution, simulated by using river morphological variables (water depth and water surface gradient), i.e., higher value (to 14 Nm− 2) in the Jingjiang Reaches, and very lower value (1–4 Nm− 2) below Chenglingji, except at a few locations in the zone of IV–VII (3–7 Nm− 2) (Fig. 6B). In contrast, the water surface gradient-dependant unit stream power shows an opposite trend in its downstream spatial distribution to the thalweg water depth (Fig. 6B). Given the boundary shear stress, the mean flow velocity was then simulated while taking into account riverbed roughness (derived from the Central along-river samples): U 30y = 5:75log U4 ks
ð1Þ
rffiffiffiffiffi τo ρ
ð2Þ
τ0 = ρgDS
ð3Þ
U4 =
where y is the 0.4 water depth where the velocity is similar to the mean velocity; ks is the riverbed roughness. For the present purpose, Mz has been used as the value of ks; U⁎ is the shear velocity which can be derived from Eq. (2); τ0 is Boundary shear stress (Nm− 2); ρ is the flow density which equals to 1000 (kg/m3); D is hydrological depth represented by the water depth; and S is the gradient of water surface. As a result, the spatial distribution of simulated mean flow velocity also denotes the higher value (mostly 2–3.2 ms− 1) in the Jingjiang Reaches, while the value below Chenglingji ranges from 0.7–1.5 ms− 1 (Fig. 6C). While comparing with the on-site measured ADP flow velocity of 1–2 ms− 1 (water column velocity at about 25,000 m3s− 1 in the middle Yangtze and 40,000 m3s− 1 in the lower Yangtze), although a slight decrease occurs in the value downstream, it was observed: 1) the simulated mean flow velocity is much higher than that of ADP measured in the Jingjiang Reaches; 2) the simulated value below
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Fig. 7. The relationship between on-site measured discharge – water depth, and discharge – flow velocity (cross-sectional, collected from the 14 hydrological gauging stations in the middle Yangtze basin; refer to Fig. 1 for locations) (Data source: Yangtze Water Resource Committee, 1950–1980).
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Fig. 8. Correlation between measured flow velocity (cross-sectional) and simulated flow velocity under discharges of 10,000–60,000 m3s− 1 in the middle Yangtze River (Yichang–Wuhan). Solid line (Y =X) denotes critical flow velocity, and dot line is deduced from the assumption that thalweg flow velocity is 1.47 times of the measured one (see insert in the figure; data collected from Yangtze Water Resource Committee, 1950–1980).
Jingjiang Reaches decreases more or less equal to the ADP value; and 3) ADP flow velocity becomes slightly higher than that of simulated value below Datong of the lower Yangtze reach (Fig. 6C). Historical velocities of mean flows on the multiannual measurements (discharges from 10,000 to 60,000 m3s− 1) was established for 14 hydrological gauging stations that are located in the middle Yangtze River (Yangtze Water Conservancy Committee, 1950–1980), and all show an exponential trend with significant correlation (Fig. 7). Comparison of historical mean flow velocity (10,000–60,000 m3s− 1 discharges, i.e. six points) with simulated one (river-morphology and grain-size-dependant; referring equation 1) denotes that most values of six points at each station are below the critical line (Y = X) (I–V; Fig. 8). This indicates that most riverbed sediments are stable even during the high-flow season. It was assessed, however, that the historical velocity of mean flow was derived from transect one. i.e. averaged from several column velocities in one cross-section (Yangtze Water Conservancy Committee, 1950–1980). This is certainly unable to represent the thalweg flow velocity used in the simulation, in which grain size is considered from the Central along-river profile. Thus, it is assumed that somehow higher velocities of historical mean flows would have to be used when plotting against the grain-sizedependant one. On-site measured flow velocity in the middle Yangtze thalweg river channel suggests that it is 1.2–1.5 times higher than that of transect one (illustrated as dot line in Fig. 8). If so, the results indicate: 1) the riverbed sediments in the Upper Jingjiang Reach (Yichang to Xinchang — sediment zone I–II; Figs. 1 and 8A) would be movable as discharge peaks to 20,000–30,000 m3s− 1; 2) the riverbed sediment remains stable even during the higher flow season in the Lower Jingjiang Reach (Shishou to Chenglingji, sediment zone III; Figs. 1 and 8B); and 3) the riverbed sediment could be movable while discharge peaks to 30,000–40,000 m3s− 1 from Luoshan to Wuhan (sediment zone IV–V, Figs. 1 and 8C). 3. Discussion 3.1. Grain-size distribution in relation to sediment zonation, river morphology and human impact The downstream distribution of riverbed sediments is often used to reflect regional fluvial geomorphological and hydrological settings (Schumm, 1977; Allen, 1985; Xu, 1986; Dade, 2000; Qian and Wu, 2003). The distribution of riverbed sediments characterized by grain size parameters is closely linked to high-low flow sediment dynamics
that shape the unique river morphology (Knighton, 1980; Ichim and Radoane, 1990; Surian, 2002). Our analytical results reveal that the middle-lower Yangtze riverbed, which crosses about 1900 km from Yichang to the river mouth area (Fig. 1), is largely characterized by medium to coarse sands, mixed with pebbles and gravels, which was also witnessed while sampling (Fig. 2). Two generalities were observed in the study area: the riverbed sediment fines downstream and sediment zonation occurs (I–XII; Figs. 2–5). It can be assumed that these are typical sedimentological characteristics of any rivers in the world (Qian, 1987; Ichim and Radoane, 1990; Harmar and Clifford, 2007). The present data, however, have recorded a relatively coarser riverbed sediment in the Upper and Lower Jingjiang reaches (zone I–III, covering about 300 km below Gezhou and 3-Gorges dam sites; Fig. 1), where the mean grain sizes in the Central along-river profile is 200–300 µm (ca. 500 µm at locales), although somewhat finer sediments occur on the North and South sides of the river (150–300 µm) (Fig. 2). In other words, the zonation of the riverbed sediments in the Jingjiang Reaches is weaker than what is observed downstream (Figs. 2 and 3). Zonation of the riverbed sediments from the Jingjiang to the river mouth area becomes more prominent, marked as IV–XII. We reason that the coarser riverbed sediment is always associated with straight river channels where flow velocity remains greater. The finer riverbed sediment tends to associate with curvy river channel (Yang and Tang, 1998). The highly-curved meandering river belt developed in the Jingjiang Reaches in the past (especially in the Lower Jingjiang) comprises several breaches and cut-offs which were developed during the high-flow seasons (Chen et al., 2007a, Fig. 1B). As instability of the river channel threatens land properties, steps were taken to straighten the channel to a large extent since 1960–70s (Yang and Tang, 1998). As a consequence, discharge expelled in the downstream direction smoothened and flow velocity accelerated. Of note, construction of dams peaked during 1960–1980s (N10,000 reservoirs impounded in the upper Yangtze basin), including Gezhou dam (completed in 1978) and all sever as major sediment barriers (Yangtze Water Conservancy Committee, 1999). Reduction in sediment load (from ca. 470 to 100 Mt/a) since the last half century has directly triggered the riverbed erosion in the Jingjiang river channel immediately below the dams, as revealed by the weakening sediment zonation (Figs. 2–4). In contrast to that below Jingjiang Reaches, the riverbed sediment transits to finer and coarser alternated sediments (sediment zones IV–XII; Figs. 2 and 3), especially below Wuhan to the river mouth area
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where an anabranching pattern predominates (VI–XII). This is supported by the distributions of grain sizes in the North, Central, and South along-river profiles. The study reveals that the variability of riverbed sediments in the Central along-river channel (100–250 µm, in general) is lesser than that at both river-sides (50–250 µm), primarily because of the strong erosion along river channel thalweg. The spatial display of the regional river morphology can indicate a relation between sediment zonation and river pattern, i.e. the more anabranching, the finer sediment is. In other words, higher curvature in an anabranching river pattern results in finer sediments on the riverbed because of flowage of water (Leopold, 1994). Unidirectional flow current plays a critical role in controlling the proportional changes in sediment, i.e. downstream fining in the middle-lower Yangtze River. This is well demonstrated by a unique change along linear trend of sand-end to 70%-silt-lean-end as shown in Fig. 4. Further, the change is closely associated with fluvial morphologies, defined as meandering, anabranching and slightly anabranching-estuary patterns from the middle to lower Yangtze (Chen et al., 2007a). The river channel thalweg can hold coarser (pebbles and/or gravelly sands) materials, while both the river sides show the alternated zones of coarser and finer sediment zones because of lateral migration. 3.2. Riverbed sediment — a multi-yearly relict facies Actually, the present distribution of riverbed sediments in the middle-lower Yangtze channel shows a long-term riverbed facies, which corresponds largely to the multiannual sediment transport processes of the high-flow seasons (Qian, 1987; Gordon et al., 2004; Chen et al., 2007a). Floods that often peak to 40,000–70,000 m3s− 1 considerably shape the riverbed (Chen et al., 2001). This is well recognized by cumulative and grain-size frequency patterns of the sediments in the present study, represented by the Central along-river profile, in which 3 modes of sediment transport (bed-load, saltation and suspension) are well established (Fig. 5). Several earlier investigations indicated that the contemporary Yangtze River is dominated by fine-grained sediment, mostly silty fine sands (Chen et al., 2001, 2007a). Coarse materials (mostly gravely sands) on the riverbed result from erosion by huge floods which exposed the relict sediments derived form local catchments since the Last Glacial
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Maximum (Zhu, 2000). The presence of sediment transport as bedload (10–15%) and saltation (40–80%) represents the dominant dynamics for sediment transport in the middle-lower river channel (above the river mouth), where suspension is extremely low except in the river mouth area (N60%; Fig. 5). 3.3. Hydrodynamical parameters in relation to river channel erodibility Grain-sizes associated hydrodynamical parameters of the present study have illustrated the riverbed morphodynamical processes and river channel characteristics (Allen, 1985; Dade, 2000; Qian and Wan, 2003). Unit stream power, boundary shear stress and simulated flow velocity (grain-size and river-morphology dependant), together with ADP flow data and river channel map of the study area (1:25,000– 1:80,000; unpublished data, archived in our project database) have provided physical insights into the changes of the middle-lower Yangtze River channel. Unit stream power is defined as the time rate of potential energy expenditure per unit weight in alluvial channel. The present simulated value of unit stream power is characterized by lowering the value towards the river coast, showing reduced energy consumption along the riverbed. Of note, the higher values in the Upper and Lower Jingjiang Reaches (sediment zone II–III) are well correlated to the greater river water surface gradient of the region (Fig. 6A), implying stronger riverbed erodibility in the meandering river channel. Smaller scale variability of unit stream power below Chenglingji can be explained by the fluctuated topography of the riverbeds, owing to the local occurrence of bedrock, river knots and former dissected deeper river channels prevailing in the region (to 70–90 m in the water depth; Chen et al., 2007a; Shi et al., 2007) (Fig. 1). These parameters also influence the riverbed erodibility. Boundary shear stress denotes the frictional force which causes flow resistance along the channel boundary. Similar to unit stream power, the overall distribution of boundary shear stress of the middlelower Yangtze River can be also associated with the river water surface gradient (Fig. 6A and B). Higher values in the Jingjiang Reaches indicate stronger riverbed erodibility, while the lower values occur in relation to weakening riverbed erodibility downstream as the water surface gradient decreases, which opposes the control by the thalweg water depth (Fig. 6B).
Fig. 9. Conceptual model — weakening zonation of riverbed sediments in the Jingjiang Reaches (Yichang to Chenglingji, Fig. 1), represents an erosive transport of riverbed sediments in response to the effect of upstream dams, and progressively enhanced downstream zonation of sediments from Chenglingji, especially from Wuhan to the river mouth area, denoting an accumulative transport of riverbed sediments in the river channel. Mean flow velocity (V), used as the Y-axis, is derived from Fig. 6C.
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Simulated flow velocity (grain-sizes, water depth and water surface gradient-dependant; referring equation 1) suggests 2– 3.2 ms− 1 in the Jingjiang Reaches as it triggers the riverbed sediment transport as bedload. In contrast, on-site measured ADP column flow velocity under ca. 25,000 m3s− 1 while surveying in 2002, is much slower than that of the simulated one (Fig. 6C). These reflect seldom bedload transport during the non-flood season, but an extensive erosive transport of riverbed sediments during the high-flow season. The lowering trend of simulated flow velocity to nearer to ADP value below Chenglingji assumes movable riverbed sediment transport either during the non-flood or flood seasons (Fig. 6C). Some higher values of simulated flow velocity than ADP one below Wuhan, where rock knots appear, also means erosive sediment transport in the high-flow season. By contrast, ADP flow velocity even becomes somewhat higher than that of simulated one below Datong in the lower Yangtze (about 500 km before reaching the river coast), assuming stronger accumulative transport of riverbed sediments.
zonation of riverbed sediments will be extending further downstream to the river coast in the near future (next 50–100 ? years; Fig. 9). This process should dramatically modify the river-channel morphology of the middle-lower Yangtze basin, and thus, more emphasis should be placed on better integrated river management in the coming years. Acknowledgement The authors are grateful to many graduate students of Department of Geography, East China Normal University, who were dedicated to the fieldworks and laboratory tests. Prof. Bandaru Hema Malini and Prof. Jack Viteck kindly reviewed the manuscript with constructive comments. This project is funded by Creative Research Groups of China (No. 40721004), The Ministry of Science and Technology, China (SKLEC-Grant No. 2008KYYW02) China National Natural Science Foundation (Grant No, 40341009), US-National Geographical Society (Grant No. 6693-00), and APN (Grant No. ARCP2008-CMY-Chen).
3.4. Simulated sediment transport under high-flow discharge The simulated flow velocities (six points for the seasonal discharge from 10,000–60,000 m3s− 1; see insert in Fig. 8A-a) to a large extent are greater than that of on-site measured ones filed in the 14 hydrological gauging stations of the middle Yangtze reach (Yangtze Water Conservancy Committee, 1950–1980) (Figs. 1 and 8 and inserted Fig. 8A-a). In that case, it would suggest unmovable riverbed sediments even during the high-flow season (40,000– 50,000 m3s− 1, Fig. 8A–C). It is emphasized, however, that the on-site measured velocity of mean flow (cross-sectional) used here is somewhat lower, which is not able to match the thalweg flow velocity for moving the Central riverbed sediments (used here for simulation; referring Eqs. (1)–(3); Fig. 8). Given higher measured velocities of flow, it would turn out a lower critical value of the initial movement of the riverbed sediments (dashed line, derived from the ratio (1.47) between measured velocity of thalweg flow and crosssectional, seeing insert in Fig. 8A-b). This helps understand that the transport of riverbed sediments from Yichang to Xinchang (Upper Jingjiang Reach) requires at least 20,000–30,000 m3s− 1 of discharge, the starting flood season as recorded in the past (Fig. 8A) (Yangtze Water Conservancy Committee, 1999, 2001; Chen et al., 2001, 2007a). On the other hand, most of the values from Xinchang to Chenglingji (Lower Jingjiang Reach) lying below the dashed line indicate stable riverbed sediments either during the non-flood or flood season, largely due to water diversion into Dongting Lake through several inlets linked to the Yangtze trunk channel (Figs. 5 and 8B). From Chenglingji to Wuhan, simulated flow velocity Vs measured values suggest that 30,000–40,000 m3s− 1 discharge is required to trigger the movement of riverbed sediments (Fig. 8C), (Yangtze Water Conservancy Committee, 1999, 2001; Chen et al., 2001, 2007a). 4. Summary The present study has revealed that the riverbed sediments fine downstream from Yichang to the river mouth area of the middle-lower Yangtze River. A series of alternate sediment zones, i.e. coarse-finegrained (medium to coarse sand, and gravelly sand) largely reflect the nature of river channel. Such sediment zonation becomes weaker, however, in the Jingjiang Reaches (from Yichang to Chenglingji), primarily from the damming effect in the last half century. Assessment of a set of grain-sizes associated hydrological parameters, including unit stream power, boundary shear stress and flow velocity, reveals an extensive erosive and accumulative transport of riverbed sediments in the Jingjiang Reaches immediately below 3-Gorges valley, especially from Wuhan to the river coast. With the construction of dams peaking during the last 50 years and taking 3-Gorges reservoir into account (being completed in 2009), we propose that the weakening in the
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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g e o m o r p h
Variations in downstream grain-sizes to interpret sediment transport in the middle-lower Yangtze River, China: A pre-study of Three-Gorges Dam Zhanqiao Wang a, Zhongyuan Chen b,⁎, Maotian Li b, Jing Chen a, Yiwen Zhao c a b c
Department of Geography, East China Normal University, Shanghai 200062, China State Key laboratory for Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China Department of Geography, University of Leeds, Leeds LS2 9JT, UK
a r t i c l e
i n f o
Article history: Accepted 30 January 2009 Available online 27 March 2009 Keywords: Grain size Hydrological parameter Erosive and accumulative riverbeds Damming effect 3-Gorges Dam Yangtze River
a b s t r a c t In 2000 and 2003 before the closure of 3-Gorges Dam, numerous sediment samples were taken from the middlelower Yangtze River channel to examine sediment transport processes and associated hydromorphological nature of the river. Analytical results show that the riverbeds consist mostly of medium to coarse sands and gravelly sands, and fine sand occurs locally, especially near the river coast. The results further indicate a downstream fining trend in riverbed sediment from Yichang to the river mouth, totaling about 1900 km long with 12 sediment zones (I-XII). These were identified as alternate coarse- and fine-grained sediment on the riverbed, although the zonation of I–III below Three-Gorges Dam site is weaker. The mode of sediment transport in the river is dominated by saltation (20–80%), followed by bed-load transport with 3–15%; transport by suspension is quite low. Grainsizes associated with hydrological parameters have greater values in the Jingjiang Reaches (from Yichang to Chenglingji; unit stream power: 5–18 N m− 1s− 1, boundary shear stress: 14 Nm− 2 and mean flow velocity: 2– 3.2 ms− 1), whereas the values obtained from Chenglingji downstream are considerably low (b 5 N m− 1s− 1, 1– 4 Nm− 2 and b 0.7–1.5 ms− 1). These values, when compared with on-site measured velocity of the ADP flow column, revealed the erosive riverbed sediment transport in the Jingjiang Reaches, and the accumulative riverbed transport downstream, from Wuhan to the river coast. Hydrological parameters together with distribution of grain-sizes indicate a coarsening riverbed in the Jingjiang river, largely because damming peaked since the last half-century. This corroborates the weakening sediment zonation in the Jingjiang Reaches, which is expected to extend further downstream towards the river coast in response to the potential impact of 3-Gorges Dam in the coming decades. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Study of riverbed sediment is a useful tool to verify the processform and environmental feedback from hierarchical natural and anthropogenic controls. Sediment formulates the riverbeds, and the grain sizes that characterize different river patterns, such as braided, meandering and anabranching, etc., result from various physical controls (Reading, 1978; Gordon et al., 2004; Harmar and Clifford, 2007). Grain sizes bridge regional hydrology and geomorphology to reflect sediment dynamical processes (Vanoni, 1975; Allen 1982; Qian and Wan, 2003). Downstream variation in riverbed grain sizes would serve as a fingerprint for a better understanding of the morphological evolution of the river channel relating to sediment transport, mitigation of the river channel, hazardous floods, and human impact, such as damming, dyking and sand-mining (William and Wolman, 1984; Dade, 2000; Surian, 2002; Eaton and Millar, 2004; Harmar and Clifford, 2007; Wang et al., 2007).
⁎ Corresponding author. E-mail address: [email protected] (Z. Wang). 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.03.009
The parameters of grain sizes, including Medium (Mz) and Sorting (SD), are traditionally useful to help examine the river-channel morphdynamical processes and geoengineerings, but the simulated hydro-parameters derived from grain sizes, such as shear stress, mean flow velocity, unit stream power, etc. have been considered more of practical significance (Allen, 1985; Qian and Wan, 2003; Gordon et al., 2004). Of note, this was confirmed by several other relevant studies in small-intermediate-scale river basins, where closer correlation exits among the simulated hydro-parameters and river morphology (cf. Schumm, 1977; Qian, 1987; Dade, 2000; Gordon et al., 2004; Harmar and Clifford, 2007). The applicability of these studies is not much relevant to the large-scale river system, like the mega-rivers of Asia, largely because of unconfined river boundaries (cf. Qian, 1987). Grain sizes can also denote the river channel morphodynamical equilibrium — the eternal topic while addressing the changes of extrinsic and intrinsic conditions in response to geomorphological and climatic fluctuations (Reading, 1978; Sam, 1987; Gupta, 2002). In most rivers, the presence of the riverbed sediment has proved as the longerterm (multiyearly) hydrodynamical facies, chiefly resulting from annual high-floods. For example, the monsoonal precipitation of Asia often peaks to 30,000–70,000 m3s− 1 in many large river
218 Z. Wang et al. / Geomorphology 113 (2009) 217–229 Fig. 1. Study area showing the locations of riverbed sampling transects (171) in the middle-lower Yangtze River, each with 3–5 samples taken from the North, Central (thalweg) and South river channels. Hydrological gauging stations used in the present study are marked. Also shown are A) the water surface gradient and riverbed topography (modified after Chen et al., 2007b); B) meandering river pattern of the middle Yangtze and C) anabranching river pattern of the lower Yangtze. I–XII denotes sediment zonation of the study area (discussed in text).
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catchments (Liebault and Piegay, 2001; Chen et al., 2007a). In this context, the riverbed distribution of grain sizes has to adapt itself to the corresponding river geometry and associated flow dynamics (Gupta, 2002). Process-form relates to hierarchical physical controls, including nature of rock, discharge, water surface and riverbed gradient. Using mono-control to interpret the process-form has proved problematic (Harmar and Clifford, 2007). The water surface gradient of basin scale determines the distribution trend in the downstream fineness in the riverbed sediment, and the riverbed topographic gradient of sub-basin scale determines the variations in the distribution of grain sizes in the along-river profile. This trend is also influenced by the local river morphology, e.g. coarser grains usually appear in a braided river channel, and finer grains occur in the meandering pattern. The recent impoundment, however, has inevitably modified the river hydrological regime. During the past few decades, artificial intervention has significantly altered the river hydomorphology no matter whether it is riverbed sediment or the sediment of the flood levees (O'connor and Grant, 2003; Gordon et al., 2004). The objectives of the present study aim to examine the downstream variation in riverbed sediment in relation to the uniqueness of river hydomorphology in the middle-lower Yangtze River channel (Fig. 1). In addition, this study attempts to test for the fitness of how the parameters derived from grain sizes are accommodated in sediment transport and associated process-form of middle-lower Yangtze River channel. This is particularly vital since the present study serves as a pre-survey of 3-Gorges Dam, being completed in 2009. A comparative study between pre- and post-dam conditions will reveal human intervention in the evolution of the river channel and the management of the integrated river basin. 1.1. Description of river channel morphology The Yangtze River originates in the Tibetan plateau and flows across its vast catchment of N106 km2 in the upper region, and enters the middle and lower reaches by Yichang (Fig. 1). The total length of the middle-lower Yangtze River channel is about 1900 km, primarily characterized by meandering and anabranching river patterns with a gentler topographic slope (Fig. 1A). The meandering river starts from the upstream Yichang (the exit of Three-Gorges Dam) to Hukou of the outlet of Poyang lake (about 955 km long; Fig. 1), and the anabranching extends from this geographic location to the mouth of the river (about 938 km long) (Chinese Academy of Science Beijing Institute of Geography, 1985). Most of the meandering river morphology in the middle Yangtze River occurs from Yichang to Dongting Lake; this course is known as ‘Jingjiang Reaches’ as indicated in the early Chinese documents (Lin, 1965, 1978) (Fig. 1). The river channel, especially the lower Jingjiang Reach (Fig. 1), remained remarkably unstable in the early of 20th century and its lateral migration extends from the northern to southern flood plain up to 20–30 km (Yang and Tang, 1998). Many meandering cut-offs are still left as polders and lakes that are still seen today (Fig. 1B). Geoengineering countermeasures, such as straightening the river course and diking along the river bank, were applied to some critical river sections to stabilize the river channel (Yang and Tang, 1998; Yangtze Water Conservancy Committee, 1999). Jingjiang Reaches are characterized by varying sizes of sandy bars that range from 1–10 km in length (Fig. 1). In the areas where these sandy bars exist, the channel becomes deeper and curvier, and in the absence of them the channel takes a straight course. The river channel of the middle Yangtze is about 1–2 km wide, and the water depth ranges generally from 20–40 m. Extensive flat topography occurs in the meandering fluvial pattern as represented by an overall water surface slope of 0.000033 (Chen et al., 2007a,b) (Fig. 1). The lower Yangtze River is termed an anabranching course because it contains a series of goose-head-like sandy islands (Fig. 1C).
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Wherever these appear, the river channel tends to be wider to some extent. In contrast, narrower river sections exist because of many individual rock exposures on the river sides, serving as ‘river-knot’ (Shi et al., 2007). Recently, to prevent river bank collapses from flood erosion, artificial river knots in the form of dykes were constructed along the curvy river belt (Tang, 1999). In the lower Yangtze River, the channel widens to 2–4 km and the water depth is about 30–40 m. At certain sections of the river where the rock knots exist the water depth can reach 60–90 m. The overall water surface slope of the lower Yangtze reach is 0.000008. 2. Method and materials In the April, 2000, a boat survey sampled riverbed sediment from the river mouth to Wuhan of the middle Yangtze basin, totaling about 1000 km (Fig. 1). About one hundred transects were selected with 10 km intervals to collect three samples, each from the North (left), Central (thalweg) and South (right) of the river channel in downstream direction. The total samples collected were 300. Sampling sites were positioned by GPS and the water depth was recorded by Echo Sounder mounted on the boat. Simultaneously sediment descriptions were made in the field. A similar survey was conducted in the May, 2003 from Wuhan to Yichang (800 km) which is located immediately below 3-Gorges Dam site. Two-hundreds and twenty (220) samples were collected from 71 transects with 10-km interval in the North, Central and South river channel. Three to five samples on average were collected from each transect. Two sets of samples were placed into plastic bags with consecutive numbering from 001 to 171 from Yichang extending to the river mouth area. Samples were tested to study the nature of grain sizes. The steps include: 1) finer-grain samples (b1 mm) were examined directly by the Laser Particle Size Analyzer (Ls-13320); 2) coarser-grain samples were sieved with 1-mm sieve, and then the sieve method applied to the coarser materials. In the latter case, the final results were integrated from both methods. Parameters of grain sizes (Mz, SD, SK, and KG) are chosen for the present study. The sediment component of sand (N63 µm), silt (4–63 µm) and clay (b4 µm) were calculated as percentage. In the present study, ordered sample cluster analysis, using Mz, SD, Sk, and KG without disordering sample number sequence, was used to identify each individual cluster, in relation to the nature of riverbed sediments and the zones of riverbed sediments in the different river patterns. Besides the percentage of sand, silt and clay, each sample is plotted against the North, Central and South along-river sampling profile. Cumulative and frequency distribution mode of grain sizes for the different zones of riverbed sediments are well marked along with the fluvial topography of the middle-lower Yangtze basin (Yangtze Water Conservancy Committee, 1999). From these modes of sediment transport patterns, namely bed-load, saltation and suspension, were defined. Another hydromorphological survey was conducted during 2002 from Yichang to the river mouth area by using ADP (Acoustic Doppler Profilers SONTEK-500). Data on flow velocity and the water depth were collected during the survey when flow discharge remained 25,000–40,000 m3s− 1. The water surface gradient of the study area was extracted from the previous river channel navigation map (1:25,000–1:80,000; unpublished data, archived in our project database, see inset in Fig. 1A). The above parameters were incorporated into the present grain sizes associated-hydrodynamical simulation, including unit stream power, boundary shear stress and flow velocity. On the basis of mean velocities of flow collected from numerous on-site measurements recorded at the 14 hydrological gauging stations of the middle Yangtze River, the mean velocities of flow were simulated for different discharges of 10,000–60,000 m3s− 1 (Yangtze Water Conservancy Committee, 1950–1980). These data enabled assessment of the sediment transport property in the river
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course immediately below 3-Gorges and Gezhou Dams under the high-flow season (Fig. 1). 2.1. Observation Analyses of five-hundred-twenty (520) samples reveal that a large extent of the medium to fine sands (Mz: 100–400 µm) constitute in large part the middle-lower Yangtze riverbed (Yichang to the river mouth area). Coarse sand (N500 µm) occurs locally and silty sand to fine sand spreads in the river mouth area. Occasionally, the riverbed is comprised of silty mud (b63 µm). Sand proportion (N63 µm) of most samples reaches as high as 70–90%, silt (63–4 µm) ranges from 4–50%, and clay (b4 µm) is less than 2–7% (Fig. 2). The distribution of
standard deviation (SD) reveals a general increase in value, ranging from 0.5 to 1.5 towards downstream of Yichang (Fig. 2). SD in the upper part of the river mouth area reduces before reaching a higher value (2–2.5) of the lower river mouth area. Skewness (SK) in alongriver distribution tends to be similar with that of SD, ranging from 0– 0.6. Increase in value is more prominent from Yichang to Wuhan than that from Wuhan downstream. Kurtosis (KG) in the along-river distribution shows a slightly lower value (~ 1–3) from Yichang to Wuhan, and remains relatively stable (2–3) from Wuhan downstream, before meeting the river mouth area where the value decreases (to 0.9). Parameter of grain sizes (Mz, SD, SK, KG) in cluster analysis helps define 12 (I–XII) sediment zones in the middle-lower Yangtze River
Fig. 2. Downstream distribution of riverbed grain-size parameters of the middle-lower Yangtze riverbed (Mz, S.D., SK, KG). The parameters are shown as the three (North, Central and South) along-river profiles.
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channel (Fig. 3). The overall spatial distribution of sediment zones highlights two major riverbeds, i.e. from Yichang to Chenglingji (~220 km upstream of Wuhan; zone I–III; Fig. 1) and from Chenglingji to the river mouth (zone IV–XII). The analysis indicates that the
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former is coarser than the latter, and shows a weaker ‘coarse-fine’ alternated riverbed; especially in the Central (thalweg) river channel (Fig. 3). This is in contrast with the riverbed sediment downstream, where zones of coarse-fine-grained riverbed sediments occur. Sand,
Fig. 3. Dendrogram of ordered cluster plots based on the parameters of grain-sizes(Mz, S.D., SK, KG). On this base, twelve (I–XII) sediment zones were defined. Sediment components (sand — N63 µm, silt — 63–4 µm and clay — b 4 µm) of the three along-river samples were denoted.
222 Z. Wang et al. / Geomorphology 113 (2009) 217–229 Fig. 4. Triangular plot of the three along-river profiles (North, Central and South) showing distribution of the components of the riverbed sediments. Based on different river morphology, each profile is divided into 3 sections, i.e. from Yichang–Wuhan (meandering river pattern — zone I–V), from Wuhan–Datong (anabranching river pattern — zone VI–VIII), and from Datong — the river mouth area (slightly anabranching river pattern — zone IX–XII).
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Fig. 5. Patterns of cumulative and frequency grain size curves (coarser and finer sediment grouped, respectively). Morphological feature of the middle-lower Yangtze River is shown. Inserted is the mode of sediment transport proportional of bedload, saltation and suspension.
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Fig. 6. Indicative hydro-parameters along the middle-lower Yangtze River channel (thalweg).
silt and clay proportions for 12 zones (I–XII) also demonstrate these riverbed characteristics (Fig. 3). The triangular plot of sand, silt and clay proportions of 520 samples from the middle-lower Yangtze River has revealed that all the samples change proportions along a linear trend from sand-end to 70%-siltlean-end (Fig. 4). This can be further differentiated from various sections of the river morphologies, i.e. meandering river pattern in the Jingjiang Reaches of the middle Yangtze basin (above Dongting Lake, Fig. 1), anabranching river pattern (Wuhan to Datong), and slightly anabranching to the river mouth area (below Datong). In the Jingjiang Reaches the coarsest sediments occur in the Central along-river profile, where sand reaches N80%, and silty sand remains lower. From Wuhan to Datong, sand lessens to 60%, and further decreases downstream to the river mouth area as silt increases to 70%. The proportion of sediment components of North and South along-river profiles is rather scattered, especially from Datong to the river mouth area, but all the samples retain the proportional change along the sand-end to 70%-silt-lean end. The mode of sediment transport in the middle-lower Yangtze River can be identified by the cumulative and frequency patterns of grain sizes (Fig. 5). In the Upper and Lower Jingjiang river channel bedload accounts for 10–15%, while saltation takes about 70–80% and suspension is quite lower. Bedload transport decreases to 5–10% downstream, i.e. below Wuhan, and 2–5% near the river mouth area (Fig. 5). This is in contrast with saltation and suspension transport, which increases to 40–70% and about 20–50% downstream, respectively. Sediment in the form of silt occurs occasionally in the riverbeds above Datong of the lower Yangtze River and also frequently from Datong to the river mouth area (Fig. 5). Unit stream power of the study area reveals that the riverbeds in the Jingjiang reaches have the highest value (5–18 N m− 1s− 1) (Fig. 6A). The value below this geographic location lowered to b5 N m− 1s− 1 downstream, in addition to the zone VI–VII (Fig. 1), where rock knots exist along the river bank (Inset C in Fig. 1). A similar trend was
observed for the boundary shear stress distribution, simulated by using river morphological variables (water depth and water surface gradient), i.e., higher value (to 14 Nm− 2) in the Jingjiang Reaches, and very lower value (1–4 Nm− 2) below Chenglingji, except at a few locations in the zone of IV–VII (3–7 Nm− 2) (Fig. 6B). In contrast, the water surface gradient-dependant unit stream power shows an opposite trend in its downstream spatial distribution to the thalweg water depth (Fig. 6B). Given the boundary shear stress, the mean flow velocity was then simulated while taking into account riverbed roughness (derived from the Central along-river samples): U 30y = 5:75log U4 ks
ð1Þ
rffiffiffiffiffi τo ρ
ð2Þ
τ0 = ρgDS
ð3Þ
U4 =
where y is the 0.4 water depth where the velocity is similar to the mean velocity; ks is the riverbed roughness. For the present purpose, Mz has been used as the value of ks; U⁎ is the shear velocity which can be derived from Eq. (2); τ0 is Boundary shear stress (Nm− 2); ρ is the flow density which equals to 1000 (kg/m3); D is hydrological depth represented by the water depth; and S is the gradient of water surface. As a result, the spatial distribution of simulated mean flow velocity also denotes the higher value (mostly 2–3.2 ms− 1) in the Jingjiang Reaches, while the value below Chenglingji ranges from 0.7–1.5 ms− 1 (Fig. 6C). While comparing with the on-site measured ADP flow velocity of 1–2 ms− 1 (water column velocity at about 25,000 m3s− 1 in the middle Yangtze and 40,000 m3s− 1 in the lower Yangtze), although a slight decrease occurs in the value downstream, it was observed: 1) the simulated mean flow velocity is much higher than that of ADP measured in the Jingjiang Reaches; 2) the simulated value below
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Fig. 7. The relationship between on-site measured discharge – water depth, and discharge – flow velocity (cross-sectional, collected from the 14 hydrological gauging stations in the middle Yangtze basin; refer to Fig. 1 for locations) (Data source: Yangtze Water Resource Committee, 1950–1980).
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Fig. 8. Correlation between measured flow velocity (cross-sectional) and simulated flow velocity under discharges of 10,000–60,000 m3s− 1 in the middle Yangtze River (Yichang–Wuhan). Solid line (Y =X) denotes critical flow velocity, and dot line is deduced from the assumption that thalweg flow velocity is 1.47 times of the measured one (see insert in the figure; data collected from Yangtze Water Resource Committee, 1950–1980).
Jingjiang Reaches decreases more or less equal to the ADP value; and 3) ADP flow velocity becomes slightly higher than that of simulated value below Datong of the lower Yangtze reach (Fig. 6C). Historical velocities of mean flows on the multiannual measurements (discharges from 10,000 to 60,000 m3s− 1) was established for 14 hydrological gauging stations that are located in the middle Yangtze River (Yangtze Water Conservancy Committee, 1950–1980), and all show an exponential trend with significant correlation (Fig. 7). Comparison of historical mean flow velocity (10,000–60,000 m3s− 1 discharges, i.e. six points) with simulated one (river-morphology and grain-size-dependant; referring equation 1) denotes that most values of six points at each station are below the critical line (Y = X) (I–V; Fig. 8). This indicates that most riverbed sediments are stable even during the high-flow season. It was assessed, however, that the historical velocity of mean flow was derived from transect one. i.e. averaged from several column velocities in one cross-section (Yangtze Water Conservancy Committee, 1950–1980). This is certainly unable to represent the thalweg flow velocity used in the simulation, in which grain size is considered from the Central along-river profile. Thus, it is assumed that somehow higher velocities of historical mean flows would have to be used when plotting against the grain-sizedependant one. On-site measured flow velocity in the middle Yangtze thalweg river channel suggests that it is 1.2–1.5 times higher than that of transect one (illustrated as dot line in Fig. 8). If so, the results indicate: 1) the riverbed sediments in the Upper Jingjiang Reach (Yichang to Xinchang — sediment zone I–II; Figs. 1 and 8A) would be movable as discharge peaks to 20,000–30,000 m3s− 1; 2) the riverbed sediment remains stable even during the higher flow season in the Lower Jingjiang Reach (Shishou to Chenglingji, sediment zone III; Figs. 1 and 8B); and 3) the riverbed sediment could be movable while discharge peaks to 30,000–40,000 m3s− 1 from Luoshan to Wuhan (sediment zone IV–V, Figs. 1 and 8C). 3. Discussion 3.1. Grain-size distribution in relation to sediment zonation, river morphology and human impact The downstream distribution of riverbed sediments is often used to reflect regional fluvial geomorphological and hydrological settings (Schumm, 1977; Allen, 1985; Xu, 1986; Dade, 2000; Qian and Wu, 2003). The distribution of riverbed sediments characterized by grain size parameters is closely linked to high-low flow sediment dynamics
that shape the unique river morphology (Knighton, 1980; Ichim and Radoane, 1990; Surian, 2002). Our analytical results reveal that the middle-lower Yangtze riverbed, which crosses about 1900 km from Yichang to the river mouth area (Fig. 1), is largely characterized by medium to coarse sands, mixed with pebbles and gravels, which was also witnessed while sampling (Fig. 2). Two generalities were observed in the study area: the riverbed sediment fines downstream and sediment zonation occurs (I–XII; Figs. 2–5). It can be assumed that these are typical sedimentological characteristics of any rivers in the world (Qian, 1987; Ichim and Radoane, 1990; Harmar and Clifford, 2007). The present data, however, have recorded a relatively coarser riverbed sediment in the Upper and Lower Jingjiang reaches (zone I–III, covering about 300 km below Gezhou and 3-Gorges dam sites; Fig. 1), where the mean grain sizes in the Central along-river profile is 200–300 µm (ca. 500 µm at locales), although somewhat finer sediments occur on the North and South sides of the river (150–300 µm) (Fig. 2). In other words, the zonation of the riverbed sediments in the Jingjiang Reaches is weaker than what is observed downstream (Figs. 2 and 3). Zonation of the riverbed sediments from the Jingjiang to the river mouth area becomes more prominent, marked as IV–XII. We reason that the coarser riverbed sediment is always associated with straight river channels where flow velocity remains greater. The finer riverbed sediment tends to associate with curvy river channel (Yang and Tang, 1998). The highly-curved meandering river belt developed in the Jingjiang Reaches in the past (especially in the Lower Jingjiang) comprises several breaches and cut-offs which were developed during the high-flow seasons (Chen et al., 2007a, Fig. 1B). As instability of the river channel threatens land properties, steps were taken to straighten the channel to a large extent since 1960–70s (Yang and Tang, 1998). As a consequence, discharge expelled in the downstream direction smoothened and flow velocity accelerated. Of note, construction of dams peaked during 1960–1980s (N10,000 reservoirs impounded in the upper Yangtze basin), including Gezhou dam (completed in 1978) and all sever as major sediment barriers (Yangtze Water Conservancy Committee, 1999). Reduction in sediment load (from ca. 470 to 100 Mt/a) since the last half century has directly triggered the riverbed erosion in the Jingjiang river channel immediately below the dams, as revealed by the weakening sediment zonation (Figs. 2–4). In contrast to that below Jingjiang Reaches, the riverbed sediment transits to finer and coarser alternated sediments (sediment zones IV–XII; Figs. 2 and 3), especially below Wuhan to the river mouth area
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where an anabranching pattern predominates (VI–XII). This is supported by the distributions of grain sizes in the North, Central, and South along-river profiles. The study reveals that the variability of riverbed sediments in the Central along-river channel (100–250 µm, in general) is lesser than that at both river-sides (50–250 µm), primarily because of the strong erosion along river channel thalweg. The spatial display of the regional river morphology can indicate a relation between sediment zonation and river pattern, i.e. the more anabranching, the finer sediment is. In other words, higher curvature in an anabranching river pattern results in finer sediments on the riverbed because of flowage of water (Leopold, 1994). Unidirectional flow current plays a critical role in controlling the proportional changes in sediment, i.e. downstream fining in the middle-lower Yangtze River. This is well demonstrated by a unique change along linear trend of sand-end to 70%-silt-lean-end as shown in Fig. 4. Further, the change is closely associated with fluvial morphologies, defined as meandering, anabranching and slightly anabranching-estuary patterns from the middle to lower Yangtze (Chen et al., 2007a). The river channel thalweg can hold coarser (pebbles and/or gravelly sands) materials, while both the river sides show the alternated zones of coarser and finer sediment zones because of lateral migration. 3.2. Riverbed sediment — a multi-yearly relict facies Actually, the present distribution of riverbed sediments in the middle-lower Yangtze channel shows a long-term riverbed facies, which corresponds largely to the multiannual sediment transport processes of the high-flow seasons (Qian, 1987; Gordon et al., 2004; Chen et al., 2007a). Floods that often peak to 40,000–70,000 m3s− 1 considerably shape the riverbed (Chen et al., 2001). This is well recognized by cumulative and grain-size frequency patterns of the sediments in the present study, represented by the Central along-river profile, in which 3 modes of sediment transport (bed-load, saltation and suspension) are well established (Fig. 5). Several earlier investigations indicated that the contemporary Yangtze River is dominated by fine-grained sediment, mostly silty fine sands (Chen et al., 2001, 2007a). Coarse materials (mostly gravely sands) on the riverbed result from erosion by huge floods which exposed the relict sediments derived form local catchments since the Last Glacial
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Maximum (Zhu, 2000). The presence of sediment transport as bedload (10–15%) and saltation (40–80%) represents the dominant dynamics for sediment transport in the middle-lower river channel (above the river mouth), where suspension is extremely low except in the river mouth area (N60%; Fig. 5). 3.3. Hydrodynamical parameters in relation to river channel erodibility Grain-sizes associated hydrodynamical parameters of the present study have illustrated the riverbed morphodynamical processes and river channel characteristics (Allen, 1985; Dade, 2000; Qian and Wan, 2003). Unit stream power, boundary shear stress and simulated flow velocity (grain-size and river-morphology dependant), together with ADP flow data and river channel map of the study area (1:25,000– 1:80,000; unpublished data, archived in our project database) have provided physical insights into the changes of the middle-lower Yangtze River channel. Unit stream power is defined as the time rate of potential energy expenditure per unit weight in alluvial channel. The present simulated value of unit stream power is characterized by lowering the value towards the river coast, showing reduced energy consumption along the riverbed. Of note, the higher values in the Upper and Lower Jingjiang Reaches (sediment zone II–III) are well correlated to the greater river water surface gradient of the region (Fig. 6A), implying stronger riverbed erodibility in the meandering river channel. Smaller scale variability of unit stream power below Chenglingji can be explained by the fluctuated topography of the riverbeds, owing to the local occurrence of bedrock, river knots and former dissected deeper river channels prevailing in the region (to 70–90 m in the water depth; Chen et al., 2007a; Shi et al., 2007) (Fig. 1). These parameters also influence the riverbed erodibility. Boundary shear stress denotes the frictional force which causes flow resistance along the channel boundary. Similar to unit stream power, the overall distribution of boundary shear stress of the middlelower Yangtze River can be also associated with the river water surface gradient (Fig. 6A and B). Higher values in the Jingjiang Reaches indicate stronger riverbed erodibility, while the lower values occur in relation to weakening riverbed erodibility downstream as the water surface gradient decreases, which opposes the control by the thalweg water depth (Fig. 6B).
Fig. 9. Conceptual model — weakening zonation of riverbed sediments in the Jingjiang Reaches (Yichang to Chenglingji, Fig. 1), represents an erosive transport of riverbed sediments in response to the effect of upstream dams, and progressively enhanced downstream zonation of sediments from Chenglingji, especially from Wuhan to the river mouth area, denoting an accumulative transport of riverbed sediments in the river channel. Mean flow velocity (V), used as the Y-axis, is derived from Fig. 6C.
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Simulated flow velocity (grain-sizes, water depth and water surface gradient-dependant; referring equation 1) suggests 2– 3.2 ms− 1 in the Jingjiang Reaches as it triggers the riverbed sediment transport as bedload. In contrast, on-site measured ADP column flow velocity under ca. 25,000 m3s− 1 while surveying in 2002, is much slower than that of the simulated one (Fig. 6C). These reflect seldom bedload transport during the non-flood season, but an extensive erosive transport of riverbed sediments during the high-flow season. The lowering trend of simulated flow velocity to nearer to ADP value below Chenglingji assumes movable riverbed sediment transport either during the non-flood or flood seasons (Fig. 6C). Some higher values of simulated flow velocity than ADP one below Wuhan, where rock knots appear, also means erosive sediment transport in the high-flow season. By contrast, ADP flow velocity even becomes somewhat higher than that of simulated one below Datong in the lower Yangtze (about 500 km before reaching the river coast), assuming stronger accumulative transport of riverbed sediments.
zonation of riverbed sediments will be extending further downstream to the river coast in the near future (next 50–100 ? years; Fig. 9). This process should dramatically modify the river-channel morphology of the middle-lower Yangtze basin, and thus, more emphasis should be placed on better integrated river management in the coming years. Acknowledgement The authors are grateful to many graduate students of Department of Geography, East China Normal University, who were dedicated to the fieldworks and laboratory tests. Prof. Bandaru Hema Malini and Prof. Jack Viteck kindly reviewed the manuscript with constructive comments. This project is funded by Creative Research Groups of China (No. 40721004), The Ministry of Science and Technology, China (SKLEC-Grant No. 2008KYYW02) China National Natural Science Foundation (Grant No, 40341009), US-National Geographical Society (Grant No. 6693-00), and APN (Grant No. ARCP2008-CMY-Chen).
3.4. Simulated sediment transport under high-flow discharge The simulated flow velocities (six points for the seasonal discharge from 10,000–60,000 m3s− 1; see insert in Fig. 8A-a) to a large extent are greater than that of on-site measured ones filed in the 14 hydrological gauging stations of the middle Yangtze reach (Yangtze Water Conservancy Committee, 1950–1980) (Figs. 1 and 8 and inserted Fig. 8A-a). In that case, it would suggest unmovable riverbed sediments even during the high-flow season (40,000– 50,000 m3s− 1, Fig. 8A–C). It is emphasized, however, that the on-site measured velocity of mean flow (cross-sectional) used here is somewhat lower, which is not able to match the thalweg flow velocity for moving the Central riverbed sediments (used here for simulation; referring Eqs. (1)–(3); Fig. 8). Given higher measured velocities of flow, it would turn out a lower critical value of the initial movement of the riverbed sediments (dashed line, derived from the ratio (1.47) between measured velocity of thalweg flow and crosssectional, seeing insert in Fig. 8A-b). This helps understand that the transport of riverbed sediments from Yichang to Xinchang (Upper Jingjiang Reach) requires at least 20,000–30,000 m3s− 1 of discharge, the starting flood season as recorded in the past (Fig. 8A) (Yangtze Water Conservancy Committee, 1999, 2001; Chen et al., 2001, 2007a). On the other hand, most of the values from Xinchang to Chenglingji (Lower Jingjiang Reach) lying below the dashed line indicate stable riverbed sediments either during the non-flood or flood season, largely due to water diversion into Dongting Lake through several inlets linked to the Yangtze trunk channel (Figs. 5 and 8B). From Chenglingji to Wuhan, simulated flow velocity Vs measured values suggest that 30,000–40,000 m3s− 1 discharge is required to trigger the movement of riverbed sediments (Fig. 8C), (Yangtze Water Conservancy Committee, 1999, 2001; Chen et al., 2001, 2007a). 4. Summary The present study has revealed that the riverbed sediments fine downstream from Yichang to the river mouth area of the middle-lower Yangtze River. A series of alternate sediment zones, i.e. coarse-finegrained (medium to coarse sand, and gravelly sand) largely reflect the nature of river channel. Such sediment zonation becomes weaker, however, in the Jingjiang Reaches (from Yichang to Chenglingji), primarily from the damming effect in the last half century. Assessment of a set of grain-sizes associated hydrological parameters, including unit stream power, boundary shear stress and flow velocity, reveals an extensive erosive and accumulative transport of riverbed sediments in the Jingjiang Reaches immediately below 3-Gorges valley, especially from Wuhan to the river coast. With the construction of dams peaking during the last 50 years and taking 3-Gorges reservoir into account (being completed in 2009), we propose that the weakening in the
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