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Assessing the potential for change in the middle Yangtze River channel following impoundment of the Three Gorges Dam
Geomorphology 147–148 (2012) 27–34
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Assessing the potential for change in the middle Yangtze River channel following impoundment of the Three Gorges Dam Wenhao Yuan a, b, Daowei Yin a, Brian Finlayson c,⁎, Zhongyuan Chen a a b c
State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China Department of Geography, East China Normal University, Shanghai 200062, China Department of Resource Management and Geography, The University of Melbourne, Victoria 3010, Australia
a r t i c l e
i n f o
Article history: Received 21 June 2010 Received in revised form 7 June 2011 Accepted 14 June 2011 Available online 26 August 2011 Keywords: Yangtze River Three Gorges Dam Channel change Downstream impacts of dams
a b s t r a c t The geomorphic impacts of dams on downstream river channels are complex, not readily predictable for specific cases, but widely reported in the literature. For the Three Gorges Dam on the Yangtze (Changjiang) River in China, no studies of the impact of the changed flow and sediment conditions below the dam on the behaviour of the channel were included in the pre-dam feasibility report. We have assembled a database of flow and sediment data for the middle Yangtze River from Yichang to Hankou and used this to analyse changes following the closure of the dam. While total flow is little affected, the operating strategy for the dam that provides for storage of part of the summer high flows to maintain hydroelectric power generation in winter (the low flow season) is reflected in changes to the seasonal distribution of flow below the dam. We calculated potential sediment carrying capacity and compared it with measured sediment concentrations for both pre- and post-dam conditions. While channel sedimentation is indicated along the middle Yangtze for pre-dam conditions, scour is indicated for post-dam conditions, highest at Yichang immediately below the dam and decreasing downstream. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The dams constructed globally on rivers in the twentieth century have generated significant hydrologic, ecological, and geomorphic adjustments at both reach and watershed scales (Magilligan et al., 2008). Although dams provide important services such as flood protection and power generation, the environmental costs of these structures are becoming apparent. The impoundment of rivers by dams and reservoirs represents a significant disruption of geomorphologic processes (Phillips, 2003). Dams typically modify the stream hydrology and sediment dynamics, which cause adjustments in the downstream channel system (Petts, 1979; Andrews, 1986; Everitt, 1993; Topping et al., 2000). The Three Gorges Dam does not greatly modify stream flow but the loss of sediment from the channel system has a substantial impact on bed mobility and the geomorphic and biotic complexity of the river, as has been shown elsewhere by, for example, Pohl (2004). Work on the downstream impacts on large river systems of multipurpose dams (Graf, 1980; Andrews, 1991; Kearsley et al., 1994; Topping et al., 2000; Li et al., 2009) indicate that downstream geomorphic responses of stream channels to dams are complex and variable and difficult to predict, mainly because the effects of local geological conditions and operational details confound and
⁎ Corresponding author. E-mail address: [email protected] (B. Finlayson). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.06.039
complicate efforts to apply models and generalisations to individual streams (Phillips, 2003). The channel immediately downstream from a dam will normally undergo bed degradation and bank erosion due to the clear water released from the dam. However, aggrading reaches or relatively stable reaches are found in some rivers with impoundments because of a sufficient sediment supply or armouring of the bed (Phillips, 2003; Gilvear, 2004). The geomorphic changes that occur downstream of dams cannot be predicted or explained without accounting for the local and regional geologic setting, geomorphic history and controls, hydrologic regime, land use and vegetation, dam/reservoir operation, time since dam construction, and other variables (Friedman et al., 1998). This difficulty in generalising the downstream response to dams has led to the development of alternative approaches such as a synoptic typology of geomorphic responses (Brandt, 2000). Brandt identified two key determinants of the qualitative geomorphic response downstream of dams: changes in flow regime, and the relationship between sediment supplied to the stream and the sediment transport capacity. Usually, there is an equilibrium channel formed in response to the flow regime, sediment supply and bed and bank materials (Tilleard, 2001; Shi et al., 2007). When the regime of flow and sediment supplied has changed the river channel will begin to adjust and become unstable during the rebuilding of a new equilibrium channel. With continuous development of hydropower and water resource exploitation in China, significant efforts have been made to investigate how the rivers downstream will respond to the impoundments.
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Scouring of the Yellow River downstream of the Xiaolangdi reservoir has been reported by Shang et al. (2008). Zhang et al. (2008) studied scouring of the downstream channel after the closing of the Danjiangkou Reservoir on the Hanjiang River and reported downstream scouring that varied with time and distance from the dam. With a number of dams constructed on the upper Yangtze since the 1980s, especially the Three Gorges Dam begun in 1992, a lot of research has focused on downstream impacts. With the completion of the Three Gorges Dam in 2003, sediment transport into the middle reaches has been reduced by about 80%, primarily during the months of high discharge (Xu and Milliman, 2009; Hu et al., 2011). Moreover, some studies report that the grain size of sediment has coarsened in the reach from Yichang to Chenglingji because of in-channel erosion (Xu et al., 2010). However, these researchers do not predict how the channel will adjust after the impoundment of the Three Gorges Dam. Further, no attempt has been made in the planning of the Three Gorges Dam to seriously consider the downstream impacts on the channel. In the feasibility study it was acknowledged that there would be degradation in the downstream channel, and while no attempt was made to estimate the magnitude, in the original feasibility study it was suggested that the degradation may be beneficial, as reported by Leopold (2002) in a critical commentary on this dam. In this study, we collected data on flow and suspended sediment from the gauging stations on the main stream of the middle Yangtze River from Yichang to Hankou in the pre- and post-dam periods and compared the hydrologic conditions and sediment transport through this reach between the pre- and post-dam periods. Also in this study, we analysed the conditions for scouring or deposition of the river bed before and after impoundment of the Three Gorges Dam. We used the difference between the maximum suspended sediment carrying capacity, calculated according to the flow velocity, water depth and sediment grain size, and the actual loads observed at the gauging stations of the Yangtze River to assess the potential for scouring or deposition in the river channel. Scouring and deposition will affect the morphology of the river channel and the channel will become unstable until a new equilibrium condition is reached. 2. Study area The Three Gorges Dam is the world's largest hydroelectric power plant and the most important water control project on the Yangtze River. Construction began in 1992 and water impoundment began
in June 2003. According to the design, the flood control capacity of the Three Gorges Dam is 22.15 × 109 m3 and the total storage capacity can reach 39.3 × 109 m 3. This storage volume is relatively small, being only 9.2% of the long term mean annual flow of the Yangtze at Yichang. The major purposes of the Three Gorges Dam are the production of electricity and flood regulation. In this climate region, with summer dominant rainfall and spring and summer flows enhanced by melting snow and ice in the upstream catchment, the dam is operated so as to maintain some spare capacity for flood mitigation in the summer flood season and store some of the summer flow to enhance power production through the winter low flow season. While the Three Gorges Dam is not designed to change the total flow volume released into the river downstream of the dam by any significant amount, the storage strategy will cause a change in the seasonal distribution of the flows. The dam will also store most of the sediment arriving from the upstream catchment. What little sediment passes the dam will be of much finer mean grain size than previously. Such changes will impact on the dynamics and morphology of the Yangtze channel downstream of the dam and it is this issue we wish to investigate in this paper. The Yangtze (Changjiang) River is the longest river in China, and the third longest river in the world. It originates from the Qinghai–Tibetan Plateau, and crosses the country from west to east, finally flowing into the East China Sea. The huge Yangtze drainage basin (Fig. 1), which is more than 6300 km in length and has a catchment area of 1.94 × 10 6 km 2, is commonly divided into upper, middle and lower. The upper Yangtze, upstream of Yichang, is 4300 km long, the middle Yangtze extends 950 km from Yichang to Hankou and below Hankou the remaining 930 km constitutes the lower Yangtze (Chen et al., 2001). The Yichang–Hankou reach is the main part of the middle Yangtze, and includes Dongting Lake and the Hanjiang tributary (Fig. 1). Below Yichang, the Yangtze River enters a flood plain tract and the slope decreases dramatically to 2–3 × 10 −5. The riverbed on this reach is alluvial sediment in contrast to the bedrock of the upper stream. This reach can be subdivided into several parts. The Yichang–Zhicheng reach is a transition zone from the bedrock gorges to the flood plain. In the Zhicheng– Luoshan reach the river is also called the Jingjiang, and has a typical meandering river pattern. Several major meander cutoffs on the Jingjiang plain occurred during the early to mid-20th century. The variations in hydrology and sediment in this reach are very complex. Some flow and sediment of the Yangtze River is diverted into
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1-5 are hydrological gauging stations: 1-Xinjiangkou 2-Shadaoguan 3-Mituosi 4-Ouchi(Kang) 5-Ouchi(Guan)
Ouchikou
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Luoshan Chenglingji
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Legend Water system Water pond Three Gorges Dam Hydrological station
Fig. 1. The middle Yangtze Basin.
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Dongting Lake through Songzikou, Taipingkou and Ouchikou. Dongting Lake is a net sediment sink within the Yangtze system, and its capacity to store flood waters has been reduced from 293 × 10 8 m 3 in 1949 to 174 × 10 8 m 3 in 1983. (Du et al., 2001). At Chenglingji the water of Dongting Lake re-enters the main stream of the Yangtze River. The channel is braided from Luoshan to Hankou.
3. Data sources and methods We have assembled a database of flow and sediment data for stations in the middle Yangtze covering the period prior to the construction of the Three Gorges Dam and for the (much shorter) period following its completion. Here we describe these databases and the analyses undertaken using them. Data from six hydrological stations on the main stream of the middle Yangtze River from Yichang to Hankou were used to construct a database of pre-dam flow and sediment load. The six hydrological stations from upstream to downstream along the river are Yichang, Zhicheng, Xinchang, Jianli, Luoshan and Hankou (Fig. 1). These stations are operated by the Changjiang Water Resources Commission. Flow, suspended sediment concentration (SSC) and suspended sediment grain sizes are measured daily, following the standard procedures of the Chinese Ministry of Water Resources and published by the Changjiang Water Resources Commission (1950–1988). Daily observations have been summed to monthly totals. Certain gaps occur in the records for these stations, particularly during the 1970s and 1980s. The water discharge and SSC data for Zhicheng station is only available from 1952 to 1959. Suspended sediment grain size at Zhicheng is only available for 1959, at Jianli and Luoshan from 1956 to 1969 and for the remaining stations from 1956 to 1974. Hydrological records for the period from 1988 to 2003 have not been released by the Changjiang Water Resources Commission. Data for the post-dam period have been sourced from the Bulletin of Yangtze River Sediment published annually by the Changjiang Water Resources Commission (2003–2007). Monthly discharge and SSC data are published for Yichang, Shashi and Hankou stations only. In order to make a comparison with the pre-dam data, we used interpolation to assemble post-dam discharge and SSC data for Zhicheng, Xinchang, Jianli and Luoshan. Also, the annual suspended sediment grain sizes at Yichang, Shashi and Hankou are sourced from the Bulletin of Yangtze River Sediment, and interpolated for the other stations. The procedure for interpolation was as follows. Flow and sediment are lost from the Yangtze channel at Songzikou, Taipingkou, and Ouchikou into Dongting Lake and these losses are estimated using discharge and SSC data from Xinjiangkou, Shadaoguan, Mituosi, Ouchi(Kang) and Ouchi(Guan) gauging stations (Fig. 1). The gauging station at Chenglingji measures the return flow from Dongting Lake into the Yangtze channel. The Hanjiang River also contributes flow and sediment to the main stream of the Yangtze River (Fig. 1). The
Fig. 3. Monthly flow duration curves at Yichang station for pre- and post-dam periods.
differences between discharge and SSC between two hydrological gauging stations were calculated taking account of these inputs and outputs. Change in discharge or SSC per unit river channel length was calculated as: Q1 ðSSC1 Þ ¼ Q m ðSSCm Þ þ ΔQ ðΔSSC ÞL þ Q i ðSSCi Þ−Q 0 ðSSC0 Þ
ð1Þ
where Q l (SSCl) is the calculated discharge (SSC) in the reach below some hydrological station, Q m (SSCm) is the discharge (SSC) data measured by the hydrological station, ΔQ (ΔSSC) is the change in discharge (SSC) per unit river channel length, and L is the river channel length between two neighbouring stations which is taken from the Hydrography of the Middle Yangtze River from Yichang to Wuhan surveyed by the People's Liberation Army Navy (1983). Q m (SSCm) is measured data at a hydrological station, Q i (SSCi) is the input of discharge (SSC) from Dongting Lake and the Hanjiang River, and Q o (SSCo) is the output of discharge (SSC) at Songzikou, Taipingkou and Ouchikou. If the Q m (SSCm) is influenced by the input or output sites, then the Qi (SSCi) and Q o (SSCo) were taken into account. With the given discharge and SSC data, annual and monthly runoff volumes and sediment loads were calculated. We also estimated the suspended sediment carrying capacity using the formula based on gravitational theory by Zhang (Qian and Wan, 2003) as follows: Svm ¼ k
U3 ghω
ω0 ¼ 1:72
!m
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ρs −ρw gD ρw
ð2Þ
ð3Þ
Fig. 2. Mean annual runoff for stations in the Yichang–Hankou reach of the Yangtze River in the pre- and post-dam periods. (Data sources : Changjiang Water Resources Commission (1950–1988, 2003–2007)).
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ω ¼ ð1−Sv Þ
ω0
ð4Þ
where Svm is the suspended sediment carrying capacity; U is the flow velocity; g is the acceleration due to gravity (9.8); h is the water depth; ω is the settling velocity of the sediment; ω0 is the settling velocity of a sphere of diameter D and density ρs; ρw and ρs are the density of water (1000 kg·m −3) and sediment (2300 kg·m−3) respectively; D is the grain diameter; Sv is the suspended sediment concentration; and the value of the coefficient k is 0.07 and the index m is 1.14 (Zhang et al., 2008; Gao et al., 2009). The water depth, h, and flow velocity, U, are taken from the simulation curves which reflect the relationships of discharge–water depth and discharge–flow velocity (Wang et al., 2009).
4. Results Following the impoundment of the Three Gorges Dam in 2003, there have been changes to the hydrological regime and the transport of sediment with flow-on consequences for the channel of the middle Yangtze downstream of the dam. The previous water and sediment controls on the behaviour of the channel have been altered and the channel can now be expected to adjust to the changed regime. The adjustment process will be complex and can be expected continue over the long term (Petts, 1979: Tilleard, 2001). Here we describe the changes in the water and sediment regimes imposed by the Three Gorges Dam and assess the likely impacts on the river channel.
Fig. 4. Monthly discharge and sediment loads at Yichang, Zhicheng, Xinchang, Jianli, Luoshan and Hankou hydrological stations in the middle Yangtze Basin for pre- and post-dam periods.
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4.1. Flow regime changes The mean annual runoff volumes for the six hydrological gauging stations from Yichang to Hankou for the pre- and post-dam periods are shown in Fig. 2. Note that the pre-dam period covers 39 years while the post-dam period is only 5 years so to some extent the differences shown in Fig. 2 are a consequence of the different length of the record period. Immediately below the Three Gorges Dam, at Yichang and Zhicheng, annual flows post-dam are slightly lower than pre-dam, presumably reflecting relatively drier conditions in the post-dam period. In the pre-dam data the effects of the losses of flow south into Dongting Lake can be seen in the progressive reduction of flow downstream at Xinchang and Jianli. No such reduction is in evidence in the post-dam data, a reflection of the changes in flow distribution through the year which we discuss further below. At Luoshan pre-dam flow consists of flow down the main Yangtze channel, via Xinchang and Jianli, together with flow from Dongting Lake that consists of water that had previously been diverted into Dongting Lake from the Yangtze plus the flow of tributaries that enter Dongting Lake from the south. The post-dam flow at Luoshan is similar to the pre-dam flow, though more of the Yangtze flow is now routed down the main channel and is not diverted through Dongting Lake. There is an increase in flow from Luoshan to Hankou in both the pre- and post-dam data, reflecting the influence of tributaries, especially the Hanjiang River. Given the small storage volume of the Three Gorges Dam in relation to total annual flow, and the lack of any significant diversions from the dam, it is not surprising that annual flow totals are not
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greatly affected by the dam. However, there are significant changes to the distribution of flow through the year and in the apportionment of flow between high and low flows, reflecting the operation of the dam to provide for flood storage and hydroelectricity generation. Fig. 3 shows flow duration curves (based on monthly flows) at Yichang and it can be seen that the highest 30% of flows are reduced in magnitude while the lowest 40% of flows have increased. The patterns of monthly flows in the pre- and post-dam periods for each station are shown in Fig. 4. Here the effect of the operating strategies of the Three Gorges Dam is revealed in the reduction of flows in the months of high flows (June to October) and the increase of flows in the months of low flows (November to May). 4.2. Sediment load changes Changes in sediment load pre- and post-dam are much more pronounced than the changes in flow at all stations (Fig. 4). The annual average sediment load at Yichang has been reduced from 479 × 10 6 t in the pre-dam period to 77 × 10 6 t in the post-dam period, a reduction of 402 × 10 6 t or 84%. The sediment loads measured in other stations showed similar sharp reductions in the post-dam period, with reductions of 407 × 10 6 t, 333 × 10 6 t, 272 × 10 6 t, 308 × 10 6 t and 290 × 10 6 t at Zhicheng, Xinchang, Jianli, Luoshan and Hankou stations respectively. Note that the magnitude of the reduction decreases in the downstream direction. However, in the pre-dam period, from Yichang to Hankou, the sediment load has a decreasing trend with 479 × 106 t at Yichang and 423 × 106 t at Hankou. However, there are still some fluctuations of sediment loads in this reach;
Fig. 5. Suspended sediment rating curves at stations in the middle Yangtze for pre- and post-dam periods grouped in pairs of adjacent stations. The top left graph contains data from all stations.
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from Zhicheng to Jianli, there is a decreasing trend in sediment load, from 505 × 106 t to 368 × 10 6 t; while at Luoshan the sediment load increased by 64 × 106 t as compared with Jianli. There was a decreasing trend from Luoshan to Hankou in the pre-dam period while in the post-dam period the sediment load increases down this reach, from 72× 106 t at Luoshan to 133 × 106 t at Hankou. The obvious differences of the suspended sediment loads pre- and post-dam can be attributed to sediment trapping by the Three Gorges Dam. Historically, the sediment load at Yichang has been 114% of the
total load of the Yangtze as measured at Datong. Sediment was lost to storage in the middle Yangtze, particularly Dongting Lake, a site of long term sediment accumulation (Chen et al., 2001) and this illustrates the importance of the catchment above the Three Gorges dam as a source of sediment. With the reduction in sediment load sourced from the upper Yangtze, the trend in the Yichang–Hankou reach has also changed. In the post-dam period, with decreased suspended sediment load in the river downstream of the dam, the sediment carrying capacity of the water has become much greater than before, and the
Fig. 6. Relationships between discharge and water depth (left), and flow velocity (right) at Yichang, Zhicheng, Xinchang, Jianli, Luoshan and Hankou hydrological stations (Adapted from Wang et al., 2009).
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riverbed has become an important source, supplying the increase in sediment load in the Yichang–Hankou reach (Fig. 4). We return to this issue later in the paper. 4.3. Relations between flow and sediment load As the runoff volume and the sediment load are changed in the post-dam period, the relationship between discharge and sediment is altered (Fig. 5). The suspended sediment concentrations (SSC) reflect the results of the adjustment between the runoff and sediment load. As the sediment loads decreased more dramatically in the postdam period than the discharge at all six stations, so the SSC has also decreased. As is shown in Fig. 5, the SSC measured in the post-dam period at all six stations is below 0.5 kg·m −3, even at discharges in excess of 40,000 m 3·s −1, while in the pre-dam period SSC typically reached about 2.0 kg·m −3 at a discharge of around 30,000 m 3·s −1. In the pre-dam period, the SSC shows an increasing trend from Yichang to Jianli, with a sharp decrease in SSC at Luoshan, and further decrease at Hankou (Fig. 5). In the post-dam period, the variations of SSC between successive stations are not obvious except in the case of Jianli–Luoshan where a decrease is observed. Further downstream, from Luoshan to Hankou, the differences in SSC are again negligible. The sediment rating curves in Fig. 5 have the form: b
SSC ¼ aQ or log SSC ¼ log a þ b log Q
ð5Þ
In the pre-dam period, the coefficients of determination (R 2) of QSSC are over 0.9 except at Luoshan (0.82). In the post-dam period, R 2 at Yichang station falls from 0.95 in the pre-dam period to 0.62 in the post-dam period. R 2 then rises downstream to reach a peak at Jianli of 0.87. However, the value at Luoshan decreases again to 0.653 and rises to 0.75 at Hankou. The sediment in the river can be divided into three kinds, wash load, transition load and bed load, with different grain sizes. The content of bed load is comparatively stable in the river, and the transition load will change as the wash load under different hydrodynamic conditions. With increasing wash load in the water, the correlation of SSC with discharge becomes poorer (Yu, 2006). After completion of the Three Gorges Dam, most of the coarse suspended sediments are trapped in the reservoir, and only the finer transition load and wash load is transported to the lower reaches. In the post-dam period, the R 2 values of the sediment rating curves decrease, and as Yichang is closer to the dam, the influence is much larger, and the R 2 value is the lowest of the six stations at only 0.62. As for the low R 2 at Luoshan, it is mainly influenced by the water flowing from Dongting Lake where most of the coarser sediments are deposited, so the suspended sediment mainly consists of finer transition and wash load. When the lake water affluxes into the main stream of the Yangtze River, the content of the wash load in the water increases, thus the R 2 of the sediment rating curve is reduced at Luoshan.
Fig. 7. Suspended sediment carrying capacity at the six hydrological stations in the preand post-dam periods.
increased sharply at Yichang and Zhicheng and then fallen to values close to the pre-dam levels at the downstream stations. The variation in sediment carrying capacity in the post-dam period is mainly affected by the change of suspended sediment grain size. The sediment carrying capacity increases as the grain size becomes smaller. As most of the coarse sediments have been deposited in the reservoir, the suspended sediment sizes in the lower river channel are smaller. This has resulted in the sediment carrying capacity increasing at Yichang and Zhicheng. With an increase in sediment size along the river from in-channel sources, the capacity eventually decreases. The sediment carrying capacity reflects the suspended sediment concentration when the relationship between water and sediment is in balance. However, in most cases, the relationship is not in balance. With the adjustment of the relationship between water and sediment, the conditions of scour or sedimentation on the riverbed have been changed as well. The difference between the SSC measured in the river and the capacity calculated by Eq. (2) can indicate scour or deposition on the riverbed. When the value of measured SSC exceeds the calculated one, sedimentation is indicated; and when the measured SSC is less than calculated, the riverbed will be scoured. The scour or sedimentation conditions of the Yichang–Hankou reach are shown in Fig. 8. It can be concluded that deposition was dominant in the pre-dam period, and this is confirmed by the sediment data (Chen et al., 2001; Xu, 2006), while in the post-dam period, the river channel is scouring. Scouring is at a maximum at Yichang, which is closest to the Three Gorges Dam and therefore most affected by it. Further from the dam, the scouring is weaker. Fluctuations are introduced by the local geomorphology and this also explains why the extent of sedimentation in the river channel was not consistent along the river in the pre-dam period. 5. Conclusion With the impoundment of the Three Gorges Dam, the conditions of flow and sediment load in the Yangtze downstream of the dam have been changed significantly. We have assembled a database of flow and sediment data for stations in the middle Yangtze covering
4.4. Sediment carrying capacity changes Because of the changes in this reach after the completion of the Three Gorges Dam, the sediment carrying capacity has also changed which causes the riverbed to be scoured in the post-dam period. The derivation of the sediment carrying capacity is defined above (Eqs. 2–4). The flow velocity and water depth are determined by the water discharge in a defined river channel (Fig. 6) and the sediment deposition rate is influenced by the sediment grain sizes and the suspended sediment concentration. As is shown in Fig. 7, the sediment carrying capacities at these six stations fluctuated in the pre-dam period. From Yichang to Jianli, the sediment carrying capacity increased, and then decreased from Jianli to Hankou. However, the sediment carrying capacities have changed in the post-dam period. They have
Fig. 8. Difference between calculated suspended sediment carrying capacity (Sm) and measured suspended sediment concentration (SSC) under pre- and post-dam conditions for stations in the middle Yangtze River. Positive values indicate the potential for scour, negative values indicate the potential for deposition.
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the period prior to the construction of the Three Gorges Dam and for the period following its completion in order to make a comparison between the pre- and post-dam periods. We have used the difference between the maximum suspended sediment carrying capacity and the observed SSC to analyse the conditions for scouring or deposition of the river bed before and after impoundment of the Three Gorges Dam. There are significant changes to the distribution of flow through the year and in the apportionment of flow between high and low flows, especially at Yichang station which is the nearest station to the dam site. At Yichang, the highest 30% of flows are reduced while the lowest 40% of flows have increased. At other stations low flows have also increased in the post-dam period. The implications of this are that more erosive flows will be maintained for longer periods because of the operating strategy of the dam. With the sediments trapped in the reservoir and clear water flowing downstream, changes in sediment load pre- and post-dam are much more pronounced than the changes in flow at all stations. The magnitude of the reduction decreases in the downstream direction. In the post-dam period, the river channel is scouring which can be confirmed by the difference between the SSC measured in the river and the calculated capacity. Further from the dam, the scouring is weaker. In the pre-dam period, the river bed experienced sedimentation and sediment was also deposited in Dongting Lake. Scour in the river channel will eventually exhaust the sediment stores and channel widening can then be expected to begin. Eventually a new channel equilibrium will develop based on a combination of bed scour and bank erosion that is difficult to predict and is likely to occur over many decades. Further monitoring and analysis will be required to determine the extent, nature and consequences of these impacts. Acknowledgements The authors sincerely thank our research group, including many graduate students, who helped establish the database. The Ministry of Education of China - 111 Project, and SKLEC (Grant No. 44KZ0051) supported this study. References Andrews, E.D., 1986. Downstream effects of Flaming Gorge Reservoir on the Green River, Colorado and Utah. Geological Society of America Bulletin 97, 1012–1023. Andrews, E.D., 1991. Sediment transport in the Colorado River basin. In: Committee on Glen Canyon Environmental Studies. Colorado River Ecology and Dam Management. National Academy Press, Washington, D.C, pp. 54–74. Brandt, S.A., 2000. Classification of geomorphological effects downstream of dams. Catena 40, 375–401. Changjiang Water Resources Commission, 1950–1988. Hydrological records on water and sediment, Changjiang Water Resources Commission, Wuhan. (unpublished, in Chinese). Changjiang Water Resources Commission, 2003–2007. Bulletin of Yangtze River Sediment. http://www.chj.com.cn (in Chinese). Chen, Z., Li, J., Shen, H., Wang, Z., 2001. Yangtze River of China: historical analysis of discharge variability and sediment flux. Geomorphology 41, 77–91.
Du, Y., Cai, S., Zhang, X., Zhao, Y., 2001. Interpretation of the environmental change of Dongting Lake, middle reach of the Yangtze River, China, by 210Pb measurement and satellite image analysis. Geomorphology 41, 171–181. Everitt, B.L., 1993. Channel responses to declining flow on the Rio Grande between Ft. Quitman and the Presidio, Texas. Geomorphology 6, 225–242. Friedman, J.M., Osterkamp, W.R., Scott, M.L., Auble, G.T., 1998. Downstream effects of dams on channel geometry and bottomland vegetation: regional pattern in the Great Plains. Wetlands 18, 619–633. Gao, Y., Fan, B., Hou, W., Hu, S., 2009. Application of different verification methods for sediment carrying capacity formulae in middle and lower Yangtze River. Journal of Yangtze River Scientific Research Institute 26, 14–17 (in Chinese). Gilvear, D.J., 2004. Patterns of the channel adjustment to impoundment of the upper River Spey, Scotland (1942–2000). River Research Applications 20, 151–165. Graf, W.L., 1980. The effect of dam closure on downstream rapids. Water Resources Research 6, 129–136. Hu, B., Wang, H., Yang, Z., Sun, X., 2011. Temporal and spatial variations of sediment rating curves in the Changjiang (Yangtze River) basin and their implications. Quaternary International 230, 34–43. Kearsley, L.H., Schmidt, J.C., Warren, K.D., 1994. Effects of Glen Canyon Dam on Colorado River sand deposits used as campsites in Grand Canyon National Park, USA. Regulated Rivers Research and Management 9, 137–149. Leopold, L.B., 2002. Sediment problems at Three Gorges Dam, MOAFS April 2002 Newsletter. http://www.moafs.org/newsletter/April%202002/3gorgluna.htm. Li, Y., Sun, Z., Liu, Y., Deng, J., 2009. Channel degradation downstream from the Three Gorges Project and its impacts on flood level. Journal of Hydraulic Engineering 9, 718–728. Magilligan, F.J., Haynie, H.J., Nislow, K.H., 2008. Channel adjustments to dams in the Connecticut River basin: implications for forested mesic watersheds. Annals of the Association of American Geographers 98, 267–284. Petts, G.E., 1979. Complex response of river channel morphology subsequent to reservoir construction. Progress in Physical Geography 3, 329–362. Phillips, J.D., 2003. Toledo bend reservoir and geomorphic response in the lower Sabine River. River Research and Applications 19, 137–159. Pohl, M., 2004. Channel bed mobility downstream from the Elwha Dams, Washington. The Professional Geographer 56, 422–431. Qian, N., Wan, Z., 2003. Mechanics of Sediment Transport. Scientific Publication, Beijing. 687 pp. (in Chinese). Shang, H., Su, Y., Zhang, M., 2008. Comparison analysis of river adjustment in different reservoir storage period in downstream of Yellow River. Journal of Water Resources and Water Engineering 19, 28–32 (in Chinese). Shi, C., Wu, B., Ma, J., 2007. Cause of formation and discrimination of channel patterns for alluvial rivers. Journal of Hydroelectric Engineering 26, 107–111 (in Chinese). Tilleard, J.W., 2001, River Channel Adjustment to Hydrologic Change, PhD Thesis, Department of Civil and Environmental Engineering, The University of Melbourne. (http://repository.unimelb.edu.au/10187/1490). Topping, D.J., Rubin, D.M., Vierra Jr., L.E., 2000. Colorado river sediment transport 1. Natural sediment supply limitation and the influence of Glen Canyon Dam. Water Resources Research 36, 515–542. Wang, Z., Chen, Z., Li, M., Chen, J., Zhao, Y., 2009. Variations in downstream grain-sizes to interpret sediment transport in the middle-lower Yangtze River, China: a prestudy of Three-Gorges Dam. Geomorphology 113, 217–229. Xu, J., 2006. Influence of variations in suspended sediment concentration and grain size on sediment deposition of Yichang–Hankou reach of Yangtze River. Advances in Water Science 17, 67–73 (in Chinese). Xu, K., Milliman, J.D., 2009. Seasonal variations of sediment discharge from the Yangtze River before and after impoundment of the Three Gorges Dam. Geomorphology 104, 276–283. Xu, X., Yang, S., Zhang, Z., 2010. Variation in grain size of sediment in middle and lower Changjiang River since impoundment of Three Gorges Reservoir. Scientia Geographica Sinica 30, 103–107 (in Chinese). Yu, F., 2006. Suspended Sediment Rating Curve of the Yangtze (Changjiang) River: Characteristics, Interpretations and Practices, Masters Dissertation, East China Normal University, 74 pp. (in Chinese). Zhang, H., Zhang, X., Wang, X., Wu, Q., Zhao, Y., 2008. Scouring characteristics of downstream channel after operation of Danjiangkou Reservoir. Journal of Yangtze River Scientific Research Institute 25, 19–22 (in Chinese).
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Assessing the potential for change in the middle Yangtze River channel following impoundment of the Three Gorges Dam Wenhao Yuan a, b, Daowei Yin a, Brian Finlayson c,⁎, Zhongyuan Chen a a b c
State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China Department of Geography, East China Normal University, Shanghai 200062, China Department of Resource Management and Geography, The University of Melbourne, Victoria 3010, Australia
a r t i c l e
i n f o
Article history: Received 21 June 2010 Received in revised form 7 June 2011 Accepted 14 June 2011 Available online 26 August 2011 Keywords: Yangtze River Three Gorges Dam Channel change Downstream impacts of dams
a b s t r a c t The geomorphic impacts of dams on downstream river channels are complex, not readily predictable for specific cases, but widely reported in the literature. For the Three Gorges Dam on the Yangtze (Changjiang) River in China, no studies of the impact of the changed flow and sediment conditions below the dam on the behaviour of the channel were included in the pre-dam feasibility report. We have assembled a database of flow and sediment data for the middle Yangtze River from Yichang to Hankou and used this to analyse changes following the closure of the dam. While total flow is little affected, the operating strategy for the dam that provides for storage of part of the summer high flows to maintain hydroelectric power generation in winter (the low flow season) is reflected in changes to the seasonal distribution of flow below the dam. We calculated potential sediment carrying capacity and compared it with measured sediment concentrations for both pre- and post-dam conditions. While channel sedimentation is indicated along the middle Yangtze for pre-dam conditions, scour is indicated for post-dam conditions, highest at Yichang immediately below the dam and decreasing downstream. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The dams constructed globally on rivers in the twentieth century have generated significant hydrologic, ecological, and geomorphic adjustments at both reach and watershed scales (Magilligan et al., 2008). Although dams provide important services such as flood protection and power generation, the environmental costs of these structures are becoming apparent. The impoundment of rivers by dams and reservoirs represents a significant disruption of geomorphologic processes (Phillips, 2003). Dams typically modify the stream hydrology and sediment dynamics, which cause adjustments in the downstream channel system (Petts, 1979; Andrews, 1986; Everitt, 1993; Topping et al., 2000). The Three Gorges Dam does not greatly modify stream flow but the loss of sediment from the channel system has a substantial impact on bed mobility and the geomorphic and biotic complexity of the river, as has been shown elsewhere by, for example, Pohl (2004). Work on the downstream impacts on large river systems of multipurpose dams (Graf, 1980; Andrews, 1991; Kearsley et al., 1994; Topping et al., 2000; Li et al., 2009) indicate that downstream geomorphic responses of stream channels to dams are complex and variable and difficult to predict, mainly because the effects of local geological conditions and operational details confound and
⁎ Corresponding author. E-mail address: [email protected] (B. Finlayson). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.06.039
complicate efforts to apply models and generalisations to individual streams (Phillips, 2003). The channel immediately downstream from a dam will normally undergo bed degradation and bank erosion due to the clear water released from the dam. However, aggrading reaches or relatively stable reaches are found in some rivers with impoundments because of a sufficient sediment supply or armouring of the bed (Phillips, 2003; Gilvear, 2004). The geomorphic changes that occur downstream of dams cannot be predicted or explained without accounting for the local and regional geologic setting, geomorphic history and controls, hydrologic regime, land use and vegetation, dam/reservoir operation, time since dam construction, and other variables (Friedman et al., 1998). This difficulty in generalising the downstream response to dams has led to the development of alternative approaches such as a synoptic typology of geomorphic responses (Brandt, 2000). Brandt identified two key determinants of the qualitative geomorphic response downstream of dams: changes in flow regime, and the relationship between sediment supplied to the stream and the sediment transport capacity. Usually, there is an equilibrium channel formed in response to the flow regime, sediment supply and bed and bank materials (Tilleard, 2001; Shi et al., 2007). When the regime of flow and sediment supplied has changed the river channel will begin to adjust and become unstable during the rebuilding of a new equilibrium channel. With continuous development of hydropower and water resource exploitation in China, significant efforts have been made to investigate how the rivers downstream will respond to the impoundments.
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Scouring of the Yellow River downstream of the Xiaolangdi reservoir has been reported by Shang et al. (2008). Zhang et al. (2008) studied scouring of the downstream channel after the closing of the Danjiangkou Reservoir on the Hanjiang River and reported downstream scouring that varied with time and distance from the dam. With a number of dams constructed on the upper Yangtze since the 1980s, especially the Three Gorges Dam begun in 1992, a lot of research has focused on downstream impacts. With the completion of the Three Gorges Dam in 2003, sediment transport into the middle reaches has been reduced by about 80%, primarily during the months of high discharge (Xu and Milliman, 2009; Hu et al., 2011). Moreover, some studies report that the grain size of sediment has coarsened in the reach from Yichang to Chenglingji because of in-channel erosion (Xu et al., 2010). However, these researchers do not predict how the channel will adjust after the impoundment of the Three Gorges Dam. Further, no attempt has been made in the planning of the Three Gorges Dam to seriously consider the downstream impacts on the channel. In the feasibility study it was acknowledged that there would be degradation in the downstream channel, and while no attempt was made to estimate the magnitude, in the original feasibility study it was suggested that the degradation may be beneficial, as reported by Leopold (2002) in a critical commentary on this dam. In this study, we collected data on flow and suspended sediment from the gauging stations on the main stream of the middle Yangtze River from Yichang to Hankou in the pre- and post-dam periods and compared the hydrologic conditions and sediment transport through this reach between the pre- and post-dam periods. Also in this study, we analysed the conditions for scouring or deposition of the river bed before and after impoundment of the Three Gorges Dam. We used the difference between the maximum suspended sediment carrying capacity, calculated according to the flow velocity, water depth and sediment grain size, and the actual loads observed at the gauging stations of the Yangtze River to assess the potential for scouring or deposition in the river channel. Scouring and deposition will affect the morphology of the river channel and the channel will become unstable until a new equilibrium condition is reached. 2. Study area The Three Gorges Dam is the world's largest hydroelectric power plant and the most important water control project on the Yangtze River. Construction began in 1992 and water impoundment began
in June 2003. According to the design, the flood control capacity of the Three Gorges Dam is 22.15 × 109 m3 and the total storage capacity can reach 39.3 × 109 m 3. This storage volume is relatively small, being only 9.2% of the long term mean annual flow of the Yangtze at Yichang. The major purposes of the Three Gorges Dam are the production of electricity and flood regulation. In this climate region, with summer dominant rainfall and spring and summer flows enhanced by melting snow and ice in the upstream catchment, the dam is operated so as to maintain some spare capacity for flood mitigation in the summer flood season and store some of the summer flow to enhance power production through the winter low flow season. While the Three Gorges Dam is not designed to change the total flow volume released into the river downstream of the dam by any significant amount, the storage strategy will cause a change in the seasonal distribution of the flows. The dam will also store most of the sediment arriving from the upstream catchment. What little sediment passes the dam will be of much finer mean grain size than previously. Such changes will impact on the dynamics and morphology of the Yangtze channel downstream of the dam and it is this issue we wish to investigate in this paper. The Yangtze (Changjiang) River is the longest river in China, and the third longest river in the world. It originates from the Qinghai–Tibetan Plateau, and crosses the country from west to east, finally flowing into the East China Sea. The huge Yangtze drainage basin (Fig. 1), which is more than 6300 km in length and has a catchment area of 1.94 × 10 6 km 2, is commonly divided into upper, middle and lower. The upper Yangtze, upstream of Yichang, is 4300 km long, the middle Yangtze extends 950 km from Yichang to Hankou and below Hankou the remaining 930 km constitutes the lower Yangtze (Chen et al., 2001). The Yichang–Hankou reach is the main part of the middle Yangtze, and includes Dongting Lake and the Hanjiang tributary (Fig. 1). Below Yichang, the Yangtze River enters a flood plain tract and the slope decreases dramatically to 2–3 × 10 −5. The riverbed on this reach is alluvial sediment in contrast to the bedrock of the upper stream. This reach can be subdivided into several parts. The Yichang–Zhicheng reach is a transition zone from the bedrock gorges to the flood plain. In the Zhicheng– Luoshan reach the river is also called the Jingjiang, and has a typical meandering river pattern. Several major meander cutoffs on the Jingjiang plain occurred during the early to mid-20th century. The variations in hydrology and sediment in this reach are very complex. Some flow and sediment of the Yangtze River is diverted into
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1-5 are hydrological gauging stations: 1-Xinjiangkou 2-Shadaoguan 3-Mituosi 4-Ouchi(Kang) 5-Ouchi(Guan)
Ouchikou
r
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Luoshan Chenglingji
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Legend Water system Water pond Three Gorges Dam Hydrological station
Fig. 1. The middle Yangtze Basin.
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Dongting Lake through Songzikou, Taipingkou and Ouchikou. Dongting Lake is a net sediment sink within the Yangtze system, and its capacity to store flood waters has been reduced from 293 × 10 8 m 3 in 1949 to 174 × 10 8 m 3 in 1983. (Du et al., 2001). At Chenglingji the water of Dongting Lake re-enters the main stream of the Yangtze River. The channel is braided from Luoshan to Hankou.
3. Data sources and methods We have assembled a database of flow and sediment data for stations in the middle Yangtze covering the period prior to the construction of the Three Gorges Dam and for the (much shorter) period following its completion. Here we describe these databases and the analyses undertaken using them. Data from six hydrological stations on the main stream of the middle Yangtze River from Yichang to Hankou were used to construct a database of pre-dam flow and sediment load. The six hydrological stations from upstream to downstream along the river are Yichang, Zhicheng, Xinchang, Jianli, Luoshan and Hankou (Fig. 1). These stations are operated by the Changjiang Water Resources Commission. Flow, suspended sediment concentration (SSC) and suspended sediment grain sizes are measured daily, following the standard procedures of the Chinese Ministry of Water Resources and published by the Changjiang Water Resources Commission (1950–1988). Daily observations have been summed to monthly totals. Certain gaps occur in the records for these stations, particularly during the 1970s and 1980s. The water discharge and SSC data for Zhicheng station is only available from 1952 to 1959. Suspended sediment grain size at Zhicheng is only available for 1959, at Jianli and Luoshan from 1956 to 1969 and for the remaining stations from 1956 to 1974. Hydrological records for the period from 1988 to 2003 have not been released by the Changjiang Water Resources Commission. Data for the post-dam period have been sourced from the Bulletin of Yangtze River Sediment published annually by the Changjiang Water Resources Commission (2003–2007). Monthly discharge and SSC data are published for Yichang, Shashi and Hankou stations only. In order to make a comparison with the pre-dam data, we used interpolation to assemble post-dam discharge and SSC data for Zhicheng, Xinchang, Jianli and Luoshan. Also, the annual suspended sediment grain sizes at Yichang, Shashi and Hankou are sourced from the Bulletin of Yangtze River Sediment, and interpolated for the other stations. The procedure for interpolation was as follows. Flow and sediment are lost from the Yangtze channel at Songzikou, Taipingkou, and Ouchikou into Dongting Lake and these losses are estimated using discharge and SSC data from Xinjiangkou, Shadaoguan, Mituosi, Ouchi(Kang) and Ouchi(Guan) gauging stations (Fig. 1). The gauging station at Chenglingji measures the return flow from Dongting Lake into the Yangtze channel. The Hanjiang River also contributes flow and sediment to the main stream of the Yangtze River (Fig. 1). The
Fig. 3. Monthly flow duration curves at Yichang station for pre- and post-dam periods.
differences between discharge and SSC between two hydrological gauging stations were calculated taking account of these inputs and outputs. Change in discharge or SSC per unit river channel length was calculated as: Q1 ðSSC1 Þ ¼ Q m ðSSCm Þ þ ΔQ ðΔSSC ÞL þ Q i ðSSCi Þ−Q 0 ðSSC0 Þ
ð1Þ
where Q l (SSCl) is the calculated discharge (SSC) in the reach below some hydrological station, Q m (SSCm) is the discharge (SSC) data measured by the hydrological station, ΔQ (ΔSSC) is the change in discharge (SSC) per unit river channel length, and L is the river channel length between two neighbouring stations which is taken from the Hydrography of the Middle Yangtze River from Yichang to Wuhan surveyed by the People's Liberation Army Navy (1983). Q m (SSCm) is measured data at a hydrological station, Q i (SSCi) is the input of discharge (SSC) from Dongting Lake and the Hanjiang River, and Q o (SSCo) is the output of discharge (SSC) at Songzikou, Taipingkou and Ouchikou. If the Q m (SSCm) is influenced by the input or output sites, then the Qi (SSCi) and Q o (SSCo) were taken into account. With the given discharge and SSC data, annual and monthly runoff volumes and sediment loads were calculated. We also estimated the suspended sediment carrying capacity using the formula based on gravitational theory by Zhang (Qian and Wan, 2003) as follows: Svm ¼ k
U3 ghω
ω0 ¼ 1:72
!m
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ρs −ρw gD ρw
ð2Þ
ð3Þ
Fig. 2. Mean annual runoff for stations in the Yichang–Hankou reach of the Yangtze River in the pre- and post-dam periods. (Data sources : Changjiang Water Resources Commission (1950–1988, 2003–2007)).
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ω ¼ ð1−Sv Þ
ω0
ð4Þ
where Svm is the suspended sediment carrying capacity; U is the flow velocity; g is the acceleration due to gravity (9.8); h is the water depth; ω is the settling velocity of the sediment; ω0 is the settling velocity of a sphere of diameter D and density ρs; ρw and ρs are the density of water (1000 kg·m −3) and sediment (2300 kg·m−3) respectively; D is the grain diameter; Sv is the suspended sediment concentration; and the value of the coefficient k is 0.07 and the index m is 1.14 (Zhang et al., 2008; Gao et al., 2009). The water depth, h, and flow velocity, U, are taken from the simulation curves which reflect the relationships of discharge–water depth and discharge–flow velocity (Wang et al., 2009).
4. Results Following the impoundment of the Three Gorges Dam in 2003, there have been changes to the hydrological regime and the transport of sediment with flow-on consequences for the channel of the middle Yangtze downstream of the dam. The previous water and sediment controls on the behaviour of the channel have been altered and the channel can now be expected to adjust to the changed regime. The adjustment process will be complex and can be expected continue over the long term (Petts, 1979: Tilleard, 2001). Here we describe the changes in the water and sediment regimes imposed by the Three Gorges Dam and assess the likely impacts on the river channel.
Fig. 4. Monthly discharge and sediment loads at Yichang, Zhicheng, Xinchang, Jianli, Luoshan and Hankou hydrological stations in the middle Yangtze Basin for pre- and post-dam periods.
W. Yuan et al. / Geomorphology 147–148 (2012) 27–34
4.1. Flow regime changes The mean annual runoff volumes for the six hydrological gauging stations from Yichang to Hankou for the pre- and post-dam periods are shown in Fig. 2. Note that the pre-dam period covers 39 years while the post-dam period is only 5 years so to some extent the differences shown in Fig. 2 are a consequence of the different length of the record period. Immediately below the Three Gorges Dam, at Yichang and Zhicheng, annual flows post-dam are slightly lower than pre-dam, presumably reflecting relatively drier conditions in the post-dam period. In the pre-dam data the effects of the losses of flow south into Dongting Lake can be seen in the progressive reduction of flow downstream at Xinchang and Jianli. No such reduction is in evidence in the post-dam data, a reflection of the changes in flow distribution through the year which we discuss further below. At Luoshan pre-dam flow consists of flow down the main Yangtze channel, via Xinchang and Jianli, together with flow from Dongting Lake that consists of water that had previously been diverted into Dongting Lake from the Yangtze plus the flow of tributaries that enter Dongting Lake from the south. The post-dam flow at Luoshan is similar to the pre-dam flow, though more of the Yangtze flow is now routed down the main channel and is not diverted through Dongting Lake. There is an increase in flow from Luoshan to Hankou in both the pre- and post-dam data, reflecting the influence of tributaries, especially the Hanjiang River. Given the small storage volume of the Three Gorges Dam in relation to total annual flow, and the lack of any significant diversions from the dam, it is not surprising that annual flow totals are not
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greatly affected by the dam. However, there are significant changes to the distribution of flow through the year and in the apportionment of flow between high and low flows, reflecting the operation of the dam to provide for flood storage and hydroelectricity generation. Fig. 3 shows flow duration curves (based on monthly flows) at Yichang and it can be seen that the highest 30% of flows are reduced in magnitude while the lowest 40% of flows have increased. The patterns of monthly flows in the pre- and post-dam periods for each station are shown in Fig. 4. Here the effect of the operating strategies of the Three Gorges Dam is revealed in the reduction of flows in the months of high flows (June to October) and the increase of flows in the months of low flows (November to May). 4.2. Sediment load changes Changes in sediment load pre- and post-dam are much more pronounced than the changes in flow at all stations (Fig. 4). The annual average sediment load at Yichang has been reduced from 479 × 10 6 t in the pre-dam period to 77 × 10 6 t in the post-dam period, a reduction of 402 × 10 6 t or 84%. The sediment loads measured in other stations showed similar sharp reductions in the post-dam period, with reductions of 407 × 10 6 t, 333 × 10 6 t, 272 × 10 6 t, 308 × 10 6 t and 290 × 10 6 t at Zhicheng, Xinchang, Jianli, Luoshan and Hankou stations respectively. Note that the magnitude of the reduction decreases in the downstream direction. However, in the pre-dam period, from Yichang to Hankou, the sediment load has a decreasing trend with 479 × 106 t at Yichang and 423 × 106 t at Hankou. However, there are still some fluctuations of sediment loads in this reach;
Fig. 5. Suspended sediment rating curves at stations in the middle Yangtze for pre- and post-dam periods grouped in pairs of adjacent stations. The top left graph contains data from all stations.
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from Zhicheng to Jianli, there is a decreasing trend in sediment load, from 505 × 106 t to 368 × 10 6 t; while at Luoshan the sediment load increased by 64 × 106 t as compared with Jianli. There was a decreasing trend from Luoshan to Hankou in the pre-dam period while in the post-dam period the sediment load increases down this reach, from 72× 106 t at Luoshan to 133 × 106 t at Hankou. The obvious differences of the suspended sediment loads pre- and post-dam can be attributed to sediment trapping by the Three Gorges Dam. Historically, the sediment load at Yichang has been 114% of the
total load of the Yangtze as measured at Datong. Sediment was lost to storage in the middle Yangtze, particularly Dongting Lake, a site of long term sediment accumulation (Chen et al., 2001) and this illustrates the importance of the catchment above the Three Gorges dam as a source of sediment. With the reduction in sediment load sourced from the upper Yangtze, the trend in the Yichang–Hankou reach has also changed. In the post-dam period, with decreased suspended sediment load in the river downstream of the dam, the sediment carrying capacity of the water has become much greater than before, and the
Fig. 6. Relationships between discharge and water depth (left), and flow velocity (right) at Yichang, Zhicheng, Xinchang, Jianli, Luoshan and Hankou hydrological stations (Adapted from Wang et al., 2009).
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riverbed has become an important source, supplying the increase in sediment load in the Yichang–Hankou reach (Fig. 4). We return to this issue later in the paper. 4.3. Relations between flow and sediment load As the runoff volume and the sediment load are changed in the post-dam period, the relationship between discharge and sediment is altered (Fig. 5). The suspended sediment concentrations (SSC) reflect the results of the adjustment between the runoff and sediment load. As the sediment loads decreased more dramatically in the postdam period than the discharge at all six stations, so the SSC has also decreased. As is shown in Fig. 5, the SSC measured in the post-dam period at all six stations is below 0.5 kg·m −3, even at discharges in excess of 40,000 m 3·s −1, while in the pre-dam period SSC typically reached about 2.0 kg·m −3 at a discharge of around 30,000 m 3·s −1. In the pre-dam period, the SSC shows an increasing trend from Yichang to Jianli, with a sharp decrease in SSC at Luoshan, and further decrease at Hankou (Fig. 5). In the post-dam period, the variations of SSC between successive stations are not obvious except in the case of Jianli–Luoshan where a decrease is observed. Further downstream, from Luoshan to Hankou, the differences in SSC are again negligible. The sediment rating curves in Fig. 5 have the form: b
SSC ¼ aQ or log SSC ¼ log a þ b log Q
ð5Þ
In the pre-dam period, the coefficients of determination (R 2) of QSSC are over 0.9 except at Luoshan (0.82). In the post-dam period, R 2 at Yichang station falls from 0.95 in the pre-dam period to 0.62 in the post-dam period. R 2 then rises downstream to reach a peak at Jianli of 0.87. However, the value at Luoshan decreases again to 0.653 and rises to 0.75 at Hankou. The sediment in the river can be divided into three kinds, wash load, transition load and bed load, with different grain sizes. The content of bed load is comparatively stable in the river, and the transition load will change as the wash load under different hydrodynamic conditions. With increasing wash load in the water, the correlation of SSC with discharge becomes poorer (Yu, 2006). After completion of the Three Gorges Dam, most of the coarse suspended sediments are trapped in the reservoir, and only the finer transition load and wash load is transported to the lower reaches. In the post-dam period, the R 2 values of the sediment rating curves decrease, and as Yichang is closer to the dam, the influence is much larger, and the R 2 value is the lowest of the six stations at only 0.62. As for the low R 2 at Luoshan, it is mainly influenced by the water flowing from Dongting Lake where most of the coarser sediments are deposited, so the suspended sediment mainly consists of finer transition and wash load. When the lake water affluxes into the main stream of the Yangtze River, the content of the wash load in the water increases, thus the R 2 of the sediment rating curve is reduced at Luoshan.
Fig. 7. Suspended sediment carrying capacity at the six hydrological stations in the preand post-dam periods.
increased sharply at Yichang and Zhicheng and then fallen to values close to the pre-dam levels at the downstream stations. The variation in sediment carrying capacity in the post-dam period is mainly affected by the change of suspended sediment grain size. The sediment carrying capacity increases as the grain size becomes smaller. As most of the coarse sediments have been deposited in the reservoir, the suspended sediment sizes in the lower river channel are smaller. This has resulted in the sediment carrying capacity increasing at Yichang and Zhicheng. With an increase in sediment size along the river from in-channel sources, the capacity eventually decreases. The sediment carrying capacity reflects the suspended sediment concentration when the relationship between water and sediment is in balance. However, in most cases, the relationship is not in balance. With the adjustment of the relationship between water and sediment, the conditions of scour or sedimentation on the riverbed have been changed as well. The difference between the SSC measured in the river and the capacity calculated by Eq. (2) can indicate scour or deposition on the riverbed. When the value of measured SSC exceeds the calculated one, sedimentation is indicated; and when the measured SSC is less than calculated, the riverbed will be scoured. The scour or sedimentation conditions of the Yichang–Hankou reach are shown in Fig. 8. It can be concluded that deposition was dominant in the pre-dam period, and this is confirmed by the sediment data (Chen et al., 2001; Xu, 2006), while in the post-dam period, the river channel is scouring. Scouring is at a maximum at Yichang, which is closest to the Three Gorges Dam and therefore most affected by it. Further from the dam, the scouring is weaker. Fluctuations are introduced by the local geomorphology and this also explains why the extent of sedimentation in the river channel was not consistent along the river in the pre-dam period. 5. Conclusion With the impoundment of the Three Gorges Dam, the conditions of flow and sediment load in the Yangtze downstream of the dam have been changed significantly. We have assembled a database of flow and sediment data for stations in the middle Yangtze covering
4.4. Sediment carrying capacity changes Because of the changes in this reach after the completion of the Three Gorges Dam, the sediment carrying capacity has also changed which causes the riverbed to be scoured in the post-dam period. The derivation of the sediment carrying capacity is defined above (Eqs. 2–4). The flow velocity and water depth are determined by the water discharge in a defined river channel (Fig. 6) and the sediment deposition rate is influenced by the sediment grain sizes and the suspended sediment concentration. As is shown in Fig. 7, the sediment carrying capacities at these six stations fluctuated in the pre-dam period. From Yichang to Jianli, the sediment carrying capacity increased, and then decreased from Jianli to Hankou. However, the sediment carrying capacities have changed in the post-dam period. They have
Fig. 8. Difference between calculated suspended sediment carrying capacity (Sm) and measured suspended sediment concentration (SSC) under pre- and post-dam conditions for stations in the middle Yangtze River. Positive values indicate the potential for scour, negative values indicate the potential for deposition.
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the period prior to the construction of the Three Gorges Dam and for the period following its completion in order to make a comparison between the pre- and post-dam periods. We have used the difference between the maximum suspended sediment carrying capacity and the observed SSC to analyse the conditions for scouring or deposition of the river bed before and after impoundment of the Three Gorges Dam. There are significant changes to the distribution of flow through the year and in the apportionment of flow between high and low flows, especially at Yichang station which is the nearest station to the dam site. At Yichang, the highest 30% of flows are reduced while the lowest 40% of flows have increased. At other stations low flows have also increased in the post-dam period. The implications of this are that more erosive flows will be maintained for longer periods because of the operating strategy of the dam. With the sediments trapped in the reservoir and clear water flowing downstream, changes in sediment load pre- and post-dam are much more pronounced than the changes in flow at all stations. The magnitude of the reduction decreases in the downstream direction. In the post-dam period, the river channel is scouring which can be confirmed by the difference between the SSC measured in the river and the calculated capacity. Further from the dam, the scouring is weaker. In the pre-dam period, the river bed experienced sedimentation and sediment was also deposited in Dongting Lake. Scour in the river channel will eventually exhaust the sediment stores and channel widening can then be expected to begin. Eventually a new channel equilibrium will develop based on a combination of bed scour and bank erosion that is difficult to predict and is likely to occur over many decades. Further monitoring and analysis will be required to determine the extent, nature and consequences of these impacts. Acknowledgements The authors sincerely thank our research group, including many graduate students, who helped establish the database. The Ministry of Education of China - 111 Project, and SKLEC (Grant No. 44KZ0051) supported this study. References Andrews, E.D., 1986. Downstream effects of Flaming Gorge Reservoir on the Green River, Colorado and Utah. Geological Society of America Bulletin 97, 1012–1023. Andrews, E.D., 1991. Sediment transport in the Colorado River basin. In: Committee on Glen Canyon Environmental Studies. Colorado River Ecology and Dam Management. National Academy Press, Washington, D.C, pp. 54–74. Brandt, S.A., 2000. Classification of geomorphological effects downstream of dams. Catena 40, 375–401. Changjiang Water Resources Commission, 1950–1988. Hydrological records on water and sediment, Changjiang Water Resources Commission, Wuhan. (unpublished, in Chinese). Changjiang Water Resources Commission, 2003–2007. Bulletin of Yangtze River Sediment. http://www.chj.com.cn (in Chinese). Chen, Z., Li, J., Shen, H., Wang, Z., 2001. Yangtze River of China: historical analysis of discharge variability and sediment flux. Geomorphology 41, 77–91.
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