Fluvial Processes in the Lower Jingjiang River: Impact of the Three Gorges Reservoir Impoundment

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FLUVIAL PROCESSES IN THE LOWER JINGJIANG RIVER: IMPACT OF THE THREE GORGES RESERVOIR IMPOUNDMENT

Xuejun SHAO1, Hong WANG2 and Zhaoyin WANG 1 3

ABSTRACT Sediment supply to the lower Jingjiang River will be subject to substantial reduction after the impoundment of the Three Gorges Reservoir, which could result in an excess of carrying capacity and serious bank erosions in the downstream alluvial channel, threatening the bank protection works and the safety of the Jingjiang Dyke. This paper presents a summary of research works concerning the fluvial processes in the lower Jingjiang River and the possible impact of the Three Gorges Reservoir impoundment on the variation of its channel pattern. Three different predictions have been put forward by researchers: 1) the Jingjiang River will evolve towards a more sinuous, meandering channel pattern, with extensive bank erosion taking place along the river; 2) the river channel will be straightened and broadened because no point bar can be formed due to reduced sediment supply while bank erosion develops in the concave bank, and 3) this river reach will maintain its present channel pattern without significant change, although the sinuosity may be slightly reduced, since: a) the Three Gorges Reservoir mainly intercept sediment particles with sizes larger than 0.025mm, and b) the complex interaction between the Yangtze River and the Dongting Lake helps to reduce the negative effect of channel erosion through certain self-adjusting mechanism in fluvial processes. Discrepancy between these predictions shows that further research efforts are needed to understand the impact of Three Gorges Reservoir operation on the downstream fluvial processes. Meanwhile, there is an urgent need to closely monitor future development in the fluvial processes of the Jingjiang River and its influence on the safety of the Jingjiang Dykes.

Key Words: Three Gorges Reservoir, Downstream impact, Fluvial processes, Yangtze River

1 INTRODUCTION The lower Jingjiang River is a 176km long section of the middle Yangtze River between Ouchikou and Chenglingji (Fig. 1), about 300 km downstream of the City of Yichang. Observations of the river channel platforms during the last 250 years indicate that the river channel has been in constant lateral migration with an obvious pattern of the single-thread meandering type (Fig. 2). Diversions into the Dongting Lake caused its sinuosity to steadily increase during the period from 1756 to 1952, until several major neck cutoffs and chute cutoffs of the meandering channel took place, naturally or by engineering measures, during the last 50 years. The sinuosity of this river section is now reduced to a level similar to that of 250 years ago, as shown in Fig. 2(g), only this time the river course has been regulated and maintained through a considerable amount of bank protection works (e.g., rock or masonry revetment), and more river training measures are being planned. According to Schumm (1971), a period in the order of a few hundred years - called the “graded time” - is the average time for alluvial river channels to achieve a stable or equilibrium channel form, which would be necessary for the river channel to adjust itself to accommodate changes of the hydrological conditions in the drainage basin. Any river exhibiting evidences of such adjustability or stability is regarded as a “graded river”, described by Mackin (1948) as “one in which, over a period of years, slope is delicately adjusted to provided, with available discharge and with prevailing channel characteristics, just the velocity required for the transportation of the load supplied from the drainage basin. The graded

1 Prof. 2 Asso. Prof., Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, China; Send correspondence to Xuejun Shao: E-mail: [email protected] 3 Prof. Dr. International Research and Training Center on Erosion and Sedimentation (IRTCES), Beijing 100044, China Note: The original manuscript of this paper was received in Sept. 2004. The revised version was received in Oct. 2004. Discussion open until June 2006. - 102 - International Journal of Sediment Research, Vol. 20, No. 2, 2005, pp. 102-108

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stream is a system in equilibrium; its diagnostic characteristics is that any change in any of the controlling factors will cause a displacement of the equilibrium in a direction that will tend to absorb the effect of the change.”

Fig. 1 A 1100 km long section of the middle and lower Yangtze River reaches to be affected by channel scour after the impoundment of the Three Gorges Reservoir. The lower Jingjiang River section is located about 300 km downstream of the City of Yichang. Observed and predicted runoff and sediment transport at stations A, B, C, D are listed in Tables 1 and 2, respectively.

(a) (b)

(e) (f)

(g)

Fig. 2 The chann el pattern evolution of the lower Jingjiang River during 1756 – 1998, showing the location of Jianli Station (i.e., station B here and in Fig.1) (Han and Yang, 2000, Pan, 2001)

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It has been noted that even a graded river does not have a static planform, an d the alluvial channel may migrate laterally all the time, but within a given region with distinct boundaries. For instance, bank erosions frequently occur in the form of rotational slip on the middle Yangtze River, which may lead to bank failures (Pan, 2001). When control factors in a drainage system are changed substantially for a long time, e.g., a large reduction in sediment supply due to engineering projects on the river, downstream bank erosion may intensify and become a threat to the bank protection works and even the flood control dykes. In addition to the common control factors for channel pattern evolution, the middle Yangtze River channel has some unique features that have strong impact on its fluvial processes, such as the diversified geomorphological conditions, the complex relations that intertwine the Yangtze River and the Dongting Lake, and human interventions, i.e., river regulation works, and bank protection measures. New uncertainties will emerge when the impoundment of the Three Gorges Reservoir at the 175m level begins in the near future, which has been subject of extensive study in the last decade.

2 SEDIMENT TRANSPORT AFTER THREE GORGES RESERVOIR IMPOUNDMENT Observed data show that during the period from 1956 to 1995, there had been almost constant sediment concentrations at typical cross-sections on the middle Yangtze River (Pan, 2001), in terms of long term averaged values, as listed in Table 1. Disturbances such as the construction of the Gezhouba Reservoir in the 1980s, the natural and artificial river meander cutoffs during the 1960s and 1970s, did not amount to any sustained impact on the sediment transport situation. The overall channel pattern of the lower Jingjiang River remains island braided, except a few places with a pattern of meandering thalweg (in the Jianli section).

Table 1 Observations at 4 gauge stations on the Middle Yangtze River before the construction of the Three Gorges Reservoir (Pan, 2001) 3 Averaged annual sediment Average sediment concentration Averaged annual runoff (km ) 3 Period transport (million tons) (kg/m ) Station Station Station Station Station Station Station Station Station Station Station Station A B D C A B D C A B D C 1956 – 1966 439 322 629 313 548 333 414 59.6 1.25 1.04 0.66 0.19 1967 –1972 416 336 631 298 493 355 431 52.5 1.18 1.06 0.68 0.18 1973 – 1980 430 360 634 279 499 394 463 38.4 1.16 1.09 0.73 0.14 1981 – 1988 439 382 633 258 555 448 482 32.7 1.27 1.17 0.76 0.13 1989 – 1995 428 387 650 270 411 356 367 27.6 0.96 0.92 0.56 0.10 Note: 1) For the locations of stations A, B, C, D please see Fig. 1; 2) Data for Station C is the confluent flow from Dongting Lake into the Yangtze River.

2.1 Reduction of Suspended Load by Reservoir Operation The long term averaged annual sediment transport at Yichang Station is 530 million tons, with a long term averaged sediment concentration of 1.21kg/m3. According to numerical model simulations (IWHR&YSRI, 1990, as cited in Lu et al 1997), the sedimentation process in the Three Gorges Reservoir will lead to a 70% reduction in both the total amount of sediment transport and the average concentration in the first 30 years of operation beginning from the year 2004. The sediment concentration at the outlet of the reservoir will recover gradually and get close to its original level in 100 years’ time, when the reservoir sedimentation process achieved a balanced state (Table 2). The simulation was based on a specially chosen 10-year hydrological series with a slightly larger amount of the total sediment transport than the long term averaged value, which was repeated 10 times to simulate a 100-year period (i.e., the graded time).

2.2 Reduction in Particle Size Simulation results indicate that the percentage of fine sediment particles (D<0.025mm) will be increased to about 80% of the total sediment transport downstream of the Three Gorges Reservoir in the first 30 years of operation, compared with the long term averaged value of just 42% (Table 3) (after Pan , 1999).

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Reduction of sediment load of such a scale will lead to river channel scour over a distance of more than 1,100 km and an average depth of scour of about 1m, according to 1-D model calculations (Han and He, 1995). Simulation results suggest that, during the first 30 year of operation, the largest amount of channel scour will occur in the lower Jingjiang River, with an average scour depth of 4.4 – 5.4m (assuming a channel width of 1800m). The extent of the scour will be of the same order of magnitude as that downstream of the 3 major dams on the Colorado River (Table 4).

Table 2 Predicted water and sediment discharge after impoundment of the Three Gorges Reservoir and comparison with observations before construction of the reservoir Annually averaged Total sediment transport Sediment concentration Years after 3 3 (3) 3 (8) impoundment discharge (´10 m /s) (%) per year (million tons) (kg/m ) (%) Before After (2) Before After Before After (7) (1) (2) (3) (4) (5) (6) (7) (8) (9) 1 – 10 14.4 14.0 97.2 556 165 1.22 0.37 30.3 11 – 20 14.4 13.9 96.5 556 156 1.22 0.36 29.5 21 – 30 14.4 13.9 96.5 556 178 1.22 0.4 32.8 31 – 40 14.4 13.9 96.5 556 207 1.22 0.47 38.5 41 – 50 14.4 13.9 96.5 556 256 1.22 0.58 47.5 51 – 60 14.4 13.9 96.5 556 323 1.22 0.74 60.7 61 – 70 14.4 13.9 96.5 556 372 1.22 0.85 69.7 71 – 80 14.4 13.9 96.5 556 408 1.22 0.93 76.2 81 – 90 14.4 13.9 96.5 556 420 1.22 0.96 78.7 91 – 100 14.4 13.9 96.5 556 427 1.22 0.97 79.5

Table 3 Predicted size distribution of discharged sediment from the Three Gorges Reservoir and observations before construction of the reservoir (after Pan, 1999) Years after Percentage by weight of each size group (particle diameter in millimeters) impoundment <0.005 0.005–0.01 0.01–0.025 0.025 –0.05 0.05–0.1 0.1–0.25 0.25–0.5 0.5–1.0 1 – 10 23.57 28.4 29.2 16.8 2.01 0.02 0 0 11 – 22 24.5 28.67 29.4 15.8 1.62 0.01 0 0 23 – 34 22.4 26.7 29.3 18.8 2.77 0.03 0 0 35 – 46 19.4 23.6 28.3 22.8 5.72 0.16 0.02 0 47 – 58 15.47 19.4 25.57 26.89 11.8 0.85 0.02 0 59 – 70 12.4 15.8 22.3 28.04 18.6 2.7 0.16 0 71 – 82 11.1 14.24 20.6 27.7 21.92 4.12 0.32 0 Before 10.0 12.0 19.6 25.7 20.7 9.1 2.8 0.1 (at Yichang)

Table 4 Channel erosion downstream of major dams on the Colorado River, USA Dam Scour period Distance of scour Depth of channel scour(m) (km) Average Maximum Minimum Hoover 1935~1948 120 5 3 Parker 1938~1975 65 4.5 3.5 Glen Canyon 1956~1975 25 4 7 2

3 OBSERVATIONS AFTER IMPOUNDMENT OF OTHER RESERVOIRS Observed scour data and recorded changes of channel pattern downstream of the Danjiangkou Reservoir and the Gezhouba Reservoir were frequently used as examples of what is going to happen downstream of the Three Gorges Reservoir, despite the fact that the two reservoirs have capacities and operation schemes differ from that of the latter. The Danjiangkou Reservoir has an effective capacity of 10 km3, equivalent to 20% of the mean annual runoff of the Hanjiang River (51km3 at Nianpanshan station). This enables its impoundment operation to continue during flood seasons, which not only led to a 99% reduction of sediment supply in the downstream river sections, but also changed the entire pattern of hydrograph downstream and resulted in a much smaller channel-forming discharge, which become a unique factor that affects the fluvial

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processes downstream. Table 5 is a summary of observations of fluvial processes downstream of the Danjiangkou Reservoir.

Table 5 Downstream fluvial processes before and after the full impoundment of Danjiangkou Reservoir (Pan et al, 1998, Han and Yang, 2000, and Tan et al, 1996) Distance from dam River pattern Observed fluvial Average depth of (km) Before After behavior channel scour Multi-thread, Multi-thread, stable General scour of river

0 ~ 82 braided thalweg, island braided channel, stabilization of planform Scour in the main Multi-thread, Multi-thread, channel, deposition on 82 ~224 meandering Stabilized thalweg, floodplains, merger of thalweg, braided, island braided bars Meandering Meandering without Widespread chute

224 ~ 410 without training training work cutoffs work Meandering with Meandering with 410 ~ 614 Very few cutoffs 1 ~ 2m training work training work

4 POSSIBLE TREND OF CHANNEL PATTERN CHANGE DOWNSTREAM THE THREE GORGES RESERVOIR Schumm (1985) proposed a classification of channel patterns based on sediment transport and other hydraulics factors of river flow (for an elaborated diagram see Knighton, 1998), which may also be used to predict how the channel pattern will respond to changes in sediment transport or hydraulics factors in the drainage basin over the graded time scale. For instance, if the original channel pattern was unstable braided or meandering thalweg, reduction of sediment supply (especially reduction of bed load transport) will cause the channel pattern to change in a direction that may finally stabilize the alluvial channel to become an island braided or meandering type. What is important about this classification is its suggestion that reduction of sediment supply in a certain section of an alluvial river channel may also lead to systematic changes of the channel pattern , accompanied by widespread bank erosions, that may sustain for a period as long as the graded time (e.g., 100 years), in addition to the scour of river bed or channel incision. In such a process, two predictions can be made for the first 30 years: a) the ratio of bed load to total load will be larger than before, as suspended load from upstream decreases and bed load resulted from bank/bed erosion increases; b) total sediment load will be much smaller, as the suspended load, used to be the largest part of the total sediment load, will now be trapped in the Three Gorges Reservoir. However, such variation of hydrological conditions may lead to different directions of river pattern change as predicted by Schumm’ s theory. Xie Jianheng (1990) noted that the amount of bank erosion will be very limited in the upper Jingjiang River, because of the hilly landform on both side of the banks and the extensive amount of bank protection work. Development of bed erosion may also be stopped because of the cobble and gravel layers on bed surface which will soon lead to an armoring process. In contrast, the lower Jingjiang River does not have sufficient bank protection works to stop bank erosion, and the thick top layer of sandy materials of the channel bed allows great possibility of bed erosion. Considering the well-accepted observation that incision of the lower Jingjiang River channel will reduce the diversion discharge into the Dongting Lake, thus leading to more channel erosion there due to more discharge in the main channel, Xie suggested that, through its self-adjustment, the alluvial channel will be subject to more bank erosion than bed erosion, as bank erosion can result in a smaller carrying capacity by extending the length of the meandering channel ( i.e., reduction of channel slope). Xie estimated that the channel sinuosity will increase from the present 2.16 to 2.41 in 50 years’ time. Experimental observations shows that water flow with an excess of carrying capacity (i.e., clear water running into a mobile bed model constructed using Mississippi river sand and silt, see Fig. 3) will lead to a steady increase of channel sinuosity, until the thalweg reaches solid boundaries. During the process the channel may be widened due to migration of the thalweg, and point and medial bars will develop. But - 106 - International Journal of Sediment Research, Vol. 20, No. 2, 2005, pp. 102-108 中国科技论文在线 http://www.paper.edu.cn

there is still doubt if such results apply to the Jingjiang River reaches, where bed material in the main channel is quite different from that on the floodplain.

Fig. 3 Meandering of thalweg and bank erosion in the well-known model study by Friedkin (1945). The experimental run shown here lasted about 600 hours of laboratory time

5 COMPLICATIONS DUE TO THE RIVER-LAKE INTERACTION To understand the impact of channel scour on variations of channel patterns in the lower Jingjiang River, Han and Yang (2000) summarized detailed observations of fluvial processes in this river reach after several major natural and artificial cutoffs. The lower Jingjiang River happens to be located downstream of the divergences into the Dongting Lake and upstream of the confluence of Yangtze River and Dongting Lake (Fig.1). It was found out that these cutoffs lead to higher discharges in the lower Jingjiang River by reducing upstream diversions into the Dongting Lake. Increased discharge and surface slope lead to intensified channel bed erosion up- and downstream the cutoff points, although sediment load from upstream remain unchanged. This has resulted in considerable reduction of water level upstream of the cutoffs. Taking into account the increase in water level at Chenglingji (near station C in Fig.1), due to sediment deposition downstream, Han and Yang concluded that the increase of stream power immediately after those cutoffs was later dissip ated through adjustment of surface profile in the channel, rather than by an increase of channel sinuosity, i.e., the bed erosion will be inevitable but there will be no significant bank erosion. Nevertheless, at certain locations, bank erosion on the concave side can still intensify, therefore bank protection works will be important in the lower Jingjiang River. The channel pattern of the lower Jingjiang River will become a sinuous one maintained by engineering efforts, rather than the natural meandering channel as it is now.

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6 CONCLUDING REMARKS Increase in sinuosity is the way of the lower Jingjiang River to respond to increases in runoff (because of reduction of diversion into the Dongting lake) in the past 250 years. Several major cutoffs in the 1960s and 1970s reduced the sinuosity of lower Jingjiang River channels to 2.0. The channel has been stabilized through “key point” bank protection works. Its channel pattern was very changeable in the last 250 years, though there have been relatively higher stability in the past 30 years. Meander cutoffs changed the hydraulic slope of the flow (which increased the carrying capacity), but did not reduce the amount of sediment in the river (which would result in even larger increase in the carrying capacity). The impoundment of the Three Gorges Reservoir will lead to significant reduction of sediment supply to the downstream alluvial channel, and the previous theoretical analysis and predictions concerning the lower Jingjiang River channel pattern is now being put to test. The longer-term trend predicted by geographers indicate serious threats that may come with the benefits from the operation of the reservoir, as far as the safety of lower Jingjiang River is concerned, though there is more generalization than quantification on this specific case. On the other hand, engineers’ experience from somewhat different situations may not apply to this case, and efforts to maintain bank stability is of crucial importance to the safely of the dykes. The impoundment of the Th ree Gorges Reservoir provides unique opportunities to both geographers and engineers. They can either test their ability to predict the nature or test their capability to change the way a fluvial system naturally works.

ACKNOWLEDGMENTS Financial support of the work presented in this paper from China’s National Key Basic Research and Development Program (Grant No. 2003CB415206) and the National Natural Science Foundation of China (Grant No. 50179015, 50221903) is gratefully acknowledged.

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