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

http://www.paper.edu.cn 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 转载中国科技论文在线 http://www.paper.edu.cn 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) (c) (d) (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) International Journal of Sediment Research, Vol. 20, No. 2, 2005, pp. 102-108 - 103 - 中国科技论文在线 http://www.paper.edu.cn 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). - 104 - 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 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).

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