ADP-flow velocity profile to interpret hydromorphological features of China s Yangtze Three-Gorges valley

Chinese Science Bulletin 2005 Vol. 50 No. 7 679 684 ADP-flow velocity profile to interpret hydromorphological features of China s Yangtze Three-Gorges valley CHEN Jing 1, CHEN Zhongyuan 2,3, XU Kaiqin 4, WEI Taoyuan 1, LI Maotian 2, WANG Zhanghua 2 & Masataka Watanabe 4 1. Key Laboratory of Geographic Information Science, East China Normal University, Shanghai 200062, China; 2. State Key Laboratory of Estuarine and Coastal Research, Shanghai 200062, China; 3. Nanjing Institute of Geography and Limnology of Chinese Academy of Sciences, Nanjing 210008, China; 4. National Institute for Environmental Studies, Tsukuba, Japan Correspondence should be addressed to Chen Zhongyuan (email: Z.Chen@ecnu.edu.cn) Abstract In late May and early June, 2002, a field investigation was conducted along the Three-Gorges valley of the upper Yangtze catchment by ADP (Acoustic Doppler Profile SONTEK-500). Data obtained when surveying were accompanied with discharge of <15000 m 3 /s in the valley and characterize the unique river-flow velocity profile and riverbed morphology. Taking into consideration the relationship between the average flow velocity and fluvial variables, four distinct river sections can be highlighted, i.e. Chongqing- Wanxian, Wanxian-Fengjie, Fengjie-Zigui and Gezhou reservoir area (upstream to downstream). The average flow velocity is in-phase with river width from Chongqing to Wanxian. High-flow velocity ranging from 3.0 to 4.0 m/s is recorded at many sites, where the wider river channel (>1000 m) and shallower water depth (<20 m) occur and large-size gravel shoals prevail. Alternated low-flow velocity (<1.5 m/s) appears at those river sections with deep water (>50 m) and U-shaped river-channel morphology. Mapping the river cross-section area at those sites can determine that smaller cross-section area accelerates the flow velocity. From Wanxian to Fengjie, the average flow velocity ranging from 3.0 to 4.5 m/s is in-phase with the water depth. The high-flow velocity is associated with narrower river-channel, where V-shaped gorges valley occurs with small cross-section area. Further downstream from Fengjie to Zigui, the low flow velocity is linked to deep river channel characterized by W-shaped valley morphology of large cross-section area, in general. The average flow velocity is 2.5 3.5 m/s, and maximum can reach 6.0 m/s near Wu-Gorge. Our survey had also detected a slow-flow velocity (mostly <1.0 m/s) in the river channel of about 100 km long in the Gezhou reservoir downstream. Heavy siltation to 20 m thick above the former riverbed and about 20 km extending upstream from the Dam site occurs above Gezhou Dam. The backwater can reach 150 km due to elevated water level to 27 m by the damming at the end of 1970s, and riverbed erosion below the dam reaches 15 20 m. In addition, our survey records the deeper water river valley from Fengjie to Yichang, ranging from 70 to 80 m (>100 m; maximum) in the gorges valley (30 40 m below the present mean sea level). This contrasts to the relative shallow water river-channel above Fengjie, i.e. 20 30 m in general and 50 60 m, maximum at gorges site. The present ADP investigation displays the hydromorphological feature in the Three-Gorges valley, and most importantly, it accumulates invaluable dataset for the post-dam study in the near future. Keywords: ADP-flow profile, Gezhou Dam, hydromophology, Three- Gorges valley, Yangtze River. DOI: 10.1360/04wd0181 Land and sea are linked closely by large rivers through runoff and sediment discharging and biochemical material transferring into the sea [1 3]. The processes carried out by energy exchange have been in a sensitive response to global change [4]. To study the mechanism of sediment transport in the river basin as to reflecting hydromorphological and climatological controls is thus becoming vital for earth scientists, at present and in the near future since any change in fluvial landform is of a great concern for promoting our human society. To examine the characteristics of river flow setting velocity, which directly influences the sediment transport from sediment sources to coastal sink, can understand the hydromophological process in large rivers. A large quantity of fluvial sediments is carried under different flow settings to the various river sections, forming unique morphologies. The flow setting closely associated with seasonality can determine river-channel erosion, siltation, flooding impact and organic and inorganic C & N supplement to the estuary and ocean [5 7] ; the latter has been acting as the driving force for climatic fluctuations at global dimension [8]. The Yangtze River, a worldwide focus on its envi- ronmental change recently, has a total length of >6300 km, a drainage area of 180 10 4 km 2 and the average annul rainfall of 800 1600 mm (maxi. 2400 mm) [9]. Due to extensive damming upstream and river-lake siltation in the middle Yangtze reaches in the past century, the annual sediment load has decreased dramatically from 4.70 10 8 to 3.50 10 8 t/a [10]. It is also noted that due to global warming effect, rainfall in the Yangtze catchment increased in the past 10 years, which would result in the high risk of frequent floods [4,11]. Furthermore, the Three- Gorges dam will be completed in 2009 and will cause great concern on environmental change, which is closely associated with various side effects relative to social and economical responses [3]. Herein, the present survey is aiming at, on the basis of on-site measurement by means of advanced physical instrument, the establishment of hydromorphological database before and during damming, apart from exploring the present features of the valley. This will shed light on post-dam study on the Yangtze drainage basin and on the impact on estuarine environ- Chinese Science Bulletin Vol. 50 No. 7 April 2005 679

ment change. 1 Material and methods From May 21 to June 2 2002, Acoustic Doppler Profiler (ADP SONTEK-500 Hz) and Echo Sounder (449DF) guided with GPS were adopted to survey the Three- Gorges valley of 650 km long in the upper Yangtze catchment. There were two datasets obtained, i.e. from Yichang to Chongqing (upstream direction, May 21 to 29), and from Chongqing to Yichang (downstream direction, May 29 to June 2). Tracklines, while surveying, remained in the main river channels of the valley. A cross-section trackline was conducted about 300 m above Gezhou Dam site. The ADP apparatus was mounted by steel rigs outside the shipboard with 50 cm spaced and 3 transducers, each with 25 degrees vertically and 120 degrees horizontally, were extended 50 100 cm below the surface water. The ADP-500 Hz of the present study can reach the maximum water depth of 100 m with the highest resolution of 1.0 m, but two blind regions occurred during surveying, each with 1.0 m at the upper and bottom water layers. Our computing interpolation in the laboratory has overcome these vacancies. The recording resolution of the ADP-flow velocity profile is produced every one minute, and each meter of water depth produces an average flow velocity. ADP apparatus was well maintained on the daily basis. Two datasets were carefully examined after surveying. Seemingly, there is no big difference between them in regard to water velocity, water depth and riverbed topography, etc. Thus, the dataset with downstream direction (Chongqing-Yichang) was used for the present study. On-site measured data of ADP-flow velocity was treated by means of spatial analysis module in Arcview on the basis of removal of false velocity recorded from acoustic reflection. The water level below Gezhou Dam of the present study referred to 1980s topographic map (1 25000), while the water level and river width data above Dam were cited from the 1950s (1 10000). Comparison between the instant water level (http://www.cjh.com.cn) and the one in the past 50 years demonstrates little change in Three-Gorges valley above Gezhou Dam. This serves as the base for deriving river-channel morphology and areas of cross-section targeted. Data of the Gezhou reservoir are derived from this field survey. 2 Results and discussion Since there had been little rainfall during surveying and few large tributaries adjoining into the Yangtze valley, it is estimated that the discharge was <15000 m 3 /s on the basis of on-site ADP cross-section measurement in the reservoir. The treated flow velocity profile clearly shows 3 layers of flow velocity, i.e. top, middle and bottom layers. In general, the average flow velocity (2.0 3.0 m/s) of the top and middle layers is faster than the bottom one (Fig. 1). In the light of fluvial variables as presented by river width, water depth and flow velocity, there are 4 sections of river-channel of Three-Gorges valley recognizable with their hydromophological implications. Clearly, the average flow velocity (grouped from the water profile) from Chongqing to Wanxian (about 300 km long, Fig. 1) is in-phase with measured river width. Approximately twenty (No. 1 20) sites with high-flow velocity ranging from 3.0 to 4.0 m/s were observed from those reaches characterized by wider (>1000 m) and shallower (<20 m) water-depth river channels. Large-sized (often >500 m long and >200 m wide) gravel shoals prevail there (seeing selective cross-sections 2, 8, 10, 13; Figs. 1 and 2). Alternatively, there are 6 sites (No. I VI; Figs. 1 and 2) of low-flow velocities (<1.5 m/s) occurring in the river section from Chongqing to Wanxian, where the water depth of river channel is deeper (>50 m), featured with U-shaped morphology (seeing selective cross-section I IV; Figs. 1 and 2). Calculation of cross-section area in relation to river flow velocity was applied to the above 26 river channel sites. The result reveals that the flow velocity of the Chongqing-Wanxian section is out-of-phase with rivercross section area (Fig. 3(a)), which is in negative correlation to the river width and positive to water depth (Fig. 3(b)). In contrast, the flow velocity from Wanxian downstream to Fengjjie (about 130 km long) is in agreement with water depth and is usually greater than that of the Chongqing-Wanxian valley. The average flow velocity in the middle and upper water layers ranges from 3.0 3.5 m/s and the maximum can reach 4.5 m/s. High-flow velocity occurs in deep water (>50 m) river channel. This is supported by 13 sites (Fig. 1; 21 33), featured with deep water depth (>50 m) and narrow (<600 m) river channel. Usually, the V-shaped morphology is dominant (see selective cross-sections 22, 26, 32 in Fig. 2). There is only one site with low-flow velocity of <1.0 m/s recorded (Fig. 1; VII). Our field observation witnesses that cobbles and gravels consist of this river section. Inverse relation between the flow velocity and water depth occurs in the downstream reaches from Fengjie to Zigui (about 100 km long), where the low-flow velocity is associated with deep valley (Fig. 1). The average flow velocity in the middle and upper water layers ranges from 2.5 to 3.5 m/s, while the maximum can reach >6.0 m/s (near Wu-Gorge, Fig. 1). Here, there are 6 sites with highflow velocities up to 6.0 m/s (Fig. 1, 34 39) and 2 sites with low-flow velocities of <2.0 m/s (Fig. 1, VIII and IX). River width is no wider than 500 m and the narrowest can be < 300 m, accompanied with W-shaped river valley of large cross-section area (seeing selective cross-section 37, Fig. 2). Similar to what is observed from the upstream river- 680 Chinese Science Bulletin Vol. 50 No. 7 April 2005

Fig. 1. Distribution of ADP flow velocity profile in Three-Gorges valley. Surface water gradient and river width above Gezhou Dam are derived from Navigation Charts of Upper Yangtze River, 1954; those below the Dam are from Navigation Charts of Middle Yangtze River, 1983 and the present field survey provides the data of Gezhou reservoir. Chinese Science Bulletin Vol. 50 No. 7 April 2005 681

Fig. 2. Fluvial morphology of representative cross-sections in Three-Gorges valley. Water depths are obtained from Navigation Charts of Upper Yangtze River, 1954. channel valley, flow velocity at above 22 sites is out-ofphase with cross-section area (Fig. 3(a)). Although the river channel from Wanxian to Fengjie is deeper, the cross-section area of its own is more controlled by narrower river width. In contrary, the river-cross section from Fengjie-Zigui is more influenced by the water depth (Fig. 3(b)). A slack-water flow velocity of <1.0 m/s on average was detached by the present ADP profiling at the river section from Zigui to Gezhou reservoir (about 100 km long, Fig. 1); the latter was closed at the end of the 1970s and is about 40 km spaced to Three-Gorges Dam site upstream. Obviously, the low-flow velocity is due to the post-dam effect. In theory, river flow velocity (v) is closely associated with cross-section area (A), other than individual fluvial variable, such as river width or water depth. The following formula usually applies to the correlation [12] Q = A*v (Q-discharge). Since there was little change in discharge during survey, it can be concluded that the flow velocity measured in the present study should vary contrarily with cross-section area from the above formula (Fig. 3(a)). The high-flow velocity (Chongqing to Wanxian) occurs in the wider and shallower river valley with many gravel shoals, resulting in smaller river-cross area. From Wanxian downstream to Fengjie, in spite of deep water, narrower and V-shaped river valley leads to small cross-section area to form the high-flow setting. The cross-section area from the further downstream Fengjie to Zigui is obviously larger than that of the above reaches due to W-shaped valley morphology. This can slow down the flow velocity (Fig. 3(b)). Thus, the flow velocity distribution in the Three- Gorges valley is closely linked with the river channel cross- section area, which is determined primarily by geological variables, such as rock formation, tectonic setting and gradient [2]. The present study using Echo Sounder also reveals the riverbed topography in Three-Gorges valley (Fig. 4). As a whole, the river channel from Fengjie to Yichang is characterized by the dramatic topographic change in riverbed and deep water. At Qutang-, Wuxia- and Xiling-Gorge the water depth reaches 70 80 m and maximum can be >100 m near Shibei (Fig. 4), where the riverbeds can be 30 40 m below the present mean sea level. In comparison, the water depth above Fengjie decreases as riverbeds elevation increases, though the water depth at some sites can still be 50 60 m, such as Fengdu, Qingxichang and Changshou (Fig. 4). Our survey also recorded 27 m elevated water level of Gezhou Dam and backwater area 682 Chinese Science Bulletin Vol. 50 No. 7 April 2005

Fig. 3. River-flow velocity in relation to hydromorphological variables in the Three-Gorges Yangtze valley. Water depths and widths are derived from Navigation Charts of Upper Yangtze River, 1954; areas are computed on the basis of the charts. Fig. 4. Riverbed morphology of Three-Gorges valley from Chongqing to Yichang. Water level is the same as that of Fig. 1. Chinese Science Bulletin Vol. 50 No. 7 April 2005 683

of 150 km long upward extending from Dam site (Figs. 1 and 4). Accordingly, heavy siltation up to 20 m above Gezhou Dam has occurred on the former riverbed, primarily along the right river-channel flank (downstream direction), which extends about 20 km upstream from Dam site (Figs. 4 and 5). Also, riverbed erosion below Dam to 15 20 m is observed (Fig. 4). Fig. 5. Changes in river-channel cross-section of the Gezhou reservoir area before and after damming. Data sources are from Navigation Charts of Upper Yangtze River, 1954 and present field survey. 3 Closing marks As is well known, the normal regulative water level after Three-Gorges to be dammed in 2009 will be elevated up to 145 175 m. This has been the world-wide focus on the potential changes in hydrography closely associated with a series of impacts on fluvial environment change, including the river mouth area. It is hoped that the present study, although by a limited time and tools, can highlight the pre-dam hydromorphological implications, which will serve as useful database for post-dam study in the near future. Acknowledgements The authors sincerely thank senior engineer Y. Z. Xue, assistant engineer J. H. Gu, Y. W. Zhao and L. Q. Li for their kind assistance in the field survey. Appreciation is extended to F. L. Yu and Y. H. Li who helped with data processing. Gratitude should also be given to Department of Geography/University College, University of Durham, for providing the visiting fellowship to Z. Y. Chen, who has the unique chance to modify this paper. This work was supported by the National Natural Science Foundation of China (Grant No. 40341009), APN/START (Grant No. 2004-06-CMY) and the Global Environment Research Fund of the Ministry of the Environment of Japan. References 1. Milliman, J. D., Meade, R. H., World-wide delivery of river sediment to the oceans, Journal of Geology, 1983, 91: 1 22. 2. Miller, A. J., Gupta, A., Introduction, in Varieties of Fluvial Form (eds. Miller, A. J., Gupta, A.), Chichester: John Wiley & Sons, 1999, 1 7. 3. Chen, Z., Yu, L. Z., Gupta, A., The Yangtze River: an introduction, Geomorphology, 2001, 41(2-3): 73 75. 4. Jiang, T., Cui, G. B., Xu, G. H., Climate change and Changjiang River floods, Journal of Lake Sciences (in Chinese), Beijing: Science Press, 2003, 1 288. 5. Shen, H. T., Material Flux of the Changjiang Estuary (in Chinese), Beijing: China Ocean Press, 2001. 6. Chen, Z., Li, J. F., Shen, H. T., Historical analysis of discharge variability and sediment flux, Geomorphology, 2001, 41(2-3): 77 91. 7. Shen, Z. L., Liu, Q., Zhang, S. M. et al., A Nitrogen budget of the Changjiang River catchment, Royal Swedish Academy of Sciences, 2003, 32: 65 69. 8. Wang, P. X., Tian, J., Cheng, X. R., Carbon reservoir changes preceded major ice-sheet expansion at the mid-brunhes event, Geology, 2003, 31(3): 239 242. 9. Changjiang River Water Resources Commission, Atlas of the Changjiang River Basin (in Chinese), Beijing: Sinomaps Press, 1999, 54 61. 10. Yang, S., Zhao, Q. Y., Belkin, I. M., Temporal variation in the sediment load of the Yangtze River and the influences of human activities, Journal of Hydrology, 2002, 263: 56 71. 11. Zhang, S. L., Tao, S. Y., Zhang, Q. Y. et al., Large and meso-a scale characteristics of intense rainfall in the mid- and lower reaches of the Yangtze River, Chinese Science Bulletin, 2002, 47(9): 779 786. 12. Pan, S. R., Wu, G. H., Chen, C. K. et al., Physical Geography (in Chinese), Beijing: Higher Education Press, 1978, 182 183. (Received June 3, 2004; accepted November 15, 2004) 684 Chinese Science Bulletin Vol. 50 No. 7 April 2005