Simulated sediment flux during 1998 big-flood of the Yangtze (Changjiang) River, China

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1 Journal of Hydrology 313 (200) Simulated sediment flux during 199 big-flood of the Yangtze (Changjiang) River, China Kaiqin Xu a, Zhongyuan Chen b, *, Yiwen Zhao c, Zhanghua Wang c, Jiqun Zhang a, Seiji Hayashi a, Shogo Murakami a, Masataka Watanabe a a National Institute for Environmental Studies, Tsukuba 30-0, Japan b State Key Laboratory for Estuarine and Coastal Research, East China Normal University, Shanghai 20002, China c Department of Geography, East China Normal University, Shanghai 20002, China Received April 2003; revised 2 February 200; accepted 10 March 200 Abstract The present study focuses on simulating sediment flux for 199 big-flood with 0-year recurrent period in the Yangtze River catchment. On the basis of close correlation between discharge and sediment load recorded on the daily base of the past decades at a series of hydrological gauging stations located in the Yangtze River, the sediment rating curve of 19/19 was selected to simulate the annual and flood season sediment fluxes of 199, when measured discharge was available in the most gauging stations. The result indicates that enormous sediment load was delivered downstream and to the estuary during the flood year. The simulated annual sediment flux was about 930 million-tonnes in the upper drainage basin, about 20 million-tonnes in the middle catchment and 20 million-tonnes in the lower drainage basin. These loads, respectively, approximate almost 1.9, 1.2, and 1. times those of the multiyearly sediment flux in the upper, middle and lower Yangtze catchments for the past decades. The result also indicates a unique pattern of sediment transport downstream through the drainage basin during the high flow season (early July to mid-september). While the upper Yangtze tributaries delivered about 0 million-tonnes of sediment downstream, the 3-Gorges valley added additional 20 million-tonnes. This totals about 0 million-tonnes that supplied the middle and lower Yangtze catchments, of which about 0 million-tonnes were silted in the middle catchments, immediately downstream of the exit of 3-Gorges. This amount is almost. times the normal flood season averaged over the last 0 years. High sediment load was also recorded in the river mouth area during the flood season, where 0 million-tonnes were delivered to the estuary and East China Sea, about 3. times that in a normal flood season. Intensifying human activity in the upper catchment is responsible for the large amount sediment sources delivered downstream and to the coastal region. q 200 Elsevier B.V. All rights reserved. Keywords: Catastrophic flood; 0-Year recurrent period; Human impact; Sediment rating curve; Simulated sediment flux; Sediment transport pattern; Yangtze (Changjiang) catchment * Corresponding author. Address: State Key Laboratory for Estuarine and Costal Research, East China Normal University, Shanghai 20002, China. Fax: C address: z.chen@ecnu.edu.cn (Z. Chen). 1. Introduction Large rivers in Asia contribute a large proportion of sediment sources to coastal depositional sink /$ - see front matter q 200 Elsevier B.V. All rights reserved. doi:10.101/j.jhydrol

2 222 K. Xu et al. / Journal of Hydrology 313 (200) and the world ocean basins as well (Milliman and Syvitski, 1992). The Yangtze River of China serves as the major sediment sources (0 million-tonnes/year; multiyearly base) area, where intensifying human activity includes impoundment of numerous reservoirs (more than 3,000; Changjiang Water Conservancy Commission, 1999) in the catchment, heavy deforestation, 3-Gorges Dam project (being completed by 2009), and on-going South North Water Transfer Project (Fig. 1; Xu et al., 2000a; Chen et al., 2001a). Recently, enormous research efforts have been made to elucidate hydrological, geomorphological and ecological aspects of the river basin in relation to rainfall and floods, sediment yield and transport, fluvial dynamics and nutrient delivery (Chen, 199; Li et al., 1999; Duan et al., 2000; He and Jiao, 2000; Chen et al., 2001a; Du et al., 2001; Higgitt and Lu, 2001; Yin and Li., 2001; Lin et al., 2002; Yang et al., 2002a,b; Lu et al., 2003; Shen et al., 2003). Many projects have also been undertaken to study the changes in fluvial environment in response to global/regional change (Chen et al., 2001a). Originated from the Qinghai-Tibet Plateau, the Yangtze River flows eastward through 11 provinces and reaches the East China Sea by-passing the metropolitan city of Shanghai. The river is longer than 300 km, with a catchment area of 1. million km 2, accounting for 1.% of the Chinese nation s total territory. The river basin consists of about % mountains and hilly regions, 11% plains and % rivers and lakes. Six major tributaries have drained the upper Yangtze plateau (Fig. 1). The Jinshajiang, Yalongjiang, Minjiang, Tuojiang and Jialingjiang all drain principally southward into the Yangtze trunk channel, while the Wujiang exclusively drains northward from the karst uplands of Guizhou Province, where agriculture prevails. The Yangtze drainage basin experiences a subtropical monsoon climate initiated from the southeast Pacific Ocean and Indian Ocean. Annual precipitation varies generally from 00 mm to more than 2200 mm between the upper and lower drainage basin, but is less than 300 mm in the westernmost plateau (Changjiang Water Conservancy Commission, 1999). The maximum Fig. 1. Yangtze drainage basin and locations of major hydrological gauging stations.

3 K. Xu et al. / Journal of Hydrology 313 (200) precipitation reaches 100 mm/year in the south central Dongting drainage basin of the middle Yangtze reach (Fig. 1). It is noteworthy that about 0% of this precipitation occurs during the wet season (May to October), and meanwhile, a large amount of runoff drains from the various sub-basins to the Yangtze trunk channel. These often generate huge floods. Historical documents recorded that large flood peaked one after another within a very short time interval (less than 1 2 days), due to shifted rainfall zones among upstream subbasins (Changjiang Water Conservancy Commission, 2001). In the last century, at least 1 large floods (more than.0!10 m 3 /s, middle Yangtze reach) were recorded and large flood tends to increase, most likely due to human alternation on river lake morphology (Changjiang Water Conservancy Commission, 1999; Xu et al., 2000b; Chen et al., 2001b; Du et al., 2001). The river carries a tremendous volume of sediments downstream to shape and alter the river basin topography, which inevitably threatens the livelihoods of people living in the densely populated river valley. During 199, a catastrophic flood event occurred as a recurrent period of about 0 years, cited on the basis of Hankou hydrological gauging station of the middle Yangtze reach (Changjiang Water Conservancy Commission, 2001). This event would have inundated the entire middle and lower Yangtze River floodplain had flood defense dykes not been elevated with time. The requirement for effective flood mitigation and prevention remains extremely critical. This situation arises because high dykes along the both riversides have been built over centuries along the middle and lower Yangtze River, where in response, the riverbed has risen through siltation until it stands presently 12 1 m above the adjacent floodplain (Chen et al., 2001a). Elevated rates due to sediment loss from river catchment in relation to human activity severely affect the change of fluvial morphology and river channel pattern (cf. Miller and Gupta, 1999; He and Jiao, 2000; Chen et al., 2001a; Gupta, 2002). To better understand the behavior of sediment transport and accumulation in the river channel, it is essential to study sediment flux particularly during the flood season. To do this, quantifying the sediment flux via measured discharge data seems a practical approach since it is almost impossible to measure systematically sediment concentration, especially for a large river system, like Yangtze during the high flow season with severe weather conditions (Qian et al., 19). The sediment rating curve, an effective method to express the close correlation between discharge and sediment load, or sediment concentration (SC), came to use almost a century ago (Ponce, 199). Using this approach, prediction on suspended sediment concentration and flux, and sediment erosion rate, etc. has become possible (cf. Walling and Webb, 19; Fuller et al., 2003; Horowitz, 2003). The present study is performed by the sediment rating curve on the basis of existing hydrological database recorded at many gauging stations sited along the river banks to simulate sediment flux for the major flood year of 199. The purpose of the effort would estimate the sediment budget for all important river sections, which will help understand associated river channel erosion or siltation, and more importantly, quantify a sediment budget for material delivered to the coast and sea during the catastrophic flooding event, since it is assumed that a considerable amount of sediment flux would be missed while measuring, during the catastrophic flood event. The quantification enhances further understanding of the sediment balance to allow improved river-basin management, including flood mitigation and prevention, and to support prediction of the potential impact of sediment being entrapped by 3-Gorges Dam and by South North Water Transfer project on the river basin and coastal zone in the near future. 2. Data sources and method In the present study, six () major hydrological gauging stations were selected from the Yangtze trunk channel, i.e. (from upstream downward): Cuntan, Yichang, Jianli, Luoshan, Hankou, and Datong (Fig. 1). Cuntan station is sited by Chongqing city in the upper Yangtze drainage basin; Yichang stands at the exit of the 3-Gorges; Jianli, Luoshan and Hankou are located in the middle Yangtze reach, and Datong, about 00 km upstream of the estuary, represents the lower Yangtze catchment. Daily measured discharges (Q) and sediment concentration for 9 years from 190s to 190s (Fig. 2) were randomly collected from these stations, documented as Internal Report on Water and Sediment (Changjiang Water Conservancy Commission, ). The sediment rating curve between discharge and sediment load (SC!Q) was derived for

4 22 K. Xu et al. / Journal of Hydrology 313 (200) Log(SS*Q) (g/s) Log(SS*Q) (g/s) Log(SS*Q) (g/s) Cuntan Yichang y = 2.x -.3 R 2 = 0.92; N= Log(Q) (m 3 /s) y = 3.191x -.0 R 2 = 0.99; N= Log(Q) (m 3 /s) Jianli..... y = 2.21x R 2 = 0.999; N= Log(Q) (m 3 /s) Log(SS*Q) (g/s) Log(SS*Q) (g/s) Log(SS*Q) (g/s) Luoshan y = 1.21x R 2 = N= Log(Q) (m 3 /s). Hankou y = 2.021x - 1. R 2 = 0.92; N= Log(Q) (m 3 /s)..... Datong y = 2.9x -.0 R 2 = 0.931; N= Log(Q) (m 3 /s) Fig. 2. Sediment rating curve established at the selected hydrological gauging stations (including, from upstream downwards, Cuntan, Yichang, Jianli, Loushan, Hankou, and Datong). Data were collected from Changjiang Water Conservancy Commission (1 1990). these all stations (Fig. 2), and the one selected from the years of 19/19 was used to simulate sediment flux (Figs. 2 and 3). The reason of selection is given in Section. From the relationship between Q and SC!Q, following correlation can be established L Z aq b (1) where L is the sediment load, Q is discharge, and a and b are the constant, given by the log (L)versusLog (Q) plot. Using this approach, the sediment flux of 199 could be simulated for the six major hydrological gauging stations (Fig. a; Changjiang Water Conservancy Commission, 199), where discharge was measured daily on site during that time. Furthermore, the discharge record for the 199 flood season (about 2. months from early July to mid-september, 199; starting and ending times can be a few days delayed in the lower catchment) was separated from the annual database. This procedure enables us to obtain

5 K. Xu et al. / Journal of Hydrology 313 (200) (a) Annual sediment load (10 ton) (b) Annual sediment load (10 ton) observed 19-simulated Cuntan Yichang Jianli Luoshang Hankou Datong Major stations 19-observed 19-simulated Cuntan Yichang Jianli Luoshang Hankou Datong Major stations Fig. 3. Observed and simulated annual sediment fluxes of 19/19 at the six hydrological gauging stations. the simulated sediment flux for the flood season for each of the study stations (Fig. b). Due to differences identified between the observed and simulated sediment fluxes in 19/19 (Fig. 3), a calibration was applied to the six major stations to optimize the simulated result both for the annual and flood season of 199. The mean error identified at each station was applied for the calibration, derived from the differences between the observed and simulated values of 19/19 listed in Table 1. Measured sediment concentrations at the Cuntan station in Chongqing, during 199 were also available (Changjiang Water Conservancy Commission, 199) for the present study (Fig. a). The daily distribution of sediment concentration for the wet season (May October) of 199 is shown in Fig.. Due to deficient sediment sources that occurred during the second flood peak of 199 recorded at Cuntan station (Fig. ), we performed a further calibration on the previously calibrated base, which produces two outputs (cases 1 and 2, Fig. a and b). Cases 1 and 2 are calculated, respectively, on the basis of: (1) the previously calibrated annual sediment flux deducted by the deficient quantity versus to the annual sediment flux, and (2) the previously calibrated flood-season sediment flux deducted by the deficient quantity versus to the sediment flux of the flood time period (about 2. months). Details are given by following steps (also indicated in Fig. ). Firstly, we set up theoretically the formula to obtain sediment deficient from Cuntan station: Sept:1 A 3 Z X ða 1 KA 2 Þ (2) Aug:1 Here, A 1 represents daily simulated sediment load of 199 by sediment rating curve 19/19 (Fig. 2); and A 2 denotes daily observed sediment load of 199, and A 3 equals the sum of overestimated value (from Aug. 1 to Sept. 1, 199, the time period of secondary flood peak). Secondly, we define case 1 as: P 1 Z A 3 = P 1 Dec: X31 Jan: 1 A 1!!100% (3) represents the ratio between overestimated value (A 3 ) and yearly sediment flux of 199;

6 22 K. Xu et al. / Journal of Hydrology 313 (200) Sediment load 10 t Cuntan Yichang Jianli Luoshan Hankou Datong Simulated Calibrated Case 1 Measured Sediment load 10 t Cuntan Yichang Jianli Luoshan Hankou Datong Simulated Calibrated Case 2 Measured Fig.. (a) Simulated sediment flux for 199-flood year; and (b) simulated sediment flux for the flood season (early July to mid-september, 199). Calibration and recalibration procedures are discussed in text. Average errors used for the calibration are listed in Table 1. Case 2 as: P 2 Z A 3 = Sept: X31 July 1 A 1!!100% () P 2 represents the ratio between overestimated value (A 3 ) and flood season sediment flux of 199. By this approach cases 1 and 2, recalibrated result both for annual and flood season at Cuntan gauging station can be obtained (Fig. a and b). Table 1 Observed and simulated sediment fluxes for 19/19 at six major hydrological stations along the Yangtze trunk channel No. Main station 19 Observed 19 Simulated Error (%) 19 Observed 19 Simulated Error (%) Average error (%) 1 Cuntan 1.!10 1.2! !10.0! Yichang 3.0!10 19.! !10 03.! Jianli 2.!10 2.! Luoshan 2.2!10 3.!10 K !10 3.0!10 K0. K.9 Hankou 1.2!10 3.!10 K10.2 K10.2 Datong 0.!10 3.0!10 K1. 3.3!10 3.!10 K2. K. Unit: tonne. Errors for each station were calculated and averaged.

7 K. Xu et al. / Journal of Hydrology 313 (200) Discharge (m 3 /s) Cuntan Station Discharge SC Sediment concentration (g/l) Date Fig.. Measured discharge and sediment concentrations at Cuntan hydrological station in the upper Yangtze drainage basin, during the wet season, 199. Thirdly, for application also using this approach, we are trying to calibrate the rest five gauging stations downstream, where only discharge data were known, by: Case 1ZC y!(1kp 1 ), and case 2ZC r!(1kp 2 ), where C y, calibrated sum of sediment flux of 199; C r, calibrated sum of sediment flux of 199. Cases 1 and 2 (in percentage), then, can be applied to the previously calibrated results of another five major hydrological stations (Fig. ), on the basis of assumption that similar hydrographic fluctuation between discharge and sediment concentration recognized at Cuntan station existed throughout the Yangtze River trunk channel during 199 big-flood. 3. Observation and results The regression analysis between discharge and sediment load using 19/19 database for the six hydrological gauging stations demonstrates high coefficients of determination (Fig. 2). Of note, correlation coefficients are generally higher in the upper Yangtze (Cuntan, Yichang and Jianli stations) than in the middle and lower Yangtze (Loushan, Hankou and Datong stations). Values of power function b in regression equation are obviously higher for the upstream stations, especially for Cuntan and Yichang (Fig. 2), and are lower for the downstream stations. Comparing the simulated sediment flux with that observed in 19/19 (Fig. 3; Table 1) reveals 1 19% estimation higher than that of observed for Cuntan and Yichang, and.9 10% lower than that below Yichang. The Jianli hydrological gauging station, about 300 km downstream from Yichang, seems the turning point, where the difference decreases to about 0.3% (Table 1). Using averaged error derived from 19/19 listed in Table 1, calibration was applied to the simulated sediment flux of 199 at each gauging station (Fig. ). Using the method of cases 1 and 2 can produce further calibrated result, showing that the annual sediment flux of 199 reaches about 0 milliontonnes at Cuntan in the upper Yangtze catchment (almost equivalent to the measured sediment flux of 20 million-tonnes; Fig. a); about 930 milliontonnes at Yichang, 0 million-tonnes at Jianli, 0 million-tonnes at Luoshan, 20 million-tonnes at Hankou and 20 million-tonnes at Daton (Fig. a; Table 2). Through the same method, calibrated sediment flux for the flood season of 199 is about 0 milliontonnes recorded at Cuntan, 0 million-tonnes at Yichang, 30 million-tonnes at Jianli, 290 milliontonnes at Luoshan, 30 million-tonnes at Hankou and 0 million-tonnes at Datong (Fig. b; Table 2). These quantities are more than 90% in proportion to the total annual sediment flux of the upper Yangtze

8 22 K. Xu et al. / Journal of Hydrology 313 (200) Cuntan station Suspended sediment concentration Sediment defficiency (A3) Discharge , 199 Method: Simulating Sediment load in 199 A1 = daily simulated sediment load of 199: A1= Q 2. / 10.3 * 2 * 300 / 10 (ton) (correlation equition derived from 19/) A2 = daily observed sedimen load of 199: A2 = Q * S.C. * 2 * 300 /10 3 (ton) A3 = sum of overestimated value (Aug.1~ Sep.1 ): Sep.1 A3= (A1-A2) Aug.1 Dec.31 Case 1: P1 = ( A 3 / A1 ) * 100 % ( P1-Overestimated value/yearly sediment flux) Jan.1 Sep.1 Case 2: P2 = ( A3 / A1 ) * 100 % (P 2 -Overestimated value/flood season sediment flux) July1 Application: Cy ~ Calibrated sum of 9 Cf ~ Calibrated flood value Case 1 = Cy * (1 - P 1 ) Case 2 = Cf * (1 - P ) 2 Fig.. Schematic diagram explaining simulation procedures for Cuntan hydrological gauging station. Details are discussed in the text and refer to Fig.. (Cuntan and Yichang), and are between 1 and 3% in proportion to the annual ones of the middle and lower Yangtze (Jianli, Luoshan Hankou and Datong stations; Table 2). The record of measured discharges at Cuntan indicates that there were two peaks in the flood event between July 1 and September 1, 199. Examining the distribution of sediment concentration throughout the flood season, we noted that the highest value (up to.0 g/l) occurred at the initiation of the first flood peak. Subsequently, sediment concentration decreased gradually to about g/l although the second flood peak pulses. When plotted against measured discharge, it is known that the sediment concentration during the second flood peak did not fully comply with discharge increase, indicating partial exhaustion of sediment sources during the second flood peak (Fig. ). Thus, the calibration is really needed for this case as the result of cases 1 and 2 shown in Table 2 and Fig..

9 K. Xu et al. / Journal of Hydrology 313 (200) Table 2 Simulated, calibrated and recalibrated (cases 1 and 2) sediment fluxes for annual and flood season (early July to mid-september) during 199 Hydrological stations Simulated (annual;!10 Calibrated (annual;!10 Case 1 (annual;! years averaged* (!10 Simulated (flood time;!10 Calibrated (flood time;!10 Case 2 (flood time;!10 Multiyearly averaged* (!10 Cuntan Yichang Jianli Luoshan Hankou Datong * Averaged sediment fluxes for last decades. Case 2/ case 1. Discussion The present study accentuates the simulating sediment flux of big 199 flood by the sediment rating curve established on the basis of former multiyear hydrological database. This method, which has been widely used to discuss sediment transport and sediment flux in river basin (cf. Asselman, 2000; Horowitz et al., 2001; Benkhaled and Remin, 2003), reveals characteristics of sediment yield, transport and associated fluvial morphological response while flooding. The close correlation between discharge and sediment load existing in the Yangtze catchment (Fig. 2) provides the feasibility of simulation of 199, when daily measured sediment concentration is inadequate. Even implicitly sometimes due to changed boundary with time, i.e. sediment sources in the upper drainage basin and flow stability, etc. this method, however, seems favoring the prediction of sediment flux (Horowitz, 2003). Also, the assumption of the present study is to stand on the base that one would never reach with satisfaction the maximum sediment flux in the light of SC measured during the flood season in hydrological gauging station. This inadequacy would lead to a considerably missing sediment budget through basin and to the coast. The sediment rating curve of 19/19 was selected to simulate the 199 sediment flux (Figs. 2 ), primarily owing to closer in time to the target year in relation to similar drainage basin setting (virtually, sediment rating curves listed in Fig. 2 are all highly correlated). Simulated sediment fluxes higher and less than that of observed above and below Jianli station may be attributed to adoption of values for the power function b in the regression equations (Figs. 2 and 3). The variation of b is much likely associated with geological and climatic controls in the different river sections in the Yangtze catchment (cf. Syvitski et al., 2000). Higher b in the upper Yangtze River could be tied with steep gradient and rock-confined valley in the 3-Gorges valley, which sustains a faster river flow with a limited range of sediment sizes (cf. Thorne et al., 199; Chen et al., 2001b). In contrast, the lower b derived from the middle and lower basin may reflect nature of meandering river pattern there, where considerably flat fluvial topography and reduced river flow widens the distribution of sediment-sizes, and wash load prevails (Qian et al., 19; Chen et al., 2001b). These were taken into account while simulating the sediment flux of 199 big-flood due to b occurrence (Table 1; Fig. ). The effect of deficient sediment concentration during the second flood peak in 199, which was included when simulating the sediment flux, has to be deducted from the annual sediment flux, as well as from the sediment flux of the flood season (Fig. a and b). It is likely that each flood generates the highest sediment concentration at the initiation of the event, and then the concentration gradually decreases to a certain level as flooding continues (Fig. ). During the 199 flood, sediment concentration in the upper Yangtze catchment was as high as 3..2 g/l in the first peak of flood (more than 0,000 m 3 /s), but it stabilized gradually to about g/l even though the discharge of the second peak increased to

10 230 K. Xu et al. / Journal of Hydrology 313 (200) ,000 m 3 /s (Fig. ). However, the coefficient of this rating curve can be still highly corrected (R 2 Z0.9) after examining. It is understood that the partial sediment insufficiency during Aug. 1 Sept. 1 (Figs. and ) will not impact largely the rating curve credibility. The simulated results for the present study demonstrate that about 20 million-tonnes sediment were supplied to the river from the 3-Gorges valley, which was delivered downstream during 2. months flood season (case 2, Table 2; the difference between Cuntan and Yichang). This high sediment yield in the upper catchment can be explained by intensifying human activity in the 3-Gorges valley due to deforestation, slope farming and changes in landuse to agricultural and industrial purposes (Fig. ). Our recent field reconnaissance witnessed that the relocation of residence from the valley base to upper mountain has lead to many large-scale geo-engineerings, i.e. housing, bridging, and highway construction. These have triggered inevitably a large quantity of sediment loss as deforesting processes. Many alluvial fans wash down slope in the valley, often extending hundreds or even to more than 1000 m long (Fig. a). Our recent investigation indicates that in the past 0 years there has been more than 130 million-tonnes/ year sediment accumulated in the Dongting Lake. The present study reveals that in 199-flood season, more than 0 million-tonnes sediment was delivered into the lake (the difference between loads at Jianli and Luoshan hydrological stations; Table 2; Fig. 1), which is almost 3. times as much as the multiyearly base. Sources witness that the lake area has shrunk largely from O000 to 22 km 2 in the last century (Changjiang Water Conservancy Commission, 2001). This finding illustrates the catastrophic nature of the flood in modifying the drainage basin morphology in ways that dwarf the impacts of processes operating during normal flood years. A large amount of sediment can be stored behind the numerous dams constructed during s in (a) (b) (c) (d) Fig.. Sediment yield processes in the 3-Gorges valley. (a) and (b) Alluvial fans formed due to deforestation. (c) and (d) Slope farming and changes in landuse.

11 K. Xu et al. / Journal of Hydrology 313 (200) the Yangtze drainage basin (more than 3,000; Changjiang Water Conservancy Commission, 1999; Yang et al., 2002a,b), including the major Gezhou dam that was completed in the end of 19 immediately downstream of the 3-Gorges. These have served as sediment reservoirs for the upper catchment (Yang et al., 2002a,b). Increases in sediment storage behind dams and lakes explain why the annual sediment supply to the lower Yangtze and estuary has largely lessened from about 0 to 30 million-tonnes over the past half-century (Changjiang Water Conservancy Commission, 1999; Chen et al., 2001b). Annual sediment fluxes of 930, 20 and 20 million-tonnes were simulated in the Yichang, Hankou and Datong stations, respectively, during the 199 flood (case 1; Table 2). These are almost 1.9, 1.2 and 1. times greater than the annually averaged sediment flux for the last 0 years (Chen et al., 2001b). Differences in sediment flux (expressed as case 2/case 1, Table 2), ranging from more than 90 (upstream) to 1 3% (downstream) actually reflect the proportion of sediment flux of flood season to annual one. Clearly, this decreasing percentage indicates the increase in sediment concentration in river water with distance downstream during the non-flood season. As river channel widens and gradient becomes gentler, fine-grained particles are getting largely concentrated in the middle and lower Yangtze catchments as suspended proportion both derived from the upper sources area and the adjacent flood plains as wash load in dry season. The sudden changed gradient (2 3!10 K ) of the middle Yangtze River course from the upper 3-Gorges region (10 0!10 K ) drives rapid siltation of sediment from the upper catchments, leading to a heavy aggradation on river bed (Chen et al., 2001b). The remainder of the sediment load was continuously carried downstream to the coast via the Datong station, in which the annual sediment budget was almost the same as recorded in the Hankou station in the middle Yangtze, when averaged from the last a few decades (Chen et al., 2001b). In contrast, the distribution pattern of the sediment flux through the Yangtze drainage basin during the 199 flood season is quite astonishing. About % of the sediment load of 0 million-tonnes (sediment load at Yichang, case 2; Fig. b; Table 2) is deposited in the middle Yangtze basin, from where the quantity of the sediment load carried downstream increases to 0 million-tonnes recorded at Datong station (Fig. b; Table 2). This is almost 3. times the average sediment load during a normal flood season. The Yangtze estuary is a huge depositional sink, where fluvial inputs interact actively with marine dynamics to form a unique ecological setting, upon which agricultural irrigation, fishery, land reclamation and industry are solely dependent. For instance, during the past 0 years, more than km 2 coastal land has been reclaimed, thanks to the rapid sedimentary progradation seaward (Chen and Zhao, 2001). Abundant nutrients adhered to fine-grained sediment delivered to the river mouth area and to further offshore attract large fish populations. Details of the sediment dispersal pattern are, therefore, crucial particularly for the case of 199-flood with a 0-year recurrent period, which transported about 20 million-tonnes sediment to the estuary (Table 2), of which a large proportion of fine-grained suspended sediment (mostly less than mm) can be further driven to the East China Sea, and even to the offshore of western Japan, about 900 km away from the Yangtze River mouth (NIES, 2002).. Summary Sediment fluxes for the 199 flood was simulated using sediment rating curve established on the basis of high correlation between discharge and sediment load of daily measured data in 19/19. However, two steps of calibration have to be taken in order to increase the accuracy of simulation: (1) optimizing difference between simulated and observed annual budget of 19/19; and (2) minimizing deficient sediment sources during the second flood peak of 199. The results demonstrate that the Yangtze River is becoming a seasonal, high-turbidity river due to intensifying human activity in the upper drainage basin, including 3-Gorges valley. The results further indicate that the sediment yield from the upper basin area amounted to 930 million-tonnes in 199 flood year, of which 0 million-tonnes was input during

12 232 K. Xu et al. / Journal of Hydrology 313 (200) the flood season. These values are nearly 1.9 and. times those for a normal flood year and flood season, respectively. About 0 million-tonnes of sediment are deposited in the middle Yangtze, particularly in the large-scale Dongting lake, which acts presently as a sediment sink. The flood event of 199 with a 0-year recurrent period greatly accelerates aggradation of the river lake beds of the middle and lower Yangtze reaches, where high dikes must be elevated with time to keep pace. Aggradation and dike raising in turn increases the potential risk of the flood plain, where it is densely populated. The simulated result recorded at Datong station indicates a sediment flux of 0 million-tonnes during the 2.-month flood season in 199, which is about 3. times that during a normal flood season. The annual sediment flux of the flood year may be as much as 20 million-tonnes, about 1. times that on the multiyearly base. These are vital reference numbers for managing coastal and marine natural resources. Being completed 3-Gorges Dam and on-going South North Water Transfer project will inevitably curtail the annual sediment budget. However, the present study would assume that sediment stored in the upper Yangtze valley and in numerous impoundments would have been flushed downstream to largely alter the catchment morphology during the catastrophic flood largely characterized by 199 case. Acknowledgements Authors would like to sincerely thank China Changjiang Water Conservancy Commission for generously providing raw data for the study. Thanks are particularly given to Professor C. R. Thorne and Dr P. Wang for their critical comments and suggestions, which largely improved the paper quality. The project is funded by State Key plan of fundamental study, China Ministry of Science and Technology (Grant No. 2002CB120); International Collaborative Research on Integrated Environmental Management in River Catchment by the Ministry of Environment of Japan, and APN/START for Global Change Research (Grant No ). References Asselman, N.E.M., Fitting and Interpretation of sediment rating cures. Journal of Hydrology 23, Benkhaled, A., Remini, B., Analysis of a sediment rating curve in Wahrane river basin (Algeria). Revue des Sciences de l Eau 1 (3), (in French). Changjiang Water Conservancy Commission, Annual Report of Changjinag Water and Sediment (interior report, unpublished, in Chinese). Changjiang Water Conservancy Commission, 199. Annual Report of Changjinag Water and Sediment (interior report, unpublished, in Chinese). Changjiang Water Conservancy Commission, Atlas of Changjiang Drainage Basin. China Map Publisher, Beijing. 2 pp. Changjiang Water Conservancy Commission, Atlas of Changjiang Flood Prevention. Scientific Publisher, Beijing. 10 pp. Chen, X.Q., 199. Changjiang (Yangtze) River Delta, China. Journal of Coastal Research 1 (3), 3. Chen, Z., Zhao, Y.W., Impact on the Yangtze (Changjiang) Estuary from its drainage basin: sediment load and discharge. Chinese Science Bulletin, 3 0. Chen, Z., Yu, L.Z., Gupta, A. (Eds.), 2001a. Yangtze River: an introduction Geomorphology, vol pp. Chen, Z., Li, J., Shen, H.T., Wang, Z., 2001b. Yangtze River of China: historical analysis of discharge variability and sediment flux. Geomorphology 1, 92. Du, Y., Cai, S., Zhang, X., Zhao, Y., Interpretation of the environmental changes of Dongting Lake, middle reach of Yangtze River, China, by 210 Pb measurement and satellite image analysis. Geomorphology 1 (2 3), Duan, S., Zhang, S., Chen, X., Zhang, X., Wang, L., Yan, W., Concentrations nitrogen and phosphorus and nutrient transport to estuary of the Yangtze River. Environmental Science 21 (1), 3. Fuller, C.W., Willett, S.D., Hovius, N., Slingerland, R., Erosion rates for Taiwan Mountain Basin: new determinations from suspended sediment records and a stochastic model of their temporal variation. Journal of Geology 111, 1. Gupta, A. (ed.), Geomorphology on Large Rivers. Geomorphology, Special Issue,, 39 pp. He, X.B., Jiao, J.R., 199. The flood and soil erosion in Yangtze River. Water Policy 1, 3. Higgitt, D.L., Lu, X.X., Sediment delivery to the three Gorges: 1 catchment controls. Geomorphology 1, Horowitz, A.J., An evaluation of sediment rating curve for estimating suspended sediment concentrations for subsequent flux calculation. Hydrological Processes 1 (1), Horowitz, A.J., Elrick, K.A., Smith, J.J., Estimating suspended sediment and trace element fluxes in large river basin: methodological consideration s as applied to the NASQAN program. Hydrological Processes 1, Li, C.A., Yin, H.F., Chen, D.X., Wang, B., Problems and strategies for flood control of middle reaches of Yangtze River:

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