Journal of Hydrology 313 (2005) 221–233
www.elsevier.com/locate/jhydrol
Simulated sediment flux during 1998 big-flood of the Yangtze
(Changjiang) River, China
Kaiqin Xua, Zhongyuan Chenb,*, Yiwen Zhaoc, Zhanghua Wangc, Jiqun Zhanga,
Seiji Hayashia, Shogo Murakamia, Masataka Watanabea
b
a
National Institute for Environmental Studies, Tsukuba 305-8506, Japan
State Key Laboratory for Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China
c
Department of Geography, East China Normal University, Shanghai 200062, China
Received 7 April 2003; revised 28 February 2005; accepted 10 March 2005
Abstract
The present study focuses on simulating sediment flux for 1998 big-flood with 60-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 1987/1988 was selected to
simulate the annual and flood season sediment fluxes of 1998, 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 520 million-tonnes in the
middle catchment and 720 million-tonnes in the lower drainage basin. These loads, respectively, approximate almost 1.9, 1.2,
and 1.8 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 580 million-tonnes of sediment
downstream, the 3-Gorges valley added additional 270 million-tonnes. This totals about 850 million-tonnes that supplied the
middle and lower Yangtze catchments, of which about 450 million-tonnes were silted in the middle catchments, immediately
downstream of the exit of 3-Gorges. This amount is almost 6.5 times the normal flood season averaged over the last 50 years.
High sediment load was also recorded in the river mouth area during the flood season, where 460 million-tonnes were delivered
to the estuary and East China Sea, about 3.8 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 2005 Elsevier B.V. All rights reserved.
Keywords: Catastrophic flood; 60-Year recurrent period; Human impact; Sediment rating curve; Simulated sediment flux; Sediment transport
pattern; Yangtze (Changjiang) catchment
1. Introduction
* Corresponding author. Address: State Key Laboratory for
Estuarine and Costal Research, East China Normal University,
Shanghai 200062, China. Fax: C86 2162 2324 16.
E-mail address: [email protected] (Z. Chen).
0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhydrol.2005.03.006
Large rivers in Asia contribute a large proportion
of sediment sources to coastal depositional sink
222
K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
and the world ocean basins as well (Milliman and
Syvitski, 1992). The Yangtze River of China serves as
the major sediment sources (470 million-tonnes/year;
multiyearly base) area, where intensifying human
activity includes impoundment of numerous reservoirs (more than 35,000; 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, 1998; 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 6300 km, with a catchment area of 1.8
million km2, accounting for 18.8% of the Chinese
nation’s total territory. The river basin consists of
about 85% mountains and hilly regions, 11% plains
and 4% 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 800 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.
K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
precipitation reaches 1800 mm/year in the south–
central Dongting drainage basin of the middle Yangtze
reach (Fig. 1). It is noteworthy that about 80% 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 15 large floods (more
than 5.0!104 m3/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
1998, a catastrophic flood event occurred as a recurrent
period of about 60 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 12–15 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
223
weather conditions (Qian et al., 1987). 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, 1989). Using this approach,
prediction on suspended sediment concentration and
flux, and sediment erosion rate, etc. has become possible
(cf. Walling and Webb, 1988; 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 1998. 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 (6) 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
600 km upstream of the estuary, represents the lower
Yangtze catchment. Daily measured discharges (Q) and
sediment concentration for 9 years from 1950s to 1980s
(Fig. 2) were randomly collected from these stations,
documented as ‘Internal Report on Water and Sediment’
(Changjiang Water Conservancy Commission,
1875–1990). The sediment rating curve between
discharge and sediment load (SC!Q) was derived for
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K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
y = 2.7856x - 4.5743
R2 = 0.9626; N=731
Cuntan
10
8
Log(SS*Q) (g/s)
9
Log(SS*Q) (g/s)
y = 1.7821x - 0.6958
R2 = 0.9196 N=731
Luoshan
9
8
7
6
8
7
7
6
6
5
5
5
4
4
3.4
3.6
3.8
4
4.2
4.4
4.6
4.8
3.6
3.8
4.0
Log(Q) (m3/s)
y = 3.1981x - 6.6066
R2 = 0.9569; N=731
Yichang
9
8
7.5
Log(SS*Q) (g/s)
Log(SS*Q) (g/s)
4.4
4.6
4.8
y = 2.021x - 1.8467
R2 = 0.9682; N=365
Hankou
8.5
7
6.5
6
5.5
8.5
8
7.5
7
6.5
6
5.5
5
4.5
4
5
4.5
4
3.4
3.6
3.8
4
4.2
4.4
4.6
4.8
3.6
3.8
4
Log(Q) (m3/s)
8.5
4.2
4.4
4.6
4.8
Log(Q) (m3/s)
y = 2.261x - 2.3591
R2 = 0.9799; N=731
Jianli
y = 2.4697x - 4.0654
R2 = 0.9431; N=731
Datong
8.5
8
8
7.5
7.5
Log(SS*Q) (g/s)
Log(SS*Q) (g/s)
4.2
Log(Q) (m3/s)
7
6.5
6
5.5
7
6.5
6
5.5
5
5
4.5
4.5
4
4
3.4
3.6
3.8
4
4.2
4.4
4.6
4.8
Log(Q) (m3/s)
3.8
4
4.2
4.4
4.6
4.8
Log(Q) (m3/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 (1875–1990).
these all stations (Fig. 2), and the one selected from the
years of 1987/1988 was used to simulate sediment flux
(Figs. 2 and 3). The reason of selection is given in
Section 4. From the relationship between Q and SC!Q,
following correlation can be established
L Z aQb
(1)
where L is the sediment load, Q is discharge, and a
and b are the constant, given by the log (L) versus Log
(Q) plot.
Using this approach, the sediment flux of 1998 could
be simulated for the six major hydrological gauging
stations (Fig. 4a; Changjiang Water Conservancy
Commission, 1998), where discharge was measured
daily on site during that time. Furthermore, the
discharge record for the 1998 flood season (about 2.5
months from early July to mid-September, 1998;
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
K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
225
Annual sediment load (106 ton)
(a)
700
1987-observed
600
1987-simulated
500
400
300
200
100
0
Cuntan
Yichang
Jianli
Luoshang
Major stations
Hankou
Datong
Annual sediment load (106 ton)
(b)
600
1988-observed
500
1988-simulated
400
300
200
100
0
Cuntan
Yichang
Jianli
Luoshang
Major stations
Hankou
Datong
Fig. 3. Observed and simulated annual sediment fluxes of 1987/1988 at the six hydrological gauging stations.
the simulated sediment flux for the flood season for each
of the study stations (Fig. 4b). Due to differences
identified between the observed and simulated sediment
fluxes in 1987/1988 (Fig. 3), a calibration was applied to
the six major stations to optimize the simulated result
both for the annual and flood season of 1998. The mean
error identified at each station was applied for the
calibration, derived from the differences between the
observed and simulated values of 1987/1988 listed in
Table 1.
Measured sediment concentrations at the Cuntan
station in Chongqing, during 1998 were also available
(Changjiang Water Conservancy Commission, 1998)
for the present study (Fig. 4a). The daily distribution
of sediment concentration for the wet season (May–
October) of 1998 is shown in Fig. 5. Due to deficient
sediment sources that occurred during the second
flood peak of 1998 recorded at Cuntan station (Fig. 5),
we performed a further calibration on the
previously calibrated base, which produces two
outputs (cases 1 and 2, Fig. 4a 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 2.5 months). Details are given by following
steps (also indicated in Fig. 6).
Firstly, we set up theoretically the formula to
obtain sediment deficient from Cuntan station:
A3 Z
Sept:15
X
ðA1 K A2 Þ
(2)
Aug:1
Here, A1 represents daily simulated sediment load
of 1998 by sediment rating curve 1987/1988 (Fig. 2);
and A2 denotes daily observed sediment load of 1998,
and A3 equals the sum of overestimated value (from
Aug. 1 to Sept. 15, 1998, the time period of secondary
flood peak).
Secondly, we define case 1 as:
P1 Z A3 =
Dec:
X31
!
A1 !100%
(3)
Jan: 1
P1 represents the ratio between overestimated
value (A3) and yearly sediment flux of 1998;
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K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
18
16
Sediment load
108t
14
12
10
8
6
Simulated
4
Calibrated
2
Case 1
Measured
0
Cuntan
Yichang
Jianli
Luoshan
Hankou
Datong
16
14
Sediment load
108t
12
10
8
6
Simulated
4
Calibrated
Case 2
2
Measured
0
Cuntan
Yichang
Jianli
Luoshan
Hankou
Datong
Fig. 4. (a) Simulated sediment flux for 1998-flood year; and (b) simulated sediment flux for the flood season (early July to mid-September,
1998). Calibration and recalibration procedures are discussed in text. Average errors used for the calibration are listed in Table 1.
Case 2 as:
P2 Z A3 =
Sept:
X31
!
A1 !100%
(4)
July 1
P2 represents the ratio between overestimated
value (A3) and flood season sediment flux of 1998.
By this approach—cases 1 and 2, recalibrated result
both for annual and flood season at Cuntan gauging
station can be obtained (Fig. 4a and b).
Table 1
Observed and simulated sediment fluxes for 1987/1988 at six major hydrological stations along the Yangtze trunk channel
No.
Main station
1987—
Observed
1987—
Simulated
Error (%)
1988—
Observed
1988—
Simulated
1
2
3
4
5
6
Cuntan
Yichang
Jianli
Luoshan
Hankou
Datong
461.8!106
534.0!106
425.4!106
428.2!106
418.2!106
404.8!106
561.2!106
619.8!106
426.6!106
388.5!106
375.6!106
345.0!106
21.5
16.1
0.3
K9.3
K10.2
K14.8
401.3!106
430.8!106
465.0!106
503.4!106
15.9
16.9
357.2!106
355.0!106
K0.6
355.3!106
346.5!106
K2.5
Unit: tonne. Errors for each station were calculated and averaged.
Error (%)
Average
error (%)
18.7
16.5
0.3
K4.9
K10.2
K8.6
K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
227
60000
Discharge (m3/s)
50000
5
40000
4
30000
3
20000
2
10000
1
0
Sediment concentration (g/l)
6
Discharge
SC
Cuntan Station
0
4-
14
-98
5-2
4-9
8
7-
3-9
8
8-
12
-98
9-2
1-9
8
10
-31
-
98
Date
Fig. 5. Measured discharge and sediment concentrations at Cuntan hydrological station in the upper Yangtze drainage basin, during the wet
season, 1998.
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 1ZCy!(1KP1), and case 2ZCr!(1KP2),
where Cy, calibrated sum of sediment flux of 1998; Cr,
calibrated sum of sediment flux of 1998.
Cases 1 and 2 (in percentage), then, can be applied
to the previously calibrated results of another five
major hydrological stations (Fig. 4), 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 1998 big-flood.
3. Observation and results
The regression analysis between discharge and
sediment load using 1987/1988 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 1987/1988 (Fig. 3; Table 1) reveals
16–19% estimation higher than that of observed for
Cuntan and Yichang, and 4.9–10% 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 1987/1988 listed in Table 1,
calibration was applied to the simulated sediment flux
of 1998 at each gauging station (Fig. 4).
Using the method of cases 1 and 2 can produce
further calibrated result, showing that the annual
sediment flux of 1998 reaches about 650 milliontonnes at Cuntan in the upper Yangtze catchment
(almost equivalent to the measured sediment flux of
620 million-tonnes; Fig. 4a); about 930 milliontonnes at Yichang, 450 million-tonnes at Jianli, 460
million-tonnes at Luoshan, 520 million-tonnes at
Hankou and 720 million-tonnes at Daton (Fig. 4a;
Table 2).
Through the same method, calibrated sediment flux
for the flood season of 1998 is about 580 milliontonnes recorded at Cuntan, 850 million-tonnes at
Yichang, 360 million-tonnes at Jianli, 290 milliontonnes at Luoshan, 350 million-tonnes at Hankou and
460 million-tonnes at Datong (Fig. 4b; Table 2).
These quantities are more than 90% in proportion to
the total annual sediment flux of the upper Yangtze
228
K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
Cuntan station
Suspended sediment concentration
Discharge
Sediment
defficiency
(A3)
7.1
8.1
Method: Simulating Sediment load in 1998
9.15, 1998
6
A1 = daily simulated sediment load of 1998: A1= Q2.7856 / 104.5743 * 24 * 3600 / 10 (ton)
(correlation equition derived from 1987/88)
A2 = daily observed sedimen load of 1998: A2 = Q * S.C. * 24 * 3600 /103 (ton)
A3 = sum of overestimated value (Aug.1~ Sep.15 ):
Sep.15
A 3=
Dec.31
Case 1: P1 = ( A 3 /
(A1-A2)
Aug.1
A1 ) * 100 % (P 1 - Overestimated value/yearly sediment flux)
Jan.1
Sep.15
Case 2: P2 = ( A3 /
A1 ) * 100 % (P2 - Overestimated value/flood season sediment flux)
July1
Application:
Cy ~ Calibrated sum of 98
Cf ~ Calibrated flood value
Case 1 = C y * (1 - P1)
Case 2 = C f * (1 - P2)
Fig. 6. Schematic diagram explaining simulation procedures for Cuntan hydrological gauging station. Details are discussed in the text and refer
to Fig. 5.
(Cuntan and Yichang), and are between 81 and 63% 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 15, 1998. Examining
the distribution of sediment concentration throughout
the flood season, we noted that the highest value (up to
4.0 g/l) occurred at the initiation of the first flood
peak. Subsequently, sediment concentration
decreased gradually to about 1.5–2.0 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. 5). Thus, the calibration is
really needed for this case as the result of cases 1 and
2 shown in Table 2 and Fig. 4.
K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
229
Table 2
Simulated, calibrated and recalibrated (cases 1 and 2) sediment fluxes for annual and flood season (early July to mid-September) during 1998
Hydrological
stations
Simulated
(annual;
!106
tonnes)
Calibrated
(annual;
!106
tonnes)
Case 1
(annual;
!106
tonnes)
30–50 years
averaged*
(!106
tonnes)
Simulated
(flood time;
!106
tonnes)
Calibrated
(flood time;
!106
tonnes)
Case 2
(flood time;
!106
tonnes)
Multiyearly
averaged*
(!106
tonnes)
Case 2/
case 1
Cuntan
Yichang
Jianli
Luoshan
Hankou
Datong
1166.152
161.438
655.639
642.122
690.201
962.771
948.081
1348.008
653.672
673.586
760.601
1045.569
652.184
927.294
449.660
463.359
523.217
719.246
442.000
516.000
429.000
435.000
421.000
451.000
1082.358
1485.105
546.769
418.809
482.779
631.214
879.957
1240.063
545.129
439.330
532.022
685.498
584.060
849.120
361.822
291.600
353.123
454.991
–
180.000
–
–
120.000
122.000
0.896
0.916
0.805
0.629
0.675
0.633
*
Averaged sediment fluxes for last decades.
4. Discussion
The present study accentuates the simulating
sediment flux of big 1998 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 1998, 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 1987/1988 was
selected to simulate the 1998 sediment flux
(Figs. 2–4), 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.,
1997; 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., 1987; Chen et al.,
2001b). These were taken into account while
simulating the sediment flux of 1998 big-flood due to
b occurrence (Table 1; Fig. 4).
The effect of deficient sediment concentration
during the second flood peak in 1998, 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. 4a
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. 5). During the
1998 flood, sediment concentration in the upper
Yangtze catchment was as high as 3.5–4.2 g/l in the
first peak of flood (more than 40,000 m3/s), but it
stabilized gradually to about 1.5–2.0 g/l even though
the discharge of the second peak increased to
230
K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
55,000 m3/s (Fig. 5). However, the coefficient of this
rating curve can be still highly corrected (R2Z0.9747)
after examining. It is understood that the partial
sediment insufficiency during Aug. 1–Sept. 15
(Figs. 5 and 6) will not impact largely the rating
curve credibility.
The simulated results for the present study
demonstrate that about 260 million-tonnes sediment
were supplied to the river from the 3-Gorges valley,
which was delivered downstream during 2.5 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. 7). 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. 7a).
Our recent investigation indicates that in the past 50
years there has been more than 130 million-tonnes/
year sediment accumulated in the Dongting Lake. The
present study reveals that in 1998-flood season, more
than 450 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 3.5 times as much as the multiyearly base.
Sources witness that the lake area has shrunk largely
from O6000 to 2625 km2 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 1950–1970s in
(a)
(b)
(c)
(d)
Fig. 7. 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.
K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
the Yangtze drainage basin (more than 35,000;
Changjiang Water Conservancy Commission, 1999;
Yang et al., 2002a,b), including the major Gezhou
dam that was completed in the end of 1988
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 470 to
350 million-tonnes over the past half-century
(Changjiang Water Conservancy Commission, 1999;
Chen et al., 2001b).
Annual sediment fluxes of 930, 520 and 720
million-tonnes were simulated in the Yichang,
Hankou and Datong stations, respectively, during
the 1998 flood (case 1; Table 2). These are almost 1.9,
1.2 and 1.8 times greater than the annually averaged
sediment flux for the last 50 years (Chen et al.,
2001b). Differences in sediment flux (expressed as
case 2/case 1, Table 2), ranging from more than 90
(upstream) to 81–63% (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!10K5) of
the middle Yangtze River course from the upper
3-Gorges region (10–40!10 K5 ) 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 1998 flood season is quite
astonishing. About 58% of the sediment load of
850 million-tonnes (sediment load at Yichang,
231
case 2; Fig. 4b; Table 2) is deposited in the
middle Yangtze basin, from where the quantity of
the sediment load carried downstream increases to
460 million-tonnes recorded at Datong station
(Fig. 4b; Table 2). This is almost 3.6 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 50 years, more than 800–1000 km2
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 1998-flood with a 60-year
recurrent period, which transported about 720
million-tonnes sediment to the estuary (Table 2), of
which a large proportion of fine-grained suspended
sediment (mostly less than 4 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).
5. Summary
Sediment fluxes for the 1998 flood was simulated
using sediment rating curve established on the basis of
high correlation between discharge and sediment load
of daily measured data in 1987/1988. 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 1987/1988; and (2) minimizing deficient
sediment sources during the second flood peak
of 1998.
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 1998 flood
year, of which 850 million-tonnes was input during
232
K. Xu et al. / Journal of Hydrology 313 (2005) 221–233
the flood season. These values are nearly 1.9 and
4.5 times those for a normal flood year and flood
season, respectively. About 450 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
1998 with a 60-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 450 million-tonnes during
the 2.5-month flood season in 1998, which is about 3.8
times that during a normal flood season. The annual
sediment flux of the flood year may be as much as 720
million-tonnes, about 1.8 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
1998 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. 2002CB412505); ‘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. 2003-12).
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