CHARACTERISTICS
OF CHANNEL EVOLUTION DOWNSTREAM FROM THE THREE GORGES PROJECT
Yitian LI, Ling JIANG and Zhaohua SUN
State Key Laboratory of Water Resource and Hydropower Engineering
Science, Wuhan University, Wuhan, 430072, China. E-mail: Ytli@whu.edu.cn
INTRODUCTION
In alluvial streams, channel process is dominated
by multi-variables such as discharge, sediment supply, geographical and
geological surroundings. Different combinations of such
factors would produce different river patterns, consequently shows different
characteristics of channel behaviors. After dam construction, dam’s
effects on downstream flow discharge and sediment supply are inevitable, which
will initiate adjustment of channel profile and planform. Moreover, such downstream adjustments will exhibit dynamical features not
only spatially but also temporally. For a certain location, those changes are
closely related to pre-dam channel pattern, resistance of bed material, distance of
the location from dam, duration since dam
construction, and so on (Williams and Wolman, 1984; Zhou et al., 2000; Surian and Rinaldi, 2003). Although accumulation of
observational data and understandings of channel behavior downstream dams has
increased, description of channel evolution is still largely empirical until
nowadays. It is difficult to predict channel adjustment with rational model.
The Yangtze
River changes its planform from meandering to multi-branch channel in the
downstream from the Three Georges Dam (TGD). Even inside a short reach the
channel form and dimensions change abruptly from meandering to straight or from
narrow-straight to wide multi-branch. These differences are not only controlled
by hydrology and hydraulics but also by geological environmental and alluvial
deposition in long historical period, thus, the complex responses of channel
evolution to the TGD will be a great challenge. Researches on channel responses
to the TGD have lasted for decades. However, the problem is far from resolved
owing to its extreme complexity, e.g. estimated channel degradation downstream
from the dam was not agree with the fact in 2003–2005 (Dai et al., 2005).
According to
current knowledge about fluvial system, the only feasible and applicable means
to determine channel adjustment under changed discharge and sediment regime is
to explore links between input variables and its impacts on channel form.
Classic examples of these kinds of cause and effects are the relationships
between downstream variation in discharge and channel dimensions. With
accumulated field sampling and adequate observations of already occurred dam
downstream adjustment on other rivers or the Yangtze River itself, it is
possible to give a qualified or even quantified estimation of fluvial process
downstream the TGD. In this paper, based on a comprehensive review of channel
features of the Yangtze River and with the help of numerical model to simulate
the post-dam flow and sediment condition, an attempt about channel evolution
responses to the TGD is presented.
STUDY AREA DESCRIPTION
Characteristics of Natural Water and Sediment
The study area is the middle and lower reach of the
Yangtze River, which start from YiChang to Jiangyin (fig. 1). Gezhouba Project (dam
impoundment in 1981) and the TGP (dam impoundment in 2003) are located upstream
the Yichang station. There are three large tributaries joining the main stream:
the Dongting Lake drainage basin, the Hanjiang River, and the Boyang Lake
drainage basin. Three diversions, named
Songzikou, Taipingkou, and Ouchikou, deliver flood flows into the Dongting
Lake, and the outlet of the lake
joins the main stream of the Yangtze River at Chenglingji station again,
forming a complex river network. Over the past 47 years before the TGP’s operation, the three diversions
had shrunk continually,
and their total annual diversion ratio of flow reduced from
29% in 1956–1966 to 14% in 1999–2002. Outflow of Boyang Lake has the
characteristic of high discharge and relatively low sediment load.
Figure 1. Scheme of the Yichang – Jiangyin Reach of the Yangtze River
Sediment
supply to the reach downstream from the TGP mainly comes from the rivers above
Yichang station. Suspended sediment load at Yichang station is about 116％ of that at Datong station,
but flow discharge at Yichang station is only 48.4% of that at Datong station.
The rest comes from the numerous tributaries of Yichang to Datong. So, water
and sediment regime is different in different reaches (tab. 1). Comparing with other large rivers in China and some rivers abroad, inner-year water
flow process of the Yangtze River
has the characteristic of less variation.
Boundary Conditions and Channel Evolution Characteristics
Downstream from the Three Georges Dam, the Yangtze River flows from mountainous
area to alluvial plain. Because water & sediment condition and channel
boundary changes along the river, different river patterns are formed and show
different channel evolution characteristics. Generally, the study reach is divided
into four segments.
Table
1. Water and sediment condition of the reach downstream from Yichang station
|
Gauging station |
Annual
average discharge (m3/s) |
Annual average
sediment concentration (kg/ m3) |
The ratio of most to least discharge of pre-dam |
||
|
pre-dam |
2003–2005 |
pre-dam |
2003–2005 |
||
|
Yichang |
13900 |
13549 |
1.14 |
0.21 |
25.67 |
|
Xinchang |
12400 |
12711 |
1.13 |
0.31 |
19.0 |
|
Jianli |
11400 |
12075 |
1.02 |
0.33 |
17.5 |
|
Luoshan |
20500 |
19833 |
0.643 |
0.221 |
19.5 |
|
Hankou |
22600 |
22807 |
0.573 |
0.216 |
15.8 |
|
Datong |
28700 |
27610 |
0.486 |
0.216 |
20.0 |
1. From
Yichang to SongZikou: This is the transitional reach from mountainous area to
alluvial plain, where channel morphology is obviously dominated by geologic structure
and lithology, forming straight or slightly curving river. The river regime
basically has been invariable for a long time.
2. From
Songzikou to Ouchikou: This reach usually is called the Upper JingJiang reach,
where the Yangtze River already flows into the alluvial plain, river bank is
formed by geological structure of three formations of clay, sand and gravel. Because
the top elevation of sand layer is commonly below low-water level, the resistance
of channel bank is strong. In addition to the restriction of man-made levees
and long-term bank protection works, the channel shows a restricted meandering
planform and changes little in long duration. Since the past century, channel
evolution observations show that local thalwegs in transitional zone between
two bends actively shift up and down, and branches develop and shrink
alternately in some channel bend with large width.
3. From
Ouchikou to Chenglingji: This reach is usually called the Lower JingJiang
reach, where the geological environment is deep alluvial deposits of double
layers of clay and sand. The range of variation of discharge in the Lower Jingjiang
reach is smaller than that in the Upper Jingjiang Reach with the influence of
three diversions, and its water profile slope is relative flat under the
influence of backwater effect of the outflow of Dongting Lake. Because of the
inadequate resistance of bed material, active concave bank erosion and frequent
avulsion is main characteristics of this reach. For those reasons, the degree
of sinuosity in the Lower JingJiang reach is much higher. Since 1984, projects
of river training works have been constructed, and activity of meandering
rivers has been restricted in a certain extent, including its total length and
lateral adjustment.
4. From
Chenglingji to Jiangyin: In this reach, zones of subsidence alternate with
uplifted regions, and controlling points of hills or rocks are distributed
widely along the river. As a result, wide
branched reaches with one or more sand bars and single narrow reaches alternately
appear, but channel lateral migration is restricted. According to previous
researches, besides the special geological environment, relatively small rang
of variation on flow hydrograph and long duration of flood season are important
conditions to keep the river branched. However, because of different
distributions of controlling points, straight, slightly curving and goose-head
multi-branched river reaches are formed, and their branches show different
evolution characteristics.
It is
obvious that riverbed boundary condition plays a crucial part on forming
different river patterns. As a large fluvial system, the Yangtze River from
Yichang to Jiangyin has already formed stable channel boundary through
long-term channel evolution. Furthermore, long history river training practice
and bank protection project has increased its stability in planform.
WATER AND SEDIMENT CONDITION AFTER
THE TGP’S OPERATION
Released Flow and Sediment Condition of the Three Gorges Reservoir
According to
operation schemes of the Three Georges Reservoir, water level behind the dam
would be kept at 135-139 m above mean sea level in 2003–2006. During this
period, inner-year flow process basically can’t be changed. After the flood
season of 2006, the water level will be elevated to 156 m above mean sea level.
During this period, although flow process in flood season will not be altered,
released flow discharge will be reduced by 25% in falling stage (especially
about October and November) to reserve water in the reservoir. In dry season,
released flow discharge can be increased to some extent because of power
generation. When water level in the reservoir is elevated to 175 m above mean
sea level, the reservoir will enter into the stage of normal operation. During
this period, flow process in flood season generally will not be changed, except
the inflow discharge of the reservoir exceeds the flood capacity of downstream
channel. The released discharge can be reduced by 42% at the end of September,
and can increase slightly from December to April of next year.
In fig. 2,
the yearly distribution about the magnitude of flow discharge is compared between
pre and post dam condition. The
ratio of the largest discharge to least discharge is 15.6 but it
is still in the range of natural discharge change. This ratio at Datong station
is
Figure 2. The yearly
distribution about the magnitude of flow discharge
After dam
closure, majority of sediment deposits in the reservoir, especially for coarse
grains. According to the computing results (CRSRI, 2002), in the first 30
years, average sediment trap efficiency of the reservoir is 70%. The input
sediment concentration is 0.4 kg/m3, which reduced by 65% after dam
construction. Suspended load finer than 0.01 mm occupies more than 50% of the
total sediment load. In 2003-2005 after impoundment, discharge changes little
but sediment discharge reduces obviously comparing with pre-dam long-time
average (tab. 1). The reduction range of sediment discharge decreases as the
distance from the dam increases, which relates to sediment supply of downstream
riverbed during degradation process.
Prediction of Sediment Transport Downstream from the TGP
After
impoundment of the reservoir, downstream channel incision will continue in long
time. In order to forecast sediment discharge recovery process downstream from
the TGP in the early period after the TGP’s operation, a 1D unsteady model for
total sediment transport (suspended load and bed load) is used in this study,
which is more rational than previous quasi-steady 1D model in theory to
simulate the sediment discharge recovery process downstream from the TGD. The
simulation reach is from Yichang to Datong, including large tributaries
(Qingjiang River and Hanjiang River), the three diversions, the Dongting Lake
and the Boyang Lake.
The results
of the model show that the estimation of sediment recovery process coincides
with the common erosion law downstream from reservoir, which is that sediment
transport rate of each grain size fraction after the reservoir’s operation
can’t exceed its original level before the reservoir’s operation (Li et al.,
2003). So the result in this study is quite reasonable.
Fig. 3 shows
erosion and deposition of riverbeds in different period after the TGP’s operation.
The sediment discharge downstream from the TGP will reduce obviously because of
the trapping sediment function of the reservoir, and annual average sediment
discharge at each station of the first twenty years post-dam will be about 42% –
60% of that of pre-dam (fig. 4(a)). In the first 20 years, the sediment release
ratio of the reservoir will change little and downstream sediment discharge is
mainly supplied from riverbed erosion. As the riverbed is coarsened gradually,
the annual sediment discharge at every station will reduce in different degrees
which relate with the development of channel scour from upstream to downstream.
Figure 3. Annual
erosion or deposition capacity of riverbeds after the TGP’s operation
As Fig. 3
and 4(a) indicate, because the riverbed from Yichang–Taipingkou will be
intensively eroded and erosion will finish roughly at the end of 15 years, the
sediment discharge at Shashi station is reduced obviously. However, till the
end of 20 years post-dam, the channel scour from Taipingkou to Ouchikou has not
finished. At that time, the reach from Ouchikou to Chenglingji will be eroded
intensively, so that annual sediment discharge at Luoshan station is stable in
the first 20 years. Because of the severe upper erosion, the reach from
Chenglingji to Datong will be deposited slightly, which is still in the scope
of natural deposition (Shi et al., 2002). Correspondingly, sediment discharge
at Hankou station will reduce. From the 15th year, the
deposition tendency from Chenglingji to Hankou starts to decrease.
|
a |
b |
Figure 4. Annual
average sediment discharge at each station during pre- and post-dam time
periods: (a) represents total sand, and (b) represents coarse sand (bigger than
For bed
material from Yichang to Datong,
sediment fraction of grain size coarser than
CHANNEL ADJUSTMENT TREND AFTER THE
TGP’S OPERATION
General tendency of channel adjustment downstream from the TGP
Considering
the post-dam changes of flow and sediment discharge, it is possible to give a
predictive description of channel responses to the TGD impoundment.
For the
reach upstream from Chenglingji, channel boundary is restricted, channel
incision is the dominate response to the impoundment. Considering the reduction
in frequency of large flood, river patterns can’t be change largely.
For the
reach downstream from Chenglingji, dam’s effects on flow regime and sediment
supply declined with distance. This means the reduction of variation range on
discharge will be small and the sediment concentration of bed load will be recovered by upstream erosion. In
addition, natural micro-geomorphology conditions still exist. The
river pattern can’t change obviously. However, local channel adjustment
resulting from the altered flow regime may be obvious.
Channel adjustment at different stage of reservoir operation
At different
stage of reservoir operation, characteristics of released discharge and
sediment are different, riverbeds erosion will occur in different location in
different time, and correspondingly channel evolution downstream from the TGP
will be different.
At present,
the reservoir operates with the behind-dam water level of 135–139 m. Under such
condition, the effect of sediment deficiency is mainly initiate adjustment in
reaches upstream from Ouchikou (fig. 3). In these reaches, although flow
process changes little, reduction of sediment load can cause erosion of shoals
and the head of central bars in multi-branched reaches. Besides these local
changes, the channel will generally be degraded under the restriction of
confined boundary. During the channel incision process, the activity of the
main flow reduces and the river channel tends to narrow and deep. For the reach
downstream from Ouchikou, because suspended sediment will be supplied due to
the upstream riverbed erosion, channel changes little during this period.
After the
TGP completely finish construction, water level behind the dam will be raised
to
A case study
The Shashi
Reach is in the sand bed reach that degradation taken place immediately after
the TGD’s operation. The river channel is straight, slightly curing and
gradually broadening. The position of the main flow is active to shift in
history. At the end of the concave bank, there is the controlling point
Guanyinji, forming a narrow segment. In duration of more than fifty years, such
channel planform is relatively stable, especially after bank protection project
was built. Historical channel evolution has shown that the channel maintains a
multi-branched planform but the point bar and the central bar shrink and expend
alternately (fig. 5). The general evolution law is that the central bar shrinks
and the point bar enlarges toward the river centre and downstream. The upper
part of the central bar changes with the law of aggradation during flow rising
and degradation during flow falling (fig. 6).
In 2003–2006
after the TGP’s operation, the central bar was eroded (fig. 7) and both
branches were lowered, which is consistent with channel adjustment trend
mentioned above. It is predicted that when the reservoir enter into normal
operation current
erosion tendency of the central bar may slower due to quickly falling of water level after flood season.
Figure 5. Variation of
sandbars’ morphology in the Shashi Reach
CONCLUSIONS
In this
paper, channel evolution characteristics of different reaches downstream from
the TGP are reviewed. Basing on prediction of sediment discharge after the
TGP’s operation, possible responses of channel evolution to the dam impoundment
are discussed. It is concluded that:
(1) In the
middle and lower reach of Yangtze River, riverbed boundary condition, including
geological structure, geomorphology and even man-made project, has important
influence to form different river patterns and channel evolution. And overall,
the channel shows a stable feature.
Figure 6. Inner-year
variation of cross section at entrance reach (2#) of both branches
Figure 7. Shoals
evolution in the initial period after the TGP’s operation
(2) After the
TGD’s operation, the change range of flow process and sediment discharge can
gradually declines as the distance from the dam increases. In the early 20
years of post-dam period, riverbeds erosion will mainly occur in upstream from
Chenglingji. Downstream from Chenglingji, the recovery efficiency of sediment
load is relatively high and the channel is deposited slightly. In the first 5
years, sediment discharge of coarse particle (grain size coarser than
(3) After the
TGP’s operation, river patterns can not change obviously but the change of sand
bar size and thalweg position will be obvious in some reaches. During the early
stage, the Upper Jingjiang reach obviously affected by channel erosion, and the
river channel tends to narrow and deep. The downstream reach will change little
due to sediment discharge recovery. When the reservoir enters into normal operation
stage, accumulative deposition will take place at shoals due to quickly falling
of water level after flood season, which is adverse to secondary branches
maintaining for the multi-branched river reach. Meanwhile, small central bars
or point bars may appear in wide bend reach.
ACKNOWLEDGEMENT
The study is
supported by National Key Basic Research and Development Program (No.
2003CB415200).
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