PRELIMINARY STUDY ABOUT RIVER-LAKE EVOLUTION EFFECT DUE
TO THREE GORGES PROJECT
Gong ping
Senior engineer, Yangtze River Scientific Research Institute, People’s republic of
China
Yang Wenjun
Lu Junyou
Wan Jianrong
1
INTRODUCTION
The full length of river course between Yichang and Datong is about 1120km
in midstream and downstream of Changjiang River(Fig 1). The river course length
between Yichang and Zhicheng is about 60km, its river course is basically straight
and light curving, with tributary stream of Qingjiang river afflux at Yidu. The river
section between Zhicheng and Chenglingji is called as Jingjiang river which is
divided into Upper Jingjiang and Lower Jingjiang with Ouchikou as border line.
Upper Jingjiang is about 170km long, belongs to curving fork river course and
Lower Jingjiang is about 170km long, belongs to wriggling river course. Songzikou,
Taipingkou and Ouchikou are three diversion passages on Jingjiang river
(Tiaoxiankou has been shut off by anchor gate installing), by which the water in
Changjiang River is divided and flows into Dongting Lake subsequently. In
addition, four rivers including Xiang, Zi, Yuan and Li merge into Dongting Lake.
The water in all these rivers is regulated and stored by Dongting Lake and flows
into Changjiang River again at Chenglingji, forming into complex river-lake
relationship. The river section between Chenglingji and Wuhan is about 230km
long, belongs to fork river course. Hanjiang, the biggest tributary stream of
Changjiang River, flows into Changjiang River at Wuhan city. The river section
between Wuhan and Datong is about 500km long, belongs to typically fork river
course, with incoming water from Poyang Lake merging into Changjiang River.
Ha
nj
ia
Three三峡水利枢纽
Gorges Reservoir
宜昌
Yichang
Gezhouba hydro project
葛洲坝水利枢纽
江
Shashi
沙市
ng
汉 R
iv
江er
Qingjiang
螺
山
城陵矶
Chenglingji
洞
庭
湖
黄
石
Huangshi
藕
池
口
Wuhan
Dongting Lake
Jianli
监利
Ouchikou
Taipingkou
松 太
滋 平
口 口
Songzikou
Zhicheng
枝
城
大
通
武
汉
Datong
清
九江
Jiujiang
鄱
阳
Poyang Lake
湖
Fig.1
Schematic drawing of main stream river section between Yichang and Datong in
Changjiang River
2
Current situation of river-lake relationship
According to river bed scouring-siltation of three diversion passages in
Jingjiang river and sediment deposition of Dongting Lake, in the condition of
sediment decreasing in upstream water basin of Changjiang River (1996~2002),
incoming sediment from these three diversion passages and four rivers including
Xiang, Zi, Yuan and Li (called as “Four Waters” for short in the following text),
which enters into Dongting Lake subsequently, decreases near 50% than its
normal value of accumlated year (1956~1995). For Dongting Lake, sediment out
of the lake zone decreases near 50% accordingly, sediment deposition in the lake
zone decreases distinctly, but silt deposit ratio hardly changes. Sediment
deposition is still the main element in river bed scouring-siltation for three
diversion passages of Jingjiang river, even in the condition of decreasing water
diversion and silt diversion distinctly. The most serious sediment deposition
happens in Ouchikou, its sediment deposition amount is more than that of
Songzikou ,but its silt diversion amount is less than that of Songzikou.
The statistical changes of water diversion and silt diversion in different times
for three diversion passages in Jingjiang river (called as “Three Passages” for
short in the following text) is from 1956, when systematic survey data can be
obtained. In nature condition, the ratio of water diversion and silt diversion tends
to decrease. With systematic curve course correction of Lower jingjiang and the
construction of Gezhouba hydro project, the decreasing process mentioned above
accelerates. The main effect comes from systematic curve course correction of
Lower jingjiang. Since Three Gorges Project is constructed, water diversion ratio
decreases lightly, silt diversion ratio changes hardly, no sudden changes happen
for now.
According to incoming water-slit time interval distribution, before Jingjiang
river curve course is corrected (1956~1966), water inflow from “Three Passages”
and “Four Rivers” is 43% and 49.5% of total water inflow into Dongting Lake
respectively. After Jingjiang river curve course is corrected in 1967 and 1973 in
succession, water inflow from “Three Passages” decreases to 28% of total water
inflow into Dongting Lake and water inflow from “Four Rivers” increases to 62.5%
of total water inflow into Dongting Lake. In the later period of 1990s, water inflow
percentage mentioned above of “Three Passages” decreases further. After Three
Gorges Reservoir is constructed, this trend still continues. For the low flow year of
2006 in particular, annual average water inflow of 2003~2006 from “Three
Passages” is 23.3% of total water inflow into Dongting Lake and water inflow
percentage of “Four Rivers” continues to increase.
When water inflow changes, silt inflow changes accordingly. Before Jingjiang
river curve course is corrected, silt inflow from “Three Passages” and “Four
Rivers” is 85.6% and 12.8% of total silt inflow into Dongting Lake respectively.
After Jingjiang river curve course is corrected, silt inflow percentage mentioned
above is 76.0% and 21.3% respectively. In the later period of 1990s, silt inflow
from “Three Passages” increases lightly to 81.1% of total silt inflow into Dongting
Lake. After Three Gorges Reservoir is constructed, with discharge silt from Three
Gorges Reservoir decreasing, silt inflow from “Three Passages” decreases, total
silt inflow into Dongting Lake decreases accordingly. In the period of 2003~2006,
silt inflow from “Three Passages” is only 58% of total silt inflow into Dongting Lake.
When total silt inflow into Dongting Lake decreases annually, silt outflow from
Dongting Lake decreases little by little too. Before Jingjiang river curve course is
corrected, for Chenglingji station which is the exit of Dongting Lake, annual
average silt outflow from Dongting Lake is 59.6 million tons, 26.1% of total silt
inflow into the lake. After Jingjiang river curve course is corrected, annual average
silt outflow is 33.1 million tons, 25.1% of total silt inflow into the lake. In the later
period of 1990s, the percentage mentioned above is 28%, hardly changes. After
Three Gorges Reservoir is constructed(2003~2006), the percentage mentioned
above increases to 60.4% rapidly, that indicates the construction of Three Gorges
Reservoir is favorable to sediment deposition decreasing in Dongting Lake zone.
Sediment deposition trend becomes slow, resulting from changed makeup of silt
suspension into the lake.
3
River-lake relationship prediction after Three Gorges Reservoir is built
One-dimension unsteady flow river network mathematical model is created to
compute and analyze, adopting one dimension Saint-Venant equation set as
basic equations set and quatre implicit difference scheme to discrete, adopting
three classes method to solve, analyzing the model’s consistency, astringency
and stability. In order to find out mathematical model simulation accuracy of flood
process, water-silt movement and quantification, the model is calibrated by flood
processes of 1996 and 1998. Mathematical model verification computation
includes river bed scouring-siltation of Jingjiang river section, long series of united
river-lake water-silt movement from 1993 to 2006. The verification computation
data indicates that simulation inaccuracy of water level and flow discharge is in
the range of rules, simulation inaccuracy
of long period river bed
scouring-siltation is less than 30%, the mathematical model is qualified for later
computation and analysis.
Computation conditions: Computation river section includes 1123km long
river section between Yichang and Datong. Computation terrain adopt river
sections between Yichang and Chenglingji of September,2002 and between
Chenglingji and Datong of May,1992~ November, 1993, Dongting Lake zone of
1995(part section using 1:5000 topographic map), Poyang Lake zone of 1998.
Computation time sequence is from 1961 to 1970.
Entrance water-silt condition: water-silt out of Three Gorges Reservoir after
regulation and storage.
Exit control condition: long-time average annual relationship curved line of
water level and flow discharge at Datong hydrological station.
During the water storage beginning period of Three Gorges Project, large
quantity sediment deposition appears in reservoir zone, silt outflow decreases
distinctly, with silt grain becoming smaller. In order to adapt changed water-silt
condition and fulfill its own silt entrainment capacity, water flow needs to scour the
original downstream river bed. So unsaturated water flow out of Three Gorges
Reservoir begins to absorb silt from downstream near dam river section, resulting
in intense scour. Scour will make pebble river bed or pebble-slit river bed
coarsening to form its own anti-scour protection layer, resulting in intense scour
moving down. For sandiness river bed, intense scour changes its section
hydraulic characteristics, resulting in water depth increasing, flow velocity
decreasing, water level going down and hydraulic slope decreasing, which can
suppress scour effect, so scour begins to move down. In this case, scour in
Yichang-Datong river section tends to develop from upstream to downstream.
With scour-siltation change happening in main stream river bed of Changjiang
River, water-silt quantity into Dongting Lake and Poyang Lake changes
accordingly, resulting in changed scour-siltation of lake zone, changed water
surface profile of river course, changed water-slit diversion in “Three Passages” of
Jingjiang and changed water-silt outflow from Dongting Lake, so the original
river-lake relationship begins to change too.
3.1
SCOUR-SILTATION CHANGE OF DIFFERENT RIVER SECTIONS
After water storage of Three Gorges Project, scour happens in upstream river
course and siltation happens in downstream river course. By 30 years after water
storage of Three Gorges Project, scour tends to balance in the upper reach river
section of Songzhikou. For the pebble-sand river section between Yichang and
Songzhikou which is 75.7km long, river bed becomes coarsening quickly, forming
a anti-scour protection layer to suppress scour development, resulting in scour
completed in several years. By 30 years after water storage of Three Gorges
Project, considering river bed width of 1000m, average scour depth in the river
section mentioned above is 1.0m. The river section between Songzhikou and
Ouchikou belongs to curved river course, with bank protection project constructed
on its concave bank, the lowest elevation of scour pit is lower than that of pebble
layer upper plate, river bed becomes coarsening due to pebble moving down or its
exposure to surface. The river section between Songzhikou and Ouchikou is
147km long, considering river bed width of 1200m,its average scour depth is 2.3m
by 30 years after water storage of Three Gorges Project. Lower Jingjiang river
section(between Ouchikou and Chenglingji) belongs to wriggling river course, is
170.2km long. In this river section, its sand layer is very thick. After water storage
of Three Gorges Project, great scour-siltation changes happen in this river section.
By 10 years after water storage of Three Gorges Project, intense scour happens
in river bed of this section. By 30 years after water storage of Three Gorges
Project, considering river bed width of 1400m, average scour depth in this river
section is 4.2m. The river section between Chenglingji and Wuhan is 230.0km
long. After water storage of Three Gorges Project, siltation and scour happens in
this river section in succession. Due to intense scour in Jingjiang river section and
greater sediment concentration of water flow, light siltation happens in early water
storage period of Three Gorges Project. With Three Gorges Reservoir operation
time extending, intense scour develops to downstream, scour is the main trend of
this river section. By 30 years after water storage of Three Gorges Project,
considering river bed width of 1900m, average scour depth in this river section is
1.23m. The river section between Wuhan and Datong is 500km long, belongs to
fork river course, which is about 623km away from Yichang city. In early water
storage period of Three Gorges Project, due to intense scour in lower reach of the
dam, water flow can absorb silt on its way, resulting in decreased silt
transportation capacity, siltation happens in the lower reach river bed of Wuhan
city. By 30 years after water storage of Three Gorges Project, total accumulated
sediment siltation quantity is 686 million tons(about 5.04×108m3), siltation quantity
of river fork is 686 million tons.
After Three Gorges Reservoir construction, with main stream river bed of
Changjiang River scoured, the entrance water level of “Three Passages” falling,
water-silt diversion quantity decreasing, scour-siltation change happens in river
bed of “Three Passages”. In the early operation stage of Three Gorges Reservoir,
scour appears in all river bed of Yichang-Ouchikou river section, bigger grain
sediment is rush away into “Three Passages” diversion courses, making their river
bed scour suppressed. By 10 years after Three Gorges Reservoir construction,
accumulated scour-siltation quantity of Songzhikou, Taipingkou and Ouchikou is
negative 30 million tons, 33 million tons and 55 million tons repectively(negative
means scour). During 30 years after Three Gorges Reservoir construction, with
increased sediment concentration of outflow water and increased sediment grain,
scour and siltation happens in Songzhikou river bed in succession, scour and
siltation happens in different zone of Taipingkou river bed, siltation in Ouchikou
river bed tends to be slow and irreversible.
Dongting Lake zone includes “Three Passages” diversion courses, western
lake zone, eastern lake zone and southern lake zone. Western lake zone includes
Ping lake, Qili lake, Songligong course and Songhuhong course etc. After Three
Gorges Reservoir construction, water diversion quantity of “Three Passages”
decreases, its silt diversion quantity decreases greatly, water into lake zone has
smaller sediment concentration. After regulation, sediment siltation is still main
trend, but siltation speed slows down.
In the early operation stage of Three Gorges Reservoir, due to decreased
water-silt diversion of “Three Passages”, sediment siltation quantity of Dongting
Lake zone decreases accordingly. By 10 years after Three Gorges Reservoir
construction, average annual sediment siltation quantity of lake zone is about 50%
of its long-time average annual value during1956~2006. By 30 years after Three
Gorges Reservoir construction, average annual sediment siltation quantity is 38%
of its long-time average annual value during1956~2006. So Three Gorges
Reservoir construction is favorable to flood control of Jingjiang river, slows down
sediment siltation in Dongting Lake, enhances its regulation function and extends
its life.
3.2
WATER-SILT DIVERSION IN “THREE PASSAGES” OF JINGJIANG RIVER
After Three Gorges Reservoir construction, main stream river bed of
Changjiang River is scoured, the entrance water level of “Three Passages” falls,
water-silt diversion quantity decreases too. Compared with long-time average
annual value of 1956~2006, average annual runoff amount of “Three Passages”
during 1~10 years after Three Gorges Reservoir construction decreases from
88.4×109m3 to 59.4×109m3, which is a one third reduction. Average annual silt
transportation quantity of “Three Passages” decreases from 120 million tons to 40
million tons, which is a two third reduction. Compared with long-time average
annual value of 1956~2006, average annual runoff amount of “Three Passages”
during 21~30 years after Three Gorges Reservoir construction decreases 50% to
48.4×109m3, average annual silt transportation quantity decreases about 25% to
23.4 million tons.
Due to river bed sediment siltation, water-silt diversion quantity of “Three
Passages” in Jingjiang river tends to decrease. For Songzhikou entrance, due to
coarsening protection layer forming in river bed quickly, entrance water level falls
lightly, which has little effect on its water-silt diversion capacity. During 30 years
after Three Gorges Reservoir construction, compared with long-time average
annual value, water diversion quantity of Songzhikou decreases only 9%~18%,
silt diversion quantity decreases only 51%~58%. During 10 years after Three
Gorges Reservoir construction, compared with long-time average annual value,
water diversion quantity of Taipingkou decreases 43%, silt diversion quantity
decreases 63%. During 21~30 years after Three Gorges Reservoir construction,
water diversion quantity of Taipingkou decreases 70%, silt diversion quantity
decreases 82%, compared with its long-time average annual value. The biggest
water-silt diversion quantity loss happens in Ouchikou. During 10 years after
Three Gorges Reservoir construction, compared with long-time average annual
value, water diversion quantity of Ouchikou decreases 64%, silt diversion quantity
decreases 80%. During 21~30 years after Three Gorges Reservoir construction,
water diversion quantity of Taipingkou decreases 96%, silt diversion quantity
decreases 98%, compared with its long-time average annual value, there is
almost no water in river bed.
4
CONCLUSION
Three Gorges Reservoir will result in great effect on river courses in upstream
and downstream dam, make river bed regulating automatically. In order to predict
river bed scour-siltation changes due to this effect, many kinds of water-silt
mathematical model, such as one-dimension unsteady flow river network
mathematical model and plane two-dimension silt mathematical model, are
developed and created. In this text, computation achievements of one-dimension
unsteady flow river network mathematical model is introduced with emphasis.
One-dimension water-silt mathematical model is created to compute river
course scour-siltation changes, analyze river-lake relationship, during the
beginning period of Three Gorges Reservoir operation(June,2003~31th,
December, 2032). The computation achievements indicate, in early stage of Three
Gorges Reservoir operation, scour will happen from upstream to downstream for
main streams in the middle and lower reaches of Changjiang River, the depth of
river beds will increase, specially for the river section upstream Zhicheng county，
river course scour will complete in a short time and extend through to its lower
reach river course. The most serious scour happens in Lower Jingjiang river
section, average river bed scour depth is 4m. Due to main stream river bed scour
effect, river bed scour-siltation of “Three Passages” in Jingjiang river changes
accordingly, siltation will be the main trend. Silt siltation quantity will decreases in
Dongting Lake zone. By 30 years after Three Gorges Reservoir construction,
average annual silt siltation quantity of lake zone is 38% of that of last 50
years(1956~2006), silt siltation speed slows down distinctly. After Three Gorges
Reservoir begins to operate, water-silt diversion of “Three Passages” decreases.
Compared with long-time average annual value of 1956~2006, by 21~30 years
after Three Gorges Reservoir construction, average annual runoff of “Three
Passages” decreases near 50%, its average annual slit transportation capacity
decreases to about 25% of long-time average annual value.
Due to complexity of silt movement and others reason, all these research
works still need to be improved and enhanced, for their better application in
scientific research and civil construction.
PRELIMINARY STUDY ABOUT RIVER-LAKE EVOLUTION EFFECT DUE TO
THREE GORGES PROJECT
ABSTRACT
Due to complex boundary conditions, multiplex river bed composition and
different river course form, river evolution and river-lake relationship are very
complicated in the middle and lower reaches of Changjiang River. After Three
Gorges Project is completely built, previous incoming water-sediment process
downstream dam will be changed, resulting in scour-siltation changes, changed
river bed evolution process and river-lake relationship adjustment for rivers and
lakes in the middle and lower reaches of Changjiang River. In order to study river
course changes and river-lake relationship adjustment after water storage of
Three Gorges reservoir, according to staging water storage process of Three
Gorges Project, preliminary study is carried out in a way combining many
technology methods such as prototype investigation data analysis and
mathematical model computation etc. The preliminary study achievements
indicate, in early stage of Three Gorges reservoir operation (June,
2003~December, 31, 2032) , scour will happen from upstream to downstream for
main streams in the middle and lower reaches of Changjiang River, the depth of
river beds will increase, specially for the river reach upstream Zhicheng county，
river course scour will complete in a short time and extend through to its lower
reach river course. Due to main stream river bed scour effect, the entrance water
level of “Three Passages” will decrease on the south bank of Jingjiang river,
resulting in corresponding scour-siltation change in its river bed, siltation will be
the main trend. After Three Gorges Reservoir construction, sediment
accumulation will decrease in Dongting Lake zone, its year average sediment
accumulation value will be 40% of its long-time average annual value after 30
years. So Three Gorges Reservoir construction can slow down sediment
accumulation in Dongting Lake zone markedly. Due to sediment movement
complexity and others reasons, more in-depth research and study about river-lake
relationship effect after Three Gorges reservoir construction are still needed to
carry out, the related study achievements can be regarded as important and
scientific basis for river-lake control research in future.
Key words Three Gorges Project River-lake relationship WATER-SILT
DIVERSION Fluvial process Stream degradation and aggradation Sediment
gradation
References
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Reservoir. Changjiang Scientific Research Institute.1972.
［2］Yang huairen Tang richang.Study on Jingjiang River Fluvial process in
Changjiang Midstream.1999.
［3］Wan jianrong,Fan beilin.Study on 1-D mathematical model computation
on deformation of River bed degradation and aggradation in zhe downstream of
Xiangjiaba Project and Xiluodu Project. 1997.