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8/10/2019 Fluvial processes and morphological response in the Yellow and Weihe Rivers to closure and operation of Sanmen
1/15
Fluvial processes and morphological response in the Yellow and
Weihe Rivers to closure and operation of Sanmenxia Dam
ZhaoYin Wang , Baosheng Wu, Guangqian Wang
State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China
Received 4 July 2006; received in revised form 27 January 2007; accepted 27 January 2007
Available online 7 February 2007
Abstract
The fluvial and morphological processes induced by impoundment of the Sanmenxia Reservoir and relevant human activities
on the Yellow River and its tributaries are complex. The long term annual sediment load of the Yellow River was 1.6 billion tons,
ranking first of all the world's rivers. In 1960, Sanmenxia Dam began filling. Sediment transport in the river then was greatly
disturbed and a new cycle of the fluvial processes was induced. First, the dam caused not only anticipated sedimentation in the
reservoir, but also serious sedimentation in the largest tributary of the river (the Weihe River). The response of fluvial process to the
dam closure varies in space and time. Second, the downstream reaches of the dam experienced erosion and resiltation, changes of
river pattern, and development of meanders. Moreover, the downstream reaches of the dam have experienced more and more water
diversion, which has induced readjustment of the longitudinal profile of the river. The study reveals that sedimentation in the
Sanmenxia Reservoir enhanced the bed elevation at Tongguan, where the Weihe River flows into the Yellow River. The rising
Tongguan's elevation caused retrogressive siltation waves in the Weihe River, which propagated at a speed of about 10 km/yr. An
equilibrium sedimentation model is proposed, which agrees well with the data of sedimentation in the Weihe River. In the reaches
below the dam the river changes from braided to wandering, or from wanderingbraided to wanderingmeandering. The discharge
released to the downstream reaches has been regulated by the reservoir and it decreases along the course because the quantity of
water diversions is more than the inflow from tributaries. The reduction in discharge causes readjustment of the longitudinal bed
profile. By using the minimum stream power theory, we prove that the riverbed profile is developing toward an Sshapeprofile.
2007 Elsevier B.V. All rights reserved.
Keywords: Fluvial processes; Sanmenxia Reservoir; River pattern; Bed profile; Water diversion
1. Introduction
According to the International Commission on Large
Dams, the world's rivers are now obstructed by more
than 40,000 large dams. From 1949 to 1990, the number
of large dams in China increased from only eight to
more than 19,000. These large dams have provided
extensive benefits during the past century and have
fueled the economy by providing cheap power, irriga-tion, and municipal water supplies. Dam construction
and reservoir operations are great disturbance to fluvial
processes of rivers. Aggradation in the reservoir and
upstream reaches and degradation in the downstream
reaches have taken place in many of the world's rivers
(Leopold, 1973; Gregory and Park, 1974; Petts, 1979;
Mahmood, 1987; Collier et al., 1996; Kondolf, 1997).
Many studies have investigated changes in river patterns
responding to dam closure, showing rivers with braided
or braidedmeandering transitional characteristics
Geomorphology 91 (2007) 6579
www.elsevier.com/locate/geomorph
Corresponding author.
E-mail address:[email protected](Z.Y. Wang).
0169-555X/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2007.01.022
mailto:[email protected]://dx.doi.org/10.1016/j.geomorph.2007.01.022http://dx.doi.org/10.1016/j.geomorph.2007.01.022mailto:[email protected]8/10/2019 Fluvial processes and morphological response in the Yellow and Weihe Rivers to closure and operation of Sanmen
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(Neill, 1973; Church, 1983; Ferguson and Werritty,
1983; Knighton and Nanson, 1993). Lateral adjustment
of river channels due to reservoir operation is more
irregular. The channel width may respond to dam
closure with trends of widening, narrowing, and no
change (Williams and Wolman, 1984). Any changesimposed on a fluvial system tend to be absorbed by the
system through a series of channel adjustments
(Schumm, 1973). The description of channel adjustment
and evolution by nonlinear decay functions are well
documented (Hey, 1979; Robins and Simon, 1983)Graf
(1977) used exponential functions to describe the
relaxation time necessary to achieve equilibrium
following a disturbance.Simon (1989)found that both
exponential and power equations were initially fitted to
the observed data.
In the upstream reaches, the primary consequence ofimpoundment of rivers is sedimentation. However,
sedimentation issues are not confined solely to the
reservoir. The backwater reach of the reservoir can
extend hundreds of kilometers upstream, as in the case
of the Three Gorges Project on the Yangtze River in
China. The current velocity and sediment carrying
capacity of the flow are reduced by reduction in energy
slope; thence sedimentation occurs in the backwater
region. The aggradation, in turn, raises the local water
surface elevation, creating additional backwater and
deposition even farther upstream and in tributaries, as in
the case of the Sanmenxia reservoir on the Yellow River.This feedback mechanism allows the depositional
environment to propagate much farther upstream than
the initial hydraulic backwater curve might suggest
(Goodwin et al., 2001).
Qian et al. (1987)have defined the wandering river as
a river pattern exhibiting very unstable channel and high
migration rate, which usually occurred in rivers carrying
high sediment load like the Yellow River in China. The
Danjiangkou Reservoir on the Hanjiang River in China
has changed the river from a braided river to a
wanderingbraided river, which is caused by strongerosion of the riverbank initiated by operation of the
reservoir. Large quantities of sediment were supplied to
the channel by bank erosion and deposited at many
midchannel bars during floods. Hence a wandering
braided channel pattern with many unstable mid
channel bars has developed. Strikingly, while the river
was developing from a braided river into a wandering
braided river, the sediment quantity measured at the
upstream station was approximately equal to that
measured at the downstream station (Xu, 1996). This
implies that a huge amount of sediment on the bed and
banks was removed, while only a small amount of
sediment was transported through the channel. Church
(1983) also reported changes of river patterns from
braided or multithread to single thread. The construction
of the Black Butte Dam on Stony Creek in the U.S. in
1963 caused the braided pattern of the reaches down-
stream of the dam to change to a singlethread, incisedmeandering pattern by 1967.
Although many case studies on fluvial processes
induced by dams have been reported, general laws on
the processes do not exist. The aim of the study is to
comprehend the complicated fluvial processes and
morphological responses in the upstream and down-
stream of the Sanmenxia Reservoir. In the upstream
sedimentation in Sanmenxia Reservoir has changed
the lower boundary of the Weihe River (the bed
elevation at the confluence), which has induced
continuous sedimentation and a new morphologicalprocess in the tributary. The laws of sedimentation
responding to varying lower boundary and equilibrium
sedimentation volume in the Weihe River for given
lower boundary conditions are studied. In the down-
stream reaches the water and sediment load have
reduced greatly since the impoundment of the
reservoir, partly because the reservoir trapped sedi-
ment and much water and sediment load were diverted
from the river along the course. The river has adjusted
itself to match the changing flow conditions. The
lower Yellow River is a wandering river. The fluvial
processes and morphological responses to the closureand operation of the Sanmenxia Reservoir and water
diversion projects along the river are complex. The
speed of channel migration did not reduce after the
impoundment as predicted, although the sand bars in
the channel had reduced, which is different from the
general morphological response to dam closure. This
paper analyzes the stability of the channel and
development trend of the bed profiles under the
changing water and sediment conditions. The research
results reported here may shed light on the manage-
ment of heavily sedimentladen rivers under changingconditions.
2. Background and data collection
As shown inFig. 1, the Yellow River has a drainage
area of 795,000 km2 and a length of 5464 km making it
the second longest river in China. The longterm
annual sediment load at the Sanmenxia Station was
1.6 billion tons before 1980, ranking first of all the
world's rivers (Qian and Dai, 1980), although the
sediment load has reduced greatly in the past 20 yr.
The huge amount of sediment is mainly from the loess
66 Z.Y. Wang et al. / Geomorphology 91 (2007) 6579
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plateau in the middle reaches of the river. In 1960, the
first dam on the river, i.e., the Sanmenxia Dam, began
filling. Sediment transport in the river then was greatly
disturbed and a new cycle of fluvial processes was
induced. Nevertheless, the dam has caused unforeseen
impacts on the fluvial processes and river morphology
both in upstream and downstream reaches, which werenot fully comprehended during the project planning
process. This is not surprising since the fluvial processes
and morphological responses to the dam closure and
reservoir operation at the basin scale are immensely
complex.
First, the dam caused not only anticipated sedimen-
tation in the reservoir, but also serious sedimentation in
the largest tributary the Weihe River, which was not
predicted. Second, the reaches downstream of the dam
experienced erosion and resiltation, channel migration,
and development of meanders. Moreover, the down-stream reaches have experienced more and more water
diversion the past decades, which has induced read-
justment of the longitudinal profile of the river. The
river flow and sediment carrying capacity have been
greatly changed, which caused a new morphological
development.
Sedimentation in the reservoir and the Weihe River
has caused severe flooding. A recent flood disaster
occurred in the lower Weihe River in the 2003 fall, which
was extremely exacerbated by sedimentation in the
Sanmenxia Reservoir on the Yellow River (Wang et al.,
2004a,b). The flood caused great economic loss and
affected million people, and has rekindled the argument
on decommission of the dam. The main cause for the
disaster was the continuously enhancing riverbed and
flood plain due to sedimentation. If there were no
Sanmenxia Dam the riverbed would be much lower and
the flood would cause no such a great disaster. Never-
theless, only 3 yr before the event, when the YellowRiver Conservancy Commission celebrated the 40 yr
anniversary of Sanmenxia Reservoir, many people spoke
highly on the reservoir and awarded Sanmenxia Res-
ervoir a great achievement in the hydroconstruction in
China. It is a great practice in the training of heavily
sedimentladen rivers (rivers carrying sediment load
often higher than 100 kg/m3). The half century safety of
the lower Yellow River reaches and development of
the river basin were attributed to the operation of the
reservoir, which played an important role in flood
control, icejam flood control, power generation, irriga-tion and water supply. The contrary opinions against
Sanmenxia Reservoir were due to a lack of under-
standing of the fluvial processes and morphological re-
sponses of the Weihe and Yellow Rivers to the reservoir
operations and insufficiency of effective management
strategies.
Sediment measurement has been performed for a long
period of time on the Yellow River; especially a
systematic and regular measurement began in 1950. In
the period from 19501980 the Yellow and Weihe Rivers
experienced a high sediment load period, and, from 1985
the sediment load in the two rivers has been remarkably
Fig. 1. The Yellow River and its tributaries (of which the Weihe River is the largest), the locations of the Sanmenxia and Xiaolangdi Dams, the
hydrological stations: Hekouzhen, Longmen, Tongguan, Huayuankou, Aishan and Lijin, and measurement crosssections: Tiexie and Gubaizui,
along the river course.
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8/10/2019 Fluvial processes and morphological response in the Yellow and Weihe Rivers to closure and operation of Sanmen
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Dam was initiated in 1957, and water impoundment
commenced in September 1960. The crest elevation of
the dam is 353 m, and the original capacity of the
reservoir was 9.705 billion m3 with a pool level of
335 m. The reservoir area extends upstream a distance of
246 km to Longmen. The lower Weihe River is affectedby the reservoir as well.
The Weihe River is 818 km long and has a
drainage area of 134,800 km2 with more than
23 million people dwelling in the river basin. The
river basin was known as the 800 li (1 li=0.5 km)
fertile Qin Valley. The most serious adverse effect of
the Sanmenxia Dam is the unanticipated sedimentation
in the lower Weihe River and, consequently, the high
flooding risk to the lower Weihe Basin and Xi'an, an
ancient capital of China. Sedimentation in the Weihe
River has changed the valley into a swamp with ahigh groundwater table. Local people complained and
some officials and scientists suggested decommission-
ing the dam. The Weihe River has been experiencing a
str iking change in f luvial processes since the
impoundment of the Sanmenxia Reservoir. The river
channel has been changing from meandering with a
sinuosity of 1.65 to straight with a sinuosity of only
1.06 and slightly meandering with a sinuosity about
1.3.
The longterm average annual runoff of the Weihe
River is 8.06 billion m3, and annual sediment load is
386.6 million tons, which compose about onefifth ofthe annual runoff and onethird of the annual sediment
load of the Yellow River at Sanmenxia. In the past
decades, the water and sediment load in the Weihe
River and the Yellow River have been reducing
mainly because of human activities. Table 1 shows
water and sediment load in the rivers in the periods
19602001 and 19862001. Water and sediment load
in the two periods are less than the average values
before 1980, but the ratios of water and sediment load
from the Weihe River to the Yellow River remain
unchanged. The majority of the sediment load consists
of silt with a median diameter of about 0.03 mm.
Before the impoundment of Sanmenxia Dam, the
Weihe River carried 386.6 million tons of sediment
into the Yellow River annually and the Weihe River
itself remained a relatively stable longitudinal bedprofile.
The elevation of Tongguan or Tongguan's elevation
is defined as a flood stage corresponding to a discharge
of 1000 m3/s at the Tongguan Hydrological Station on
the Yellow River, which acts as the base level of the bed
profile of the Weihe River. Before the Sanmenxia Dam
Tongguan's elevation was about 323.5 m. Since
impoundment of the Sanmenxia Reservoir, sediment
has been depositing in the reservoir, which causes
Tongguan's elevation to increase. The energy slope and
sediment carrying capacity of the flow in the WeiheRiver have been reduced. The sediment load could not
be transported into the Yellow River and sedimentation
occurred in the lower Weihe River. In other words, the
rising Tongguan's elevation has changed the lower
boundary of the Weihe River, hence inducing a new
cycle of fluvial processes.
The filling of Sanmenxia Reservoir began in
September 1960 and the pool level reached the highest
pool level of 332.58 m on Feb. 9, 1961. The reservoir
functioned as a storage basin until March 1962. Severe
sedimentation problem became evident immediately
after impoundment. During the first 18 months, 93% ofthe incoming sediment load was trapped in the reservoir
and caused 17% of capacity loss below an elevation of
335 m or 26% of capacity loss below an elevation of
330 m. Tongguan's Elevation had risen about 5.5 m in
Oct. 1961. A flood with a discharge of 2700 m3/s from
the Weihe River was blocked, which flooded 17,000 ha
of farmland in the lower Weihe Plain and caused a great
economic lose. To mitigate the sedimentation, the
operation scheme was changed to detain only flood
water in flood seasons. However, the floodreleasing
Table 1
Water and sediment load of the Yellow and Weihe Rivers in the past decades
River/hydrologic
station
Distance to the
Yellow River
mouth L (km)
Annual runoff
(19602001)
(bil. m3)
Annual sediment
load (19602001)
(mil. tons)
Average sediment
concentration
(19602001) (kg/m3)
Annual runoff
(19862001)
(bil. m3)
Annual sediment
load (19862001)
(mil. tons)
Weihe/Huaxian 1177 6.79 312 46.04 4.66 248
Yellow/Tongguan 1092 34.61 1043 30.13 25.16 722
Yellow/Sanmenxia 996 34.69 1009 29.09 24.62 712
Yellow/Huayuankou 734 37.44 910 24.20 25.88 610
Yellow/Aishan 374 33.07 770 25.00 19.16 440
Yellow/Lijin 100 28.56 700 36.80 13.56 350
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capacity of the outlet structures was limited. In order to
increase the capacity to discharge sediment from the
reservoir a project of reconstruction of outlet structures
was carried out in two stages during the period 1964
1973. Ten bottom outlets were reopened, thus, sediment
deposit can be sluiced out off the reservoir through theseoutlets with the lowest elevation at 280 m.
The operation scheme of the Sanmenxia Reservoir
has been substantially changed to achieve a balance
between sediment inflow and outflow in the following
three reservoir operation modes (Wu and Wang, 2004):
(i) storage from September 1960 to March 1962, the
reservoir was operated at a high storage level the whole
year round; (ii) detaining flood water and sluicing
sediment from March 1962 to October 1973, the
reservoir was operated at a low storage level throughout
the year, detaining floods only during flood seasons andsluicing sediment with the largest possible discharges;
and (iii) storing clear water and releasing turbid water
from November 1973 to the present, the reservoir has
been operated at a high level (315320 m) to store
relatively clear water in nonflood seasons (November
June) and at a low level (302305 m) to release high
sediment concentrations in flood seasons (JulyOcto-
ber), as shown inFig. 2. Moreover, the bottom outlets
have also been used to discharge high concentration
density currents. As the concentration is higher than
200 kg/m3, density current occurs in the reservoir. The
density current may be directly released from thereservoir though the bottom outlets even if the pool
level remains high.
The longitudinal profile in the reservoir has varied
with the changes of operation modes as shown inFig. 3.
During the storage operation period the reservoir was
severely silted. The measured sedimentation volume in
1964 was about 1.95 billion tons, representing 70% of
the incoming sediment load in the first 4 yr. Changing
the operation modes has reduced the sedimentation
volume, and the bed profiles have been relatively stable
since the 1970s. The cross
section CS 41 is at Tongguan, at which the Weihe River flows into the
reservoir.
Fig. 4shows the variations in Tongguan's elevation
over time from 1960 to 2001. Three ascending periods
are denoted by I, II, and III; and two descending periods
are denoted by 1 and 2. The abrupt rise and fall in 1960
and 1962 were caused by the impoundment in 1960 and
change of the operation mode from storage to flood
detention. The time of high elevation (329 m in Fig. 3)
was short and its influence on the Weihe River
sedimentation was temporary, although it caused an
obvious flood stage rise in 1961. Therefore, the periodof 19601962 is not separated from the ascending
period I.
The ascent and descent of Tongguan's elevation were
results of reservoir sedimentation and erosion, which in
turn were caused by variations in the pool level of the
reservoir. Generally speaking, sedimentation in the
lower Weihe River occurred during the periods when
Tongguan's Elevation rose, and erosion occurred during
the periods when it fell. The total volume of sediment
Fig. 2. Variation of pool level of Sanmenxia Reservoir in differentoperation modes.
Fig. 3. Longitudinal profiles at Sanmenxia Reservoir during different
periods of operation (in which CS12CS48 are the measurement cross
sections on the reservoir reach of the Yellow River).
Fig. 4. Variation of Tongguan's elevation (watersurface elevation at
Tongguan for a flow of 1000 m3/s).
70 Z.Y. Wang et al. / Geomorphology 91 (2007) 6579
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deposited in the lower Weihe River up to the year 2001
was about 1.3 billion m3. The sedimentation was
distributed mainly in a 100kmlong reach from the
confluence. The accumulated deposition volume per
unit length was high near the confluence, reduced
upstream, and to nearly zero near Xi'an. Fig. 5 shows
the transect of the profiles of the channel bed and
floodplain in the lower Weihe River measured in 1960
and 2001 at the crosssections WY2 and WY7, which
are 21 and 59 km from Tongguan, respectively. Thefloodplain elevation had risen by 3 to 5 m from
sedimentation, and the main channel had shrunken and
become more unstable. The flood discharge capacity of
the channel was hence reduced and the flood stage at the
same discharge was substantially enhanced.
Simon (1989) and Simon and Thorne (1996) studied
channel response in disturbed alluvial channels and
found that the changes imposed on a fluvial system tend
to be absorbed by the system through several stages of
channel adjustment and following exponential decay
equations. The response of the Weihe River to theSanmenxia Dam closure is more complex because the
raised Tongguan's elevation is not stable and the effect
has transmitted from the confluence to Xianyang Station
(180 km upstream from Tongguan. Erosion and
sedimentation caused by the ascending and descending
of Tongguan's elevation propagated upstream in retro-
gressive waves.Fig. 6ac shows the distribution of the
deposition rate per unit river length in the periods 1960
1969, 19691973, and 19731980, respectively, in
which the horizontal axis is the number of the
measurement crosssections on the Weihe River; the
average distance between the neighboring crosssections
is about 6 km. In the period from 1960 to 1969,
Tongguan's Elevation rose abruptly from 323.5 to
328.5 m (seeFig. 4). As a result, sedimentation occurred
in the reach around Huaxian at a rate of up to
2.5 million tons/km/yr (Fig. 6a). The mark I indicates
that the sedimentation corresponding to the first ascend-ing period of Tongguan's elevation. In the period from
1969 to 1973, the sedimentation wave moved upward to
the reach between Huaxian and Lintong, but the rate of
sedimentation decreased to about 0.75 million tons per
km per year (Fig. 6b). In the meantime, the first erosion
wave occurred near the river mouth, which corresponded
to the first descending period of Tongguan's elevation,
indicated by the mark 1. In 19731980, the first
sedimentation wave had moved upstream to Lintong, the
first erosion wave had moved to Huaxian and the peaks
had obviously decreased too. During this period, thesecond sedimentation wave occurred in the reach
between the river mouth and Huaxian indicated by the
mark II. This wave of sedimentation was associated
Fig. 5. Aggradation of the lower Weihe River measured at cross-
sections WY2 (21 km from Tongguan) and WY7 (59 km fromTongguan) from 1960 to 2001 (Wang and Li, 2003).
Fig. 6. Erosion () and sedimentation (+) per unit length per year
showing retrogressive waves in the lower Weihe River as a result of
ascending and descending of Tongguan's elevation. (The cross
sections are numbered from the river mouth. Huaxian, Lintong, and
Xianyang are hydrological stations by the river and are about 50 km,
128 km and 180 km upstream from Tongguan. The distance betweenneighboring crosssections is about 6 km). (Wang and Li, 2003).
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with the second ascending period of Tongguan's
elevation. The ascending and descending of Tongguan's
Elevation generated erosion and sedimentation waves,
which propagated retrogressively along the Weihe River,
at a speed of about 10 km/yr.
4. Equilibrium sedimentation model
Two questions to be answered about the fluvial
processes in the Weihe River induced by the Sanmenxia
Dam are: Is there any equilibrium of sedimentation in
the Weihe River? And whether the sedimentation has
reached the equilibrium? We propose a simple model to
answer the questions (Wang et al., 2004a,b). Assume
there is an equilibrium sedimentation volume, Ve for a
given increment of Tongguan's elevation. If the real
sedimentation volume,V, is much less than Ve the rateof sedimentation in the river is high. The rate of
sedimentation is proportional to the difference between
the equilibrium and real sedimentation volume:
dV
dt K Ve V 1
in which K is a constant with dimension of [1/T]. The
solution of the equation is
V ektR
KVeeKtdt const
2
The equilibrium sedimentation volume Veis propor-tional to the enhancement of Tongguan's elevation Zt,
which is given by Zt =Zt323.5, in which Zt is
Tongguan's elevation at time t and 323.5 m is the
Tongguan's elevation before the dam. Simply, the
equilibrium sedimentation volume can be imagined to
have a shape like a cone, then it may be assumed
Ve ADZt=2 3
in which A is a representative area of riverbed and
floodplain on which sedimentation occurs. Substituting
Eq. (3) into (2) yields
V 1
2AKeKt
Z t0
DZteKtdt DZt
4
in whichtis the time from 1960 when Sanmenxia Dam
begin to fill and Tongguan's elevation began to rise. The
parameters in the equation are determined from data as
A =5.30108(m2) and K= 0.15/yr. Fig. 7 shows the
calculation result of the sedimentation volume (solid
curve) in comparison with the real sedimentation
volume (pyramids). The dashed curve in the figure is
the calculation result with the value ofZt remaining
unchanged at 5 m (Zt,=328.5323.5= 5), which
shows that the equilibrium sedimentation volume is
around 1.3 billion m3.
As shown in Fig. 7, the model agrees well with the data
of sedimentation, which proves that for a givenZt, there is
indeed an equilibrium sedimentation volume. If the in-crement in Tongguan's elevation remains unchanged, the
sedimentation of the lower Weihe River may reach
equilibrium in about 25 yr. At present, the sedimentation
of the lower Weihe River is approaching to the equilibrium
volume, and there will be no great volume of accumulated
sedimentation if Tongguan's elevation stops rising.
Nevertheless, the equilibrium sedimentation volume is
dynamic and increases with rising lower boundary. If
Tongguan's Elevation continues to rise the equilibrium
sedimentation volume will be greater than 1.3 billion m3
and longer time is needed to reach the equilibrium.Sanmenxia Dam not only caused retrogressive sedi-
mentation and erosion in the lower Weihe River, but also
changed the river patterns. Before the reservoir began to
be used, the lower Weihe River was a meandering river,
with a value of sinuosity of about 1.65, in which sinuosity
is defined as the ratio of the length of the channel to the
length of the river valley. The closure of the dam reduced
the sinuosity to 1.06 in 1968, as shown in Fig. 8a. Very
quick sedimentation in this period buried the meandering
channel. In the meantime a straight channel developed
which was affected mainly by the reservoir operation. In
the period from 1970 to 1975 the Weihe River expe-rienced erosion and the channel developed gradually from
straight to meandering. The sinuosity had gradually
increased to 1.2. In the following period more and more
meanders have developed and the lower Weihe River has
been developing toward meandering with a sinuosity
about 1.3.
Moreover, the river channel has become quite un-
stable since the closure of the dam.Fig. 8b shows the
Fig. 7. Calculated cumulative sedimentation volume with Eq. (4) (solid
curve) in comparison with the real sedimentation volume (pyramids).
The dashed curve in the figure is the calculation result with the value ofZtremaining unchanged at 5 m (Wang and Li, 2003).
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migration distances of the stream channel measured at
crosssections WY535 during the first ascending and
descending periods of Tongguan's elevation. The
migration distance was up to 1.8 km at the cross
sections near Huaxian (WY11). The dam had less effect
in the reaches farther upstream, and the migration
distance was
8/10/2019 Fluvial processes and morphological response in the Yellow and Weihe Rivers to closure and operation of Sanmen
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remains unchanged, which is different from the
morphological responses to dam closure on other rivers.To control the migration of the channel, local people
have constructed many spur dykes by the channel. The
spur dykes have, to a certain degree, fixed the channel
and concentrated the flow and cause sediment deposi-
tion between the spur dykes. The channel, therefore, was
deepened and relatively stabilized. A channelization
degree is defined as the ratio of the total length of the
spur dykes to the length of the channel, or the length of
spur dykes per channel length. Fig. 10 shows the
distribution of channelization degree along the river
downstream from Sanmenxia Reservoir. From the 1970s
to 2002, the degree has increased from 0.20.8 to 0.8
1.35. Nevertheless, the natural fluvial processes tend tobreak the constraint of the spur dykes, and the flow
scours the dykes and causes them to collapse.
Fig. 11 shows the probability of collapse of each
dyke as a function of the channelization degree. The
probability is calculated with the total times of collapse
per year over the number of spur dykes. The probability
is low as if the channelization degree is 0.8, however, the probability of
dyke collapse abruptly increases from 10% to 30%. The
high probability of dyke collapse is due to the conflict
between the natural fluvial processes and the constraint
Fig. 10. Distribution of channelization degree (the ratio of the length of
the spur dykes to the length of the channel) along the lower YellowRiver (Wang et al., 2004a,b).
Fig. 9. The lower Yellow River channel wandered within the grand levees during the storage operation (19601964) and late operation periods
(19801984) of Sanmenxia Dam. (The curves are the thalweg of the channels) (Wang et al., 2004a,b).
Fig. 11. Probability of dyke collapse as a function of the channelizationdegree (Wang et al., 2004a,b).
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of channelization. In fact, the strongest conflict occurs
for channelization degrees in the range of 0.81.0,
therefore, dyke failure has a corresponding high
probability. Nevertheless, if the channelization degreeapproaches to 2 (both sides of the channel are
completely controlled with spur dykes), the channel
motion will change from lateral to vertical. The channel
will be deepened, resulting in an increase in the bankfull
discharge.Fig. 12shows the probability of dyke failure
against the bankfull discharge. Following an increase in
bankfull discharge, the probability of dyke failure
decreases.
Sanmenxia Reservoir has caused the lower Yellow
River to change from a wanderingbraided into a
wanderingsingle thread channel. Fig. 13 shows the
channel morphology of the TiexiePeiyu reach, which
is about 157189 km downstream from Sanmenxia
Dam, before and after the construction of the dam (Yang
et al., 1994). There were many sand bars before closure
of the dam; the number of bars had decreased 3 yr after
the dam was used for impoundment. The river had
become a singlethread channel by 1964. In themeantime, the sinuosity of the river increased. The
number of meanders in a 300kmlong river reach (150
450 km downstream from the dam) had increased from
16 to 22. This reach developed from a wandering
braided channel to a wanderingmeandering channel.
Meanders have generally developed after the San-
menxia Dam. The reach from the dam to Tiexie (0 to
157 km directly below the dam) is constrained by
mountains and no meanders develop within it. Statistics
are made for a 400kmlong reach, from 150 to 550 km
below the dam, which was an active fluvial reach. Beforethe impoundment of the dam, only 16 meanders were
located in the 400kmlong reach, and more meanders
have generally developed after the impoundment. Fig.
14 shows the numbers of meanders with different
wavelength in the reach in the 1970s, 1980s, and 1990s.
The meander wavelength is defined as the distance
from one turning point of the channel on one side of the
valley to the next turning point on the same side. As
shown inFigs. 14, 17small meanders exist in the reach
in the 1970s. Some meanders were separated by straight
sections and some other meanders connected with each
other and form small meandering sections. Between two
Fig. 12. Probability of dyke failure as a function of bankfull discharge
(Wang et al., 2004a,b).
Fig. 13. Channel morphology of the TiexiePeiyu reach (157189 km from Sanmenxia) preand postSanmenxia Dam.
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small meandering sections was a section with straight
channel. Lately the reservoir operation became stable,more meanders developed and the meandering sections
became longer. In the 1980s, however, 22 meanders
with a wavelength from 3 to 30 km had developed in the
reach. In the 1990s, the number of meanders continued
increasing and the meanders became regular; 31 of them
have meander wavelength within the range of 615 km.
The river became more and more meandering. In the
process, manmade spur dykes affected, more or less, the
development of meanders.
6. Impacts of discharge reduction on fluvial
processes
Sanmenxia Reservoir regulates, more or less, the
flow discharge for the downstream reaches, and the
peak discharge of extreme events was cut down.
Moreover, the annual runoff released to the down-
stream reaches has been reduced. The average
precipitation of the Yellow River basin is 476 mm,
but the pan evaporation is 10003000 mm/yr. The
total surface runoff of the watershed is 58 billion m3,
about 2% of the total of China. The downstream
channel is a perched river, with its riverbed 10 m
higher than the surrounding land. This poses aflooding risk but also provides the potential for
water diversion to farmland and numerous cities and
towns within and outside of the Yellow River basin.
For instance, residents in Tianjin and Qingdao cities,
which are several hundred kilometers from the Yellow
River and outside of the river basin, are drinking water
from the Yellow River. Currently more than 4500
Fig. 14. Numbers of meanders with different wavelengths in a 400km
long reach downstream of Sanmenxia Reservoir in the 1970s, 1980s,
and 1990s (Wang et al., 2004a,b).
Fig. 15. Variation of annual runoff and sediment load in the period from 1960 to 1997 at Xiaolangdi (130 km from Sanmenxia Dam) and Lijin
(900 km from Sanmenxia Dam). The differences between the two stations are due to the inflow from tributaries and water diversions ( Wang et al.,2004a,b).
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water diversion projects and 29,000 pumping stations
have been completed for irrigation and water supply
since the dam construction. The irrigation area has
increased from 0.8 million ha in 1950 to 7 million ha
in 1995.Water diversion inevitably affects the fluvial pro-
cesses. Water diversion may even change a section of a
perennial stream to an ephemeral river section (Fogg
and Muller, 1999). For the first time in its history, the
Yellow River failed to reach the ocean in 1972. In 1997,
the Yellow River failed to reach the sea twothirds of the
year because most of the river water was diverted. While
water diversion projects have become a popular and
important strategy to meet increasing water demand, the
stream flow, sediment transport, and fluvial processes of
rivers are increasingly affected.
Fig. 15(a) and (b) shows the variation of the annu-al water and sediment load from 1960 to 1997 at the
Xiaolangdi (130 km downstream from Sanmenxia) and
Lijin (900 km downstream from Sanmenxia) hydrologic
stations, in which the horizontal lines represent the
average runoff and sediment load. The differences
between the figures at the two stations are due to the
inflow from tributaries and outflow by water diversions
along the course from Xiaolangdi to Lijin. From 1960
1969, more water flowed through Lijin than Xiaolangdi
because water diversion was less than the inflow from
tributaries. From 19701985, the annual runoff at Lijinwas equal to or slightly less than at Xiaolangdi because
more water had been diverted. From 1986 to the present,
however, the total volume of water diverted was much
more than the inflow from tributaries, and the water
runoff decreased along the course. The annual runoff was
about 11 billion m3 less at Lijin than at Xiaolangdi. The
reduction in runoff over a long stretch of the river elicited
a sharp reduction in the flow's sedimentcarrying
capacity. Therefore, the annual load was much less at
Lijin than at Xiaolandi from 1986 to the present.
Table 1 also shows the decrease of water and
sediment load along the course. From 1986, water and
sediment loads increased along the course and reached
their maximum values at Huayuankou and then
decreased farther downstream from diversion. The
sediment load at Lijin is less than that at Sanmenxia
by more than 300 million tons, which must have been
deposited in the reach between Sanmenxia and Lijinand consequently changed the morphology of the
river.
One of the impacts of the runoff reduction on the
fluvial processes was the shrinkage of the channel.
Fig. 16 shows the bankfull discharge of the lower Yel-
low River during different periods. Water diversion has
reduced the discharge and sedimentcarrying capacity,
and sediment has been deposited in the channel, which
has made the channel shallow and unstable. As a result,
the bankfull discharge has decreased steadily. The
bankfull discharge was about 9000 m
3
/s in 1958 and1964; it decreased to about 6000 m3/s in 1985, and to
only 3000 m3/s in 1999. The shallow channel cannot
accommodate floodwater as before; so the flood stage
has become extremely high and the phenomenon known
as the little flood with high flooding disasters has
occurred in the river basin.
The second important impact of water diversions is
the adjustment of the riverbed profile. Field evidence
from natural streams shows that variations in successive
processes and forms result from a system's tendency to
minimize the rate of energy dissipation with time
(Simon, 1992). According to the minimum streampower theory, the morphology of fluvial rivers develops
to reach the minimum stream power (Yang, 1996). This
can be described by the following equation:
dP
dx
d
dx csQ c Q
ds
dxs
dQ
dx
0 6
in whichPis the stream power in ton/s, is the specific
weight of water in ton/m3, s is the riverbed slope, x is
Fig. 16. Bankfull discharge along the lower Yellow River course
during different periods (Wang et al., 2004a,b).
Fig. 17. Longitudinal bed profiles of the lower Yellow River in 1977and 1997 (Wang et al., 2004a,b).
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the distance along the river course in km, and Q is the
discharge in m3/s. For most rivers, the discharge
increases along the course from the inflow from
tributaries; thus, the termsdQ / dxis positive. According
to Eq. (6), the term Qds / dx must be negative, or the
slope of the riverbed decreases along the course, so thatthese rivers exhibit concave riverbed profiles. Eq. (6)
indicates the direction of morphological processes and
equilibrium state of longitudinal river profile. Sediment
load plays an important role in the speed of morpho-
logical process but does not change the direction and the
final equilibrium of the profile. The higher the sediment
load the faster is the morphological process. For a low
sediment load river the riverbed profile often does not
meet Eq. (6) because it takes a very long time to reach
the minimum stream power profile.
The Yellow River carries heavy sediment load and themorphological processes are fast. The large quantity of
water diverted along the course of the Yellow River
makes the term sdQ / dx negative. For instance, since
1986, the average discharge has decreased along the
Yellow River course in the reach downstream of
Huayuankou, i.e., dQ / dx
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