fluvial processes and morphological response in the yellow and weihe rivers to closure and operation...

8/10/2019 Fluvial processes and morphological response in the Yellow and Weihe Rivers to closure and operation of Sanmen

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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]


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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.

67Z.Y. Wang et al. / Geomorphology 91 (2007) 6579


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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).



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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


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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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fluvial processes and morphological response in the yellow and weihe rivers to closure and operation...

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