reservoir hazards

8/13/2019 Reservoir Hazards

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Reservoir Sedimentation with the Sanmenxia Reservoir as a

Case Study

Baosheng Wu

Tsinghua University

August 24, 2007, Incheon, South Korea


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Outline of Lecture

Part I: Basic Concepts of ReservoirSedimentation

Part II: Sedimentation Management of theSanmenxia Reservoir


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Part I

Basic Concepts of ReservoirSedimentation


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What is a Dam?

What is a Reservoir?


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What is a Dam or Reservoir? A dam is a barrier across flowing water that obstructs,

directs or slows down the flow, often creating a reservoir,lake or impoundments.

A dam is a structure that blocks the flow of a river, stream,or other waterway.

A reservoir is the storage tank or wholly or partly artificiallake for storing water.

A reservoir is a large tank or natural or artificial lake usedfor collecting and storing water for human consumption oragricultural use.

In Australian and South African English, the word "dam" can alsorefer to the reservoir as well as the structure.


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Why are dams important? Dams store water in the reservoir during times of excess flow,

so that water can be released from the reservoir during thetimes that natural flows are inadequate to meet the needs ofwater users.

Dams are important because they help people have water todrink and provide water for industry, water for irrigation, water

for fishing and recreation, water for hydroelectric power production, water for navigation in rivers, and other needs.Dams also serve people by reducing or preventing floods.

Hoover Dam Itaipu Dam Sanmenxia Dam


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Worldwide Construction up to 1990

There was a sharp upsurge in dam constructionfrom 1950 onward.

1 < 1900

2 1900-1909

3 1910-1919

4 1920-1929

5 1930-1939

6 1940-1949

7 1950-19598 1960-1969

9 1970-1979

10 1980-1989


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Construction Rate of Large Dams in USA and China

The United States undertook large scale dam construction earlier than most countries, with the most suitable sites developed, therate of dam construction has declined to low levels. A similar

patterns is evident in China and other countries.

4 1 2 1 5 3

177

479

583

220

150

0

100

200

300

400

500

600

700

< 1 9 0 0

1 9 0 0

1 9 1 0

1 9 2 0

1 9 3 0

1 9 4 0

1 9 5 0

1 9 6 0

1 9 7 0

1 9 8 0

1 9 9 0

Decade

N u m

b e r

o

f

D d

a m s

USAChina

Over 15m Over 15m


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Why Sedimentation in Reservoir ? Preimpoundment sediment balance:

Prior to dam construction, most natural river reaches are

approximately balanced with respect to sediment inflowsand outflows.(Sediment inflow = Sediment outflow)

Sediment deposition occurs as the flow enters theimpounded reach of a reservoir due to a decrease in flowvelocity and drop in transport capacity of the flow. Coarsesediment is deposited first in the upper part of the reservoir,

while finer sediment is transported further into thereservoir. The impounded reach will accumulate sedimentand lose storage capacity until a new balance with respectto sediment inflow and outflow is again achieved.


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Rate of Storage Loss

The worldwide average annual rate of storage lossdue to reservoir sedimentation is on the order of 0.5to 1% of total storage capacity (Mahmood, 1987).

66 reservoirs in the United States had an averagestorage loss rate of 0.71%.

20 reservoirs in the China had an average storageloss rate of 2.26%.

The Sanmenxia Reservoir had a reservoir capacityof 9.75 109m3 in 1960, reduced to 5.7 109m3 by1964.


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Need for Sediment Management Reservoirs have traditionally been planned,

designed, and operated on the assumption that theyhave a finite life, frequently as short as 100 years,which will eventually be terminated by sedimentaccumulation.

Little thought has been given to reservoirreplacement when todays impoundments are lostto sedimentation, or to procedures to maintainreservoir services despite continued sedimentinflow.

There has been the tacit assumption that somebodyelse, members of a future generation, will find asolution when todays reservoirs become seriouslyaffected by sediment.


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Need for Sediment Management

Sedimentation problems are growing astodays inventory of reservoirs ages, andsevere sediment problems are starting to beexperienced at sites worldwide, includingmajor projects of national importance.

Sediment management in reservoirs is nolonger a problem to be put off until the future;it has become a contemporary problem.

Sustainable development: Meeting the needsof the present without compromising theability of future generations to meet their

own needs.


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Need for Sediment Management

New Projects:The sustainability criteria suggested for new

reservoirs is to design for a minimum of 1000 yearsof operation. (Morris & Fan, 1998)

Existing reservoirs:At existing reservoirs, sustainable sedimentmanagement should seek to balance sediment

inflow and outflow across the impounded reachwhile maximizing long-term benefits.


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Reservoir Characteristics

Dead storage is the volume that is below the invert of the lowest-level outletand which cannot be drained by gravity.

Inactive storage is the lower part of the conservation pool that is normally not

used. Active or conservation storage is the volume that can be manipulated for

beneficial use, but excluding flood storage. It lies above the minimum operatinglevel and below the bottom of the flood storage pool.

Live storage is the total volume below full reservoir level less dead storage. flood storage is the upper portion of the pool dedicated to flood detention.


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Reservoir Operation

1) Top of gates2) Guide curve for

maximum poollevel

3) Flood control

storage4) Drawdown period

for sediment

flushing5) Flood detention

and release


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Geomorphic Stages of Reservoir Life Reservoir life can be described in

geomorphic terms as a three-stage process:

Stage 1 Continuous Sediment Trapping Stage 2 Main Channel and Growing Floodplain Stage 3 Sediment Balance


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Stage 1 Continuous Sediment Trapping

Coarse bed material load is deposited as soon as streamvelocity diminishes as a result of backwater from the dam,creating delta deposits at points of tributary inflow.

Most finer sediments are carried further into the reservoir by

either stratified or nonstratified flow and accumulatedownstream of the delta deposits. These finer sediments firstfill in the submerged river channel, after which continueddeposition produces horizontal sediment beds extending

across the width of the pool. Sediments are trapped during all flood events.


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Stage 2 Main Channel and Growing Floodplain

When sedimentation reaches the spillway crest, thereservoir transits from continuous deposition to a mixedregime of deposition and scour. A main channel will bemaintained by scour, and its base level will be established

by the spillway. Sediment deposition continues onfloodplain areas on either side of the channel, causing thefloodplain elevation to rise above the spillway elevation.


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Stage 2 Main Channel and Growing Floodplain

The channel-floodplain configuration may also be created by reservoir drawdown for sediment routing or flushing,in which case both the main channel and adjacentfloodplains will be submerged during normal impoundingand the base level of the main channel at the dam will beestablished by the elevation of the low-level outlet.

Sediments will be deposited in both channel andfloodplain areas during impounding. Scouring duringdrawdown will remove sediment from the channel but notthe flood plains, which will gradually rise in elevation assediment continues to accumulate. .


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Stage 3 Sediment Balance

Sediment inflow and outflow are essentially in full long-term balance whenthe amount and grain size distribution of sediment entering the reservoir is

balanced by the material passing the dam. Considerable upstream aggradation may occur above the spillway crest, and

delta deposits must reach the dam before this balance is reached. Sediments of all sizes may accumulate upstream of the dam during smaller

events, but major floods can wash out large volumes of accumulated

sediment. In reservoirs subject to hydraulic flushing, the sediment releasemay be asynchronous with respect to the seasonality of sediment inflow.

Note:Sediment movementthrough the reach is

not necessarily thesame as preimpoundmentconditions


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Deposition Pattern

Generalized depositional zones in a reservoir 2. Foreset deposits represent the face of the delta advancinginto the reservoir and are differentiated from topset beds by

an increase in slope and decrease in grain size.3. Bottomset beds consist of fine sediments which are

deposited beyond the delta by turbidity currents or

nonstratified flow.

1. Topset beds correspond todelta deposits of rapidly

settling sediment. reservoir.

Generalized depositionalzones in a reservoir


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Longitudinal Deposit Geometry

Longitudinal deposition patterns will vary dramaticallyfrom one reservoir to another as influenced by pool

geometry, discharge and grain size characteristics of theinflowing load, and reservoir operation. Deposits can exhibit four basic types of patterns depending

on the inflowing sediment characteristics and reservoiroperation

Longitudinal patterns of

sedimentdeposition inreservoirs


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Complex Depositional Patterns

Longitudinal profile in Sakuma Reservoir,Japan, after 24 years of operation.

Reservoirs may exhibit different depositional processesfrom one zone to another, resulting in a complexdepositional patter. As shown in the figure, the thalweg

profile consists of both delta and turbidity currentdeposits.

Kulekhani Reservoir, Nepal


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Turbidity Currents

Turbidity currents occur when sediment-laden waterenters an impoundment, plunges beneath the clearwater, and travels downstream along the submerged

thalweg.

Schematic diagramof the passage of aturbid density currentthrough a reservoirand being vented

trough a low-leveloutlet.


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Part II Sedimentation Management of the

Sanmenxia Reservoir


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1. Introduction

Sanmenxia Dam, located in the Yellow River, was completedin 1960.

Severe sedimentation problems became evident immediatelyafter impoundment. To cope with the sedimentation problems:

The dam has been reconstructed to provide high sediment releasing-capacity of outlet structures.

The reservoir operation has been substantially changed to achieve a balance between sediment inflow and outflow.

The purposes are to report: the complex sedimentation processes in response to the dam

reconstruction and operation, and the engineering experience and lessons learned for sedimentation

management.


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2. Sanmenxia Dam

Drainage area above dam: 690,000 km 2

Annual mean discharge: 1,400 m3

/s Height of dam: 106 m Normal pool level: 335 m Storage capacity: 9.6 billion m 3

Yellow River Basin

Original Face of the Damsite


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Original Face of the Damsite


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Overview of Sanmenxia Reservoir


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Overview of Sanmenxia Reservoir


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4. Sedimentation Problems in the Reservoir

Annual sediment load: 1.6 billion tons

Average sediment concentration: 35 kg/m3

Max sediment concentration: 911 kg/m 3

Sanmenxia DamLoess Plateau


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Sedimentation Problems in the Reservoir

Tongguan

DamWei River

The river is constricted from a width of more than 10 km toless than 1 km at Tongguan, forming a naturally constrictedriver reach.

The bed elevation at Tongguan servers as a hydraulic controlfor both the Yellow and Wei Rivers upstream.

Loss of 20% of the effective storage capacity within one and half years,60% in 6 years.

Rapid upstream extension of sediment deposition in the backwater zone:

1960 to 1962, the channel bed at Tongguan station was raised 4.5 m.

Backwater sediment deposition extended over about 74 km in the lowerWei River, upstream of Tongguan.


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Abandoned Drainage Outlet to Wei River


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Abandoned Drainage Outlet to Wei River

3m


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Channel bed aggradation in the lower Wei River

(b) Cross section no. 7

325

330

335

340

345

500 800 1100 1400 1700 2000 2300 2600 2900 3200Distance (m)

E l e v a t i o n

( m

19602001

(a) Cross section no . 2318

322

326

330

334

338

4700 4900 5100 5300 5500 5700 5900 6100 6300 6500

Distance (m)

E l e v a t

i o n

( m

19602001


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A 2-year flood on Sept.1, 2003

peak discharge of 3,570 m 3/s the highest stage record of 342.76 m

Five levee breaches occurred around Huaxian city;

More than 20,000 people had to be evacuated;

Loss of 280 million US dollars.

2003 Flood in the lower Wei River

A 20 m wide of dike


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A 30 m wide of dike breach occurred onSept. 1, 2003, at the

right bank of FangshanRiver

A 20 m wide of dike breach occurred on Sept.

1, 2003, at the right bankof Luowen River, atributary of the Wei River


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A part of theinundated Weinan

city, Sept. 2, 2003

Inundated floodplains


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Inundated Huaxianhydrologic station,Sept. 1, 2003

5 D R i


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First stage reconstruction from 1965 to 1968

Second stage reconstruction from 1970 to 1973 Supplementary works from 1984 to 2000

5. Dam Reconstruction

First stage of reconstruction from


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Two tunnels at elevation 290 m were added, andfour penstocks remodeled into outlets for sluicing

sediment. The discharge capacity had been increased from

3,080 m 3/s to 6,100 m 3/s at a water level of 315 m.

However, the sills of the outlet structures were toohigh and the capability of the reservoir to releasefloodwater was inadequate.

The ratio of outflow-inflow sediment reached 80%,the amount of backwater deposition increasedaccordingly and the bed elevation at Tongguancontinued to rise.

g

1965 to 1968

Second stage of reconstruction from


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8 bottom outlets at elevation 280 m previously usedfor diversions were reopened for sluicing sediment;

the intake elevation of the penstocks No. 1-5 werelowered from 300 m to 287 m. The releasing capacity of all the outlets increased

from 6,100 m 3/s to about 9,100 m 3/s at an elevation of315 m. No significant backwater could accumulate immediately

behind the dam in medium or minor flood conditions, andthe outflow and inflow ratio of sediment had reached 105%. A part of the reservoir capacity was restored, and the bed

elevation at Tongguan dropped by 2 m.

g1970 to 1973

Supplementary works from 1984


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Due to surface abrasion and cavitation, bottom sluicesno. 1 to 8 underwent repairs from 1984 - 1988.

the total discharge capacity of 8 bottom sluices wasreduced by about 471 m 3/s due to compression.

Two more bottom sluices, no. 9 and 10, were opened

in 1990 to compensate for the reduction resulting from bottom sluice repairs. Penstocks no. 6 and 7 were converted back to power

generation in 1994 and 1997, respectively. The last two bottom sluices, no. 11 and 12, were

opened in 1999 and 2000, respectively.

pp y

to 2000

Front View of the Outlet Structures


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Before

After

12 deep holes 8 penstocks

2 tunnels

12 deep holes

12 bottom sluices5 penstocks

2 penstocks1 flushing pipe

300 300

290

280

300


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27 Outlets at Sanmenxia DamOutlet Dimensions Total

number Serial

number Invert

elevation (m)Q at 335m

(m3/s)

Deep holes w d=3 8 m 12 1-12 300 503/eachBottom sluices w d=3 8 m 12 1-3 280 497/each

Tunnels D = 11 m 2 1-2 290 1,410Flushing pipe D = 7.5 m 1 8 300 290

2 6-7 300 230/each5 1-5 287 210/each

Penstocks

27 16,620

Reservoir Outlet Capacity


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Reservoir Outlet Capacity

280

290

300

310

320

330

340

E l e v a

t i o n

( m )

0 2000 4000 6000 8000 10000 12000 14000 16000Outlet Discharge (m^ 3/s)

1960 1968 1973

3080 91016100

315m

Two Tunnel Outlets


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Two Tunnel Outlets

Four Remodeled Penstocks


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Four Remodeled Penstocks

Eight Bottom Outlets at 280 m


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Eight Bottom Outlets at 280 m

Releasing Sediment Through the Bottom Sluices


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No significant backwater would occurimmediately behind the dam in case of mediumand minor floods

6 Dam Operation


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In addition to the increased the dischargecapacity by reconstruction of outlet structures,the Sanmenxia Reservoir has adopted threedifferent modes of operation:

Storage ( Sep. 1960 to Mar. 1962 ) Flood detention ( Mar. 1962 to Oct. 1973 ) Controlled release (Nov. 1974 to present)

6. Dam Operation

Different Modes of Operation


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1. Storage ( Sep. 1960 to Mar. 1962 )Initial period of reservoir impoundment,when the reservoir was operated at highstorage level throughout the whole year.

2. Flood detention ( Mar. 1962 to Oct. 1973 )Period of flood detention and sedimentsluicing, water being released without anyrestrictions. The reservoir was operated atlow storage level throughout the year.




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3. Controlled release (Nov. 1974 to present)Period of impounding relatively clear water

in non-flood seasons (Nov.-June) anddischarging the turbid water in flood seasons(July-Oct.). The reservoir has been operatedat high water level in non-flood seasons, andat low storage level during flood seasons, andall outlets were to be opened in time of flood

peaks to sluice the sediment as much as possible.


Typical Operation Schemes


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Typical Operation Schemes

295

300

305

310

315

320

325330

335

340

0 50 100 150 200 250 300 350

Time (day)

E l e v a

t i o n

( m )

11 12 1 2 3 4 5 6 7 8 9 10

1974-2001

1960-1961

1963-1973

Non-flood season Flood season

Variation of the Pool Levels


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285

290

295

300

305310

315

320

325

330

335

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005Time / year

P o o

l l e v e

l / m

Non-flood season

Flood season

Water year

Flood detention Controlled releaseStorage

I II III

Accumulated Deposition in the Reservoir


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Accumulated Deposition in the Reservoir

0.00.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time (year)

A c c u m u l a t e d

d e p o s

i t i o n

( 1 0

9 m

3 )

Tongguan to dam

Longmen to Tongguan

Lower Wei River

I II III

Variation of Reservoir Capacity


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Variation of Reservoir Capacity

0

1

2

3

4

5

6

7

1960 1965 1970 1975 1980 1985 1990 1995

Time (y ea r)

R e s e r v o

i r s t o r a g e c a p a c

i t y

( 1 0 9

m 3

)

Pool level < 330 m

Pool level < 323 m

I II III

Cross-Sectional Profiles


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Cross Section No. 22 Cross Section No. 31

295

300

305

310

315

320

325

330

335

340

345

1700 1900 2100 2300 2500 2700 2900

Distance m

E l e v a t

i o n m

April, 1960

Oct., 1961Oct., 1964

Sept., 1973

Oct., 1995

305

310

315

320

325

330

335

340

345

500 1000 1500 2000 2500 3000 3500 4000

Distance (m)

E l e v a t

i o n m

April, 1960

Oct., 1961Oct., 1964

Sept ., 1973

Oct., 1995

Longitudinal Profiles


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g

280

290

300

310

320

330

340

0255075100125150

Distance from dam (km)

A v e r a g e c h a n n e

l b o

t t o m e l e v

a t i o n

( m

April, 1960

Oct. , 196 1

Oct. , 196 4

Sept ., 197 3

Oct. , 199 5

CS 31

CS 48

CS 41

CS 37

CS 22

CS 12

Variation of elevation of Tongguan


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323

324

325

326

327

328

329

330

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time (year)

E l e v a

t i o n

o f T o n g g u a n

( m )

I II III

Operation scheme: I. Storage II. Flood detension III. Controlled

The river is constricted from awidth of more than 10 km toless than 1 km at Tongguan ,forming a naturally constricted

river reach. The bed elevationat Tongguan servers as ahydraulic control for both theYellow and Wei Riversupstream.

7. Influence of Water Resources


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Mean annual runoff: 58 billion m 3

Water diversion (1990s) : 40 billion m 3

Sediment flushing : 20-24 billion m 3

Development

If the water consumption keeps growing, there

will not be enough flow to carry the sediment outthe reservoir, and all the way to sea.

S i i f ff d di l d


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Statistics of water runoff and sediment load atTongguan station

Year Runoff (10 9 m3)

Sed. Load (10 9 tons)

1960-1969 45.07 1.42

1970-1979 35.66 1.32

1980-1989 36.75 0.78

1990-1999 25.12 0.79

Decrease of annual runoff and flood flows


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The peak discharge and the frequency offloodwaters entering the reservoir have beendecreasing.

0

20

40

60

80

100

120

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time (year)

N u m

b e r o f

d a y s

no. of days with Q>3000m^3/s

average number of days

0

10

20

30

40

50

60

70

80

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time (year)

A n n u a l r u n o

f f ( 1 0

9 m

3 )

(a) Annual runoff (b) Number of flood flows

Percentages of flow and sediment in flood


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g

seasons at Tongguan station

Year Runoff (%) Sed. Load (%)

1960-1969 58.6 84.7

1970-1979 54.9 84.9

1980-1989 56.8 80.3

1990-1999 43.2 73.8The temporal distribution of runoff within a year has

been changed.



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323

324

325

326

327

328

329

330

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time (year)

E l e v a

t i o n o f

T o n g g u a n

( m

)

I II III

Operation scheme: I. Storage II. Flood detension III. Controlled


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Lanes Geomorphic Relationship

Q Q s

S D 50

50~ sQS Q D

( )sQ f QS =

329

330

/ m

1969 - 19731969 - 1973 1974 - 2001


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317

318

319

320

321

322

323

324

325

326

327

328

329

1965 1970 1975 1980 1985 1990 1995 2000 2005

Year

A v e r a g e p o o l

l e v e

l ( m

)

A n n u a l r u n o

f f ( 1 0 9 m

3 )

T o n g g u a n

' s e l e v a t

i o n

( m )

Tongguan's elevation

Annual runoff

Pool level

295

305

310

300

20

30

40

50

315

325

326

327

328

329

10 20 30 40 50 60

Annual runoff / 109

m3

E l e v a

t i o n o

f T o n g g u a n

/ m

Z tg = -0.0005W a + 328.95

(b)

325

326

327

328

329

294 296 298 300 302 304 306 308

Average pool level in flood season / m

E l e v a

t i o n o f

T o n g g u a n

/

1969 - 1973

1974 - 2001

3.2

3.4

( 1 0 9 m

3 )

318

321

324

v e l ( m ) lAccumulated deposition

3.2

3.3

G ( 1 0 9 m

3 )

1969-1973 1974-2001


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2.0

2.2

2.4

2.6

2.8

3.0

1965 1970 1975 1980 1985 1990 1995 2000 2005

Year


d e p o s i o n

b e l o w

T G

300

303

306

309

312

315

318

A n n u a l w e i g

h t e d - a v e r a g e p o o

l l e v

Pool level

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

1965 1970 1975 1980 1985 1990 1995 2000 2005Year


d e p o s i o n

b e l o w

T G ( 1 0 9 m

3 )

300

303

306

309

312

315

318

321

F i v e y e a r s '

s u p e r i m p o s e

d p o o l

l e v e

l ( m ) tAccumulated deposition

Superimposed pool level

2.7

2.8

2.9

3.0

3.1

300 302 304 306 308 310 312 314

Annual discharged-weighted average pool level (m)

A c c u

m u l a t e d

d e p o s i o n

b e l o w

T G

2.7

2.8

2.9

3.0

3.1

3.2

3.3

300 302 304 306 308 310 312 314

Five years' superimposed pool level (m)


d e p o s

i o n

b e l o w

T G ( 1 0 9 m

3 )

1969-1973 1974-2001

(b)821.1~0153.0 5 = d s Z V

126.27~

0973.0 5 = d s Z V

5

5

8( 308.60 )

0.0153 1.821

0 .0 10 25 l n 1 d s d

Z

V Z

e =

+ +

(a)

(b)

(c)

(d)

Adjusting the pool level


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The operation level (controlled release scheme)was determined based on earlier inflow conditionswhich are no longer relevant.

The current level is no longer compatible with the

changed inflow conditions, notably the reducedannual runoff that has been recorded since 1986. Lowering the pool level could compensate, at least

partially, for the negative effect of reduced runoff,and could also prevent backwater extension.



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The best operation scheme Preventing backwater deposition, Having minimum effect on the newly formed

ecological system in the reservoir area belowTongguan, and

Having minimum lose of hydropowergeneration.



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The preliminary plan Allowing the flood flows to pass through the

dam without any control in flood seasons; Lowering the maximum pool level from 321-

323 m to 318 m in non-flood seasons, with theaverage pool level not higher than 315 m ;

Decommissioning the dam


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Decommissioning the Sanmenxia Dam andallowing the flow to run freely withoutcontrol. Lowering the elevation of Tongguan by 2 4 m.

Slowing down the sedimentation process in thelower reaches of the Wei River, and reducingthe flooding risk.

Other Countermeasures


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River training in the river reaches upstreamand downstream of the backwater zone;

Dredging the channel in the vicinity of the backwater zone;

Strengthening soil conservation practices inthe Loess Plateau;

Building sediment detention reservoirs onriver reaches above Sanmenxia Dam.

Jet ship used for sediment flushing


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A close look of the jet ship


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A close look of the jet shipejector nozzle


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0.6-0.8m60-90

8. Concluding Remarks


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The original reservoir planning and designwere conducted according to the standardand experiences for clear water.

The principle of streamflow regulationadopted was basically the same as that usedin reservoirs on non-sediment-laden rivers.



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Difficulties of resettlement of inhabitants livedin the reservoir area was underestimated.

In the original design, anticipating that thesediment inflow of the reservoir would be

rapidly reduced was too optimistic andunrealistic, say, by 50% in less than 15 years,

by the proposed sediment retention reservoirs to be built in the upland area and soil conservationworks.



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Both water and sediment need to be regulated Enough outlet discharge capacity is necessary The operational mode of controlled release

should be adopted The dams operation level should be adjusted

timely in accordance with changing inflows of

water and sediment


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Thank You !

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