Reservoir Sedimentation – with the Sanmenxia Reservoir as a

Case Study

Baosheng Wu

Tsinghua University

August 24, 2007, Incheon, South Korea

Outline of Lecture

• Part I: Basic Concepts of Reservoir Sedimentation

• Part II: Sedimentation Management of the Sanmenxia Reservoir

Part I

• Basic Concepts of Reservoir Sedimentation

• What is a Dam?

• What is a Reservoir?

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 artificial lake for storing water.

• A reservoir is a large tank or natural or artificial lake used for collecting and storing water for human consumption or agricultural use.

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

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 the times that natural flows are inadequate to meet the needs of water users.

• Dams are important because they help people have water to drink 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

Worldwide Construction up to 1990

• There was a sharp upsurge in dam construction from 1950 onward.

1 < 19002 1900-19093 1910-19194 1920-19295 1930-19396 1940-19497 1950-19598 1960-19699 1970-1979

10 1980-1989

Construction Rate of Large Dams in USA and China

The United States undertook large scale dam construction earlierthan 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

<1900

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

DecadeNum

ber

of D

dams

USA China

Over 15m Over 15m

Why Sedimentation in Reservoir ?• Preimpoundment sediment balance:

Prior to dam construction, most natural river reaches are approximately balanced with respect to sediment inflows and outflows.

(Sediment inflow = Sediment outflow)• Sediment deposition occurs as the flow enters the

impounded reach of a reservoir due to a decrease in flow velocity and drop in transport capacity of the flow. Coarse sediment is deposited first in the upper part of the reservoir, while finer sediment is transported further into the reservoir. The impounded reach will accumulate sediment and lose storage capacity until a new balance with respect to sediment inflow and outflow is again achieved.

Rate of Storage Loss• The worldwide average annual rate of storage loss

due to reservoir sedimentation is on the order of 0.5 to 1% of total storage capacity (Mahmood, 1987).

• 66 reservoirs in the United States had an average storage loss rate of 0.71%.

• 20 reservoirs in the China had an average storage loss rate of 2.26%.

• The Sanmenxia Reservoir had a reservoir capacity of 9.75×109m3 in 1960, reduced to 5.7 ×109m3 by 1964.

Need for Sediment Management• Reservoirs have traditionally been planned,

designed, and operated on the assumption that they have a finite “life,” frequently as short as 100 years, which will eventually be terminated by sediment accumulation.

• Little thought has been given to reservoir replacement when today’s impoundments are lost to sedimentation, or to procedures to maintain reservoir services despite continued sediment inflow.

• There has been the tacit assumption that somebody else, members of a future generation, will find a solution when today’s reservoirs become seriously affected by sediment.

Need for Sediment Management• Sedimentation problems are growing as

today’s inventory of reservoirs ages, and severe sediment problems are starting to be experienced at sites worldwide, including major projects of national importance.

• Sediment management in reservoirs is no longer a problem to be put off until the future; it has become a contemporary problem.

• Sustainable development: Meeting the needs of the present without compromising the ability of future generations to meet their own needs.

Need for Sediment Management• New Projects:

The sustainability criteria suggested for new reservoirs is to design for a minimum of 1000 years of operation. (Morris & Fan, 1998)

• Existing reservoirs:At existing reservoirs, sustainable sediment management should seek to balance sediment inflow and outflow across the impounded reach while maximizing long-term benefits.

Reservoir Characteristics

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

• Inactive storage is the lower part of the conservation pool that is normally notused.

• Active or conservation storage is the volume that can be manipulated for beneficial use, but excluding flood storage. It lies above the minimum operating level 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.

Reservoir Operation

1) Top of gates2) Guide curve for

maximum pool level

3) Flood control storage

4) Drawdown period for sediment flushing

5) Flood detention and release

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

Stage 1 – Continuous Sediment Trapping

• Coarse bed material load is deposited as soon as stream velocity 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 accumulate downstream of the delta deposits. These finer sediments first fill in the submerged river channel, after which continued deposition produces horizontal sediment beds extending across the width of the pool.

• Sediments are trapped during all flood events.

Stage 2 – Main Channel and Growing Floodplain

• When sedimentation reaches the spillway crest, the reservoir transits from continuous deposition to a mixed regime of deposition and scour. A main channel will be maintained by scour, and its base level will be established by the spillway. Sediment deposition continues on floodplain areas on either side of the channel, causing the floodplain elevation to rise above the spillway elevation.

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 adjacent floodplains will be submerged during normal impounding and the base level of the main channel at the dam will be established by the elevation of the low-level outlet.

• Sediments will be deposited in both channel and floodplain areas during impounding. Scouring during drawdown will remove sediment from the channel but not the flood plains, which will gradually rise in elevation as sediment continues to accumulate. .

Stage 3 – Sediment Balance

• Sediment inflow and outflow are essentially in full long-term balance when the 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 release may be asynchronous with respect to the seasonality of sediment inflow.

Note:Sediment movement through the reach is not necessarily the same as preimpoundmentconditions

Deposition Pattern

Generalized depositional zones in a reservoir2. Foreset deposits represent the face of the delta advancing into 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 to delta deposits of rapidly settling sediment. reservoir.

Generalized depositional zones in a reservoir

Longitudinal Deposit Geometry

• Longitudinal deposition patterns will vary dramatically from one reservoir to another as influenced by pool geometry, discharge and grain size characteristics of the inflowing load, and reservoir operation.

• Deposits can exhibit four basic types of patterns depending on the inflowing sediment characteristics and reservoir operation

Longitudinal patterns of sediment deposition in reservoirs

Complex Depositional Patterns

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

• Reservoirs may exhibit different depositional processes from one zone to another, resulting in a complex depositional patter. As shown in the figure, the thalwegprofile consists of both delta and turbidity current deposits.

Kulekhani Reservoir, Nepal

Turbidity Currents

• Turbidity currents occur when sediment-laden water enters an impoundment, plunges beneath the clear water, and travels downstream along the submerged thalweg.

Schematic diagram of the passage of a turbid density current through a reservoir and being vented trough a low-level outlet.

Part II• Sedimentation Management of the

Sanmenxia Reservoir

1. Introduction

• Sanmenxia Dam, located in the Yellow River, was completed in 1960.

• Severe sedimentation problems became evident immediately after 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 sedimentationmanagement.

2. Sanmenxia Dam

• Drainage area above dam: 690,000 km2

• Annual mean discharge: 1,400 m3/s• Height of dam: 106 m• Normal pool level: 335 m• Storage capacity: 9.6 billion m3

Yellow River Basin

Original Face of the Damsite

Overview of Sanmenxia Reservoir

Overview of Sanmenxia Reservoir

4. Sedimentation Problems in the Reservoir

• Annual sediment load: 1.6 billion tons• Average sediment concentration: 35 kg/m3

• Max sediment concentration: 911 kg/m3

Sanmenxia DamLoess Plateau

Sedimentation Problems in the Reservoir

Tongguan

DamWei River

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

• The bed elevation at Tongguan servers as a hydraulic control for 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 lower Wei River, upstream of Tongguan.

Aggradation in a Tributary of Wei River

The bridge piers have been increased two times, adding total height of 6.4m.1969

1974

Abandoned Drainage Outlet to Wei River

3m

Channel bed aggradation in the lower Wei River

(b) Cross sect ion no. 7

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345

500 800 1100 1400 1700 2000 2300 2600 2900 3200

Distance (m)

Elev

atio

n (m

)

19602001

(a) Cross sect ion no. 2318

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330

334

338

4700 4900 5100 5300 5500 5700 5900 6100 6300 6500

Distance (m)

Elev

atio

n (m

)

19602001

• A 2-year flood on Sept.1, 2003– peak discharge of 3,570 m3/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 30 m wide of dike breach occurred on

Sept. 1, 2003, at the right bank of Fangshan

River

A 20 m wide of dike breach occurred on Sept. 1, 2003, at the right bank of Luowen River, a tributary of the Wei River

A part of the inundated Weinancity, Sept. 2, 2003

Inundated floodplains

Inundated Huaxianhydrologic station, Sept. 1, 2003

• First stage reconstruction from 1965 to 1968 • Second stage reconstruction from 1970 to 1973 • Supplementary works from 1984 to 2000

5. Dam Reconstruction

• Two tunnels at elevation 290 m were added, and four penstocks remodeled into outlets for sluicing sediment.

• The discharge capacity had been increased from 3,080 m3/s to 6,100 m3/s at a water level of 315 m.

• However, the sills of the outlet structures were too high and the capability of the reservoir to release floodwater was inadequate.

• The ratio of outflow-inflow sediment reached 80%, the amount of backwater deposition increased accordingly and the bed elevation at Tongguan continued to rise.

First stage of reconstruction from 1965 to 1968

• 8 bottom outlets at elevation 280 m previously used for diversions were reopened for sluicing sediment; the intake elevation of the penstocks No. 1-5 were lowered from 300 m to 287 m.

• The releasing capacity of all the outlets increased from 6,100 m3/s to about 9,100 m3/s at an elevation of 315 m. – No significant backwater could accumulate immediately

behind the dam in medium or minor flood conditions, and the 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.

Second stage of reconstruction from 1970 to 1973

• Due to surface abrasion and cavitation, bottom sluices no. 1 to 8 underwent repairs from 1984 - 1988.– the total discharge capacity of 8 bottom sluices was

reduced by about 471 m3/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.

Supplementary works from 1984 to 2000

Front View of the Outlet StructuresBefore

After

12 deep holes 8 penstocks

2 tunnels

12 deep holes

12 bottom sluices5 penstocks

2 penstocks1 flushing pipe

300 300

290

280

300

27 Outlets at Sanmenxia DamOutlet Dimensions Total

numberSerial

numberInvert

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

280

290

300

310

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330

340

Elev

atio

n (m

)

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

1960 1968 1973

3080 91016100

315m

Two Tunnel Outlets

Four Remodeled Penstocks

Eight Bottom Outlets at 280 m

Releasing Sediment Through the Bottom Sluices

No significant backwater would occur immediately behind the dam in case of medium and minor floods

• In addition to the increased the discharge capacity by reconstruction of outlet structures, the Sanmenxia Reservoir has adopted three different 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

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

2. Flood detention ( Mar. 1962 to Oct. 1973 ) Period of flood detention and sediment sluicing, water being released without any restrictions. The reservoir was operated at low storage level throughout the year.

Different Modes of Operation

3. Controlled release (Nov. 1974 to present) Period of impounding relatively clear water in non-flood seasons (Nov.-June) and discharging the turbid water in flood seasons (July-Oct.). The reservoir has been operated at high water level in non-flood seasons, and at low storage level during flood seasons, and all outlets were to be opened in time of flood peaks to sluice the sediment as much as possible.

Different Modes of Operation

Typical Operation Schemes

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0 50 100 150 200 250 300 350Time (day)

Ele

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1974-2001

1960-1961

1963-1973

Non-flood season Flood season

Variation of the Pool Levels

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1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time / year

Pool

leve

l / m

Non-flood season

Flood season

Water year

Flood detention Controlled releaseStorage

I II III

Accumulated Deposition in the Reservoir

0.0

0.5

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)

Acc

umul

ated

dep

ositi

on (1

09 m

3 )

Tongguan to dam

Longmen to Tongguan

Lower Wei River

I II III

Variation of Reservoir Capacity

0

1

2

3

4

5

6

7

1960 1965 1970 1975 1980 1985 1990 1995

Time (year)

Res

ervo

ir st

orag

e ca

paci

ty (1

09 m3 )

Pool level < 330 m

Pool level < 323 m

I II III

Cross-Sectional Profiles

Cross Section No. 22 Cross Section No. 31

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Distance (m)

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April, 1960Oct., 1961Oct., 1964Sept., 1973Oct., 1995

Longitudinal Profiles

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290

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310

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340

0255075100125150

Distance from dam (km)

Ave

rage

cha

nnel

bot

tom

ele

vatio

n (m

)

April, 1960Oct ., 1961Oct ., 1964

Sept ., 1973Oct ., 1995

CS 31

CS 48

CS 41CS 37

CS 22

CS 12

Variation of elevation of Tongguan

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327

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330

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time (year)

Elev

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Ton

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n (m

)

I II III

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

The river is constricted from a width of more than 10 km to less than 1 km at Tongguan, forming a naturally constricted river reach. The bed elevation at Tongguan servers as a hydraulic control for both the Yellow and Wei Rivers upstream.

• Mean annual runoff: 58 billion m3

• Water diversion (1990s) : 40 billion m3

• Sediment flushing : 20-24 billion m3

7. Influence of Water Resources Development

If the water consumption keeps growing, there will not be enough flow to carry the sediment out the reservoir, and all the way to sea.

Statistics of water runoff and sediment load at Tongguan station

Year Runoff(109 m3)

Sed. Load(109 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

The peak discharge and the frequency of floodwaters entering the reservoir have been decreasing.

0

20

40

60

80

100

120

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time (year)

Num

ber o

f day

s

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

average number of days

0

10

20

30

40

50

60

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1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time (year)

Annual runoff (10

9m3)

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

Percentages of flow and sediment in flood 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 73.8

The temporal distribution of runoff within a year has been changed.

Variation of elevation of Tongguan

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324

325

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327

328

329

330

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Time (year)

Elev

atio

n of

Ton

ggua

n (m

)

I II III

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

Lane’s Geomorphic Relationship

Q Qs

S D50

50~ sQS Q D

( )sQ f QSγ=

317

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326

327

328

329

1965 1970 1975 1980 1985 1990 1995 2000 2005

Year

Ave

rage

poo

l lev

el (m

) A

nnua

l run

off (

109 m

3 )

Ton

ggua

n's

elev

atio

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 / 109m3

Elev

atio

n of

Ton

ggua

n /

m

Ztg = -0.0005Wa + 328.95

(b)

325

326

327

328

329

330

294 296 298 300 302 304 306 308

Average pool level in flood season / m

Elev

atio

n of

Ton

ggua

n / m

1969 - 1973

1974 - 2001

1969 - 19731969 - 1973 1974 - 2001

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

Acc

umul

ated

dep

osio

n be

low

TG

(109 m

3 )

300

303

306

309

312

315

318

321

324

Ann

ual w

eigh

ted-

aver

age

pool

leve

l (m

)lAccumulated deposition

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

Acc

umul

ated

dep

osio

n be

low

TG

(109 m

3 )

300

303

306

309

312

315

318

321

Five

yea

rs' s

uper

impo

sed

pool

leve

l (m

)tAccumulated deposition

Superimposedpool level

2.7

2.8

2.9

3.0

3.1

3.2

3.3

300 302 304 306 308 310 312 314

Annual discharged-weighted average pool level (m)

Acc

umul

ated

dep

osio

n be

low

TG

(109 m

3 )

Δ 1969-1973× 1974-2001

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)

Acc

umul

ated

dep

osio

n be

low

TG

(109 m

3 )

Δ 1969-1973× 1974-2001

(b)821.1~0153.0 5 −= ds ZV

126.27~0973.0 5 −= ds ZV

5

5

8( 308.60 )

0.0153 1.821

0.01025 ln 1 d

s d

Z

V Z

e −

= −

⎡ ⎤+ +⎣ ⎦

(a)

(b)

(c)

(d)

Adjusting the pool level

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

• The current level is no longer compatible with the changed inflow conditions, notably the reduced annual 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.

Adjusting the pool level

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

ecological system in the reservoir area below Tongguan, and

– Having minimum lose of hydropower generation.

Adjusting the pool level

• 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 the average pool level not higher than 315 m;

Decommissioning the dam

• Decommissioning the Sanmenxia Dam and allowing the flow to run freely without control. – Lowering the elevation of Tongguan by 2 – 4 m. – Slowing down the sedimentation process in the

lower reaches of the Wei River, and reducing the flooding risk.

Other Countermeasures

• River training in the river reaches upstream and downstream of the backwater zone;

• Dredging the channel in the vicinity of the backwater zone;

• Strengthening soil conservation practices in the Loess Plateau;

• Building sediment detention reservoirs on river reaches above Sanmenxia Dam.

Jet ship used for sediment flushing

A close look of the jet ship

A close look of the jet ship

0.6-0.8m60-90º

ejector nozzle

• The original reservoir planning and design were conducted according to the standard and experiences for clear water.

• The principle of streamflow regulation adopted was basically the same as that used in reservoirs on non-sediment-laden rivers.

8. Concluding Remarks

• Difficulties of resettlement of inhabitants lived in the reservoir area was underestimated.

• In the original design, anticipating that the sediment inflow of the reservoir would be rapidly reduced was too optimistic and unrealistic, say, by 50% in less than 15 years, by the proposed sediment retention reservoirs to be built in the upland area and soil conservation works.

8. Concluding Remarks

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

should be adopted• The dam’s operation level should be adjusted

timely in accordance with changing inflows of water and sediment

8. Concluding Remarks

Thank You !