Morris & Fan. Reservoir Sedimentation Handbook

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CHAPTER 3

ENGINEERING FEATURES OF DAMS AND

RESERVOIRS

This chapter describes basic characteristics of dams and reservoirs and introduces

nomenclature and concepts concerning their design and operation.

3.1 HISTORICAL PERSPECTIVE

3.1.1 Dams in Antiquity

The oldest reservoir in operation today is the 100-Mm

Afengtang reservoir,

constructed west of Shanghai during 589 to 581 B.C. The original dam, reportedly

constructed of alternating layers of earth and straw held into place with chestnut piles, no

longer exists and has been reconstructed. However, the record for longest service is

apparently held by the Mala'a reservoir in Egypt, originally constructed by King

Amenemhet III (1842 to 1798/95 B.C.) in the Faiyum depression 90 km southwest of

Cairo and reconstructed in the third century B.C. with a dam 8 km long and 7 m high. It

stored up to 275 Mm

of water diverted from the Nile and remained in operation until the

eighteenth century, a span of 3,600 years. The oldest continuously operating dam still in

use is the Kofini flood control diversion dam and channel constructed in 1260 B.C. on the

Lakissa River upstream of the town of Tiryns, Greece, which it continues to protect. The

original structure is still in operation, and while floodwaters have on occasion approached

the crest, it has never been overtopped. Remains of other ancient dams have been located

at various sites bordering the Mediterranean, the Near and Middle East, Sri Lanka, and in

Central America where the first dam is dated at 700 B.C. (Schnitter, 1994).

Dam construction was undertaken on a wide scale by the Romans. Schnitter lists 79

Roman dams, the dimensions of which are known today: 47 in Spain, Portugal, and

France; 12 in North Africa; 18 in the Near and Middle East; but only 2 in Italy. The

20.5m-tall Harbaqa Dam and its heavily sedimented reservoir near Palmyra, Syria,

constructed by the Romans for irrigation supply, is an example of the relative

permanence of sediment deposits upstream of a dam (Fig. 3.1). Although the dam was

breached centuries ago, the gullies that traverse the deposits have removed only a fraction

of the total sediment deposits. In contrast, the Prosperina dam in Mérida, Spain, also built

by the Romans during either the first or second century, has remained in essentially

continuous service for more than 1800 years. This 6-Mm

reservoir has only a 7-km

tributary watershed and is augmented by small canals diverting flow from an additional

13 km

of watershed. Because of its large storage capacity relative to watershed area,

equivalent to 860 mm of soil denudation, the lack of development or intensive

agricultural activity in its gently rolling watershed, and small diversion canals, it was not

ENGINEERING FEATURES OF DAMS AND RESERVOIRS 3.2

FIGURE 3.1 Harbaqa Dam constructed by the Romans 80 km from Palmyra, Syria. Despite

centuries of erosion, most of the sediment still remains trapped upstream of the dam(DeBoysson).

heavily silted. The small diversion canals cannot transport significant flow into the reser-

voir during major sediment-producing runoff events. During the early 1990s, the reser-

voir was drained, the Roman dam rehabilitated, sediment excavated, and the reservoir

was returned to service.

Little is known about sediment control measures in ancient dams. The earliest record

of specialized structures and techniques for sediment management mentioned by Smith

(1971) or by Schnitter (1994) are associated with the Almansa Dam in Alicante, Spain,

constructed in 1384 for irrigation and provided with a low-level outlet more than 3m in

area for emptying and flushing. The flushing tunnel was closed at its upstream end with

timbers and the reservoir was impounded for a period of years until a significant amount

of sediment had accumulated. To initiate flushing, a worker was sent into the tunnel to

cut away the supports restraining the wooden gate. If the sediment accumulation before

the dam was sufficiently deep and compacted, the worker could exit the tunnel once the

gate was cut away, then pierce through the sediments from the top of the dam with a long

rod to initiate the flow that would empty the reservoir and scour away the sediments.

However, this was a hazardous procedure since inadequate sediment depth or strength in

front of the bottom outlet caused premature blowout, killing anyone in the tunnel. In the

absence of flushing the reservoir would fill completely with sediment after several

decades (Fernandez Ordonez,1984). Utilization of a low-level outlet for intermittent

flushing was incorporated into many historic Spanish dams and is occasionally referred to

as the Spanish method of flushing (McHenry, 1974).

The histories of dam building compiled by both Schnitter (1994) and Smith (1971)

underscore the potential longevity of these structures, if sedimentation can be controlled.

Most ancient dams fell into ruin because of lack of structural maintenance as a result of

changing economic or political conditions, or accumulation of sediments. A minority ex-

perienced catastrophic failure due to floods, and this often after a long period of neglect.

ENGINEERING FEATURES OF DAMS AND RESERVOIRS 3.3

3.1.2. Modern Dam Construction

Large-scale impounding of the world's rivers was not undertaken until the twentieth

century, and most dams have been constructed since 1950. The number of large dams

(over 15 m tall) worldwide in the inventory maintained by the International Committee

on Large Dams (ICOLD, 1988) grew as follows: 427 in 1900, 5268 in 1950, and about

39,000 in 1986. The average annual rate of dam construction for various periods

summarized in Table 3.1 shows the slow rate of construction prior to 1950, the frenzy of

dam construction from about 1950 to about 1980, especially in China, and the reduced

rate of dam construction since 1980.

TABLE 3.1 Rate of Large Dam Completions Since 1950

Period

Large dams commissioned per year

Excluding China China World

1900-50 97 0 97

1951-77 357 611 968

1978-82 335 420 755

1983-86 209 56 265

1987-90 NA NA NA

1991-94 187 122 309

Source: ICOLD. Data for 1987-90 not available.

Rates of dam construction within each country reflect greater variation than the world

totals. Data from the United States and Mexico are presented in Fig. 3.2 and illustrate a

period of rapid dam construction followed by a decline. The United States undertook

large-scale da

m construction earlier than most countries and today, with the most suitable

FIGURE 3.2 Rates of large dam construction in the United States and Mexico (data from

U.S. Army Corps of Engineers and ICOLD, 1984 and 1988).

ENGINEERING FEATURES OF DAMS AND RESERVOIRS 3.4

sites developed, the rate of dam construction has declined to low levels. A similar

patterns is evident in the data from China (Table 3.1) and in many other countries.

3.2 CHARACTERISTICS OF DAMS

3.2.1 Functions of Dams

The term dam can refer to any barrier erected to obstruct or control the flow of water.

Three functional classes of dams may be defined: (1) storage dams, (2) barrages, and (3)

diversion dams. These structures regulate, respectively: (1) head upstream and discharge

released downstream, (2) head but not discharge, and (3) neither head nor discharge.

Storage dams impound water to create a permanent pool, or regulate discharge by varying

pool level as in the case of water supply, flood control, and storage-based hydro. A

barrage is a low structure in which gate operation is used to control water level but does

not regulate discharge by storing and then releasing water. A barrage does not have

significant storage capacity and its gates are operated to balance inflow and outflow to

maintain a target upstream water level, typically for delivery to an intake or for

navigation (Fig. 3.3). Diversion dams are constructed for the sole purpose of diverting

water, without the intent of regulating either discharge rate or water level.

Both storage dams and barrages elevate the water surface profile, deepen the flow

depth, reduce flow velocity, and cause sediment to accumulate upstream of the barrier.

Both types of structures can be adversely affected by sediment accumulation. However,

the gates in a barrage extend to the riverbed level and both suspended and bed loads are

passed through the structure. Because of their small hydrologic size, limited impact or

flood flows, and gate placement, the impounded river reach upstream of a barrage may

reach a new sediment equilibrium within a few years or decades, whereas storage

reservoirs may require many centuries.

3.2.2 Embankment Dams

Approximately 80 percent of all dams are constructed as massive structural embankments

FIGURE 3.3 Section view of Saint Pierre de Beouf Barrage, Rhone River, France, showing:

(1) radial gate; (2) overspill flap gate; (3) armor for abrasion protection; (4) energy dissipatio

block; (5) scour protection; (6) chain hoist for gate; (7) hoist for stop logs.

ENGINEERING FEATURES OF DAMS AND RESERVOIRS 3.5

of earth or rock designed to resist the lateral forces imposed by the reservoir pool at its

maximum stage, plus an impermeable barrier such as clay or concrete usually located

within the embankment, but sometimes constructed as an impermeable membrane on the

upstream face. A cross-section view of an embankment dam is shown in Fig. 3.4.

Embankment dams are extensively used for structures in the 15- to 30-m height range,

where they represent nearly 90 percent of the total number, but they represent only 25

percent of the dams exceeding 200 m in height. However, the world's tallest dam is an

embankment dam. The largest dams are listed in Table 3.2 for various categories.

3.2.3 Concrete Dams

Dams constructed of concrete may employ a variety of methods to withstand hydrostatic

forces. A concrete gravity dam resists overturning and sliding forces by the gravitational

mass of the concrete structure. Figure 3.5 illustrates the configuration of a concrete

gravity dam with a hydropower station. The most recent major advancement in the

construction of concrete dams is the application of roller-compacted concrete (RCC), a

zero slump mixture of cement, aggregate, water, and, optionally, pozzolan, which is

normally placed continuously in lifts of about 0.5 m and compacted with a vibratory

roller. It combines the benefits and safety of a concrete gravity dam with the rapid and

economical placement methods used in embankment dam construction (Hillebrenner,

1993). This technology has several important advantages over conventional concrete

construction including lower cost, less potential for damage by flooding, and shorter

construction time (Prendergast, 1992).

In narrow gorges having competent rock on either abutment, an arch dam may be

constructed in which the hydrostatic forces are transmitted along the axis of the dam and

into the rock abutments. Arch dams have much thinner sections than gravity dams.

Although constituting less than 5 percent of dams worldwide, they account for half of all

dams over 150 m high.

Embankment Dam

FIGURE 3.4 Cross-sectional view of an embankment dam. The upstream face is protected wit

stone or other material to prevent erosion by wave action, and internal drains control seepage an

avoid piping. A selective withdrawal or multilevel intake tower is also shown.

ENGINEERING FEATURES OF DAMS AND RESERVOIRS 3.6

TABLE 3.2 Largest Dams for Various Categories

Category and name River

Country/

state

Height,

Storage

capacity,

Power

capacity,

MW Year

World

hest dams

Rogun Vakhsh Russia

335 13,300 - UC

Nure

Vakhsh Ta

ikistan

300 10,500 2,700 1980

Grand Dixence Dixence Switzerland

285 401 - 1961

uri In

uri Geor

272 1,100 - 1980

Largest reservoirs*

Brats

ara U.S.S.R.

125 169,000 4,500 1964

h Aswan

ile E

gypt

111 162,000 2,100 1970

Kariba Zambezi Zimbabwe

128 160,368 - 1959

Akosombo

Volta

Ghana

134 147

960 - 1965

Largest hydropower projects

Itai

u Parana Brazil

196 29,000 12,600 1983

Guri Caroni Venezuela

162 135,000 10,300 1986

Sayano Yenisei Russia

245 31,300 6,400 1989

Grand Coulee Columbia USA

168 11,795 6,180 1942

Highest dams

Droville

Feather

California 230 4,670 644 1968

Hoover

Colorado

Nevada 221 34,850 1,434 1936

Dworsha

N.Fork Clearwate

aho 219 4

260 400 1973

Glen Canyon

Colorado

Arizona 216

33,300 1042 1966

Largest reservoir

Hoove

Colorado Nevada 230 34

850 1

434 1936

Glen Can

on Colorado Arizona 216 33

300 1

042 1966

Garrison Missouri North Dakota 64 27

920 517 1954

Oahe Missouri South Dakota 75 27,430 640 1958

Largest hydropower projects

Grand Coulee

Columbia

Washington 168 11,795 6,180 1942

Chief Jose

h Columbia Washin

ton 72 752 2

467 1955

John Da

Columbia Ore

on 70 617 2

160 1968

Bath County Upper

Little Back Ck

Virginia 143 44 2,100 UC

United States

UC = unco

mpleted.

*208.8-km

Owen Falls on White Nile, Uganda, not included, as it impounds a natural lake.

Source: ICOLD (1984, 1988).

3.3 RESERVOIR CHARACTERISTICS AND OPERATION

3.3.1 Reservoir Size

Reservoir volume may be expressed in absolute units, such as millions of cubic meters

(Mm

), or relative to mean annual flow. From the standpoint of sediment management

the relative size, termed hydrologic size or capacity to inflow (C:I) ratio, is more

important than absolute size and is computed as the ratio of total reservoir volume to

mean annual inflow.

ENGINEERING FEATURES OF DAMS AND RESERVOIRS 3.7

FIGURE 3.5 Section view of concrete gravity dam with a hydropower station, showing the various

terms used to describe system.

Hydrologic size is a primary factor influencing the rate of sediment accumulation

(Brune, 1953) and is also a primary determinant of the types of sediment management

techniques that can be used. Hydrologically small reservoirs have short residence times

and typically spill a significant part of the streamflow downstream during floods. They

can be manipulated in a variety of ways to encourage the release of sediments together

with that part of the annual discharge that is spilled. A reservoir having a capacity inflow

ratio exceeding 50 percent may be considered hydrologically large and may have

significant year-to-year carryover storage capacity. Having a large capacity relative to

runoff, these reservoirs spill little water, and there is limited opportunity to periodically

draw down or empty these larger reservoirs for sediment management because the

associated water loss would be unacceptable. However, as large reservoirs fill with

sediment and their capacity decreases, a wider range of sediment management techniques

will become applicable. At some sites it appears feasible to maintain a reservoir capacity

of approximately 20 to 30 percent of mean annual flow using sediment flushing (Tolouie,

1993; Xia, 1987).

The terms hydraulic residence time, turnover time, and detention period all refer to

the time required to replenish the water in the reservoir computed from the actual storage

volume (which varies as a function of pool elevation) and the inflow rate over a specified

time period (rather than the long-term mean inflow). The hydraulic residence time in a

hydrologically small reservoir can vary from under an hour during a major flood to many

months during the dry season. Because of hydraulic short-circuiting, the actual hydraulic

residence time for a particular parcel of water may be very different from the average

value computed from the inflow rate and reservoir storage.

3.3.2 Pool Geometry

Reservoir pool geometry has a major influence on hydraulic behavior and the pattern of

sediment transport within the impoundment. The pool configurations shown in Fig. 3.6

illustrate the wide range of pool planforms that occur.

ENGINEERING FEATURES OF DAMS AND RESERVOIRS 3.8

FIGURE 3.6 Pool configurations for various reservoirs: (a) La Plata, Puerto Rico; (b) Itaip

Brazil/Paraguay; (c) Shasta, California; (d) San Luis, California. The San Luis configuration is

typical o; reservoirs in areas of low relief requiring a long dam and creating a relatively

shallow lake.

3.3.3 Pool Allocation

Several water levels may be defined for reservoirs. The full reservoir level (FRL) is

determined from the crest elevation of the ungated spillway or the design normal water

level against the crest gate. During floods the water level in the reservoir may rise above

the full level, and this higher design level is termed the maximum water level. The

minimum operating level is determined from the requirements of the particular intake

design and is the minimum level at which the lowest-level intake can be operated.

The total storage volume created upstream of a dam may be divided into several zone

based on the placement of outlet structures and operational assignments (Committee or

Hydropower Intakes, 1995). As illustrated in Fig. 3.7, dead storage is the volume that is

below the invert of the lowest-level outlet and which cannot be drained by gravity. Live

storage is the total volume below full reservoir level less dead storage. In projects with a

flood control component, flood storage is the upper portion of the pool dedicated to flood

detention. 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. The size of the flood storage pool may be

varied seasonally depending on flood hazard, and the joint use pool is the volume that

may be dedicated to flood control part of the year and for conservation storage during

other periods. Inactive storage is the lower part of the conservation pool that is normally

not used. For example, drawdowns may normally be limited to benefit recreational users,

but may become necessary during a prolonged drought.

3.3.4 Stage-Storage Relationships

Reservoir storage characteristics are quantified using stage-storage and stage-storage

graphs. The accumulation of sediment displaces volume within the reservoir, causing

both the storage and area curves to shift, as shown in Fig. 3.7. Sediment tends to

accumulate at all levels within the reservoir, with the specific accumulation pattern

varying from one site to another. Although the dead storage volume is often termed a

sediment storage pool, this is inaccurate because only a portion of the sediment inflow

will be deposited in the dead storage zone. In some cases, such as in long reservoirs

ENGINEERING FEATURES OF DAMS AND RESERVOIRS 3.9

FIGURE 3.7 Shift of stage-storage and stage-area curves due to sedimentation.

absent turbidity currents or where most sediments are deposited in delta areas, virtually

all of the initial sedimentation will be focused in the higher-elevation active pool area,

with little accumulation in dead storage.

3.3.5 Types of Reservoir Operation

A reservoir storage pool operated for water conservation captures irregular runoff flows

to make subsequent deliveries to users at scheduled rates. A hydrologically small

municipal water supply reservoir will normally remain at a high level, except during

drought when it will drop to low levels. A hydrologically small irrigation reservoir may

deliver all stored water to farmers on an annual basis and may be emptied at the end of

each irrigation season. In contrast, hydrologically large reservoirs having carryover

storage are characterized by water levels that gradually fluctuate over a wide range in

response to series of wet and dry years. In recreation reservoirs it is normally desirable to

have a continuously high and stable pool level, although some water manipulation may

be desirable for purposes such as the control of aquatic vegetation, enhancement of

fisheries, or consolidation of muddy sediments by desiccation (Cooke et al., 1993).

Operation for hydropower production seeks to balance two conflicting objectives. To

maximize energy yield per unit of water, the pool should be maintained at the highest

possible level, yet the pool elevation should be low enough to capture all inflowing flood

runoff for energy generation. The resultant operation represents a compromise between

high-head and storage requirements, and will often result in significant seasonal water

level variations as water captured during the flood season is released to generate power

during periods of low water inflow.

A reservoir operating rule guides the allocation of variable inflow over time and

among different uses. The concept of a rule curve is presented in Fig. 3.8, which

illustrates a seasonally variable flood control pool with an annual drawdown at the

beginning of the flood season for sediment removal. The seasonally variable flood

ENGINEERING FEATURES OF DAMS AND RESERVOIRS 3.10

FIGURE 3.8 Conceptual reservoir operating curve showing: (1) top of gates; (2) guide curve for

maximum pool level; (3) flood control storage; (4) drawdown period for sediment flushing; (5)

flood detention and release.

cont

rol pool provides flood storage only in the season of flood hazard, and allows this

portion of the pool to be filled for conservation storage at the end of the flood season.

Many variables can influence operations, and the operating schedule of a hydropower

reservoir may be determined by factors including actual and anticipated power load,

water level, anticipated inflow (e.g., snowmelt), current and projected cost of power from

alternative sources, recreational demands (e.g., lake boating or downstream kayaking and

rafting), and environmental requirements affecting downstream releases and upstream

pool levels. Multipurpose reservoirs have an operating regime that reflects the evolving

compromise between the potentially conflicting uses, and nominally single-purpose

projects may significantly alter their operation to meet new demands (e.g., recreation) or

to address environmental problems. Management issues can become highly complex and

politicized in systems containing multiple reservoirs extending over many hundreds of

kilometers along rivers shared by several states, such as the controversy between upper-

basin and lower-basin states along the Missouri River in the United States. Upper-basin

states prefer higher water levels in upper-basin reservoirs for recreational purposes,

whereas lower- basin interest prefer releases to enhance navigation.

Morris &amp; Fan. Reservoir Sedimentation Handbook

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Morris & Fan. Reservoir Sedimentation Handbook