CENG 6607

Planning & Design of

Dams
Dr. Netsanet Nigatu
Course content

• Planning and design of dams in the context of hydropower

development.

• The course covers the basics in dam engineering for

Hydraulic engineers, including embankment and concrete
dams, planning and operation of dams projects.

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Outline of the presentation
• General definitions
• Classification of Dams
• Embankment Dams
• Concrete Dams
• Selection of Type of Dam
• Selection of Dam site

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1.1 General
1.1.1 Definitions
 A dam may be defined as an obstruction across a river. Through the obstruction,
storage is formed, which can be utilized for various uses. The retained body of water is
referred as a reservoir, and the retaining structure is the dam.

Benefits:
• Increased head (m)
• Developing storage (volume of
water)

 Small Dams:
Height < 15 m

 Large Dams:
Height > 15 m or
Storage capacity > 1Mm3 or
Spillway capacity > 2000 m3/s

AAiT, Planning, Dr. Netsanet NIgatu

 Definition continued…
 Large dam
 For the purpose of inclusion in the World Register of Dams, a large dam is
defined as (ICOLD definition)
1. any dam above 15 meters in height (measured from the lowest point of
foundation to top of dam) or,
2. any dam between 10 to 15 meters in height which meets at least one of
the following conditions;
a. The crest length is not less than 500 meters
b. The capacity of the reservoir formed by the dam is not less
than one million cubic meters (1Mm3)
c. The maximum flood discharge dealt with by the dam is not less
than 2000 cubic meters per second (Spillway capacity > 2000
m3/s)
d. The dam had specially difficult foundation problems
e. The dam is of unusual design

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 Historical perspectives
 The history of dam building dates back to ancient time, and is bound up
with the earlier civilizations of the middle east and the far east. Countless
small dams, invariably simple embankment structures, were constructed for
irrigation purposes in for example, China, Japan, India and Sri Lanka.

 Sadd El-Kafara, Egypt

• 2600-2500 BC
• Oldest known dam of real significance
• Constructed with an earth fill central
zone flanked by rock shoulders and
with rubble masonry face protection,
built for flood control.
• Completed to a height of ca 14 m
• Crest length ca. 110 m
• The dam breached, probably in
consequence of flood overtopping,
after a short period of service.

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AAiT, Planning, Dr. Netsanet NIgatu 7
 Dams with largest capacity reservoirs

Capacity Year of
Rank Name Country (M m3) Completion
1 Kakhovska CIS 182,000 1955 (June 6,2023
2 Kariba Zimbabwe/Zambia 180,000 1959
3 Bratsk Russia 169,270 1964
4 Aswan High Dam Egypt 168,900 1970
5 Akosombo Ghana 153,000 1965
6 Daniel Johnson Canada 141,850 1968
7 Guri Venezuela 138,000 1986
8 Krasnoyarsk Russia 73,300 1967
9 W A C Bennett Canada 70,310 1967
?? Grand Ethiopian Ethiopia ≈ 70,000 Under
Renaissance construction
Dam (GERD)
?? Gibe III Ethiopia 14,000 2015

AAiT, Embankment, 2022, Dr.

Netsanet Nigatu 8
 World’s Tallest Dams

Height Year of
Rank Name Country (m) Completion
1 Jinping-I China 305 2013
2 Nurek Tadjikistan 300 1980
3 Xiaowan China 292 2010
4 Baihetan China 289 2021
5 Xiluodu China 285.5 2013
6 Grand Dixence Switzerland 285 1964
7 Enguri Gorgeia 272 1978
8 Yusufeli Turkey 270 2021
14 Gibe III Ethiopia 243 2015
?? Tekeze Ethiopia 189 2009
?? GERD Ethiopia 145

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Netsanet Nigatu 9
 Earth fill Dam

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 Rock fill Dam

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 Concrete Gravity Dam

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Itapu Dam, Brazil, Uragay

 Concrete Gravity Dam

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 Concrete Gravity Dam

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 Concrete Single Arch Dam

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 Concrete Single Arch Dam

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 Concrete Single Arch Dam

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 Concrete Multiple Arch Dam

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 Concrete Buttress Dam

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Roselende Dam
1.1.2 Roles of dams
 Dams are very expensive structures:
 Grand Renaissance Dam: USD 5 bn
 Gilgel Gibe III: USD 1.8 bn
 The functions of dams and reservoirs include
1. Storage
 Water supply
Gefersa, Legedadi
 Irrigation water for agriculture
Fincha, Beles,
 Hydropower
Gilgel Gibe, Tekeze,
 Flood control
2. Creating head
 Hydropower
3. Maintaining water level
Navigation
 Recreation
4. Multipurpose: multiple benefits from a single
investment. AAiT, Planning, Dr. Netsanet NIgatu
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 The world Register of Dams counts 58,402 large dams, of these 28,684 ( 49.1 %)
are single purpose dams

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 Major Dams in Ethiopia
Hydropower Irrigation
Gilgel Gibe III 1870 MW
Fincha 134 MW ??
Melka Wakana 153 MW
Gilgel Gibe I, II 180 MW +420 MW
Tekeze 300 MW
Beles 460 MW 140,000 Ha
Fincha Amerti Nesse 97 MW 6000 Ha
Under Construction
Grand Ethiopian 6000 MW
Renaissance

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1.1.3 Impacts of Dams

The presence of a dam interferes with natural systems. The main disadvantages
includes Three Gorges Dam in Yangetz river in
 Resettlement and relocation China
 over 1,000,000 dams so far built  181 m high, 2.335 Km long
Gravity dam
 40 to 80 million displaced  Reservoir stores 39 Km3,
 Destruction of fauna and flora surface area 1024 Km2 ,
 Dam failure causes catastrophic damage Reservoir length 600 Km
 Uses
(Banqiao Dam - China) (Banqiao Dam - China), 1975,typhoon Produces nina,
22,000theMW,third
deadliest flood in history, 26,000 to 240,000 flood protection
 Change in groundwater level Navigation
Impacts
 groundwater level rise sometimes up to 90m 1.24 million people
 damage to infrastructure displaced
 damage to farmland (water logging)  Submerged 13 cities,
140 towns, 1350 villages
 Influence on sediment balance Over 6000 species of
 (sediment transport capacity) plants
 Influence on fish  The reservoir is blamed
for an increase in landslide
(many people in the world live on coastal and earthquake in the
areas and depend on fish for daily dietary) region.
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AAiT, Planning, Dr. Netsanet NIgatu 24
1.1.4 Dam Structure and Reservoirs
1. Reservoir
2. Dam
3. Service Intake
4. Bottom outlet
5. Spill structure
6. Diversion structure
7. Service road

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 1.1.5 Storage Components of reservoir
MWL 1. Active storage: Usable
FRL 2. Inactive storage: Unusable
3 Live storage: Non sediment
4 Dead storage: Sediment
5 Flood storage: (multipurpose dam)

MOL

6. Reservoir capacity; gross capacity of reservoir; gross storage; storage capacity

7. Full reservoir level (FRL); normal water level; the maximum elevation to which the reservoir water rise
during normal condition
8. Flood surcharge; the volume of water stored between the normal pool level and maximum pool level
9 .Maximum water level (MWL); is the maximum level to which the water rises during the worst design
flood.
10. Minimum operating level (MOL); top of inactive storage,the lowest water surface elevation which has
to be kept under normal operating conditions 26

AAiT, Planning, Dr. Netsanet NIgatu

1.2 Classification of Dams
 On the following slides the different classifications of dams is reviewed,
however bear in mind that
Geophysical environment, foundation geology, catchment flood
hydrology, material characteristics, etc. are all site specific.

This results in that every dam

is quite unique

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 Classification of dams continued…

 In the World Register of Dams,  Dams can also be viewed or

dams are classified by classified :
• Type (structural behavior) • By age
• Spillway capacity • By size (height and\or
• Reservoir capacity volume
• Installed capacity with • By function
energy • By material (dam type by
• Irrigated area material)
• Volume for flood protection • By hydraulic design
• Resettled persons • According to their hazard
potential,
• Hazard potential classification for a
dam may change over time
• Periodically review and update of the
hazard classification is required

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 Classification of dams continued…
 Dams are numerous types and there are various ways of classification

1.2.1 Classification based on purpose

A. Storage dams: They are constructed to impound water in periods of surplus

supply for use in period of deficient supply
B. Diversion Dams: are constructed to divert flow of water from its natural
course or to by pass water, for example around a dam site during construction.
they are constructed to provide head for carrying water in to ditches or other
conveyance systems to the place of use
C. Detention Dams: They are constructed to retard flood runoff and minimize
the effect of floods.

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1.2.2 Classification based on hydraulic design

A. Overflow dams: are those designed to carry discharge over their crests.
Generally dams are not designed as overflow dams over its entire length.
Concrete gravity dams.
B. Non overflow dams: are those designed not to be overtopped. The dam crest
is at a higher elevation than the expected maximum flood level. Earhtfill, rockfill,
masonry, concrete, etc. Usually a spillway is provided to permit overflow of
surplus water.
C. Composite dams:

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1.2.3 Classification according to materials of construction

Dams

Concrete Embankment

Gravity Arch Buttress Earth fill Rock fill

11% 4% 1% 82%

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1.3 Embankment Dams
1.3.1 Definition

 Any dam constructed of excavated materials placed without addition of binding

materials other than those inherent in the natural material.
 The material is usually obtained at or near dam site.

 Accounts for over 82% of all large dams

A typical Embankment dam consists of three basic parts:

 Shell,
Core and
 Foundation.

 Additional appurtenances may be present depending on type of dam.

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 Transition filter

Water Core
2 2
1
Upstream impervious blanket
Cutoff wall
Pervious foundation
3

Impervious stratum

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Foundation
Provides support both to vertical and horizontal load
Resist seepage underneath the embankment

 Core
To prevent seepage of water through the embankment
Can be placed at the center or upstream from the center
It may extend to impervious stratum in pervious foundation
 Materials
 Earth (cheap)
Permeability (cm/s)
Gravel 1 to 100
Sand 1 to 10-3
Silt 10-3 to 10-5
Clay < 10-6

Concrete (less flexible)

Shell
To provide structural support for the core and to distribute the load to foundation34
AAiT, Planning, Dr. Netsanet NIgatu
1.3.2 Types of Embankment Dams
 Embankment dams are classified in to two types:
Earth fill / Earth dams and
Rock fill dams.

1.3.2.1 Earthfill dams / earth dams

 An earth dam is an embankment dam, constructed primarily of compacted earth or
soil, and containing more than 50% of earth material.

Soils: Clay, Silt, Sand, and Gravel

 Three main types of earth dam

Homogeneous earth dam
Zoned earth dam
Diaphragm earth dam

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A Homogeneous Earth dam
It is composed of only one kind of material
The material must be sufficiently impervious to provide an adequate water barrier
The upstream face should be flat to guard against sloughing due to rapid drawdown
The downstream slope should be flat so that it resists sloughing when saturated to high
level
Regardless of d/s slope and permeability, seeping water will come out in d/s face
Drainage filters are provided in the d/s
These types of dams are recommended in localities
The locally available soil shows little variation in strength & permeability
Soils of contrasting permeability are available in small quantities / high cost
 These types of dams are not recommended
 Dispersive and erodible soils such as dispersive clay soils

36
B . Zoned Earth Dam
 It is the most commonly constructed type
 Three zones. An impervious core flanked by pervious shells
1. Impervious core controls seepage
2. The pervious shell provides structural support and protects the core
 The upstream shell provides stability against rapid draw-down
 The downstream shell acts as a drain to control seepage and lower the
phreatic surface
3. Transition filter (drains and drainage layers) control washing out of the core

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The pervious shells are constructed from sands and gravels
 Common materials used for impervious core include clay, silty clay, sandy clay, clayey
sand, silty sand, possibly with some gravel. Usually greater than 15% passing 0.075 mm,
preferably more.

The core width for a central impervious core-type embankment should be established
using types of material available, the filter design, and seismic considerations.

In general, the minimum width of the core at the base or cutoff should be equal to or
greater than 25% to 50% of the height of the maximum reservoir elevations.

The maximum core width will usually be controlled by stability and availability of
impervious materials.

A core top width of 3 m is considered to be the minimum for construction purposes.

AAiT, Embankment, 2022, Dr. Netsanet

Nigatu
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C. Diaphragm Earthfill Dam
 Three zones: Upstream and downstream shell of permeable material
Thin diaphragm / Impermeable material
 Placed centrally / upstream
 Rolled clay, cement concrete, asphalt concrete
The dam is a diaphragm type
 If the thickness of the impervious core is < 3m
 At any elevation the thickness is less than the height above the elevation
Potential for cracking of the core due to differential movement induced by
Embankment consolidation
Reservoir level fluctuation
Non uniform foundation settlement

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 Diaphragm
Clay diaphragm
The construction of an internal earth diaphragm with the necessary filter
requires higher degree of precision

 Concrete diaphragm
 Internal diaphragm made of concrete is not available for inspection or
emergency repair if ruptured

If the availability of core material is so limited that zoned dam can not be constructed, a
diaphragm type is considered

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AAiT, Planning, Dr. Netsanet NIgatu

1.3.2.2 Rock fill embankment dams
A rock fill dam is an embankment dam, constructed primarily of rock, as a major
structural element and containing more than 50% of rock. (link)

 Rocks: Cobbles (> 76mm), boulders, crashed rocks

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AAiT, Planning, Dr. Netsanet NIgatu 42
 An impervious membrane is used as a water barrier and can be placed either with in the
embankment (Internal) or on the upstream slope (external)

 Various materials have been used for impervious membrane (impervious core) including
 Clay,
Cement Concrete,
Asphaltic concrete

 Generally rock fill dams can prove economical

 Large quantities of rock are readily available & Earth materials are difficult to obtain
Excessive wet climatic condition limit the placement of large quantities of earth fill
materials
Short construction seasons prevail

 Depending on the location of the impervious membrane

 Internal membranes
Central
 Sloping
External

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1.3.3 Advantages and Disadvantages of Embankment Dams

A . Advantages
 Suitable to wide valley and steep gorges alike
 Adaptable to wide range of foundation condition (more potential sites)
 Insensitive to small settlements
100 m high dam
 Simple construction, high mechanized process (load to the foundation)
 Use of natural materials, minimizing cost of transportation Emankment 1.8-2.1 Mpa
 Increasing dam height is simple Gravity dam 3.2 – 4.0 Mpa
Buttress dam 5.5 – 7.5 Mpa
 Can be well integrated to the landscape.
Arch dam 7.5-10.0 MPa

B. Disadvantage
 Sensitive to wave action
 Overtopping is not allowed, higher free board
 Erosion danger on the d/s unless berms are provided
 Construction material and progress affected by weather
 Huge mass
50m high 500 m crest length
Embankment dam: 3.4 million m3 soil
Gravity dam: 687,500 m3 concrete 20% of Embankment volume
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1.4 Concrete Dams

1.4.1 Concrete Gravity Dams

1.4.1.1 Definition

 ICOLD definition: A dam constructed of concrete and or masonry which relies on its
weight for stability.

 A concrete gravity dam is a massive concrete structure, roughly triangular in shape,

and designed so that its weight ensures structural stability against the hydrostatic
pressure of the impounded water and other forces that may act on the dam.

 In the earlier periods, gravity dams were constructed from masonry. In recent years,
however, gravity dams are constructed from concrete.
 Gravity dams are
 permanent structures that require little maintenance
 constructed to greater heights

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 The tallest concrete gravity Dam
Height 285 m
Length 700 m
Base width 200 m
Volume 6,000,000 m3

Reservoir
Capacity 400,000,000 m3
Catchment area 46 km2
Surface area 4 km2
Max. water depth 284 m
Power station
Installed capacity 2,069 MW

Grande Dixence Dam, Switzerland

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 Crest: Top of the dam
Heel: Dam contact with foundation on the u/s
side
Toe: Dam contact with foundation on the d/s
side
Abutment: sides of the valley which the
structure of the dam meets
Gallery: opening or passage left in the body of
the dam for inspection and drainage purposes
Outlets: opening to discharge water
Headwater: Impounded water

Headwater Outlet

Gallery Tailwater

Heel Toe
Foundation 47

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1.4.2 Arch Dams
1.4.2.1 Definition

ICOLD definition: Arch dam. A concrete or masonry dam which is curved in plan
so as to transmit the major part of the water load to the abutments.

More economical than gravity dams since less material is required. (less self weight).

According to ICOD: thickness of the dam is less than 0.6 times its height (t < 0.6h)

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The Largest arch dam Type of dam Arch, double-curvature
Height 292 m
Xiaowan Dam, China
Length 902 m
Crest width 13 m
Base width 69 m

Thickness: height = 69/292 = 0.24

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1.4.3 Buttress Dams
1.4.3.1 Definition (1%)
 ICOLD definition: Buttress Dam
consisting of a watertight part supported at
intervals on the downstream side by a series Buttress walls
of buttresses. Road
 A buttress dam consists of two principal
elements Upstream
A sloping upstream deck slab that
retains the water
 Buttress walls that support the deck Downstream
slab and transfer the loads to the
foundation Deck slab
 They were first developed to store
water in regions where materials were
expensive or scarce (Hollow gravity
dams).

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 The main advantage is its versatility. Doesn’t
require strong rock foundation needed for arch,
and does not require the large amount of building
materials for gravity and buttresses, can be
modified to the specific situation.

 Main disadvantages is the height limitation and

it is harder to design and build than others.

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1.5 Classification based on hazard potential

 Reservoirs retained by one or more dams, constitute a potential hazard to the

downstream area: life, property and environment (society, economy and
environment).
 Failure of a dam, resulting in flooding downstream, can result in unacceptable
fatalities and economic damage.

 Hazard classification doesn't indicate the structural integrity of the dam itself
but rather the effects if a failure should occur.
 The hazard potential assigned to a dam is based on consideration of the effect
of a failure during both normal and flood flow conditions.

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Classification based on hazard potential

 Three classification is for example adopted by FERC (USA)

 Low, significant, and high.

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Some major dam disasters 1959- 1993

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1.5 Selection of type of dam
 Two main questions during the early planning and design stages ,
 where to build? i.e., selection of the site and
 what type? selection of the type of dam should be carefully considered.
 Note
 It is only in exceptional circumstances that only one type of dam is suitable for
a given dam site.
Generally, preliminary designs and estimates for several types of dams and
appurtenant structures are required before one can be proved the most suitable
and economical.

 The selection of the type of dam requires cooperation among experts representing
several disciplines-including
hydrologists;
geotechnical, hydraulic, and structural engineers and
engineering geologists
to ensure economical and appropriate designs for the physical factors, such as
topography, geology and foundation conditions, available materials, hydrology, and
seismicity. AAiT, Planning, Dr. Netsanet NIgatu 56
 Some of the factors that are considered in selection of dam type are
 topography Others
 foundation condition  Outlet works
 availability of construction material  Diversion structures
 spillway size and location  Accessibility
1.5.1 Topography  Earthquake
Topography consideration include :  Environment
 surface configuration of the dam site and reservoir area
 accessibility to the site and construction materials
 Topography, in large measure, dictates the first choice of type of dam
 A narrow stream flowing between high, rocky walls would suggest a concrete
dam
A narrow V-shaped valley with abutments strong enough to resist the arch
thrust is best suited for an arch dam.
 For a moderately wide valley with small depth of overburden and strong
foundations at the base, a gravity or buttress dam is indicated;
 A low, rolling plain would suggest an earth dam with a separate spillway
 For intermediate conditions, other conditions takes on importance location of
spillway

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1.5.2 Geology and foundation conditions
 Geologic considerations include the various types of rock and soil present and their
suitability as foundation and construction materials.
The foundation geology at a dam site often dictates the type of dam suitable for that
site. The strength, thickness, and inclination of strata; permeability; fracturing; jointing; and
faulting are all important considerations when considering a dam type.
A. Solid rock foundation
Main feature
 high bearing capacity > 5MPa
 high shearing strength
 resistance to erosion and seepage
Suitable: all types of dam

B. Gravel foundation
Main features:
medium bearing capacity 200-600 Kpa
serious percolation / seepage problem thus require special precaution with cutoffs
Suitable: if well compacted, it is suitable for earth fill, rock fill, small concrete gravity
dam
Unsuitable: large concrete dams 59

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C. Silt and fine sand foundations
Main features
 Low bearing capacity (100-300 kpa)
 Collapse up on saturation (Silt LL=27%, PL=20% & PI=7%)
Susceptible to cracking (Non plastic fine)
 Piping
 Percolation losses
Suitable: if properly designed, they are suitable for earth fill and small concrete gravity
dams
Unsuitable: rock fill and large concrete dams

D. Clay foundation
Main features
Low bearing capacity (75 -300 kpa)
 Low foundation shearing strength
Excessive settlements of unconsolidated and high mositure content

Suitable: earthfill dams after special treatment

E. Non uniform foundation 60

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1.5.3 Availability of Materials

 Materials for dams of various types which may sometimes be available at or near a dam
site include
Soil for embankments
Rock for embankments and rip rap
Concrete aggregates (sand, gravel, crushed stone)

The most economical type of dam will often be the one for which materials in sufficient
quantity are found onsite or within a reasonable haul distance from the site.

Elimination or reduction of transportation expenses for construction materials,

particularly those which are used in great quantity, will effect a considerable reduction in
the total cost of the project.

Availability of sand and gravel for concrete at a reasonable cost locally -> Concrete
Dam
Availability of suitable soils for an earth fill in nearby borrow pits -> Earth fill dams

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1.5.4 Spillway size and location

 Spillways, as a safety structure, are an important components of dams.

The size, type and natural restriction in locations will be a controlling factor in the
choice of the type of dam

 Location:
 Embankment dams would require a separate spillway:- > presence of saddle
 In concrete dams the spillway can be integrated with the dam within the valley

 Size and type

The selection of a specific spillway type will be influenced by the magnitude of
the floods to be passed (hydrology of the area).
Thus in streams with large floods, the spillway will become the dominant
structure and the selection of the dam type will become secondary.
The cost of the spillway will become a considerable portion of the total cost
of the dam project. In such cases combining the spillway with the dam as in
the case of concrete overflow dams will become advantageous.
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1.5.5 Other Factors
A. Limitations of outlet works
In embankment dams, outlet works can not pass through the dam body. In
concrete dams that is not the problem. This means that, In dam sites where the
site condition is not suitable for digging of tunnels through abutments, provision of
outletworks for embankment dams would be difficult. In such cases, concrete
dams may be favoured.

B. Problem of diverting the river during construction

The situtation is similar for the construction of diversion structures. River
diversion is one of the major activities that is carried out in dam construction. Two
approaches exist. In the first approach, If site condition permit, the river can be
diverted by constructing open channels or tunnels that circumvent the dam site.
In the second approach, if the site condition does not permit for construction of
channels and tunnels, the diversion can be accomplished with the help of
diversion conduits that pass through the dam site/body or by other construction
procedure such as staged construction. The first approach can be used for both
embankment and concrete dams. The second approach can be used with
concrete dams only.
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C. Accessibility of the site
Embankment dams usually involve transportation of large volumes of dam
material. They require 4 to 6 times as much material as gravity dam. This means
that accessibility of the dam site is critical for embankment dams

D. Earthquake
Embankment dams are less sensitive to earth quake than concrete dams

E. Environmental impact:

AAiT, Planning, Dr. Netsanet NIgatu 64

1.6 Selection of Dam Sites
Two requirements,
 Functional
Technical

1.6.1 Functional Requirement

 The functional requirement refers to a balance between the sites natural physical
characteristics (supply) with the purpose of the reservoir (demand)
a. Catchment Area and hydrology
The catchment area, catchment characteristics, and catchment hydrology
should be of such size and characteristics that the runoff produced from the
catchment and stored behind the dam is capable of meeting the demand
imposed on the dam.
b. Available head
One of the hydraulic parameter in the study of hydropower potential of a site is
the head available.
P=Qρgh
A great saving in hydropower development can be gained, if the head for
power generation is obtained from the natural topography ofNetsanet
AAiT, Planning, Dr. a site.NIgatu 65
c. Valley forms
For storage at economic costs a bottleneck configuration is required.

At bottleneck
Dam width is minimum
Storage volume is big

A site where the river has a U-bend 'or an S-curve is advantageous in layout as
tunnels needed for diversion or conveyance can be aligned across the bend and thus be
shorter in length.

Dam axis
Dam axis

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AAiT, Planning, Dr. Netsanet NIgatu

1.6.2 Technical
 Geology of dam site:
A dam is a massive structure and imposes not only the heavy load of the dam
body but also high water pressures on the foundations. It is necessary that the
foundations be able to sustain these loads, after suitable treatment where necessary,
without such deformations or stress concentrations as may cause damage to or failure
of the dam.
 Reservoir water tightness:
 Abutment stability (avoid landslide) and water tightness
 Availability of construction materials
 Availability of suitable site for spillway

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