Chapter 8: Dams Flashcards
The purpose of a dam is to
impound (store) water for any of several reasons
The purpose of a dam is to impound (store) water for any of several reasons, e.g.,
flood control, water supply for humans or livestock, irrigation, energy generation, recreation, or pollution control.
Water from rainfall or snowmelt naturally runs
downhill into a stream valley and then into larger streams or other bodies of water
The “watershed system” refers to
the drainage process through which rainfall or snowmelt is collected into a particular stream valley during natural runoff (directed by gravity)
Water from rainfall or snowmelt naturally runs downhill into a stream valley and then into larger streams or other bodies of water. The “watershed system” refers to the drainage process through which rainfall or snowmelt is collected into a particular stream valley during natural runoff (directed by gravity).
Dams constructed across such a valley then impound the
runoff water and release it at a controlled rate.
Dams constructed across such a valley then impound the runoff water and release it at a controlled rate. During periods of high runoff, …………………………………. typically increases
water stored in the reservoir
Dams constructed across such a valley then impound the runoff water and release it at a controlled rate. During periods of high runoff, water stored in the reservoir typically increases, and ……………………………. may occur.
overflow through a spillway
Dams constructed across such a valley then impound the runoff water and release it at a controlled rate. During periods of high runoff, water stored in the reservoir typically increases, and overflow through a spillway may occur. During periods of low runoff, reservoir levels usually
decrease
During periods of low runoff, reservoir levels usually decrease. The owner can normally control the reservoir level to some degree by
adjusting the quantity of water released
During periods of low runoff, reservoir levels usually decrease. The owner can normally control the reservoir level to some degree by adjusting the quantity of water released. Downstream from the dam, the stream continues to
exist
The owner can normally control the reservoir level to some degree by adjusting the quantity of water released. Downstream from the dam, the stream continues to exist, but because the quantity of water flowing is
normally controlled, very high runoffs (floods) and very low runoffs (drought periods) are avoided.
Dams may either be
human-built or result from natural phenomena
Dams may either be human-built or result from natural phenomena, such as
landslides or glacial deposition
Dams may either be human-built or result from natural phenomena, such as landslides or glacial deposition. The majority of dams are
human structures normally constructed of earthfill, rocks or concrete
The majority of dams are human structures normally constructed of earthfill, rocks or concrete. Naturally occurring lakes may also be modified by
adding a spillway to allow for safe, efficient release of excess water from the resulting reservoir.
Dam owners should be aware of:
■ the different types of dams
■ essential components of a dam
■ how the components function, and
■ physical conditions likely to affect a dam.
Human-built dams may be classified according to
the type of construction materials used,
the methods used in construction,
their slope or cross-section,
the way they resist the forces of the water pressure behind them,
the means of controlling seepage, and occasionally, their purpose.
Components of dams
- foundation
- core or membrane
- shell
- transition filter
- internal drain
- toe drain
FOUNDATION
It consists of
earth or rock
FOUNDATION
It consists of either earth or rock and provides a support for
the embankment
. FOUNDATION
It consists of either earth or rock and provides a support for the embankment and resists
both vertical and horizontal loads
. FOUNDATION
It consists of either earth or rock and provides a support for the embankment and resists both vertical and horizontal loads. It also resists
under seepage on the flow of water beneath the dam.
CORE OR MEMBRANE
It holds back
the free water of the dam reservoir
CORE OR MEMBRANE
It holds back the free water of the dam reservoir. It is located either at
the centre or upstream from the centre of the dam.
CORE OR MEMBRANE
It holds back the free water of the dam reservoir. It is located either at the centre or upstream from the centre of the dam. In case of rock fill dams, the core is
provided on the upstream face.
SHELL
It provides
structural support for the core and distributes the load over the foundation.
SHELL
It provides structural support for the core and distributes the load over the foundation. The dams which are constructed of the same materials are called
homogeneous dams.
In core dams, a transition filter between the
core and shell
In core dams, a transition filter between the core and shell is generally provided to
prevent the migration of the fine grained core materials into the pores of the coarse grained shell materials.
An internal drain is provided on
the downstream side of the dam
INTERNAL DRAIN
An internal drain is provided on the downstream side of the dam to
carry away the seepage, and also to prevent the saturation of the upper part of the downstream shell by rain on the dam.
A toe drain is provided at
the downstream face of the shell
A toe drain is provided at the downstream face of the shell. A grating is provided to
cover the upstream face to prevent erosion or wash by waves
Construction Materials— the materials used for construction of dams include
earth, rock, tailings from mining, concrete, steel, and any combination of those materials.
Embankment Dams—embankment dams, the most common type in use
today
Embankment Dams—embankment dams, the most common type in use today, have the general shape shown in Figure 1. Their side slopes typically have
a grade of two to one (horizontal to vertical) or flatter
. Embankment Dams—embankment dams, the most common type in use today, have the general shape shown in Figure 1. Their side slopes typically have a grade of two to one (horizontal to vertical) or flatter. Their capacity for water retention is due to
the low permeability of the entire mass (in the case of a homogeneous embankment) or of a zone of low-permeability material (in the case of a zoned embankment dam).
Embankment Dams—embankment dams, the most common type in use today, have the general shape shown in Figure 1. Their side slopes typically have a grade of two to one (horizontal to vertical) or flatter. Their capacity for water retention is due to the low permeability of the entire mass (in the case of a homogeneous embankment) or of a zone of low-permeability material (in the case of a zoned embankment dam). Materials used for embankment dams include
natural soil or rock obtained from borrow areas or nearby quarries, or waste materials obtained from mining or milling
. Embankment Dams—embankment dams, the most common type in use today, have the general shape shown in Figure 1. Their side slopes typically have a grade of two to one (horizontal to vertical) or flatter. Their capacity for water retention is due to the low permeability of the entire mass (in the case of a homogeneous embankment) or of a zone of low-permeability material (in the case of a zoned embankment dam). Materials used for embankment dams include natural soil or rock obtained from borrow areas or nearby quarries, or waste materials obtained from mining or milling. If the natural material has a high permeability, then
a zone of very low-permeability material must be included in the dam to retain water
Materials used for embankment dams include natural soil or rock obtained from borrow areas or nearby quarries, or waste materials obtained from mining or milling. If the natural material has a high permeability, then a zone of very low-permeability material must be included in the dam to retain water. An embankment dam is termed an
“earthfill” or “rockfill” dam
If the natural material has a high permeability, then a zone of very low-permeability material must be included in the dam to retain water. An embankment dam is termed an “earthfill” or “rockfill” dam depending on
whether it is composed mostly of compacted earth or mostly of compacted or dumped pervious rock.
If the natural material has a high permeability, then a zone of very low-permeability material must be included in the dam to retain water. An embankment dam is termed an “earthfill” or “rockfill” dam depending on whether it is composed mostly of compacted earth or mostly of compacted or dumped pervious rock. The ability of an embankment dam to resist the hydrostatic pressure caused by
reservoir water
An embankment dam is termed an “earthfill” or “rockfill” dam depending on whether it is composed mostly of compacted earth or mostly of compacted or dumped pervious rock. The ability of an embankment dam to resist the hydrostatic pressure caused by reservoir water is primarily the result of
the mass, weight, and strength of its materials.
concrete dams may be categorized into
gravity and arch dams
concrete dams may be categorized into gravity and arch dams according to
the designs used to resist the stress due to reservoir water pressure.
Concrete Dams — concrete dams may be categorized into gravity and arch dams according to the designs used to resist the stress due to reservoir water pressure. A concrete gravity dam (shown in Figure 2.) is the
most common form of concrete dam.
Concrete Dams — concrete dams may be categorized into gravity and arch dams according to the designs used to resist the stress due to reservoir water pressure. A concrete gravity dam (shown in Figure 2.) is the most common form of concrete dam. In it, the mass weight of the concrete and friction resist
the reservoir water pressure.
A recently developed method for constructing concrete gravity dams involves the use of
a relatively weak concrete mix which is placed and compacted in a manner similar to that used for earthfill dams.
A recently developed method for constructing concrete gravity dams involves the use of a relatively weak concrete mix which is placed and compacted in a manner similar to that used for earthfill dams. Roller-compacted concrete has the advantages of
decreased cost and time
A recently developed method for constructing concrete gravity dams involves the use of a relatively weak concrete mix which is placed and compacted in a manner similar to that used for earthfill dams. Roller-compacted concrete has the advantages of decreased cost and time. In addition, there are no joints where
seepage could occur
An arch dam is
a solid dam made of concrete that is curved upstream in plan
An arch dam is a solid dam made of concrete that is curved upstream in plan. The arch dam is designed so that the force of the water against it, known as
hydrostatic pressure
An arch dam is a solid dam made of concrete that is curved upstream in plan. The arch dam is designed so that the force of the water against it, known as hydrostatic pressure, presses against
against the arch, compressing and strengthening the structure as it pushes into its foundation or abutments.
various construction techniques could be used in a single dam. For example, a dam could include an earthen or
rockfill embankment as well as a portion made of concrete.
various construction techniques could be used in a single dam. For example, a dam could include an earthen or rockfill embankment as well as a portion made of concrete. A recent design for low-head dams (with a ……………………………..) uses
(with a minimal height of water behind the dam) uses inflatable rubber or plastic materials anchored at the bottom by a concrete slab.
A recent design for low-head dams (with a minimal height of water behind the dam) uses inflatable rubber or plastic materials anchored at the bottom by a concrete slab. Some dams are constructed for special purposes, such as
diversion of water, or permit construction of other facilities in river valleys.
Some dams are constructed for special purposes, such as diversion of water, or permit construction of other facilities in river valleys. These dams are called
diversion dams and cofferdams, respectively
Because the purpose of a dam is to retain water effectively and safely, its water-retention ability is of prime importance. Water may pass from the reservoir to the downstream side of a dam by:
(1) Seeping through the dam.
(2) Seeping through the abutments.
(3) Seeping under the dam.
(4) Overtopping the dam.
(5) Passing through the outlet works.
(6) Passing through or over a service (primary) spillway
(7) Passing over an emergency spillway.
The first three modes are considered undesirable, particularly if
the seepage is not limited in area or volume
The first three modes are considered undesirable, particularly if the seepage is not limited in area or volume. Water normally leaves a dam by
passing through an outlet works or a service spillway
The first three modes are considered undesirable, particularly if the seepage is not limited in area or volume. Water normally leaves a dam by passing through an outlet works or a service spillway; it should pass over
an emergency spillway only during periods of very high reservoir levels and high water inflow
Seepage through a dam—all embankment dams and most concrete dams allow
some seepage.
Seepage through a dam—all embankment dams and most concrete dams allow some seepage. The earth or other material used to construct embankment dams has
some permeability
Seepage through a dam—all embankment dams and most concrete dams allow some seepage. The earth or other material used to construct embankment dams has some permeability, and water under pressure from the reservoir will eventually
seep through
Seepage through a dam—all embankment dams and most concrete dams allow some seepage. The earth or other material used to construct embankment dams has some permeability, and water under pressure from the reservoir will eventually seep through. However, it is important to
control the quantity of seepage
Seepage through a dam—all embankment dams and most concrete dams allow some seepage. The earth or other material used to construct embankment dams has some permeability, and water under pressure from the reservoir will eventually seep through. However, it is important to control the quantity of seepage by using
low-permeability materials in construction and by channeling and restricting the flow
Seepage through a dam—all embankment dams and most concrete dams allow some seepage. The earth or other material used to construct embankment dams has some permeability, and water under pressure from the reservoir will eventually seep through. However, it is important to control the quantity of seepage by using low-permeability materials in construction and by channeling and restricting the flow so that embankment materials
do not erode
. Seepage through a dam—all embankment dams and most concrete dams allow some seepage. The earth or other material used to construct embankment dams has some permeability, and water under pressure from the reservoir will eventually seep through. However, it is important to control the quantity of seepage by using low-permeability materials in construction and by channeling and restricting the flow so that embankment materials do not erode. Seepage through a concrete dam is usually
minimal
Seepage through a concrete dam is usually minimal and is almost always through
joints between blocks, or through cracks which may have developed
Seepage through a concrete dam is usually minimal and is almost always through joints between blocks, or through cracks which may have developed. Maintenance of these joints and cracks is
therefore essential
Seepage through a concrete dam is usually minimal and is almost always through joints between blocks, or through cracks which may have developed. Maintenance of these joints and cracks is therefore essential. The seepage water should be
collected and channelized
Seepage through a concrete dam is usually minimal and is almost always through joints between blocks, or through cracks which may have developed. Maintenance of these joints and cracks is therefore essential. The seepage water should be collected and channelized, so that its quantity can be
measured and erosion minimized
Seepage around a dam
seepage under a dam, through the dam foundation material, or around the ends of a dam
Seepage around a dam—seepage under a dam, through the dam foundation material, or around the ends of a dam through the
abutment materials
. Seepage around a dam—seepage under a dam, through the dam foundation material, or around the ends of a dam through the abutment materials may become
a serious problem if the flow is large or of sufficient velocity to cause erosion.
Seepage around a dam—seepage under a dam, through the dam foundation material, or around the ends of a dam through the abutment materials may become a serious problem if the flow is large or of sufficient velocity to cause erosion. Seepage under a dam also creates
high hydrostatic uplift (pore-water) pressure
Seepage around a dam—seepage under a dam, through the dam foundation material, or around the ends of a dam through the abutment materials may become a serious problem if the flow is large or of sufficient velocity to cause erosion. Seepage under a dam also creates high hydrostatic uplift (pore-water) pressure, which has the effect of
diminishing the weight of the dam
Seepage around a dam—seepage under a dam, through the dam foundation material, or around the ends of a dam through the abutment materials may become a serious problem if the flow is large or of sufficient velocity to cause erosion. Seepage under a dam also creates high hydrostatic uplift (pore-water) pressure, which has the effect of diminishing the weight of the dam, making it
less stable
Seepage around a dam—seepage under a dam, through the dam foundation material, or around the ends of a dam through the abutment materials may become a serious problem if the flow is large or of sufficient velocity to cause erosion. Seepage under a dam also creates high hydrostatic uplift (pore-water) pressure, which has the effect of diminishing the weight of the dam, making it less stable. Seepage through abutments or foundations can
dissolve the constituents of certain rocks
Seepage around a dam—seepage under a dam, through the dam foundation material, or around the ends of a dam through the abutment materials may become a serious problem if the flow is large or of sufficient velocity to cause erosion. Seepage under a dam also creates high hydrostatic uplift (pore-water) pressure, which has the effect of diminishing the weight of the dam, making it less stable. Seepage through abutments or foundations can dissolve the constituents of certain rocks
such as limestone or gypsum
Seepage around a dam—seepage under a dam, through the dam foundation material, or around the ends of a dam through the abutment materials may become a serious problem if the flow is large or of sufficient velocity to cause erosion. Seepage under a dam also creates high hydrostatic uplift (pore-water) pressure, which has the effect of diminishing the weight of the dam, making it less stable. Seepage through abutments or foundations can dissolve the constituents of certain rocks such as limestone or gypsum so that any cracks or joints in the rock become
progressively larger and in turn allow more seepage.
Abutment or foundation seepage may also result in
“piping” internal erosion
Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is
fast enough to erode away small particles of soil
Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is fast enough to erode away small particles of soil. This erosion progresses from the water exit
point backward to the entrance point
Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is fast enough to erode away small particles of soil. This erosion progresses from the water exit point backward to the entrance point. When that point is reached, water may then flow
without restriction
Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is fast enough to erode away small particles of soil. This erosion progresses from the water exit point backward to the entrance point. When that point is reached, water may then flow without restriction, resulting in even
greater erosion and probable dam failure
Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is fast enough to erode away small particles of soil. This erosion progresses from the water exit point backward to the entrance point. When that point is reached, water may then flow without restriction, resulting in even greater erosion and probable dam failure. Obviously, large, unrestricted seepage is
undesirable
Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is fast enough to erode away small particles of soil. This erosion progresses from the water exit point backward to the entrance point. When that point is reached, water may then flow without restriction, resulting in even greater erosion and probable dam failure. Obviously, large, unrestricted seepage is undesirable. To minimize this possibility, dams are constructed with
internal impermeable barriers and internal drainage facilities
Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is fast enough to erode away small particles of soil. This erosion progresses from the water exit point backward to the entrance point. When that point is reached, water may then flow without restriction, resulting in even greater erosion and probable dam failure. Obviously, large, unrestricted seepage is undesirable. To minimize this possibility, dams are constructed with internal impermeable barriers and internal drainage facilities such as
drainpipes or filter systems, or other drainage systems
Abutment or foundation seepage may also result in “piping” internal erosion, in which the flow of water is fast enough to erode away small particles of soil. This erosion progresses from the water exit point backward to the entrance point. When that point is reached, water may then flow without restriction, resulting in even greater erosion and probable dam failure. Obviously, large, unrestricted seepage is undesirable. To minimize this possibility, dams are constructed with internal impermeable barriers and internal drainage facilities such as drainpipes or filter systems, or other drainage systems such as
toe, blanket, or chimney drains
In summary, the overall water retention ability of a dam depends on its
permeability, the abutments, the foundation, and the efforts made to reduce that permeability or restrict the flow of water through these components.
Intentional release of water, as stated earlier, is confined to water releases through
a service spillway or outlet works or over emergency spillways
Service (principal) or mechanical spillway—the service (principal) or mechanical spillway maintains
the normal water level in the reservoir
Service (principal) or mechanical spillway—the service (principal) or mechanical spillway maintains the normal water level in the reservoir. Its function is to
pass expected flood flows past the dam safely and without erosion
Service (principal) or mechanical spillway—the service (principal) or mechanical spillway maintains the normal water level in the reservoir. Its function is to pass expected flood flows past the dam safely and without erosion. It may consist of a
pipe through the dam or a system of gates
Service (principal) or mechanical spillway—the service (principal) or mechanical spillway maintains the normal water level in the reservoir. Its function is to pass expected flood flows past the dam safely and without erosion. It may consist of a pipe through the dam or a system of gates that discharge water over
the top into a concrete spillway.
Drawdown facility—all dams should have some type of drawdown facility which can:
- Quickly lower the water level if failure of the dam is imminent.
- • Serve the operational purposes of the reservoir.
- • Lower the water level for dam repairs.
- • Purposely fluctuate the pool level to kill weeds and mosquitoes.
The valve regulating the drawdown facility should be on the
upstream end of the conduit
The valve regulating the drawdown facility should be on the upstream end of the conduit to
minimize the risk to the dam
The valve regulating the drawdown facility should be on the upstream end of the conduit to minimize the risk to the dam posed by
a possible internal rupture of the pipe.
Emergency (auxiliary) spillway — as the name implies, an emergency spillway functions during emergency conditions to
prevent overtopping of a dam
Emergency (auxiliary) spillway — as the name implies, an emergency spillway functions during emergency conditions to prevent overtopping of a dam. A typical emergency spillway is an
excavated channel in earth or rock near one abutment of a dam
Emergency (auxiliary) spillway — as the name implies, an emergency spillway functions during emergency conditions to prevent overtopping of a dam. A typical emergency spillway is an excavated channel in earth or rock near one abutment of a dam. An emergency spillway should always discharge
away from the toe of a dam to avoid its erosion.
Emergency (auxiliary) spillway — as the name implies, an emergency spillway functions during emergency conditions to prevent overtopping of a dam. A typical emergency spillway is an excavated channel in earth or rock near one abutment of a dam. An emergency spillway should always discharge away from the toe of a dam to avoid its erosion. Furthermore, the spillway should be constructed in such a manner that the spillway itself will
not seriously erode when it is in use
Emergency (auxiliary) spillway — as the name implies, an emergency spillway functions during emergency conditions to prevent overtopping of a dam. A typical emergency spillway is an excavated channel in earth or rock near one abutment of a dam. An emergency spillway should always discharge away from the toe of a dam to avoid its erosion. Furthermore, the spillway should be constructed in such a manner that the spillway itself will not seriously erode when it is in use. Obviously, erosional failure of the spillway could be as catastrophic as
failure of the dam itself
