Chapter 2 Introduction to Dams

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Chapter 2 Introduction to Dams Powered By Docstoc
					Chapter 2:

Introduction to Dams

2.0 General                                           water flowing is normally controlled, very               ■    how the components function, and
                                                      high runoffs (floods) and very low runoffs               ■  important physical conditions likely to
      The purpose of a dam is to impound
                                                      (drought periods) are avoided.                              affect a dam.
(store) water for any of several reasons,
e.g., flood control, water supply for                                                                               Human-built dams may be classified
humans or livestock, irrigation, energy               2.2 Types of Dams                                        according to the type of construction
generation, recreation, or pollution                       Dams may either be human-built or                   materials used, the methods used in construc-
control. This manual primarily concen-                result from natural phenomena, such as                   tion, their slope or cross-section, the way
trates on earthen dams, which constitute              landslides or glacial deposition. The                    they resist the forces of the water pressure
the majority of structures in place and               majority of dams are human structures                    behind them, the means of controlling
under development in Texas.                           normally constructed of earthfill or                     seepage, and occasionally, their purpose.
                                                      concrete. Naturally occurring lakes may                  A. Components—The components of a
                                                      also be modified by adding a spillway to                    typical dam are illustrated in Figure 2.1.
2.1 The                                               allow for safe, efficient release of excess                 Nearly all dams possess the features
Watershed System                                      water from the resulting reservoir.                         shown or variations of those features.
     Water from rainfall or snowmelt                       Dam owners should be aware of:                         Definitions of the terms are given in the
naturally runs downhill into a stream           ■ the different types of dams                                     Glossary. The various dam components
valley and then into larger streams or other ■ essential components of a dam                                      are discussed in greater detail later on.
bodies of water. The “watershed system”
refers to the drainage process through
                                                                  Figure 2.1
which rainfall or snowmelt is collected
                                                     Parts of an Earthen Dam
into a particular stream valley
during natural runoff (directed by                                                                                              Spillway

gravity). Dams constructed across                                                  Shoreline

such a valley then impound the                                                                                  Natural Ground
                                                                                          Left Abutment Area
runoff water and release it at a
controlled rate. During periods of

high runoff, water stored in the                                                  Wave Protection

reservoir typically increases, and                                                       Riprap

                                                                                                                     Embankment (Fill)

                                                                                                                                                       Groin Area
overflow through a spillway may

                                                                                                                                                        & Riprap

occur. During periods of low                    Spillway Riser
                                                 & Trashrack
runoff, reservoir levels usually                                                                         Chimney Drain & Filter
                                                                                                                        Blanket Drain & Filter
decrease. The owner can normally
control the reservoir level to some      Bottom                               Upstream
                                                                                                          Downstream                Toe Drain
                                                                                                                                     & Filter        Spillway
                                          Drain                                                              Shell                                    Outlet
degree by adjusting the quantity of                                                          Core                                              Toe   Stilling
                                                                Spillway Conduit                                                                      Basin
water released. Downstream from                                Foundation                                       Cut-Off (Core) Trench
the dam, the stream continues to                                                       Impervious Stratum                                            Riprap

exist, but because the quantity of              Source: North Carolina Department of Environmental and Natural Resources (1989).

                                                           Guidelines for Operation and Maintenance of Dams in Texas

B. Construction Materials— The                                                 Figure 2.2
   materials used for construction of dams                                Concrete Gravity Dam
   include earth, rock, tailings from mining
   or milling, concrete, masonry, steel, and
   any combination of tho se materials.                                                     Resistance to Movement
                                                                                             Offered by Abutment
     1. Embankment Dams—Embankment
        dams, the most common type in use
        today, have the general shape shown
        in Figure 2.1. Their side slopes
        typically have a grade of two to one
                                                Pressure of
        (horizontal to vertical) or flatter.     Reservoir
        Their capacity for water retention is
        due to the low permeability of the
        entire mass (in the case of a homoge-
        neous embankment) or of a zone of
        low-permeability material (in the
        case of a zoned embankment dam).         Support Offered                   Resistance to Movement
                                                  by Foundation                  Offered by Key in Foundation
           Materials used for embankment
                                                              Resistance to Movement
       dams include natural soil or rock               (Friction Between Dam & Foundation)
       obtained from borrow areas or
       nearby quarries, or waste materials
       obtained from mining or milling. If        dam is a specific type of gravity dam             A recently developed method for
       the natural material has a high            in which the large mass of concrete is        constructing concrete gravity dams
       permeability, then a zone of very-         reduced, and the forces are diverted          involves the use of a relatively weak
       low-permeability material must be          to the dam foundation through                 concrete mix which is placed and
       included in the dam to retain water.       vertical or sloping buttresses. Gravity       compacted in a manner similar to
                                                  dams are constructed of non-                  that used for earthfill dams. Roller-
           An embankment dam is termed
                                                  reinforced vertical blocks of concrete        compacted concrete has the advan-
       an “earthfill” or “rockfill” dam
                                                  with flexible seals in the joints             tages of decreased cost and time. In
       depending on whether it is com-
       posed mostly of compacted earth or         between the blocks.                           addition, there are no joints where
       mostly of compacted or dumped                  Concrete arch dams are typically          seepage could occur.
       pervious rock.                             rather thin in cross-section (Figure        3. Other Types—Various construction
           The ability of an embankment           2.3). The reservoir water forces               techniques could be used in a single
       dam to resist the hydrostatic pressure     acting on an arch dam are carried              dam. For example, a dam could
       caused by reservoir water is primarily     laterally into the abutments. The              include an earthen or rockfill embank-
       the result of the mass, weight, and        shape of the arch may resemble a               ment as well as a portion made of
       strength of its materials.                 segment of a circle or an ellipse, and         concrete. In such a case, the concrete
     2. Concrete Dams—Concrete dams may           the arch may be curved in the                  section would normally contain the
        be categorized into gravity and arch      vertical plane as well. Such dams are          spillway or other outlet works.
        dams according to the designs used        usually built from a series of thin            A recent design for low-head dams
        to resist the stress due to reservoir     vertical blocks that are keyed                (with a minimal height of water
        water pressure. A concrete gravity        together, with water stops between            behind the dam) uses inflatable
        dam (shown in Figure 2.2) is the          the blocks. Variations of arch dams           rubber or plastic materials anchored
        most common form of concrete              include multi-arch dams, in which             at the bottom by a concrete slab.
        dam. In it, the mass weight of the        more than one curved section is used,          Some dams are constructed for
        concrete and friction resist the          and arch gravity dams, which combine          special purposes, such as diversion of
        reservoir water pressure. A buttress      some features of the two types.               water, or permit construction of

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Guidelines for Operation and Maintenance of Dams in Texas

                                     Figure 2.3                                                 B. Seepage Around a Dam—Seepage
                                 Concrete Arch Dam                                                 under a dam, through the dam
                                                                                                   foundation material, or around the
                                                                                                   ends of a dam through the abutment
                                                    Resistance to Movement                         materials may become a serious
                                                     Offered by Abutment
                                                                                                   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
    Pressure of
     Reservoir                                                                                     weight of the dam, making it less stable.
                                                                                                       Seepage through abutments or
                                                                                                   foundations can dissolve the constitu-
                                                                                                   ents of certain rocks such as limestone,
                                                                                                   dolomite, or gypsum so that any cracks
                                                                                                   or joints in the rock become progres-
          Support Offered                  Resistance to Movement                                  sively larger and in turn allow more
           by Foundation                 Offered by Key in Foundation
                                                                                                   seepage. Abutment or foundation
                                                                                                   seepage may also result in “piping”
     other facilities in river valleys. These   mally leaves a dam by passing through an           internal erosion, in which the flow of
     dams are called diversion dams and         outlet works or a service spillway; it should      water is fast enough to erode away small
     cofferdams, respectively.                  pass over an emergency spillway only               particles of soil. This erosion progresses
                                                during periods of very high reservoir levels       from the water exit point backward to
2.3 Water-                                      and high water inflow.                             the entrance point. When that point is
                                                A. Seepage Through a Dam—All                       reached, water may then flow without
Retention Ability                                  embankment dams and most concrete               restriction, resulting in even greater
     Because the purpose of a dam is to                                                            erosion and probable dam failure.
                                                   dams allow some seepage. The earth or
retain water effectively and safely, its
                                                   other material used to construct                    Obviously, large, unrestricted
water-retention ability is of prime impor-
                                                   embankment dams has some perme-                 seepage is undesirable. To minimize this
tance. Water may pass from the reservoir
                                                   ability, and water under pressure from          possibility, dams are constructed with
to the downstream side of a dam by:
                                                   the reservoir will eventually seep              internal impermeable barriers and
(1) Seeping through the dam.
                                                   through. However, it is important to            internal drainage facilities such as
(2) Seeping through the abutments.                 control the quantity of seepage by using        drainpipes or filter systems, or other
(3) Seeping under the dam.                         low-permeability materials in construc-         drainage systems such as toe, blanket,
(4) Overtopping the dam.                           tion and by channeling and restricting          or chimney drains.
(5) Passing through the outlet works.              the flow so that embankment materials               Flow through a dam foundation
(6) Passing through or over a service              do not erode.                                   may be diminished by grouting known
   (primary) spillway.                                 Seepage through a concrete dam is           or suspected highly permeable material,
(7) Passing over an emergency spillway.            usually minimal and is almost always            constructing a cutoff wall or trench
     The first three modes are considered          through joints between blocks, or               below a dam, or constructing an
undesirable, particularly if the seepage is        through cracks or deteriorated concrete         upstream impermeable blanket. Figure
not limited in area or volume. Overtop-            which may have developed. Mainte-               2.1 illustrates a cutoff trench.
ping of an embankment dam is also very             nance of these joints and cracks is                  In summary, the overall water
undesirable because the embankment                 therefore essential. The seepage water          retention ability of a dam depends on
material may be eroded away. Additionally,         should be collected and channelized, so         its permeability, the abutments, the
only few concrete dams have been                   that its quantity can be measured and           foundation, and the efforts made to
designed to be overtopped. Water nor-              erosion minimized.                              reduce that permeability or restrict the

T E X A S C O M M I S S I O N O N E N V I R O N M E N TA L Q U A L I T Y                                                                  11
                                                             Guidelines for Operation and Maintenance of Dams in Texas

     flow of water through these compo-          reservoir reaches a certain level, water     spillway functions during emergency
     nents. Should high permeability occur,      flows into a standpipe or riser pipe or      conditions to prevent overtopping of a
     seepage can lead to piping, which will      over a gate. Intake structures for           dam. A typical emergency spillway is an
     likely result in failure.                   spillways must have systems that             excavated channel in earth or rock near
                                                 prevent clogging by trash or debris.         one abutment of a dam. An emergency
                                                                                              spillway should always discharge away
2.4 Release                                    B. Drawdown Facility—All dams should
                                                                                              from the toe of a dam to avoid its
                                                  have some type of drawdown facility
of Water                                          which can:                                  erosion. Furthermore, the spillway
     Intentional release of water, as stated                                                  should be constructed in such a manner
                                                 • Quickly lower the water level if
earlier, is confined to water releases                                                        that the spillway itself will not seriously
                                                   failure of the dam is imminent.
through a service spillway or outlet works                                                    erode when it is in use. Obviously,
                                                 • Serve the operational purposes of the
or over emergency spillways.                                                                  erosional failure of the spillway could
A. Service (Principal) or Mechanical                                                          be as catastrophic as failure of the dam
                                                 • Lower the water level for dam repairs.
   Spillway—The service (principal) or                                                        itself. An emergency spillway should be
   mechanical spillway maintains the             • Purposely fluctuate the pool level to      sized to convey the so-called “design
   normal water level in the reservoir. Its        kill weeds and mosquitoes.                 flood”—the rare, large-magnitude flood
   function is to pass expected flood flows          The valve regulating the drawdown        used to establish design criteria. The
   past the dam safely and without               facility should be on the upstream end       spillways of many existing dams are
   erosion. It may consist of a pipe             of the conduit to minimize the risk to       now considered undersized because
   through the dam or a system of gates          the dam posed by a possible internal         standards for the design flood have
   that discharge water over the top into a      rupture of the pipe     .                    increased over the years.
   concrete spillway . Either method uses      C. Emergency (Auxiliary) Spillway—As
   the overflow principle. When the               the name implies, an emergency

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