**DAM'S**

***Types and Considerations***

A **dam** (a common Teutonic word, cf. Swedish and German *damm*, and
the Gothic verb *faurdammjan*, to block up) is a barrier built across
flowing water in order to hold it back, often creating a water reservoir
or lake behind the dam. Dams may be built to provide water for
irrigation or town water supply, control the amount of water in rivers
or to provide hydroelectric power. Dams may also be built to control
effluent from industrial work sites such as miness or factories.

The best place for building a dam is a narrow part of a deep river
valley; the valley sides can then act as natural walls. The primary
function of the dam\'s structure is to fill the gap in the natural
reservoir line left by the stream channel. The most desirable sites are
usually those where the gap becomes a minimum for the required storage
capacity. The best economical arrangement is often a composite structure
such as a masonry dam flanked by earth embankments. The current use of
the land to be flooded should be dispensable.

The dam also serves to divert water to the intake works.

Dams may be classified according to their height. A *large dam* is used
to describe a dam higher than 15 metres, a *major dam* for constructions
over 150 metres.

Dams may be classified as follows:

1\. Timber dams

2\. Rock-fill dams

3\. Earth dams

1\. plain

2\. core wall

3\. hydraulic fill

4\. Masonry dams

1\. gravity, solid and hollow

2\. arch, single and multiple

The timber dam is rarely used by humans because of its short lifespan
and the limitation in height to which it can be built. The locations
where timber dams are most economical to build are those where timber is
plentiful, cement is costly and difficult to transport, and only a
submerged diversion dam is required. Timber is the basic material used
by beavers, often with the addition of mud or stones.

Rock-fill dams are embankments of loose rock with either a watertight
upstream face of concrete slabs or timber or a watertight core. Where
suitable rock is at hand, a minimum of transportation of materials can
be realized with this type of dam. Like the earth embankment, rock-fill
dams resist damage from earthquakes quite well.

Handout No. Four

Earth dams are constructed as a simple homogeneous embankment of
well-compacted earth, sometimes with a watertight concrete core or
upstream face, or sometimes with a hydraulic [fill]{.underline} to
produce a watertight core. A type of temporary earth dam occasionally
used in high lattitudes is the frozen-core dam, in which a coolant is
circulated through pipes inside the dam to maintain a watertight region
of permafrost within it.

Masonry dams are of either the gravity or the arch type. In a gravity
dam, stability is secured by making it of such a size and shape that it
will resist overturning, sliding and crushing at the toe. The dam will
not overturn provided the resultant force falls within the base.
However, in order to prevent tension at the upstream face and excessive
compression at the downstream face, the dam cross section is usually
designed so that the resultant falls within the middle third at all
elevations of the cross section. In the arch dam stability is obtained
by a combination of arch and gravity action. If the upstream face is
vertical the entire weight of the dam must be carried to the foundation
by gravity, while the distribution of the normal hydrostatic pressure
between vertical cantilever and arch action will depend upon the
stiffness of the dam in a vertical and horizontal direction. When the
upstream face is sloped the distribution is more complicated. The normal
component of the weight of the arch ring may be taken by the arch
action, while the normal hydrostatic pressure will be distributed as
described above. Hence, for the gravity type, good impervious
foundations are essential, but for the arch type, firm reliable supports
at the abutments (either buttress or canyon side wall) are more
important. The most desirable place for an arch dam is a narrow canyon
with steep side walls composed of sound rock. When situated on a
suitable site, a gravity dam inspires more confidence in the layman than
any other type; it has mass that lends an atmosphere of permanence,
stability, and safety. When built on a carefully studied foundation with
stresses calculated from completely evaluated loads, the gravity dam
probably represents the best developed example of the art of dam
building. This is significant because the fear of flood is a strong
motivator in many regions, and has resulted in gravity dams being built
in some instances where an arch dam would have been more economical.

Gravity dams are classified as \"solid\" or \"hollow.\" The solid form
is the more widely used of the two, though the hollow dam is frequently
more economical to construct. Gravity dams can also be classified as
\"overflow\" and \"non-overflow.\" If the dam is meant to serve as a
spillway section, its downstream face is ordinarily made an ogee curve
with the curvature such that there will be no tendency of the water to
leave the surface of the concrete, even with the maximum water elevation
at the crest.

Two types of single-arch dams are in use, namely the constant-angle and
the constant-radius dam. The constant-radius type employs the same face
radius at all elevations of the dam, which means that as the channel
grows narrower towards the bottom of the dam the central angle subtended
by the face of the dam becomes smaller. In a constant-angle dam, this
subtended angle is kept a constant and the variation in distance between
the abutments at various levels is are taken care of by varying the
radii. The safety of an arch dam is dependent on the strength of the
side wall abutments, hence not only should the arch be well seated on
the side walls but also the character of the rock should be carefully
inspected. The multiple-arch dam consists of a number of single-arch
dams with concrete buttresses as the supporting abutments. The
multiple-arch dam does not require as many buttresses as the hollow
gravity type, but requires good rock foundation because the buttress
loads are heavy.

Significant engineering considerations when building a dam include
permeability of the surrounding rock or soil, earthquake faults, peak
flood flows, reservoir silting, environmental impacts on river
fisheries, forests and wildlife, impacts on human habitations and
compensation for land being flooded as well as population resettlement,
and removal of toxic materials and buildings from the proposed reservoir
area.

Dam failures are generally catastrophic if the structure is breached or
significantly damaged. Routine monitoring of seepage from drains in, and
around, larger dams is necessary to anticipate any problems and permit
remedial action to be taken before structural failure occurs. Most dams
incorporate mechanisms to permit the resevoir to be lowered or even
drained in the event of such problems. Another solution can be rock
grouting - pumping cement slurry into weak fractured rock under
pressure.

Handout No. Three