Connecticut Water Trails Association
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Connecticut Water Trails Association |
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Connecticut Water Trails Program Dams
How A Dam Works
A typical dam is a wall of solid material built across a river to block
the flow of the river thus storing water in the lake that will form
upstream of the dam as water continues to flow from the river upstream
of the dam.
The main purpose of most dams is to create a permanent reservoir of
water for use at a later time. The dam must be watertight (ie
impermeable or impervious to water) so that water does not leak out of
the dam and escape downstream. An essential part of a dam is therefore
the "impermeable membrane", ie the watertight part of the dam that
prevents water leaking out. As we shall see later, it is not necessary
that the entire dam wall be watertight. The natural earth or rock on
which the dam is built (ie the dam foundation) must also be watertight
as must the river valley in which the storage reservoir forms. If these
natural areas (dam foundation and storage area) are not watertight then
water could leak out of the reservoir even if the dam itself is
watertight.
As well as being watertight a dam must also be stable ie the dam wall
must have sufficient strength to firstly, stand permanently under its
own weight especially when at least part of the dam wall is saturated
with water and secondly, resist the water pressure in the lake upstream
of the dam. This water pressure exerts a force on the dam wall tending
to push it downstream. The higher the dam, the greater the depth of
water stored behind the dam and the greater the water pressure on the
dam wall. The dam must also have sufficient strength to resist other
forces to which it may be subjected from time to time eg shaking from
earthquakes. The threat that earthquakes pose to dams varies widely
depending on the region of the world in which the dam is located.
A dam must have some way of releasing water in controlled amounts as it
is needed i.e. an outlet valve of some type. Depending on the purpose of
the dam the water may be released into a pipeline to supply a city with
water, or into a hydro-electric power station to generate electricity or
the water may simply be released into the river bed downstream of the
dam and allowed to flow naturally down the river, eventually to be
pumped out and used for irrigation of crops further downstream. The
outlet valve must be connected via a pipe or tunnel to some type of
intake structure where the water is actually drawn from the storage
reservoir.
When the river on which the dam has been built floods a very large
volume of flood water will flow into the storage reservoir. Usually this
is very, very much more water than can be released through the outlet
valve. A dam must have some means whereby these large volumes of flood
water can flow around the dam without causing damage to the dam itself; i.e. a spillway which, in most cases, is an open cut channel large enough
to carry the flood water around the dam. If the dam is built of concrete
the spillway may form part of the dam wall itself. However, if the dam
is built of earth and/or rock fill (i.e. soil and broken rock) the
spillway must be a separate structure because flood waters cannot be
allowed to flow over the top of a fill (or embankment) dam which would
be quickly washed away by the flood water if this was to happen.
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