Dams and
Reservoirs
Overview of
Dams and Reservoirs
By Amanda Briney
A dam is any barrier that
holds back water; dams are primarily used to save, manage, and/or prevent the
flow of excess water into specific regions.
In addition, some dams are
used to generate hydropower.
This article examines
man-made dams but dams can also be created by natural causes like mass wasting
events or even animals like the beaver.
Another
term often used when discussing dams is reservoir.
A reservoir is a man-made lake that is primarily used for
storing water.
They
can also be defined as the specific bodies of water formed by the construction
of a dam.
For
example, the Hetch Hetchy Reservoir in California’s Yosemite National Park is
the body of water created and held back by the O’Shaughnessy Dam.
Types of Dams
Today,
there are several different types of dams and the man-made ones are classified
by their size and structure.
Typically a large dam is
classified as being higher than 50-65 feet (15-20 meters) while major dams are
those over 492-820 feet (150-250 meters).
One of the most common types
of major dams is the arch dam.
These masonry or concrete
dams are ideal for narrow and/or rocky locations because their curved shape
easily holds back water via gravity without the need for a lot of construction
materials.
Arch dams can have one large
single arch or they can have multiple small arches separated by concrete
buttresses.
The Hoover Dam which is on the border of the U.S. states of
Arizona and Nevada is an arch dam.
Another
type of dam is the buttress dam. These can have multiple arches, but unlike a
traditional arch dam, they can be flat as well.
Normally buttress dams are
made of concrete and feature a series braces called buttresses along the
downstream side of the dam to prevent the natural flow of water.
The Daniel-Johnson Dam in Quebec, Canada is a multiple arch
buttress dam.
In
the U.S., the most common type of dam is the embankment dam.
These are large dams made out
of soil and rock which use their weight to hold back water.
To prevent water from moving
through them, embankment dams also have a thick waterproof core.
The Tarbela Dam in Pakistan
is the world’s largest embankment dam.
Finally,
gravity dams are huge dams that are constructed to hold back water using only
their own weight.
To do this, they are
constructed using extensive amounts of concrete, making them difficult and
expensive to build.
The Grand Coulee Dam in
the U.S. state of Washington is a gravity dam.
Types of
Reservoirs and Construction
Like
dams, there are different types of reservoirs as well but they are classified
based on their use.
.
The three types are called: a valley dammed
reservoir, a bank-side reservoir, and a service reservoir.
.
Bank-side reservoirs are those formed when water
is taken from an existing stream or river and stored in a nearby reservoir.
.
Service reservoirs are mainly constructed to
store water for later use. They often appear as water towers and other elevated
structures.
The first and usually largest type of reservoir
is called a valley dammed reservoir.
.
These are reservoirs that are located in narrow valley areas where
tremendous amounts of water can be held in by the valley’s sides and a dam.
The
best location for a dam in these types of reservoirs is where it can be built
into the valley wall most effectively to form a water tight seal.
To
construct a valley dammed reservoir, the river must be diverted, usually
through a tunnel, at the start of work.
The first step in creating
this type of reservoir is the pouring of a strong foundation for the dam, after
which construction on the dam itself can begin.
These steps can take months
to years to complete, depending on the size and complexity of the project.
Once finished, the diversion
is removed and the river is able to flow freely toward the dam until it
gradually fills the reservoir.
Dam Controversy
In
addition to the high cost of construction and river diversion, dams and
reservoirs are often controversial projects because of their social and
environmental impacts.
.
Dams themselves affect many
different ecological components of rivers such as fish migrations, erosion,
changes in water temperature and therefore changes in oxygen levels, creating
inhospitable environments for many species.
In addition, the creation of
a reservoir requires the flooding of large areas of land, at the expense of the
natural environment and sometimes villages, towns and small cities.
The construction of China’s
Three Gorges Dam, for example, required the relocation of over one million
people and flooded many different archaeological and cultural sites.
Main Uses of Dams and Reservoirs
Despite
their controversy, dams and reservoirs serve a number of different functions
but one of the largest is to maintain an area’s water supply.
.
Many of the world’s largest
urban areas are supplied with water from rivers that are blocked via dams.
San Francisco, California for
example, gets the majority of its water supply from the Hetch Hetchy Reservoir
via the Hetch Hetchy Aqueduct running from Yosemite to the San Francisco Bay
Area.
Another major use of dams is
power generation as hydroelectric power is one of the world’s major sources of
electricity.
Hydropower is generated when
the potential energy of the water on the dam drives a water turbine which
in then turns a generator and creates electricity.
To best make use of the
water’s power, a common type of hydroelectric dam uses reservoirs with
different levels to adjust the amount of energy generated as it is needed.
When demand is low for
instance, water is held in an upper reservoir and as demand increases, the
water is released into a lower reservoir where it spins a turbine.
Some
other important uses of dams and reservoirs include a stabilization of water
flow and irrigation, flood prevention, water diversion and recreation.
Amanda Briney has a Master of Arts degree in geography from
California State University, East Bay (CSUEB) and Bachelor of Arts degrees in
both English and geography from California State University, Sacramento (CSUS).
Amanda previously served as the first Geography Intern for Geography at
About.com and her first article for the website was the “Geography of Coffee.”
Experience
As a
geographer, Amanda loves to travel. Some of her favorite places are the Pacific
Northwest, California’s Sierra Nevada, Idaho and Utah. She also enjoys hiking,
camping and kayaking and loves visiting our nation’s national parks. Amanda has
recently been to Crater Lake, Grand Teton, Lassen, Yellowstone, Yosemite and
Zion national parks. She hopes to someday visit Denali, Glacier, and the Grand
Canyon.
Education
Amanda
began her college education at Diablo Valley College in Pleasant Hill,
California where she took courses from across the disciplines for her general
education requirements. Among those courses was an introduction to physical
geography course with a lab and she immediately fell in love with geography
after doing labs focused on topics such as air photo interpretation and
learning about the formation of the Earth's landscapes. Once she transferred to
CSUS however, she continued to major in English because of an interest in
writing, but she kept taking geography courses as well.
After
deciding she wanted to be a professional writer in geography, Amanda declared
herself as a double major in both geography and English. After graduating from
CSUS, Amanda began graduate school at CSUEB to learn more about geography and
fine tune her research and writing skills. She graduated with her M.A. degree
in 2012.
Amanda's
favorite subjects within the geography discipline are biogeography, urban
planning, and climate/meteorology.
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