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Lecture 5

EARTH SCIENCE 2WW3-lecture 5 Dams & Increasing Supplies.pdf


Department
Earth Sciences
Course Code
EARTHSC 2WW3
Professor
Luc Bernier
Lecture
5

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Dams & Increasing Supplies
1) Which province is currently facing opposition against a proposal for a dam and
hydroelectric generating station, built on its Peace River?
a. Alberta
b. Quebec
c. New Brunswick
d. British Columbia
Dams & Unintended Impacts
Orange are negative green are positive
Water in the reservoir can evaporate more easily
There is a net loss because water taken for irrigation is not usually returned to the river
World Commissions on Dams estimate that about 60% of effects of dams are unintended
by planners
Part 1 - Dams: A Few Fundamentals
Dams: Tools of Water Engineering
Dams such as the Condit Dam in Washington State (~1910) have great historical
significance
They have allowed irrigated agriculture by storing water in periods of god supply
therefore converting flood hazard into a usable resource by accumulating river flows until
further needed
They have also contributed to urban development
In the modern world they are essential developments large-scale water diversions

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Dams & Increasing Supplies
Structure of a Dam
The upstream face is facing the reservoir where the water is being stored
The downstream face is pointing towards the direction the water would normally follow
as it is flowing down the river
The maximum upward limit is the crest of the dam
If water would flow above the crest this would lead to conditions of inundation
To regulate the flow of water adjustments can be made through the principle spillway
however under conditions where the volume of water in the reservoir would be
exceptional it could lead to inundation and water will follow the left and right abutment
and flow down the emergency spillway and flow in the normal direction of the river.
Principal Storage Zones in Reservoir
Under normal river discharge conditions the volume of water stored in the reservoir will
be found in the active storage with the minimum level of water corresponding to the
minimum pool level
Under higher discharge conditions the excess water that is found in the reservoir will
correspond to the flood pool
Under these conditions the volume of water in the reservoir can be controlled through the
principle outlet
Under exceptional flood conditions (high river discharge) the maximum level of water
that can be stored in the reservoir will correspond to the maximum water surface and
approximately to the freeboard of the crest of the dam
To prevent any additional water to create inundation downstream by spilling over the top
of the dam the volume of water can be controlled through the emergency spill way
Rivers with high sediments loads have high dead storage. That is sediments carried by the
river settle out of suspension once they encounter the wall of the dam and fall at the
bottom of the reservoir therefore reducing the active storage and the volume of water that
can be stored in a reservoir
Dead storage is filled by sediments

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Dams & Increasing Supplies
Dams Worldwide
This map illustrates the location of some of the major dams worldwide
Halfoftheworld’srivershaveoneormorelargedams
The proportion is even greater if we consider only the largest rivers
Then2/3oftheworld’srivers have one or more large dams
More than 45,000 large dams are found in over 150 countries
Large dams are under construction in China, Turkey, Japan and Iran
Large dams store around 1/6oftheworld’sriverflows
Role of Dams
Irrigation is the primary functionofhalfoftheworld’sdams
As a result they are responsible for 12-16% of the world food supplies
271 million hectares of irrigated lands depends on these dams
Therefore 30-40% of irrigated lands worldwide depend on dams for their water supply
1/3oftheworld’sdamsareusedforhydropowergeneration
This means some dams are used both for hydropower generation and irrigation
The remainder of dams can be used for public water supply
Part 2 - Dams and Controversies
Dams and River Regulation
The last 50 years dams have become an important tool is water management
Used to control floods, build the water, and release it later after the flood has passed
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