FLOOD05005

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: State Of Colorado Dam Safety Manual
• Date: 6/1/1983
• Prepared By: Colorado State Engineers Office
• Floodplain - Doc Type: Educational/Technical/Reference Information

15

3.3

in some cases to produce a useful storage facil-
ity. Many factors are involved in selecting the
dam site.

The dam site must be able to:

1. Hold up the dam.

2. Hold the dam in place.

3. Hold back water.

Many times the type of dam selected is a result
of the site conditions. Even though the best
efforts are employed in site selection, design,
and construction, all dams will experience
some seepage.

Three types of dams are discussed in this chap-
ter: earth, rocklill, and concrete. The discussion
includes typical sections identifying the parts of
each, the role of each part, and how each type of
dam functions to achieve the primary purpose
of holding back water safely.

REVIEW
The following list is provided for easy review and quick
reference when looking at the details of the three types
of dams:

Required Functions of a Dam
All dams must:

1. Contain the water and resist leakage.
2. Maintain shape.
3. Resist downstream movement.

The foundation, abutments, and reservoir basin must:

1. Hold up the dam.
2. Hold the dam in place.
3. Hold back the water.

Important Physical Principles
1. The greater the depth of water is above any point,
the greater the pressure is at that point.

2. Water travels in streams, drops, and tiny droplets.
Because the water molecu'e is small, special pre-
cautions must be taken to prevent excessive flow of
water through earthen materials.

3. The amount of water that will move through a soil
is determined by:

a. The height of water above the point being
considered.
b. The type of material.
c. The amount of space available for the water to
flow through the soil after compaction of that
material.
d. The length of the flow path.

4. Proper moisture control allows an earthen mate-
rial to be compacted efficiently. Proper compac-
tion results in the fewest paths for seepage and the
greatest strength.

5. Steel, concrete, and asphalt provide almost total
resistance to the flow of water.

3.4

EARTH DAMS
Understanding the physical principle of how water
behaves under pressure is primary to understanding
the behavior of an earth dam. Let us look at a single
drop of water at some depth in the reservoir. The
pressure trying to force the drop of water through the
dam is proportional to the height of water above the
drop. The sketch below shows a droplet of water at
Point A in the reservoir. The pressure forces the drop-
let through the dam to Point B. SECTION
2.

-

==
...
..
..
Q
PRESSURE

......
A '....

FIGURE 3.4-1
SEEPAGE PATH

As the drop of water is pushed horizontally through the
soil of the dam, gravity is also pulling the drop down-
ward. The path is similar to that shown in the above
sketch. Now, if we follow the paths of several drops of
water at different depths in the reservoir, the pattern
traced by the droplets is similar to that shown in the
following sketch.

PRESSURE

DROP

PRESSURE

PRESSURE

FIGURE 3.4-2
FLOW PATH

The sketch above illustrates the concept of seepage
through an earth dam. If the area available for flow in a
given soil structure is changed, the paths will change.
The items that will influence the area available for flow
are material grain size and amount of compaction. If
the material selected has larger grain size, or if it is not
well compacted, the result is an unacceptable amount
of seepage as shown in the following sketch.