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

16

~

CORE
I!I rl~CREST. ~ [j

0PSTREA~ :iK
SLOPE DOWNSTREAM
CORE SLOPE
TOE
a. HOMOGENEOUS b. ZONED e. ZONED
W IURAIN SYSTEM

a. GRAVEL

FIGURE 3.4-5
TYPICAL CROSS SECTIONS

b. SAND

c.CLAY

FIGURE 3.4-3
MATERIALS INFLUENCE FLOW PATH

The first two sketches represent undesirable conditions
because uncontrolled seepage is exiting on the down-
stream slope of the dam. The situation develops
because the structure does not provide sufficient resist-
ance to the flow of water. Gravel and sand, because of
their larger grain size, have more area available for
flow than finer-grained material such as clay. Com-
pacting clay soils further reduces the area available for
flow. As the water moves through the soil structure,
some of the pressure which is pushing it through the
dam is dissipated. The path of the water must be made
long enough to use up all the pressure pushing on the
drop at point A by the time the drop reaches point B.

Type:

Homogeneous

Zoned

Zoned w (Drain
System
Drain system of
granular material
allows the con-
trolled exit of
seepage.

Characteristics

Constructed of
one material.

Constructed of
two or more
materials. Central
core or midsec-
tion has greater
resistance to flow.

FIGURE 3.4-4
RESISTANCE TO FLOW

We have seen how the grain size ofthe material and the
amount of compaction influences the area available for
flow. For a given material, the length of the flow path
will determine the amount of water that flows through
as seepage. In the next section we will see how various
configurations are used to control seepage, allow the
dam to maintain its shape, and prevent downstream
movement.

The three different cross sections are designed
from knowledge of the principles used to con-
trol the path of the water through the dam:

1. Selection of material by grain size.

2. Compaction.

3. Elongation of the path by increasing the
base width.

The homogeneous dam, for example, controls
the flow of water by lengthening the path. The
path is lengthened by increasing the base width;
however, without drains seepage can exit the
dam above the toe. The zoned dam provides
control ofthe flow of water with a core having a
very high resistance to flow.

The sketches below show the difference in the
paths of water between a homogeneous dam
and a zoned dam.
HEIGHT
VARIES
UP TO 1/3 THE
RESERVOIR
STORAGE HEIGHT.:
I

TOE

.......i!!
.=tl
'"
PRESSURE

'.~

__ B RESISTANCE TO FLOW
- - BY SOIL STRUCTURE

ia. HOMOGENEOUS

TOE

b. ZONED

3.4-1 TYPICAL CONFIGURATIONS
The selection of shape and materials allows an
~ffective dam to be constructed of locally avail-
able material. The following cross-sections repre-
sent three most common earth embankments.

FIGURE 3.4-.
TYPE OF CROSS SECTION INFLUENCES FLOW PATH

The middle section (core) ofthe zoned dam has
more resistance to flow and dissipates the pres-
sure caused by the reservoir in a short distance.

In the case ofthe homogeneous dam, the height
of visible seepage water near the toe can exit as
high as 1/3 the height of the reservoir water
surface. The appearance of water on the down-
stream slope can endanger the safety of the
structure. Therefore, every effort is made in
design of a dam to minimize the chance for
water to exit on the downstream face.

In the case of the zoned dam with drain, a
coarse-grained material is installed which has
little resistance to the flow of water.