Those Dammed Lakes - Hydro-Environmental Analysis: Freshwater Environments - page 295

For federal facilities, the extent of allowable modiications depends on the original congressional
authorization. Major changes may require congressional reauthorization. Therefore, legal require-
ments are a major consideration if there is a desired change in operations. One example that will
be discussed in later sections is what was required to alter the releases from reservoirs in northwest
Arkansas (White River, etc.) to protect the downstream (tailwater) isheries.

10.3 TYPES OF DAMS
One of the primary factors affecting the physical, chemical, and biological characteristics of reservoirs
is the design and operation of dams. A dam is designed to hold or impound water and resist the forces
acting on it, including the force of water (weight and pressure), waves, gravity, and seismic forces.
Dam design considerations include:

• Project purpose

• Material availability and life expectancy (design life)

• Foundations and materials (bearing, sliding, seepage, liquefaction settlement, seepage,

leakage, piping, etc.)

• Stability (overturning and slope stability; seismic resistance, water barriers, and drains)

Basically, dams can be subdivided into three types: gravity structures, ill dams, and structural
dams. Of the dams included in the 2009 NID, by far the majority were earth-illed dams (Figure 10.8).

10.3.1 G raVIty S tructureS
Gravity structures rely on their mass for stability. Typical gravity structures are composed of con-
crete, masonry, or roller-compacted concrete (RCC). Concrete gravity dams often run in a straight
line across a valley, as illustrated by Grand Coulee Dam (Figure 10.9).

Dams by primary type

73,423

2,391 2,688

1,824

1,451

203 424 505 696

169 18 206

136

FIGURE 10.8 U.S. Dams in the 2009 National Inventory of Dams database. (From U.S. Army
Corps of Engineers National Inventory of Dams, National Data available at: http://geo.usace.army.mil/
pgis/f?p=397:5:0::NO.)