348 CHAPTER TWELVE
DAMS
A structure that bars or detains the flow of water in an open channel or water-
course. Dams are constructed for several principal purposes. Diversion dams
divert water from a stream; navigation damsraise the level of a stream to
increase the depth for navigation purposes; power damsraise the level of a
stream to create or concentrate hydrostatic head for power purposes; and stor-
age damsstore water for municipal and industrial use, irrigation, food control,
river regulation, recreation, or power production. A dam serving two or more
purposes is called a multiple-purposedam. Dams are commonly classified by
the material from which they are constructed, such as masonry, concrete, earth,
rock, timber, and steel.*
Gravity Dams†
Any dam that does not depend on arch action to resist the forces imposed on
it might be termed a gravity dam. The term, however, is customarily
restricted to solid masonry or concrete dams of roughly triangular section
which are straight or only slightly curved in plan. Dams of this type depend
for their stability almost entirely upon their own weight and, in spite of their
impressive bulk, have a small factor of safety. This fact must be kept con-
stantly in mind at all stages in the design and construction of a gravity dam,
(Fig. 12.29).
Water Pressure. The unit pressure of water increases in proportion to its
depth. The horizontal force due to water pressure can thus be represented by a
triangular load whose resultant is at two-thirds of the distance from the water
surface to the base of the section under consideration. The formula for this water
pressure, Fig. 12.29, is:
(12.182)where Htotal height of dam from base to crest, ft (m)
hheight of section considered to water surface, depth of water, ft (m)
bbase or thickness of dam from the face to the back measured
horizontally, ft (m)
eeccentricity, distance from point of application of resultant to
center of base, ft (m)
y 1 distance from center of base to downstream face, ft (m)
y 2 distance from center of base to upstream face, ft (m)
wdensity of water, 62.5 lb/cu ft (kg/cu m)
Rresultant, foundation reaction, or equilibrant, lb (N)
Ftotal force, see Fig. 12.29 for subscripts, lb (N)
F 2 ^1 / 2 wh^2*Encyclopedia of Science and Technology, McGraw-Hill.
†Davis—Handbook of Applied Hydraulics, McGraw–Hill.