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50 Chapter 2 Water Sources: Surface Water
timber sheathing once much used on the upstream face can be dangerously stressed and
fail as the fill itself, or its foundation, settles. They are no longer in favor.

2.10.2 Masonry Dams
In the construction of gravity dams, cyclopean masonry and mass concrete embedding
great boulders have, in the course of time, given way to poured concrete; in the case of
arched dams rubble has also ceded the field to concrete. Gravity dams are designed to be
in compression under all conditions of loading. They will fit into almost any site with a
suitable foundation. Some arched dams are designed to resist water pressures and other
forces by acting as vertical cantilevers and horizontal arches simultaneously; for others,
arch action alone is assumed, thrust being transmitted laterally to both sides of the valley,
which must be strong enough to serve as abutments. In constant-radius dams, the up-
stream face is vertical or, at most, slanted steeply near the bottom; the downstream face is
projected as a series of concentric, circular contours in plan. Dams of this kind fit well
into U-shaped valleys, where cantilever action is expected to respond favorably to the
high-intensity bottom loads. In constant-angle dams, the upstream face bulges upvalley;
the downstream face curves inward like the small of a man’s back. Dams of this kind fit
well into V-shaped valleys, where arch action becomes their main source of strength at all
horizons.
Concrete buttresses are designed to support flat slabs or multiple arches in buttress dams.
Here and there, wood and steel structures have taken the place of reinforced concrete. Their
upstream face is normally sloped one on one and may terminate in a vertical cutoff wall.
All masonry dams must rest on solid rock. Foundation pressures are high in gravity
dams; abutment pressures are intense in arched dams. Buttress dams are light on their
foundations. Making foundations tight by sealing contained pockets or cavities and seams
or faults with cement or cement-and-sand grout under pressure is an important responsibil-
ity. Low-pressure grouting (up to 40 psig or 278 kPa) may be followed by high-pressure
grouting (200 psig or 1,390 kPa) from permanent galleries in the dam itself, and a curtain
of grout may be forced into the foundation at the heel of gravity dams to obstruct seepage.
Vertical drainage holes just downstream from the grout curtain help reduce uplift.

2.11 SPILLWAYS

Spillways have been built into the immediate structure of both embankment and masonry
dams, in each instance as masonry sections (see Fig. 2.5). Masonry dams may indeed serve
as spillways over their entire length. In general, however, spillways are placed at a distance
from the dam itself to divert flow and direct possible destructive forces—generated, for
example, by ice and debris, wave action, and the onward rush of waters—away from the
structure rather than toward it. Saddle dams or dikes may be built to a lower elevation than
the main impounding dam in order to serve as emergency floodways.
The head on the spillway crest at time of maximum discharge is the principal compo-
nent of the freeboard, namely, the vertical distance between maximum reservoir level and
elevation of dam crest. Other factors are wave height (trough to crest), wave runup on slop-
ing upstream faces, wind setup or tilting of the reservoir surface by the drag exerted in the
direction of persistent winds in common with differences in barometric pressure, and (for
earth embankments only) depth of frost.
Overflow sections of masonry and embankment dams are designed as masonry struc-
tures and separate spillways as saddle, side channel, and drop inlet or shaft structures.
Spillways constructed through a saddle normally discharge into a natural floodway leading