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48 Chapter 2 Water Sources: Surface Water
The cost and difficulty of maintaining adequate coverage of the water surface have op-
erated against the widespread use of such substances. Small and light plastic balls have
also been used to retard evaporation from water surfaces of reservoirs.

2.10 DAMS AND DIKES

Generally speaking, the great dams and barrages of the world are the most massive structures
built by man. To block river channels carved through mountains in geologic time periods,
many of them are wedged between high valley walls and impound days and months of flow
in deep reservoirs. Occasionally, water reservoirs reach such levels that their waters would
spill over low saddles of the divide into neighboring watersheds if saddle dams or dikes were
not built to complement the main structure. In other ways, too, surface topography and sub-
surface geology are of controlling influence. Hydraulically, they determine the siting of
dams; volumes of storage, including subsurface storage in glacial and alluvial deposits; and
spillway and diversion arrangements. Structurally, they identify the nature and usefulness of
foundations and the location and economic availability of suitable construction materials.
Soils and rock of many kinds can go into the building of dams and dikes. Timber and steel
have found more limited application. Like most other civil engineering constructions, there-
fore, dams and their reservoirs are derived largely from their own environment.
Structurally, dams resist the pressure of waters against their upstream face by gravity,
arch action, or both. Hydraulically, they stem the tides of water by their tightness as a
whole and the relative imperviousness of their foundations and abutments. They combine
these hydraulic and structural properties to keep seepage within tolerable limits and chan-
neled such that the working structures remain safe. Various materials and methods of con-
struction are used to create dams of many types. The following are the most common
types: (a) embankment dams of earth, rock, or both and (b) masonry dams (today largely
concrete dams) built as gravity, arched, or buttressed structures.

2.10.1 Embankment Dams

Rock, sand, clay, and silt are the principal materials of construction for rock and earth
embankments. Permeables provide weight, impermeables watertightness. Optimal exca-
vation, handling, placement, distribution, and compaction with special reference to se-
lective placement of available materials challenge the ingenuity of the designer and
constructor. Permeables form the shells or shoulders, impermeables the core or blanket
of the finished embankment. Depending in some measure on the abundance or scarcity
of clays, relatively thick cores are centered in a substantially vertical position, or rela-
tively thin cores are displaced toward the upstream face in an inclined position. Common
features of an earth dam with a central clay core wall were illustrated earlier in Fig. 2.4.
Concrete walls can take the place of clay cores, but they do not adjust well to the move-
ments of newly placed, consolidating embankments and foundations; by contrast, clay is
plastic enough to do so. If materials are properly dispatched from borrow pits, earth
shells can be ideally graded from fine at the watertight core to coarse and well draining
at the upstream and downstream faces. In rock fills, too, there must be effective transi-
tion from core to shell, the required change in particle size ranging from a fraction of a
millimeter for fine sand through coarse sand (about 1 mm) and gravel (about 10 mm) to
rock of large dimensions.
Within the range of destructive wave action, stone placed either as paving or as
riprap wards off erosion of the upstream face. Concrete aprons are not as satisfactory,
sharing as they do most of the disadvantages of concrete core walls. A wide berm at the
foot of the protected slope helps to keep riprap in place. To prevent the downstream face