Page 56 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 2
Water Sources: Surface Water
2.9.1 Quality Control
suitable construction materials. Soils and rock of many kinds
can go into the building of dams and dikes. Timber and steel
Of concern in the quality management of reservoirs is the
have found more limited application. Like most other civil
control of water weeds and algal blooms; the bleaching of
engineering constructions, therefore, dams and their reser-
color; the settling of turbidity; destratification by mixing or
voirs are derived largely from their own environment.
aeration; and, in the absence of destratification, the selec-
Structurally, dams resist the pressure of waters against
tion of water of optimal quality and temperature by shifting
their upstream face by gravity, arch action, or both. Hydrauli-
intake depths in order to suit withdrawals to water uses or to
cally, they stem the tides of water by their tightness as a
downstream quality requirements.
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-
2.9.2 Evaporation Control
neled such that the working structures remain safe. Various
The thought that oil spread on water will suppress evapora-
materials and methods of construction are used to create
tion is not new. It is well known that
dams of many types. The following are the most common
types: (a) embankment dams of earth, rock, or both and (b)
1. Certain chemicals spread spontaneously on water as
masonry dams (today largely concrete dams) built as gravity,
layers no more than a molecule thick.
arched, or buttressed structures.
2. These substances include alcohol (hydroxyl) or fatty
acid (carboxyl) groups attached to a saturated paraffin
chain of carbon atoms.
2.10.1 Embankment Dams
3. The resulting monolayers consist of molecules ori-
ented in the same direction and thereby offering more Rock, sand, clay, and silt are the principal materials of con-
resistance to the passage of water molecules than do struction for rock and earth embankments. Permeables pro-
thick layers of oil composed of multilayers of hap- vide weight, impermeables watertightness. Optimal excava-
hazardly oriented molecules. tion, handling, placement, distribution, and compaction with
4. The hydrophilic radicals (OH or COOH) at one end special reference to selective placement of available materi-
of the paraffin chain move down into the water phase, als challenge the ingenuity of the designer and constructor.
while the hydrophobic paraffin chains themselves Permeables form the shells or shoulders, impermeables the
stretch up into the gaseous phase. Examples of suit- core or blanket of the finished embankment. Depending in
able chemicals are alcohols and corresponding fatty some measure on the abundance or scarcity of clays, rel-
acids. atively thick cores are centered in a substantially vertical
position, or relatively thin cores are displaced toward the
The cost and difficulty of maintaining adequate coverage upstream face in an inclined position. Common features of
of the water surface have operated against the widespread an earth dam with a central clay core wall are illustrated
use of such substances. Small and light plastic balls have earlier in Fig. 2.4. Concrete walls can take the place of clay
also been used to retard evaporation from water surfaces of cores, but they do not adjust well to the movements of newly
reservoirs. placed, consolidating embankments and foundations; by con-
trast, clay is plastic enough to do so. If materials are prop-
erly dispatched from borrow pits, earth shells can be ideally
2.10 DAMS AND DIKES
graded from fine at the watertight core to coarse and well
Generally speaking, the great dams and barrages of the world draining at the upstream and downstream faces. In rock fills,
are the most massive structures built by man. To block river too, there must be effective transition from core to shell, the
channels carved through mountains in geologic time peri- required change in particle size ranging from a fraction of a
ods, many of them are wedged between high valley walls millimeter for fine sand through coarse sand (about 1 mm)
and impound days and months of flow in deep reservoirs. and gravel (about 10 mm) to rock of large dimensions.
Occasionally, water reservoirs reach such levels that their Within the range of destructive wave action, stone placed
waters would spill over low saddles of the divide into neigh- either as paving or as riprap wards off erosion of the upstream
boring watersheds if saddle dams or dikes were not built face. Concrete aprons are not as satisfactory, sharing as they
to complement the main structure. In other ways, too, sur- do most of the disadvantages of concrete core walls. A wide
face topography and subsurface geology are of controlling berm at the foot of the protected slope helps to keep riprap in
influence. Hydraulically, they determine the siting of dams; place. To prevent the downstream face from washing away,
volumes of storage, including subsurface storage in glacial it is commonly seeded with grass or covering vines and pro-
and alluvial deposits; and spillway and diversion arrange- vided with a system of surface and subsurface drains. Berms
ments. Structurally, they identify the nature and usefulness break up the face into manageable drainage areas and give
of foundations and the location and economic availability of access to slopes for mowing and maintenance. Although they