Page 45 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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8 Chapter 1 Introduction to Water Systems

Figure 1.2 A Watershed Lake in
Western Missouri Provides Water
Supply
(Courtesy of the National Resources
Conservation Service and USDA)

discharge must equal or exceed the demands of the community they serve for a reasonable
number of years in the future. Because their dry season flows generally fall short of con-
current municipal requirements, their floodwaters must usually be stored in sufficient vol-
ume to ensure an adequate supply. Necessary reservoirs are impounded by throwing dams
across the stream valley. In this way, amounts up to the mean annual flow can be utilized.
The area draining to an impoundment is known as the catchment area or watershed. Its
economical development depends on the value of water in the region, but it is a function,
too, of runoff and its variation, accessibility of catchment areas, interference with existing
water rights, and costs of construction. Allowances must be made for evaporation from
new water surfaces generated by the impoundage (Fig. 1.2) and also often for release of
agreed-on flows to the valley below the dam (compensating water). Increased ground
storage in the flooded area and the gradual diminution of reservoir volumes by siltation
must also be considered.
Intake structures are incorporated in impounding dams or kept separate. Other impor-
tant components of impounding reservoirs are (a) spillways safely passing floods in excess
of reservoir capacity and (b) diversion conduits safely carrying the stream past the con-
struction site until the reservoir has been completed and its spillway can go into action.
Analysis of flood records enters into the design of these ancillary structures.
Some impounded supplies are sufficiently safe, attractive, and palatable to be used
without treatment other than protective disinfection. However, it may be necessary to re-
move high color imparted to the stored water by the decomposition of organic matter in
swamps and on the flooded valley bottom; odors and tastes generated in the decomposi-
tion or growth of algae, especially during the first years after filling; and turbidity (finely