Page 70 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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2.1 Sources of Surface Water 33

Figure 2.5 Concrete Spillway in Dam
(Courtesy of the Department of Water Supply, Maui County, Hawaii)

the construction site until the reservoir has been completed and its spillway can go into ac-
tion. 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 treat-
ment other than protective disinfection. However, it may be necessary to remove (a) high
color imparted to the stored water by the decomposition of organic matter in swamps and
on the flooded valley floor; (b) odors and tastes generated in the decomposition or growth
of algae, especially during the first years after filling; and (c) turbidity (finely divided clay
or silt) carried into streams or reservoirs by surface wash, wave action, or bank erosion.
Recreational uses of watersheds and reservoirs may endanger the water’s safety and call
for treatment of the flows withdrawn from storage.
Much of the water entering streams, ponds, lakes, and reservoirs in times of
drought, or when precipitation is frozen, is seepage from the soil. Nevertheless, it is
classified as surface runoff rather than groundwater. Water seeps from the ground when
surface streams are low and to the ground when surface streams are high. Release of
water from ground storage or from accumulations of snow in high mountains is a deter-
mining factor in the yield of some catchment areas. Although surface waters are derived
ultimately from precipitation, the relations between precipitation, runoff, infiltration,
evaporation, and transpiration are so complex that engineers rightly prefer to base calcu-
lations of yield on available stream gaugings. For adequate information, gaugings must
extend over a considerable number of years.
In the absence of adequate natural storage, engineers construct impounding reser-
voirs (Fig. 2.6). More rarely they excavate storage basins in lowlands adjacent to
streams. Natural storage, too, can be regulated. Control works (gates and weirs or sills)
at the outlets to lakes and ponds are examples. Some storage works are designed to serve
a single purpose only; others are planned to perform a number of different functions and
to preserve the broader economy of natural resources. Common purposes include:
1. Water supply for household, farm, community, and industry
2. Dilution and natural purification of municipal and industrial wastewaters
3. Irrigation of arable land