Page 54 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 2
Water Sources: Surface Water
curve by planimetering the area enclosed between the curve
and its ordinate.
confined to the marginal lands of water courses, especially
In reservoir operation, a small amount of water lies
those closest to water intakes themselves. Yet water quality
management need not be neglected. Scattered habitations can
below the invert of the reservoir outlet. Constituting the dregs
be equipped with acceptable sanitary facilities; wastewaters
of the impoundage, this water is of poor quality. The asso-
can be adequately treated or, possibly, diverted into neighbor-
ciated reduction in useful storage is offset, in general, by
bank storage released from the soil as the reservoir is drawn
ing drainage areas not used for water supply; swamps can be
down. Moreover, the water below the outlet sill does form a
drained; and soil erosion can be controlled. Intelligent land
conservation pool for fish and wildlife.
management of this kind can normally be exercised most eco-
Surface areas and volumes enter not only into the solu-
nomically when water is drawn from upland sources where
small streams traverse land of little value and small area.
tion of hydrologic problems but also into the management of
However, some upland watersheds are big enough to satisfy
water quality, such as the control of algae by copper sulfate increases, land holdings of water utilities are understandably
and destratification by pumping or aeration. the demands of great cities. The water supplies of Boston,
New York, and San Francisco are examples.
2.7 MANAGEMENT OF CATCHMENT
AREAS
2.7.2 Lowland Areas
The comparative advantage of developing surface rather
When water is drawn from large lakes and wide rivers that,
than underground waters is offset, in large measure, by the
without additional storage, yield an abundance of water, man-
unsteadiness of surface runoff, in both quantity and qual-
agement of their catchments ordinarily becomes the concern
ity, and the recurrence of flow extremes. Those hydrologic
of more than one community (examples are the Ohio and
factors that enter strongly into the development of surface-
Mississippi Rivers) and sometimes of more than a single
water supplies must, therefore, be kept clearly in mind in
state (the Delaware River is a notable example) and even of a
their design and operation, with special reference to
single country (e.g., the Great Lakes are shared with Canada
1. The principles of selecting, preparing, and control- and the Colorado River is shared with Mexico). Regional,
ling catchment areas. interstate, and international authorities must be set up to
manage and protect land and water resources of this kind.
2. The choice and treatment of reservoir areas and the
management of natural ponds and lakes as well as
impounding reservoirs.
2.7.3 Quality Control
3. The siting, dimensioning, construction, and main-
tenance of necessary engineering works, including To safeguard their sources, water utilities can fence and post
dams and dikes, intake structures, spillways, and their lands, patrol watersheds, and obtain legislative author-
diversion works. Also keep in mind that river systems ity for enforcing reasonable rules and regulations for the
may have to be developed for multiple purposes, not environmental management of the catchment area. When the
just for municipal uses. cost of policing the area outweighs the cost of purifying
its waters in suitable treatment works, purification is often
The gathering grounds for public water supplies vary preferred. It is likewise preferred when lakes, reservoirs, and
in size from a few hundred acres to thousands of square streams become important recreational assets and their enjoy-
miles, and in character from sparsely inhabited uplands to ment can be encouraged without endangering their quality. It
densely populated river valleys. The less developed they are, goes without saying that recreation must be properly super-
the better, relatively, they lend themselves to exploitation for vised and recreational areas suitably located and adequately
steady yields and the production of water of high quality. equipped with sanitary facilities.
2.7.1 Upland Areas
2.7.4 Swamp Drainage
Occasionally, a water utility can, with economic justifica-
tion, acquire the entire watershed of its source and manage Three types of swamps may occur on catchment areas:
solely for water supply purposes, excluding habitations and
factories to keep the water safe and attractive; letting arable 1. Rainwater swamps where precipitation accumulates
lands lie fallow to prevent wasteful runoff and high turbidi- on flat lands or where rivers overflow their banks in
ties; draining swamps to reduce evaporation and eliminate times of flood
odors, tastes, and color; and cultivating woodlots to hold 2. Backwater swamps or reaches of shallow flowage in
back winter snows and storm runoff and help preserve the sluggish, often meandering streams where bends or
even tenor of stream flow. As competition for water and land other obstructions can hamper flow