Civil Engineering Reference
In-Depth Information
For development of scattered wells feeding individually into the distribution system
through short pipelines, and for other projects that are readily adaptable to construction
in stages, shorter design periods of 10 to 20 years are appropriate.
SURFACE WATER SUPPLIES
Safe Yield
The following is a brief discussion describing safe yield concepts for streams and
reservoirs and lakes.
Flowing Streams
When no storage is provided, the minimum available (considering
any competing water rights) stream flow of record must exceed the estimated future
water demand on the maximum day. The best hydrologic data on minimum stream
flows are those recorded on the specific watershed in question. In the absence of such
data or with a short period of record, it may be necessary to use estimating methods.
For such estimates, stream flow and weather records of contributing or adjacent wa-
tersheds should be used. Empirical formulas and ratios contained in published literature
vary widely for watersheds of the same size; therefore, they should not be considered
satisfactory criteria for judging the adequacy of a source unless they are supported by
hydrologic data obtained from the specific watershed.
In making estimates, a careful study should be made of all factors that affect and
determine the safe yield of a proposed surface water source, including such data as:
geographical location, storm paths, prevailing winds, type and intensity of precipita-
tion, topography and size of basin, orientation of basin, types of soil, types of vege-
tation, condition of ground surface, type and extent of artificial drainage, extent of
surface storage in lakes and swamps, condition and slope of stream channel, average
slope of basin, character of drainage net, and evaporation, infiltration, and other losses.
There are many methods of estimating runoff. The accuracy of the various methods
depends upon the ability and the experienced judgment of the estimator in finding and
supplying the correlating factors that will produce a realistic synthetic record. Con-
sultation with the U.S. Weather Bureau and the U.S. Geological Survey is recom-
mended.
Reservoirs and Lakes
When the demand for water is greater than the minimum
rate of flow in the stream from which the water is to be taken, an impounding reservoir
may be required. The development of reservoir sites is discussed later in this chapter.
In general, the ideal topographic conditions for a reservoir are a narrow gorge in which
a dam may be built at minimum expense, and an expanding valley immediately above
the gorge that will afford a large amount of storage per unit of surface area. This
minimizes evaporation loss and the growth of algae and aquatic vegetation.
A reservoir will yield only part of the long-term average runoff of the watershed
that it controls. The rest either will go over the spillway in times of flooding or will
be lost to evaporation, bank storage, seepage, or siltation. Up to a certain point, in-
creasing the amount of storage space in the reservoir will cause additional water to be
on hand in drought times, and will enable the project to produce a greater dependable
yield. However, the volume of water lost to lake surface evaporation also increases
