Page 46 - Water Engineering Hydraulics, Distribution and Treatment

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Water Sources: Surface Water
Chapter 2
developments. The choice of single-purpose storage systems
should indeed be justified fully.
Storage is provided when stream flow is inadequate or
rendered unsatisfactory by heavy pollution. Release of stored
waters then swells flows and dilutes pollution. Storage itself
also affects the quality of the waters impounded. Both desir-
able and undesirable changes may take place. Their identifi-
cation is the responsibility of limnology, the science of lakes
or, more broadly, of inland waters.
If they must receive wastewaters, stream flows should
be adjusted to the pollution load imposed on them. Low-
water regulation, as such, is made possible by headwater
or upstream storage, but lowland reservoirs, too, may aid
dilution and play an active part in the natural purification of
river systems. Whether overall results are helpful depends on
Figure 2.5 Concrete spillway in dam (Courtesy of the the volume and nature of wastewater flows and the chosen
Department of Water Supply, Maui County, HI). regimen of the stream.
only; others are planned to perform a number of different 2.2 SAFE YIELD OF STREAMS
functions and to preserve the broader economy of natural
In the absence of storage, the safe yield of a river system is
resources. Common purposes include
its lowest dry-weather flow; with full development of stor-
age, the safe yield approaches the mean annual flow.The
1. Water supply for household, farm, community, and
economical yield generally lies somewhere in between. The
industry
attainable yield is modified by (a) evaporation, (b) bank stor-
2. Dilution and natural purification of municipal and
age, (c) seepage out of the catchment area, and (d) silting.
industrial wastewaters
Storage–yield relations are illustrated in this chapter by cal-
3. Irrigation of arable land culations of storage to be provided in impounding reservoirs
4. Harnessing water power for water supply. However, the principles demonstrated are
also applicable to other purposes and uses of storage.
5. Low-water regulation for navigation
6. Preservation and cultivation of useful aquatic life
7. Recreation, for example, fishing, boating, and bathing 2.3 STORAGE AS A FUNCTION OF DRAFT
AND RUNOFF
8. Control of destructive floods
A dam thrown across a river valley impounds the waters of
The greatest net benefit may accrue from a judi- the valley. Once the reservoir has filled, the water drawn
cious combination of reservoir functions in multipurpose from storage is eventually replenished by the stream, pro-
vided runoff, storage, and draft are kept in proper balance.
The balance is struck graphically or analytically on the basis
of historical records or replications generated by suitable
statistical procedures of operational hydrology.
Assuming that the reservoir is full at the beginning of
2
a dry period, the maximum amount of water S (MG/mi or
2
ML/km ) that must be withdrawn from storage to maintain
2
2
a given average draft D (MG/mi or ML/km ) equals the
maximum cumulative difference between the draft D and the
2
2
runoff Q (MG/mi or ML/km ) in a given dry period, or
∑
S = maximum value of (D − Q) (2.1)
To find S, ∑ (D − Q) is summed arithmetically or graph-
ically. The mass diagram method illustrated in Fig. 2.7 is
a useful demonstration of finding ∑ (D − Q) = ∑ D − ∑ Q.
The shorter the interval of time for which runoff is recorded,
Figure 2.6 Hoover dam, Clark County, NV (Courtesy of the the more exact the result. As the maximum value is
National Resources Conservation Service and USDA). approached, therefore, it may be worthwhile to shift to short