Page 46 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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1.5 Surface Water 9
divided clay or silt) carried into streams or reservoirs by surface wash, wave action, or
bank erosion. Recreational uses of watersheds and reservoirs may call for treatment of the
flows withdrawn from storage.
Much of the water in 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 determining 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 calculations of
yield on available stream gaugings. For adequate information, gaugings must extend
over a considerable number of years.

EXAMPLE 1.2 ESTIMATES OF YIELDS FROM WATER SHEDS AND STORAGE REQUIREMENTS
Certain rough estimates of the yield of surface water sheds and storage requirements are shown in
Fig. 1.1. Rainfall is used as the point of departure, merely to identify the dimensions of possible
rainfall–runoff relationships. Determine:
1. The yields from the water sheds,
2. The storage requirements,
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2
3. The number of people who can be supported by a drainage area of 100 mile (259 km ) if
there is adequate impoundage for water storage, and
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2
4. The number of people who can be supported by a drainage area of 100 mile (259 km ) if
there is no impoundage for water storage.
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2
The following assumptions are made: (a) Rainfall 20 in./mile annually 19.6 cm/km , (b) a stream
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3
flow of about 1 MGD/mile (million gallons per day per square mile) or (1.547 ft /s)/mile [or 1.46
2
MLD/mile (million liters per day per square mile)] is a good average for the well-watered sections of
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2
North America, (c) for 75% development (0.75 1 MGD/mile or 0.75 1.46 MLD/mile ), about
half a year’s supply must generally be stored. In semiarid regions storage of three times the mean an-
nual stream flow is not uncommon, that is, water is held over from wet years to supply demands during
dry years, (d) average water consumption 150 gpcd 567.8 Lpcd, (e) for water supply by continu-
ous draft, low water flows rather than average annual yields govern. In well-watered sections of North
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America, these approximate 0.1 ft /s or 64,600 gpd/mile (or 28.32 L/s, or 0.094316 MLD/km ).
Solution 1 (U.S. Customary System):
1. The following conversion factors and approximations are being employed:
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1 in. rainfall/mile 17.378 MG
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Hence, 20 in./mile annually 20 17.378 348 MG or 348>365 0.952 MGD.
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2. A stream flow of about 1 MGD/mile is a good average for the well-watered sections of
North America. Not all of it can be adduced economically by storage.
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For 75% development (0.75 MGD/mile , or 750,000 gpd/mile ), about half a year’s
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supply must generally be stored. For a catchment area of 100 mile , therefore:
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Storage (0.75 MGD/mile )(100 mile ) (0.5 365 days) 13,688 MG 13.5 BG
(billion gallons) approximately
In semiarid regions storage of three times the mean annual stream flow is not uncommon,
that is, water is held over from wet years to supply demands during dry years.