Page 62 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 2
Water Sources: Surface Water
Downspout
Downspout
Asphaltic
seal
Flapper valve
from roof
Screen
Overflow
12" Min. Pyramid galvanized
screen
12" Min.
Roof washer
Manhole cover
Filter sand
receives first
Faucet
20" Min.
runoff from
Effective size 0.3 mm
roof
Caulking
3" Min.sand 1 /8" to coarse
2" Min.
3" Min. 1/8" to 3/8" gravel
4" Min. 3/4" to 11/4" gravel
Sand filter (may be used
in place of roof washer)
Maximum water level
Basement
Screen
Screened drain To pump
Figure 2.17 Cistern equipped with sand filtration for collection of surface water in rural area for drinking. Conversion
′′
factor: 1 = 1in. = 2.54 cm.
water contains most of the dust and other undesirable wash- the runoff is collected in a cistern, water loss due to evapo-
ings from the catchment surfaces and may have to be wasted. ration, replacement of soil moisture deficit, and infiltration
The combined loss is particularly great during the dry season is small. As a general rule, losses from smooth concrete or
of the year. A cutoff, switch, or deflector in the downspout asphalt-covered ground catchments average less than 10%.
permits selection of the quality of water to be stored. Sand For shingled roofs or tar and gravel surfaces, losses should
filters (Fig. 2.17) are successfully employed to cleanse the not exceed 15%; and for sheet metal roofs the loss is negli-
water and prevent its deterioration (a) by growth of unde- gible. A conservative design can be based on the assumption
sirable organisms and (b) by the bacterial decomposition that the amount of water that can be recovered for use is
of organic materials, both of which may give rise to tastes, three-fourths of the total annual rainfall. The location of the
odors, and other changes in the attractiveness and palatability cistern should be governed by both convenience and quality
of the water. protection. A cistern should be as close to the point of ulti-
Storage to be provided in cisterns depends on seasonal mate use as practical. A cistern should not be placed closer
rainfall characteristics and commonly approximates one- than 50 ft (15 m) to any part of a sewage-disposal installation
third to one-half of the annual needs in accordance with and should be on higher ground.
the length of dry spells. If the water is to be filtered before Cisterns collecting water from roof surfaces should be
storage, standby capacity in advance of filtration must be pro- located adjacent to the building, but not in basements sub-
vided if rainfalls of high intensity are not to escape. Because ject to flooding. They may be placed below the surface
of the relatively small catchment area available, roof drainage of the ground for protection against freezing in cold cli-
cannot be expected to yield an abundant supply of water, and mates and to keep water temperatures low in warm cli-
a close analysis of storm rainfalls and seasonal variations mates, but should be situated on the highest ground prac-
in precipitation must be made if catchment areas, standby ticable, with the surrounding area graded to provide good
tanks, filters, and cisterns are to be proportioned and devel- drainage.
oped properly. The size of cistern needed will depend on the size of
A properly located and constructed controlled catchment the family and the length of time between periods of heavy
and cistern, augmented by satisfactory treatment facilities, rainfall. The size of the catchment area will depend on the
will provide safe water. A controlled catchment is a defined amount of rainfall and the character of the surface. The safety
surface area from which rainfall runoff is collected. For these factor allowed should be for lower than normal rainfall lev-
controlled catchments, simple guidelines to determine water els. Designing for two-thirds of the mean annual rainfall
yield from rainfall totals can be established. When the con- will result usually in a catchment area of adequate capacity
trolled catchment area has a smooth surface or is paved and (Fig. 2.18).