Page 94 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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2.14 Collection of Rainwater 57

Downspout
Downspout Asphaltic
Screen from roof seal Flapper valve
Overflow
12" Min. Pyramid galvanized
12" Min. screen
Roof washer
Manhole cover receives first Filter sand
runoff from Faucet 20" Min.
Effective size 0.3 mm
2" Min. Caulking roof 3" Min.sand 1 /8" to coarse
3" Min. 1/8" to 3/8" gravel
4" Min. 3/4" to 11/4" gravel
Sand filter (may be used
Maximum water level in place of roof washer)
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 1 in. 2.54 cm
Storage to be provided in cisterns depends on seasonal rainfall characteristics and
commonly approximates one-third to one-half of the annual needs in accordance with the
length of dry spells. If the water is to be filtered before storage, standby capacity in ad-
vance of filtration must be provided if rainfalls of high intensity are not to escape. Because
of the relatively small catchment area available, roof drainage cannot be expected to yield
an abundant supply of water, and a close analysis of storm rainfalls and seasonal variations
in precipitation must be made if catchment areas, standby tanks, filters, and cisterns are to
be proportioned and developed properly.
A properly located and constructed controlled catchment and cistern, augmented by satis-
factory treatment facilities, will provide safe water. A controlled catchment is a defined surface
area from which rainfall runoff is collected. For these controlled catchments, simple guidelines
to determine water yield from rainfall totals can be established. When the controlled catchment
area has a smooth surface or is paved and the runoff is collected in a cistern, water loss due to
evaporation, replacement of soil moisture deficit, and infiltration is small. As a general rule,
losses from smooth concrete or asphalt-covered ground catchments average less than 10%. For
shingled roofs or tar and gravel surfaces, losses should not exceed 15%; and for sheet metal
roofs the loss is negligible. A conservative design can be based on the assumption that the
amount of water that can be recovered for use is three-fourths of the total annual rainfall. The
location of the cistern should be governed by both convenience and quality protection. A cistern
should be as close to the point of ultimate use as practical. A cistern should not be placed closer
than 50 ft (15 m) to any part of a sewage-disposal installation and should be on higher ground.
Cisterns collecting water from roof surfaces should be located adjacent to the build-
ing, but not in basements subject to flooding. They may be placed below the surface of the
ground for protection against freezing in cold climates and to keep water temperatures low
in warm climates, but should be situated on the highest ground practicable, with the sur-
rounding area graded to provide good drainage.