Page 61 - Water Engineering Hydraulics, Distribution and Treatment

2.14 Collection of Rainwater
2. The stream is diverted to one side of its valley, the
other side being laid dry by a more or less semicir-
Rain is rarely the immediate source of municipal water
cular cofferdam. After construction has progressed
supplies—a notable example is the water supply of the com-
far enough in the protected zone, stream flow is redi-
munities in the islands of Bermuda, on which streams are
verted through a sluiceway in the completed section
lacking and groundwater is brackish. The use of rainwater
of the dam, and a new cofferdam is built to pump out
is generally confined (a) to farms and towns in semiarid
the remaining portion of the construction site.
regions devoid of satisfactory groundwater or surface-water
supplies, and (b) to some hard-water communities in which,
Diversion conduits are built as grade aqueducts and tun-
because of its softness, roof drainage is employed principally
nels, or as pressureconduits andtunnels. As amatter of safety,
for household laundry work and general washing purposes,
however, it should be impossible for any conduit passing
while the public supply satisfies all other requirements. In
through an earth embankment dam to be put under pressure;
most hard-water communities, the installation and operation
a leak might bring disaster. Accordingly, gates should be 2.14 COLLECTION OF RAINWATER 39
installed only at the inlet portal, never at the outlet portal. of municipal water-softening plants can ordinarily be justi-
If a pipe must work under pressure, it should be laid within fied economically. Their introduction is desirable and does
a larger access conduit. To discourage seepage along their away with the need for supplementary rainwater supplies and
outer walls, conduits passing through earth dams or earth the associated objection of their possible cross-connection
foundations are often given projecting fins or collars that with the public supply.
increase the length of path of seepage (by, say, 20% or more) For individual homesteads, rainwater running off the
and force flow in the direction of minimum as well as maxi- roof is led through gutters and downspouts to a rain barrel or
mum permeability. At their terminus near the toe of the dam, cistern situated on the ground or below it (Fig. 2.16). Barrel
moreover, emerging conduits should be surrounded by rock, or cistern storage converts the intermittent rainfall into a
through which residual seepage waters can escape safely. continuous supply. For municipal service, roof water may be
The capacity of diversion conduits is determined by combined with water collected from sheds or catches on the
flood-flow requirements. Variations in the head and volume surface of ground that is naturally impervious or rendered so
of floodwater impounded behind the rising dam are important by grouting, cementing, paving, or similar means.
factors in this connection. Rising heads normally increase The gross yield of rainwater supplies is proportional to
the capacity of diversion conduits, and increasing storage the receiving area and the amount of precipitation. Some rain,
reduces the intensity of floods. At the same time, however, however, is blown off the roof by wind, evaporated, or lost in
dangers to the construction site and the valley below mount wetting the collecting area and conduits and in filling depres-
higher. sions or improperly pitched gutters. Also, the first flush of
Catchment area

Manhole
access
Roof washer

Water hauler’s
filler pipe
Line to house pump
(below frost line)
Screened
intake
Overflow

Figure 2.16 Rainwater collection.