Page 26 - Water Engineering Hydraulics, Distribution and Treatment

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Introduction to Water Systems
Chapter 1
strongly saline waters by evaporation and con-
sea level, making it the height of a 55 story building. Despite
densation.
their size, icebergs move an average of 17 km (about 10 mi)
3. Groundwater:
a day. These icebergs originate from glaciers and may have
(a) From natural springs.
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an interior temperature of –15 Cto −20 C(5 Fto − 4 F).
(b) From wells.
Municipal supplies may be derived from more than
(c) From infiltration galleries, basins, or cribs.
one source, the yields of available sources ordinarily
being combined before distribution. Dual public water sup-
(d) From wells, galleries, and, possibly, springs, with
plies (see Chapter 8) of unequal quality are unusual in
flows augmented from some other source (i)
North America. However, they do exist, for example, as a
spread on the surface of the gathering ground,
high-grade supply for general municipal uses and a low-
(ii) carried into charging basins or ditches, or (iii)
grade supply for specific industrial purposes or firefighting.
led into diffusion galleries or wells. iceberg in the North Atlantic was 168 m (about 551 ft) above
Unless the low-grade (nonpotable) supply is rigorously dis-
(e) From wells or galleries with flows maintained
infected, its existence is frowned on by health authorities
by returning to the ground the water previously
because it may be cross-connected, wittingly or unwittingly,
withdrawn from the same aquifer for cooling or
with the high-grade (potable) supply. A cross-connection is
similar purposes.
defined as a junction between water supply systems through
which water from doubtful or unsafe sources may enter an
Several schemes have been proposed to make use of
otherwise safe supply.
icebergs as a water source; to date, however, this has only
been done for novelty purposes. One of the serious moves
toward the practical use of icebergs is the formation of an
Arabian–American investment group to search for the opti- 1.4 RAINWATER
mal way to transport and melt icebergs for use as a source of
drinking water supply without the need for on-land storage. Rain is rarely the immediate provenance of municipal water
Glacier runoff is considered to be surface water. supplies. Instead, the capture of rainwater is confined to farms
An iceberg is a large piece of freshwater ice that has and rural settlements usually in semiarid regions devoid of
broken off from a snow-formed glacier or ice shelf and is satisfactory ground or surface waters. On homesteads, rain-
floating in open water. Because the density of pure ice is water running off roofs is led through gutters and downspouts
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about 920 kg/m and that of sea water about 1025 kg/m , to rain barrels or cisterns situated on or in the ground. Storage
typically only one-tenth of the volume of an iceberg is above transforms the intermittent rainfall into a continuous supply.
water. The shape of the rest of the iceberg under the water can For municipal service, sheds or catches on ground that is
be difficult to surmise from looking at what is visible above naturally impervious or made tight by grouting, cementing,
the surface. Icebergs generally range from 1 to 75 m (about paving, or similar means must usually be added.
3–250 ft) above sea level and weigh 100,000–200,000 metric The gross yield of rainwater is proportional to the receiv-
tonne (about 110,000–220,000 short ton). The tallest known ing area and the amount of precipitation. However, some rain
EXAMPLE 1.1 CALCULATING THE VOLUME OF RAINFALL THAT CAN BE COLLECTED FROM A
BUILDING ROOF
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Make a rough estimate of the volume in gallons or liters of water that can be caught by 3,000 ft (278.7 m ) of horizontally projected
roof area (the average area of American farm buildings) in a region where the mean annual rainfall is 15 in. (38.1 cm).
Solution 1 (US Customary System):
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Gross yield = 3,000 ft × (15∕12 ft) × 7.48 gal∕ft = 28,100 gal annually = 28,100 gal∕365 days
= 77 gpd.
Net yield approximates two-thirds gross yield = 18,800 gal annually = 51 gpd.
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About half the net annual yield, or 9,400 gal = 1,250 ft , must normally be stored to tide the supply over dry spells.
Solution 2 (SI System):
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Gross yield = (278.7m )(38.1∕100 m)(1,000 L∕m ) = 106,178 L annually = 291 L∕day
= 291 L∕d.
Net yield approximates two-thirds gross yield = 291 L∕d(2∕3) = 194 L∕d = 70,790 L∕year.
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About half the net annual yield = 0.5 (70,790 L∕year) = 35,395 L = 35.4m must be stored to tide the supply over dry spells.