Page 44 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch01_001-028.qxd 8/2/10 9:12 PM Page 7

1.5 Surface Water 7

Solution 2 (SI System):
3
2
Gross yield (278.7 m ) (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.
3
About half the net annual yield 0.5 (70,790 L/year) 35,395 L 35.4 m stored to tide the
supply over dry spells.

1.5 SURFACE WATER
In North America by far the largest volumes of municipal water are collected from surface
sources. The quantities that can be gathered vary directly with the size of the catchment
area, or watershed, and with the difference between the amounts of water falling on it and
the amounts lost by evapotranspiration. The significance of these relationships to water
supply is illustrated in Fig. 1.1. Where surface water and groundwater sheds do not coin-
cide, some groundwater may enter from neighboring catchment areas or escape to them.

1.5.1 Continuous Draft
Communities on or near streams, ponds, or lakes may take their supplies from them by
continuous draft if stream flow and pond or lake capacity are high enough at all seasons of
the year to furnish requisite water volumes. Collecting works ordinarily include (a) an in-
take crib, gatehouse, or tower; (b) an intake conduit; and (c) in many places, a pumping
station. On small streams serving communities of moderate size, an intake or diversion
dam may create sufficient depth of water to submerge the intake pipe and protect it against
ice. From intakes close to the community the water must generally be lifted to purification
works and thence to the distribution system.
Most large streams are polluted by wastes from upstream communities and industries.
Purification of their waters is then a necessity. Cities on large lakes usually guard their sup-
plies against their own and their neighbor’s wastewater and spent industrial-process waters
by moving their intakes far away from shore and purifying both their water and waste-
water. Diversion of wastewater from lakes will retard the lakes’ eutrophication.

1.5.2 Selective Draft
Low stream flows are often left untouched. They may be wanted for other downstream pur-
poses, or they may be too highly polluted for reasonable use. Only clean floodwaters are
then diverted into reservoirs constructed in meadow lands adjacent to the stream or other-
wise conveniently available. The amount of water so stored must supply demands during
seasons of unavailable stream flow. If draft is confined to a quarter year, for example, the
reservoir must hold at least three-fourths of the annual supply. In spite of its selection and
long storage, the water may have to be purified.

1.5.3 Impoundage
In their search for clean water and water that can be brought and distributed to the com-
munity by gravity, engineers have developed supplies from upland streams. Most of them
are tapped near their source in high and sparsely settled regions. To be of use, their annual