Page 27 - Water Engineering Hydraulics, Distribution and Treatment

is blown off the roof, evaporated, or lost in wetting the col-
lecting surfaces and conduits and in filling depressions or
Low stream flows are often left untouched. They may be
improperly pitched gutters. Also, the first flush of water may
wanted for other downstream purposes or they may be too
have to be wasted because it contains dust, bird droppings,
highly polluted for reasonable use. Only clean floodwaters
and other unwanted materials. The combined loss may be
are then diverted into reservoirs constructed in meadow lands
high. A cutoff, switch, or deflector in the downspout permits
adjacent to the stream or otherwise conveniently available.
selective diversion of unwanted water from the system. Sand
The amount of water so stored must supply demands during
filters will cleanse the water as it enters the cistern and prevent
seasons of unavailable stream flow. If draft is confined to a
its deterioration via the growth of undesirable organisms and
quarter year, for example, the reservoir must hold at least
consequent tastes, odors, and other changes in attractiveness
three-fourths of the annual supply. In spite of its selection
and palatability.
and long storage, the water may have to be purified.
The storage to be provided in cisterns depends on the 1.5.2 Selective Draft 1.5 Surface Water 5
distribution of rainfall. Storage varies with the length of dry
spells and commonly approximates one-third to one-half the 1.5.3 Impoundage
annual consumption. If rainfalls of high intensity are to be
captured, standby capacity must exist in advance of filtration. In their search for clean water and water that can be brought
Because their area is small, roofs seldom yield much water. and distributed to the community by gravity, engineers have
A careful analysis of storm rainfalls and seasonal variations developed supplies from upland streams. Most of them are
in precipitation is, therefore, required. tapped near their source in high and sparsely settled regions.
To be of use, their annual discharge must equal or exceed
the demands of the community they serve for a reasonable
1.5 SURFACE WATER
number of years in the future. Because their dry season flows
In North America by far the largest volumes of municipal generally fall short of concurrent municipal requirements,
water are collected from surface sources. The quantities that their floodwaters must usually be stored in sufficient vol-
can be gathered vary directly with the size of the catch- ume to ensure an adequate supply. Necessary reservoirs are
ment area, or watershed, and with the difference between impounded by throwing dams across the stream valley. In this
the amounts of water falling on it and the amounts lost by way, amounts up to the mean annual flow can be utilized. The
evapotranspiration. The significance of these relationships to area draining to an impoundment is known as the catchment
water supply is illustrated in Fig. 1.1. Where surface water area or watershed. Its economical development depends on
and groundwater sheds do not coincide, some groundwater the value of water in the region, but it is a function, too,
may enter from neighboring catchment areas or escape to of runoff and its variation, accessibility of catchment areas,
them. interference with existing water rights, and costs of construc-
tion. Allowances must be made for evaporation from new
water surfaces generated by the impoundage (Fig. 1.2) and
1.5.1 Continuous Draft
also often for release of agreed-on flows to the valley below
Communities on or near streams, ponds, or lakes may take the dam (compensating water). Increased ground storage in
their supplies from them by continuous draft if stream flow the flooded area and the gradual diminution of reservoir vol-
and pond or lake capacity are high enough at all seasons of umes by siltation must also be considered.
the year to furnish requisite water volumes. Collecting works Intake structures are incorporated in impounding dams
ordinarily include (a) an intake crib, gatehouse, or tower; (b) or kept separate. Other important components of impounding
an intake conduit; and (c) in many places, a pumping station. reservoirs are (a) spillways safely passing floods in excess of
On small streams serving communities of moderate size, an reservoir capacity and (b) diversion conduits safely carrying
intake or diversion dam may create sufficient depth of water the stream past the construction site until the reservoir has
to submerge the intake pipe and protect it against ice. From been completed and its spillway can go into action. Analysis
intakes close to the community the water must generally be of flood records enters into the design of these ancillary
lifted to purification works and thence to the distribution structures.
system. Some impounded supplies are sufficiently safe, attrac-
Most large streams are polluted by wastes from upstream tive, and palatable to be used without treatment other than
communities and industries. Purification of their waters is protective disinfection. However, it may be necessary to
then a necessity. Cities on large lakes usually guard their sup- remove high color imparted to the stored water by the decom-
plies against their own and their neighbor’s wastewater and position of organic matter in swamps and on the flooded
spent industrial-process waters by moving their intakes far valley bottom; odors and tastes generated in the decomposi-
away from shore and purifying both their water and wastew- tion or growth of algae, especially during the first years after
ater. Diversion of wastewater from lakes will retard the lakes’ filling; and turbidity (finely divided clay or silt) carried into
eutrophication. streams or reservoirs by surface wash, wave action, or bank