THE WATER PROBLEM OF A LARGE CITY
193. It is by no means unusual for the residents of a large city or
town to receive through the newspapers a notification that the city
water supply is running low and that economy should be exercised in
its use. The problem of supplying a large city with an abundance of
pure water is among the most difficult tasks which city officials have
to perform, and is one little understood and appreciated by the
average citizen.
Intense interest in personal and domestic affairs is natural, but
every citizen, rich or poor, should have an interest in civic affairs
as well, and there is no better or more important place to begin than
with the water supply. One of the most stirring questions in New York
to-day has to do with the construction of huge aqueducts designed to
convey to the residents of the city, water from the distant Catskill
Mountains. The growth of the population has been so phenomenally rapid
that the combined output of all available near-by sources does not
suffice to meet the increasing consumption.
Where does your city obtain its water? Does it bring it to its
reservoirs in the most economic way possible, and is there any
legitimate excuse for the scarcity of water which many communities
face in dry seasons?
194. Two Possibilities. Sometimes a city is fortunate enough to be
situated near hills and mountains through which streams flow, and in
that case the water problem is simple. In such a case all that is
necessary is to run pipes, usually underground, from the elevated
lakes or streams to the individual houses, or to common reservoirs
from which it is distributed to the various buildings.
[Illustration: FIG. 148.--The elevated mountain lake serves as a
source of water.]
Figure 148 illustrates in a simple way the manner in which a mountain
lake may serve to supply the inhabitants of a valley. The city of
Denver, for example, is surrounded by mountains abounding in streams
of pure, clear water; pipes convey the water from these heights to the
city, and thus a cheap and adequate flow is obtained. Such a system is
known as the gravity system. The nearer and steeper the elevation, the
greater the force with which the water flows through the valley pipes,
and hence the stronger the discharge from the faucets.
Relatively few cities and towns are so favorably situated as regards
water; more often the mountains are too distant, or the elevation is
too slight, to be of practical value. Cities situated in plains and
remote from mountains are obliged to utilize the water of such streams
as flow through the land, forcing it to the necessary height by means
of pumps. Streams which flow through populated regions are apt to be
contaminated, and hence water from them requires public filtration.
Cities using such a water supply thus have the double expense of
pumping and filtration.
195. The Pressure of Water. No practical business man would erect a
turbine or paddle wheel without calculating in advance the value of
his water power. The paddle wheel might be so heavy that the stream
could not turn it, or so frail in comparison with the water force that
the stream would destroy it. In just as careful a manner, the size and
the strength of municipal reservoirs and pumps must be calculated. The
greater the quantity of water to be held in the reservoir, the heavier
are the walls required; the greater the elevation of the houses, the
stronger must be the pumps and the engines which run them.
In order to understand how these calculations are made, we must study
the physical characteristics of water just as we studied the physical
characteristics of air.
When we measure water, we find that 1 cubic foot of it weighs about
62.5 pounds; this is equivalent to saying that water 1 foot deep
presses on the bottom of the containing vessel with a force of 62.5
pounds to the square foot. If the water is 2 feet deep, the load
supported by the vessel is doubled, and the pressure on each square
foot of the bottom of the vessel will be 125 pounds, and if the water
is 10 feet deep, the load borne by each square foot will be 625
pounds. The deeper the water, the greater will be the weight sustained
by the confining vessel and the greater the pressure exerted by the
water.
[Illustration: FIG. 149.--Water 1 foot deep exerts a pressure of 62.5
pounds a square foot.]
Since the pressure borne by 1 square foot of surface is 62.5 pounds,
the pressure supported by 1 square inch of surface is 1/144 of 62.5
pounds, or .43 pound, nearly 1/2 pound. Suppose a vessel held water to
the depth of 10 feet, then upon every square inch of the bottom of
that vessel there would be a pressure of 4.34 pounds. If a one-inch
tap were inserted in the bottom of the vessel so that the water flowed
out, it would gush forth with a force of 4.34 pounds. If the water
were 20 feet deep, the force of the outflowing water would be twice as
strong, because the pressure would be doubled. But the flow would not
remain constant, because as the water leaves the outlet, less and less
of it remains in the vessel, and hence the pressure gradually sinks
and the flow drops correspondingly.
In seasons of prolonged drought, the streams which feed a city
reservoir are apt to contain less than the usual amount of water,
hence the level of the water supply sinks, the pressure at the outlet
falls, and the force of the outflowing water is lessened (Fig. 150).
[Illustration: FIG. 150.--The pressure at an outlet decreases as the
level of the water supply sinks.]
196. Why the Water Supply is not uniform in All Parts of the City.
In the preceding Section, we saw that the flow from a faucet depends
upon the height of the reserve water above the tap. Houses on a level
with the main supply pipes (Figs. 148 and 151) have a strong flow
because the water is under the pressure of a column _A_; houses
situated on elevation _B_ have less flow, because the water is under
the pressure of a shorter column _B_; and houses at a considerable
elevation _C_ have a less rapid flow corresponding to the diminished
depth _(C)_.
Not only does the flow vary with the elevation of the house, but it
varies with the location of the faucet within the house. Unless the
reservoir is very high, or the pumps very powerful, the flow on the
upper floors is noticeably less than that in the cellar, and in the
upper stories of some high building the flow is scarcely more than a
feeble trickle.
[Illustration: FIG. 151.--Water pressure varies in different parts of
a water system.]
When the respective flows at _A_, _B_, and _C_ (Fig. 151) are measured,
they are found to be far lower than the pressures which columns of
water of the heights _A_, _B_, and _C_ have been shown by actual
demonstration to exert. This is because water, in flowing from place
to place, expends force in overcoming the friction of the pipes and
the resistance of the air. The greater the distance traversed by the
water in its journey from reservoir to faucet, the greater the waste
force and the less the final flow.
In practice, large mains lead from the reservoir to the city, smaller
mains convey the water to the various sections of the city, and
service pipes lead to the individual house taps. During this long
journey, considerable force is expended against friction, and hence
the flow at a distance from the reservoir falls to but a fraction of
its original strength. For this reason, buildings situated near the
main supply have a much stronger flow (Fig. 152) than those on the
same level but remote from the supply. Artificial reservoirs are
usually constructed on the near outskirts of a town in order that the
frictional force lost in transmission may be reduced to a minimum.
[Illustration: FIG. 152.--The more distant the fountain, the weaker
the flow.]
In the case of a natural reservoir, such as an elevated lake or
stream, the distance cannot be planned or controlled. New York, for
example, will secure an abundance of pure water from the Catskill
Mountains, but it will lose force in transmission. Los Angeles is
undertaking one of the greatest municipal projects of the day. Huge
aqueducts are being built which will convey pure mountain water a
distance of 250 miles, and in quantities sufficient to supply two
million people. According to calculations, the force of the water will
be so great that pumps will not be needed.
197. Why Water does not always flow from a Faucet. Most of us have
at times been annoyed by the inability to secure water on an upper
story, because of the drawing off of a supply on a lower floor.
During the working hours of the day, immense quantities of water are
drawn off from innumerable faucets, and hence the quantity in the
pipes decreases considerably unless the supply station is able to
drive water through the vast network of pipes as fast as it is drawn
off. Buildings at a distance from the reservoir suffer under such
circumstances, because while the diminished pressure is ordinarily
powerful enough to supply the lower floors, it is frequently too weak
to force a continuous stream to high levels. At night, however, and
out of working hours, few faucets are open, less water is drawn off at
any one time, and the intricate pipes are constantly full of water
under high pressure. At such times, a good flow is obtainable even on
the uppermost floors.
In order to overcome the disadvantage of a decrease in flow during the
day, standpipes (Fig. 153) are sometimes placed in various sections.
These are practically small steel reservoirs full of water and
connecting with the city pipes. During "rush" hours, water passes from
these into the communicating pipes and increases the available supply,
while during the night, when the faucets are turned off, water
accumulates in the standpipe against the next emergency (Figs. 151 and
154). The service rendered by the standpipe is similar to that of the
air cushion discussed in Section 184.
[Illustration: FIG. 153.--A standpipe.]
198. The Cost of Water. In the gravity system, where an elevated
lake or stream serves as a natural reservoir, the cost of the city's
waterworks is practically limited to the laying of pipes. But when the
source of the supply is more or less on a level with the surrounding
land, the cost is great, because the supply for the entire city must
either be pumped into an artificial reservoir, from which it can be
distributed, or else must be driven directly through the mains (Fig.
154).
[Illustration: FIG. 154.--Water must be got to the houses by means of
pumps.]
A gallon of water weighs approximately 8.3 pounds, and hence the work
done by a pump in raising a gallon of water to the top of an average
house, an elevation of 50 feet, is 8.3 × 50, or 415 foot pounds. A
small manufacturing town uses at least 1,000,000 gallons daily, and
the work done by a pump in raising that amount to an elevation of 50
feet would be 8.3 × 1,000,000 × 50, or 415,000,000 foot pounds.
The total work done during the day by the pump, or the engine driving
the pump, is 415,000,000 foot pounds, and hence the work done during
one hour would be 1/24 of 415,000,000, or 17,291,666 foot pounds; the
work done in one minute would be 1/60 of 17,291,666, or 288,194 foot
pounds, and the work done each second would be 1/60 of 288,194, or
4803 foot pounds.
A 1-H.P. engine does 550 foot pounds of work each second, and
therefore if the pump is to be operated by an engine, the strength of
the latter would have to be 8.7 H.P. An 8.7-H.P. pumping engine
working at full speed every second of the day and night would be able
to supply the town with the necessary amount of water. When, however,
we consider the actual height to which the water is raised above the
pumping station, and the extra pumping which must be done in order to
balance the frictional loss, it is easy to understand that in actual
practice a much more powerful engine would be needed. The larger the
piston and the faster it works, the greater is the quantity of water
raised at each stroke, and the stronger must be the engine which
operates the pump.
In many large cities there is no one single pumping station from which
supplies run to all parts of the city, but several pumping stations
are scattered throughout the city, and each of them supplies a
restricted territory.
199. The Bursting of Dams and Reservoirs. The construction of a safe
reservoir is one of the most important problems of engineers. In
October, 1911, a town in Pennsylvania was virtually wiped out of
existence because of the bursting of a dam whose structure was of
insufficient strength to resist the strain of the vast quantity of
water held by it. A similar breakage was the cause of the fatal
Johnstown flood in 1889, which destroyed no less than seven towns, and
in which approximately 2000 persons are said to have lost their lives.
Water presses not only on the bottom of a vessel, but upon the sides
as well; a bucket leaks whether the hole is in its side or its bottom,
showing that water presses not only downward but outward. Usually a
leak in a dam or reservoir occurs near the bottom. Weak spots at the
top are rare and easily repaired, but a leak near the bottom is
usually fatal, and in the case of a large reservoir the outflowing
water carries death and destruction to everything in its path.
If the leak is near the surface, as at _a_ (Fig. 155), the water
issues as a feeble stream, because the pressure against the sides at
that level is due solely to the relatively small height of water
above _a_ (Section 195). If the leak is lower, as at _b_, the issuing
stream is stronger and swifter, because at that level the outward
pressure is much greater than at _a_, the increase being due to the
fact that the height of the water above _b_ is greater than that above
_a_. If the leak is quite low, as at _c_, the issuing stream has a
still greater speed and strength, and gushes forth with a force
determined by the height of the water above _c_.
[Illustration: FIG. 155.--The flow from an opening depends upon the
height of water above the opening.]
The dam at Johnstown was nearly 1/2 mile wide, and 40 feet high, and
so great was the force and speed of the escaping stream that within an
hour after the break had occurred, the water had traveled a distance
of 18 miles, and had destroyed property to the value of millions of
dollars.
If a reservoir has a depth of 100 feet, the pressure exerted upon each
square foot of its floor is 62.5 × 100, or 6250 pounds; the weight
therefore to be sustained by every square foot of the reservoir floor
is somewhat more than 3 tons, and hence strong foundations are
essential. The outward lateral pressure at a depth of 25 feet would be
only one fourth as great as that on the bottom--hence the strain on
the sides at that depth would be relatively slight, and a less
powerful construction would suffice. But at a depth of 50 feet the
pressure on the sides would be one half that of the floor pressure, or
1-1/2 tons. At a depth of 75 feet, the pressure on the sides would be
three quarters that on the bottom, or 2-1/4 tons. As the bottom of the
reservoir is approached, the pressure against the sides increases, and
more powerful construction becomes necessary.
Small elevated tanks, like those of the windmill, frequently have
heavy iron bands around their lower portion as a protection against
the extra strain.
Before erecting a dam or reservoir, the maximum pressure to be exerted
upon every square inch of surface should be accurately calculated, and
the structure should then be built in such a way that the varying
pressure of the water can be sustained. It is not sufficient that the
bottom be strong; the sides likewise must support their strain, and
hence must be increased in strength with depth. This strengthening of
the walls is seen clearly in the reservoir shown in Figure 152. The
bursting of dams and reservoirs has occasioned the loss of so many
lives, and the destruction of so much property, that some states are
considering the advisability of federal inspection of all such
structures.
[Illustration: FIG. 156.--The lock gates must be strong in order to
withstand the great pressure of the water against them.]
200. The Relation of Forests to the Water Supply. When heavy rains
fall on a bare slope, or when snow melts on a barren hillside, a small
amount of the water sinks into the ground, but by far the greater part
of it runs off quickly and swells brooks and streams, thus causing
floods and freshets.
When, however, rain falls on a wooded slope, the action is reversed; a
small portion runs off, while the greater portion sinks into the soft
earth. This is due partly to the fact that the roots of trees by their
constant growth keep the soil loose and open, and form channels, as it
were, along which the water can easily run. It is due also to the
presence on the ground of decaying leaves and twigs, or humus. The
decaying vegetable matter which covers the forest floor acts more or
less as a sponge, and quickly absorbs falling rain and melting snow.
The water which thus passes into the humus and the soil beneath does
not remain there, but slowly seeps downward, and finally after weeks
and months emerges at a lower level as a stream. Brooks and springs
formed in this way are constant feeders of rivers and lakes.
In regions where the land has been deforested, the rivers run low in
season of prolonged drought, because the water which should have
slowly seeped through the soil, and then supplied the rivers for weeks
and months, ran off from the barren slopes in a few days.
Forests not only lessen the danger of floods, but they conserve our
waterways, preventing a dangerous high-water mark in the season of
heavy rains and melting snows, and then preventing a shrinkage in dry
seasons when the only feeders of the rivers are the underground
sources. In the summer of 1911, prolonged drought in North Carolina
lowered the rivers to such an extent that towns dependent upon them
suffered greatly. The city of Charlotte was reduced for a time to a
practically empty reservoir; washing and bathing were eliminated,
machinery dependent upon water-power and steam stood idle, and every
glass of water drunk was carefully reckoned. Thousands of gallons of
water were brought in tanks from neighboring cities, and were emptied
into the empty reservoir from whence it trickled slowly through the
city mains. The lack of water caused not only personal inconvenience
and business paralysis, but it occasioned real danger of disease
through unflushed sewers and insufficiently drained pipes.
The conservation of the forest means the conservation of our
waterways, whether these be used for transportation or as sources of
drinking water.