General Science Part 17

CHAPTER XIX

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.

[Ill.u.s.tration: FIG. 148.--The elevated mountain lake serves as a source of water.]

Figure 148 ill.u.s.trates 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 munic.i.p.al reservoirs and pumps must be calculated. The greater the quant.i.ty 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.

[Ill.u.s.tration: 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).

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 munic.i.p.al projects of the day. Huge aqueducts are being built which will convey pure mountain water a distance of 250 miles, and in quant.i.ties 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 quant.i.ties of water are drawn off from innumerable faucets, and hence the quant.i.ty 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 circ.u.mstances, 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 pa.s.ses from these into the communicating pipes and increases the available supply, while during the night, when the faucets are turned off, water acc.u.mulates 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.

[Ill.u.s.tration: 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).

[Ill.u.s.tration: 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 quant.i.ty 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 quant.i.ty 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_.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 pa.s.ses 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 gla.s.s 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.