=131. Cantilever and Stiffened Suspension Bridges.=—There are two other
types of bridges of later development which have, in recent years,
become prominent by remarkable examples of both completed structure
and design; they are known as the cantilever and stiffened suspension
bridges. Both are adapted to long spans, although the latter may be
applied to much longer spans than the former. A cantilever structure,
with a main span of 1800 feet between centres of piers, is now in
process of construction across the St. Lawrence River at Quebec, while
the well-known Forth Bridge across the Firth of Forth in Scotland has
a main span of 1710 feet. The longest stiffened suspension bridge yet
constructed is the New York and Brooklyn Bridge, with a river span
of about 1595.5 feet between centres of towers, but the stiffened
suspension system has been shown by actual design to be applicable to
spans of more than 3200 feet, with material now commercially produced.
[Illustration: FIG. 41.]
[Illustration: FIG. 42.—Monongahela Bridge—Pittsburgh, Carnegie &
Western Railroad (Wabash), at Pittsburgh.]
=132. Cantilever Bridges.=—Figs. 41 and 42 exhibit in skeleton outline
two prominent cantilever designs for structures in this country.
That shown in Fig. 41 was intended for a bridge across the Hudson
River between Sixtieth and Seventieth streets, New York City. The
main central opening has a span of 1800 feet, and a length of 2000
feet between centres of towers. Fig. 42 shows the Monongahela River
cantilever bridge,[3] now being built at Pittsburgh, Penn. Both figures
exhibit the prominent features of the cantilever system. The main parts
are the towers, at each end of the centre span, which are 534.5 feet
high in the North River Bridge and 135 feet high in the Monongahela
River structure, and the central main or river span with its simple
non-continuous truss hung from the ends of the cantilever brackets or
arms which flank it on both sides. These cantilever arms are simply
projecting trusses continuous with the shore- or anchor-arms. They
rest on the piers at either end of the main span, as a lever rests
on its fulcrum. This arrangement requires the shore extremities or
the anchor-arms to be anchored down by a heavy weight formed by the
masonry piers at those points. Recapitulating and starting from the two
shore ends of the structure, there are the anchor-spans, continuous
at the towers, with the cantilever arms projecting outward toward the
centre of the main opening and supporting at their ends the suspended
truss, which is a simple, non-continuous one. It is thus evident that
the cantilever bridge is a structure composed of continuous trusses
with points of contraflexure permanently fixed at the ends of the
suspended span. The greatest bending moments are at the towers, and
the great depth at that point is given for the purpose of affording
adequate resistance to those moments by the members of the structure.
The following statement shows some elements of the more prominent
cantilever bridges of this country and of the Forth Bridge:
[3] This bridge was designed by and is being constructed under the
direction of Messrs. Boller and Hodge, Consulting Engineers, New York
City.
Length of Cantilever
Name. Opening, Centre to Total Length.
Centre of Towers.
Pittsburgh 812 feet. 1504 feet.
Red Rock (Colo.) 660 ” 990 ”
Memphis (Tenn.) 790.48 ” 2378.2 ”
Forth 1710 ” 5330 ”
The arrangement of web members of cantilever structures is designed to
be such as will transfer the loads from the points of application to
the points of support in the shortest and most direct paths. Both Figs.
41 and 42 show these general results accomplished by an advantageous
arrangement of web members.
It is interesting to note that the first cantilever bridge designed
and built in this country was constructed in 1871. This structure was
designed and erected by the late C. Shaler Smith, a prominent civil
engineer of his day.
=133. Stiffened Suspension Bridges.=—The stiffened suspension bridge is
a structure radically different in its main features and its mode of
transferring load to points of support from any heretofore considered,
except arched ribs. When a load is supported by a beam or truss, the
stresses, either in the web members of the truss or in the solid web
of the beams, travel up and down those members in zigzag directions
with a relatively large amount of metal required for that kind of
transference. That metal is represented by the weight of the web
members of the truss and of the solid web of the beam. Again, there are
two sets of truss members—the chords or flanges, one of which sustains
tension and the other an equal amount of compression. The greater part
of this metal must be so placed and used that the working intensities
of stress are comparatively small. This is particularly the case in
compression members of both chords and webs which constitute the
greater portion of the weight of the truss. All compression members
are known as long columns which sustain not only direct compression
but bending, and the amount of stress or load which they carry per
square inch is relatively small, decreasing as the length increases.
For all these reasons the amount of metal required for both beams and
trusses is comparatively large. In suspension bridges, however, the
conditions requiring the employment of a relatively large amount of
metal with relatively small unit stresses are absent. The main members
of a suspension bridge are the cables and the stiffening trusses, the
latter being light in reference to the length of span. The cables are
subjected to tension only, which is the most economical of all methods
of using metal. A member in tension tends to straighten itself, so
that it is never subjected to bending by the load which it carries.
The opposite condition exists with compression members. Again, grades
of steel possessing the highest ultimate resistance may be used in the
manufacture of cables. It is well known that wire is the strongest form
in which either wrought-iron or steel can be manufactured. While the
ultimate tensile resistance of ordinary structural steel will seldom
rise above 70,000 pounds per square inch, steel wire, suitable to be
used in suspension-bridge cables, may be depended upon, at the present
time, to give an ultimate resistance of at least 180,000 pounds per
square inch. The elastic limit of ordinary structural steel is but
little above half its ultimate resistance, while the elastic limit
of the steel used in suspension-bridge cables is probably not less
than three fourths of its ultimate resistance. It is seen, therefore,
that the high resistance of steel wire makes the steel cable of the
suspension bridge a remarkably economical application of metal to
structural purposes.
The latest example of stiffened suspension-bridge is the new East River
Bridge reaching across the East River from Broadway in Brooklyn to
Delancey Street, New York City, now being built, with a main span of
1600 feet between centres of towers. The entire length of the metal
structure is 7200 feet, and the elevation of the centres of cable at
the tops of the towers is 333 feet above mean high water.
Fig. 43 shows a view of this bridge. Its three principal divisions are
the cables, the stiffening trusses, and the towers. The latter afford
suitable points of support for the cables, which not only extend over
the river span, but are carried back to points on the land where they
are securely attached to a heavy mass of anchorage masonry. These
anchorages must be sufficiently heavy to prevent any load which may
come upon the bridge from moving them by the pull of the cables. It is
usual to make these masses so great that they are capable of resisting
from two to two and a half times the pull of the cables.
[Illustration: FIG. 43.—New East River Bridge.]
=134. The Stiffening Truss.=—The function of the stiffening trusses
is peculiar and imperatively essential to the proper action of the
whole system. If they are absent and a weight should be placed upon
the cable at any point, a deep sag at that point would result. If a
moving load should attempt to pass along a roadway supported by a cable
only, the latter would be greatly distorted, and it would be impossible
to use such a structure for ordinary traffic. Some means must then
be employed by which the cable shall maintain essentially the same
shape and position, whatever may be the amount of loading. It can be
readily shown that if any perfectly flexible suspension-bridge cable
carries a load of uniform intensity over the span from one tower to
the other, the curve of the cable will be a parabola, with its vertex
at the lowest point. Furthermore, it can also be shown that if any
portion of the span be subjected to a uniform load, the corresponding
portion of the cable will also assume a parabolic curve. It is assumed
in all ordinary suspension-bridge design that the total weight of the
structure, including the cables and the suspension-rods which connect
the stiffening trusses to the cable, is uniformly distributed over
the span, and that assumption is essentially correct. So far as the
weight of the structure is concerned, therefore, the curve of the cable
will always be parabolic. It only remains, therefore, to devise such
stiffening trusses as will cause any moving load passing on or over
the bridge to be carried uniformly to the cables throughout the entire
span. This condition means that if any moving load whatever covers any
portion of the span, the corresponding pull of the suspension-rods on
the cables must be uniform from one tower to the other, and that result
can be practically accomplished by the proper design of stiffening
trusses; it is the complete function of those trusses to perform just
that duty.
=135. Location and Arrangement of Stiffening Trusses.=—It has been,
and is at the present time to a considerable extent, an open question
as to the best location and arrangement of the stiffening trusses. The
more common method in structures built is that illustrated by the New
York and Brooklyn and the new East River bridges. Those stiffening
trusses are uniform in depth, extending from one tower to the other, or
into the land spans, and connected with the cables by suspension-rods
running from the latter down to the lower chords of the trusses. It is
obvious that the floor along which the moving load is carried must have
considerable transverse stiffness, and hence it may appear advisable
to place the stiffening trusses so that the floor may be carried by
them. On the other hand, some civil engineers maintain that it is a
better distribution of stiffening metal to place it where the cables
themselves may form members of the stiffening trusses, with a view to
greater economy of material.
Figs. 44, 45, and 46 illustrate some of the principal proposed methods
of constructing stiffening trusses in direct connection with the
cables. The structure shown in Fig. 44 illustrates the skeleton design
of the Point Bridge at Pittsburgh. The curved member is a parabolic
cable composed of eye-bars. This parabolic cable carries the entire
weight of the structure and moving load when uniformly distributed. If
a single weight rests at the centre, the two straight members of the
upper chord may be assumed to carry it. If a single weight rests at any
other point of the span, it will be distributed by the bracing between
the straight and curved members of the stiffening truss. Obviously the
most unbalanced loading will occur when one half of the span is covered
with moving load. In that case the bowstring stiffening truss in either
half of Fig. 44 will make the required distribution and prevent the
parabolic tension member from changing its form.
[Illustration: FIG. 44.]
[Illustration: FIG. 45.]
[Illustration: FIG. 46.]
The type of bracing shown in Fig. 45 possesses some advantages of
a peculiar nature. Each curved lower chord of the stiffening truss
corresponds to the position of the perfectly flexible cable with
the moving load covering that half of the span which belongs to the
greatest sag of the cable. The two parabolic cables thus cross each
other in a symmetrical manner at the centre of the span. If the moving
load covers the entire span, the line of resistance or centre line of
imaginary cable will be the parabola, shown by the broken line midway
along each crescent stiffening truss. The diagonal bracing placed
between the cables is so distributed and applied as to maintain the
positions of cables under all conditions of loading.
The mode of constructing the stiffening truss between two cables, shown
in Fig. 46, is that adopted by Mr. G. Lindenthal in his design for a
proposed stiffened suspension bridge across the Hudson River with a
span of about 3000 feet. The two cables are parabolic in curvature and
may be either concentric or parallel. This system of stiffening bracing
possesses some advantages of uniformity and is well placed to secure
efficient results. The same system has been used in suspension bridges
of short span by Mr. Lindenthal at both St. Louis and Pittsburgh. The
stiffening bracing produces practically a continuous stiffening truss
from one tower to the other, whereas the systems shown in Figs. 44 and
45 involve practically a joint at the centre of the span.
In all these three types of vertical stiffness the floor is designed
to meet only the exigencies of local loading, being connected with the
stiffening truss above by suspension bars or rods, preferably of stiff
section.
When stiffening trusses are placed along the line of the floor, as in
the case of the two East River bridges, to which reference has already
been made, those trusses need not necessarily be of uniform depth, and
they may be continuous from tower to tower or jointed at the centre,
like those of the New York and Brooklyn suspension bridge. This centre
joint detracts a little from the stiffness of the structure, but in a
proper design this is not serious.
=136. Division of Load between Cables and Stiffening Truss.=—In a case
where continuous stiffening trusses are employed it is obvious that
they may carry some portion of the moving load as ordinary trusses.
The portion so carried will be that which is required to make the
deflection of the stiffening truss equal to that of the cable added
to the stretch of the suspension-rods. In the old theory of the
stiffening truss constructed along the floor of the bridge this effect
was ignored, and the computations for the stresses in those trusses
were made by the aid of equations of statical equilibrium only. That
assumption, that the cable carried the entire load, was necessary
to remove the ambiguity which would otherwise exist. In modern
suspension-bridge design those trusses may be assumed continuous from
tower to tower with their ends anchored at the towers, or they may be
designed to be carried continuously through portions of the land spans
and held at their extremities by struts reaching down to anchorages,
so that those ends may never rise nor fall, but move horizontally if
required. If there are no pin-joints in the trusses at the centre
and ends of the main span, equations of statical equilibrium are not
sufficient to enable the reactions under the trusses and the horizontal
component of cable tension to be found.
One of the best methods of procedure for such cases is that of
least work, in which the horizontal component of cable tension is
so found that the total work performed in the elastic deflection of
the stiffening trusses, suspension-rods, cables, and towers is a
minimum. After having found this horizontal component of the cable
tension and the reactions under the stiffening trusses, the stresses
in all the members of the entire structure can be at once determined.
It is obvious that the stiffening truss and the cables must deflect
together. It is equally evident that the deeper the stiffening trusses
are the more load will be required to deflect them to any given
amount, and hence that the deeper they are the more load they will
carry independently of the cable. It is desirable to throw as much of
the duty of carrying loads upon the cables as possible. It therefore
follows that the stiffening trusses should be made as shallow as the
proper discharge of their stiffening duties will permit.
=137. Stresses in Cables and Moments and Shears in Trusses.=—The
necessary limits of this discussion will not permit even the simplest
analyses to be given. It is evident, however, that the greatest
cable stresses will exist at the tops of the towers, and that if the
horizontal component of cable tension be found by any proper method,
the stress at any other point will be equal to that horizontal
component multiplied by the secant of cable inclination to a horizontal
line, it being supposed that the suspenders are found in a vertical
plane.
If the stiffening trusses are jointed at the centre of the main span,
as well as at the ends, the simple equations of statical equilibrium
are sufficient in number to make all computations, for the reason that
the centre pin-joint gives the additional condition that, whatever
may be the amount or distribution of loading, the centre moment must
be zero. If _l_ is the length of main or centre span and _p_ the
moving load per linear foot of span, and if the stiffening trusses run
from tower to tower, the following equations will give their greatest
moments and shears both by the old and new theory of the stiffening
truss.
_p_ = load per lin. ft., _l_ = length of span in ft.,
Old theory. New theory.
Max. moment _M_ = 0.01856_pl_² _M_ = 0.01652_pl_²}no centre
Max. shear _S_ = ⅛_pl_ _S_ = ⅛_pl_ } hinge.
With centre hinge _M_ = 0.01883_pl_² and _S_ = ⅛_pl_
The details of the theory of stiffening trusses for suspension
bridges have been well developed during the past few years and are
fully exhibited in modern engineering literature. The long spans
requiring stiffened suspension bridges are usually found over navigable
streams, and hence those bridges must be placed at comparatively
high elevations. This is illustrated by the clear height of 135 feet
required under the East River suspension-bridge structures already
completed and in progress. Furthermore, the heights of towers above
the lowest points of the cables usually run from one eighth to one
twelfth of the span. These features expose the entire structure to
comparatively high wind pressures, which must be carefully provided
against. This is done by the requisite lateral bracing between the
stiffening trusses and by what is called the cradling of the cables.
The latter expression simply means that the cables as they are built
are swung out of a vertical plane and toward the axis of the structure,
being held in that position by suitable details. The cables on opposite
sides of the bridge are thus moved in toward each other so as to
produce increased stability against lateral movement. Occasionally
horizontal cables are stretched between the towers in parabolic curves
in order to resist horizontal pressures, just as the main cables carry
vertical loads. This matter of stability against lateral wind pressures
requires and receives the same degree of careful consideration in
design as that accorded to the effects of vertical loading. The same
general observation applies also to the design of the towers.
=138. Thermal Stresses and Moments in Stiffened Suspension
Bridges.=—All material used in engineering structures expands and
contracts with rising and falling temperatures to such an extent
that the resulting motions must be provided for in structures of
considerable magnitude. In ordinary truss-bridges one end is supported
upon rollers, so that as the span changes its length the truss ends
move the required amount upon the rollers. In the case of stiffened
suspension bridges, however, the ends of the cables at the anchorages
are rigidly fixed, so that any adjustment required by change of
temperature must be consistent with the change of length of cable
between the anchorages. The backstays, which are those portions of
the cables extending from the anchorages to the tops of the towers,
expand and contract precisely as do the portions of the cable between
the tops of the towers. As the cables lengthen, therefore, the sag or
rise at the centre of the main span will be due to the change in the
entire length of cable from anchorage to anchorage. In order to meet
this condition it is usual to support the cables at the tops of the
towers on seats called saddles which rest upon rollers, so as to afford
any motion that may be required. Designs have been made in which the
cables are fixed to the tops of steel towers. In such cases changes
of temperature would subject the towers to considerable bending which
would be provided for in the design.
The rise and fall at the centres of long spans of stiffened suspension
bridges is considerable; indeed, for a variation of 120° Fahr. the
centre of the New York and Brooklyn Bridge changes its elevation by
4.6 feet if the saddles are free to move, as intended. In the case of
a stiffened suspension bridge designed to cross the North River at New
York City with a main span of 3200 feet a variation of 120° Fahr. in
temperature would produce a change of elevation of the centre of the
span of 6.36 feet. Such thermal motions in the structure obviously will
produce stresses of considerable magnitude in various parts of the
stiffening trusses, all of which are invariably recognized and provided
for in good design.
=139. Formation of the Cables.=—At the present time suspension-bridge
cables are made by grouping together in one cylindrical mass a large
number of so-called strands or individual small cables, each composed
of a large number of parallel wires about one sixth of an inch in
diameter. The four cables of the New York and Brooklyn Bridge are each
composed of 19 strands, each of the latter containing 332 parallel
wires, making a total of 6308 wires, the cables themselves being
15½ inches in diameter. The wire is No. 7 gauge, i.e., 0.18 inch in
diameter. In the new East River Bridge each of the four cables is 18¼
inches in diameter and contains 37 strands, each strand being composed
of 208 wires all laid parallel to each other, or a total of 7696 wires.
The size of the wire is No. 6 (Roebling) gauge, i.e., 0.192 inch in
diameter. These strands are formed by laying wire by wire, each in
its proper place. The strands are then bound together into a single
cable, around which is tightly wound a sheathing or casing of smaller
wire, 0.134 inch in diameter for the New York and Brooklyn Bridge. The
tightness of this binding wire insures the unity of the whole cable,
each wire having been placed in its original position so as to take a
tension equal to that of each of the other wires. The suspension-rods
are usually of wire cables and are attached by suitable details to the
lower chords of the stiffening truss, also by specially designed clamps
to the cable. The stiffening trusses are usually built with all riveted
joints, so as to secure the greatest possible stiffness from end to
end. The stiffened suspension bridge has been shown by experience, as
well as by theory, to be well adapted to carry railroad traffic over
long spans.
=140. Economical Limits of Spans.=—In the past, suspension bridges
have, in a number of cases, been built for comparatively short spans,
but it is well recognized among engineers that their economical use
must be found for spans of comparatively great length. While definite
lower limits cannot now be assigned to such spans, it is probable that
with present materials of construction and with available shop and
mill capacities the ordinary truss-bridge may be economically used up
to spans approximately 700 to 800 feet, and that above that limit the
cantilever system is economically applicable to lengths of span not yet
determined but probably between 1600 and 2000 feet. The special field
of economical employment of the long-span stiffened suspension bridge
will be found at the upper limit of the cantilever system. So far as
present investigations indicate, the stiffened suspension type of
structure may be employed to advantage from about 1800 feet up to the
maximum practicable length of span not yet assignable, but perhaps in
the vicinity of 4000 feet. Obviously such limits are approximate only
and may be pushed upward by further improvements in the production of
material and in the enlargement of both shop and mill capacity.
PART III.
_WATER-WORKS FOR CITIES AND TOWNS._
CHAPTER XIV.
=141. Introductory.=—A preceding lecture in this course has shown to
what an advanced state the public supply of water to large cities was
developed in ancient times. The old Romans, Greeks, Egyptians, and
other ancient peoples evidently possessed an adequate appreciation of
the value of efficient systems of public water-supply. Very curiously
that appreciation diminished so greatly as almost to disappear
during the middle ages. The demoralization of public spirit and the
decrease of national power which followed the fall of Rome induced, in
their turn, among other things, a neglect of the works of the great
water system of Rome, entailing their partial destruction. The same
retrogression in civilization seemed to affect other ancient nations
as well, until probably the lowest state of the use of public waters
and the construction of public water systems was reached somewhere
between A.D. 1000 and A.D. 1300 or 1400. Without reasonable doubt the
terrible epidemics or plagues of the middle ages can be charged to the
absence of suitable water-supplies and affiliated consequences. During
that middle period of the absence of scientific knowledge and any
apparent desire to acquire it, sanitary works and consequently sanitary
conditions of life were absolutely neglected. No progress whatever was
made toward reaching those conditions so imperative in large centres of
population for the well-being of the community. Grossly polluted waters
were constantly used for public and private supplies, and no efforts
whatever were made among the masses toward the suitable disposition
of refuse matters or, in a word, to attain to sanitary conditions of
living.
A few important works were completed, particularly in Spain, but
nothing indicative of general relief from the depths of ignorance and
sanitary demoralization to which the greater portion of the civilized
world had sunk at that time. The city of Paris took all its water from
the Seine, except that which was supplied by a small aqueduct built in
1183. So small was the supply, aside from the water obtained from the
river, that in 1550 it is estimated that the former amounted to about
one quart only per head of population per day. The situation in London
was equally bad, for it was only in the first half of the thirteenth
century that spring-water was brought to the city by means of lead
pipes and masonry conduits. Public water-works began to be constructed
in Germany on a small scale in the early part of the fifteenth century.
Obviously no pumps were available in those early days of water-supply,
so that the small systems which have been mentioned were of the
gravity class; that is, the water flowed naturally in open or closed
channels from its sources to the points of consumption. Pumps of a
simple and crude type first began to be used at a point on the old
London Bridge in 1582, and in Hanover in 1527. Subsequently to those
dates other pumps were set up on London Bridge, and installations
of the same class of machinery were made in Paris in 1608, usually
operated by water-power in some simple manner, as by the force of the
water-currents. In 1624 the Paris supply received a reinforcement of
200,000 gallons per day by the completion of the aqueduct Arcueil. The
New River Company was incorporated in 1619 for the partial supply of
the city of London, and it began to lay its pipes at that time. As its
name indicates, it took its supply from New River, and the inception of
its business is believed to mark the first application of the principle
of supplying each house with water. This company is still in existence
and furnishes a considerable portion of the present London supply.
=142. First Steam-pumps.=—The application of steam to the creation or
development of power by Watt, near the end of the eighteenth century,
stimulated greatly the construction of water-works, as it offered a
very convenient and economical system of pumping. It seems probable
that the first steam-pumps were used in London in 1761. Twenty years
later a steam-pump was erected in Paris, while another was installed in
1783. The second steam-pump in London was probably constructed in 1787.
In all these earlier instances of the use of steam-pumps river supplies
were naturally used.
=143. Water-supply of Paris and London.=—After the early employment of
steam pumping-machinery demonstrated its great efficiency for public
water-supplies, the extension of the latter became more rapid, and
since 1800 the supplies of the two great cities of London and Paris
have been greatly increased. As late as 1890 the Paris supply amounted
to about 65 gallons per head of population, one fourth of which was
used as potable, being drawn from springs, while three fourths, drawn
from rivers, was used for street-cleaning or other public purposes.
This supply, however, was found inadequate and was re-enforced in 1892
by an addition of 30,000,000 gallons per day of potable water brought
to the city by an aqueduct 63 miles long. Another addition of about
15,000,000 gallons has been provided more recently.
Rather curiously the water-supply of London is afforded by eight
private companies, one of which is the old New River Company already
mentioned. These companies, with one exception, draw their supply
mainly from the rivers Thames and Lea, all such water being filtered.
The remaining company draws its water from deep wells driven into the
chalk. The total population supplied amounts to about 5,500,000, the
rate of supply being thus less than 45 gallons per head per day.
=144. Early Water-pipes.=—Inasmuch as the use of cast-iron for pipes
was only begun about the year 1800, other materials were used prior
to that date. As is well known, the pipes used in ancient water-works
were either of lead or earthenware. In the eighteenth century wooden
pipes made of logs with their centres bored out were used, sometimes
6 or 7 inches in diameter. As many lines of these log pipes were used
as needed to conduct a single line of supply. In the earlier portion
of the nineteenth century such log pipes, usually of pine or spruce,
were used by the old Manhattan Company for the supply of New York
City. A section of such a wooden pipe, with a bore of about 2½ inches
is preserved in the museum of the Department of Civil-Engineering of
Columbia University. Large quantities of such pipes were formerly used.
=145. Earliest Water-supplies in the United States.=—The earliest
system of public water-supply in this country was completed for the
city of Boston in 1652. This was a gravity system. It is believed
that the first pumping-machinery for such a supply was set up for the
town of Bethlehem, Pa., and put in operation in 1754. Subsequently
water-supplies were completed for Providence, R. I., 1772, and for
Morristown, N. J., in 1791; the latter has maintained a continuous
existence since that date. The first use of steam pumping-machinery in
this country was in Philadelphia in 1800. This machinery, curiously
enough, was largely of wood, including some portions of the boiler; it
was necessarily very crude and would perform with 100 pounds of coal
only about one twenty-fifth or one thirtieth of what may be expected
from first-class pumping-machinery at the present time. Other cities
and towns soon began to follow the lead of these earlier municipalities
in the construction of public water-supplies, but the principal
development in this class of public works has taken place since about
1850.
It is estimated that the total population supplied in 1880 was about
12,000,000, which rose to about 23,000,000 in 1890, and it is probably
not less than 50,000,000 at the present time.
=146. Quality and Uses of Public Water-supply.=—Advances in the public
supplies in this country have been made rather in the line of quantity
than quality. Insufficient attention has been given both to the
quality of the original supply and to the character of the reservoirs
in which it is gathered until within possibly the past decade. A few
cities like Boston have scrutinized with care both the quality of the
water and the character of the bottom and banks of reservoirs, and
have spared neither means nor expense to acquire a high degree of
excellence in their potable water. The same observations can be applied
to a few other large cities, but to a few only. The realization of
the dependence of public health upon the character of water-supply,
however, has been rapidly extending, and it will doubtless be but a
short time before the care exercised in collecting and preparing water
for public use will be as great in this country as in Europe, where few
large cities omit the filtration of public waters.
The distribution of water supplied for public use is not limited to
domestic purposes, although that class of consumption controls public
health so far as it is affected by the consumption of water. The
applications of water to such public purposes as street-cleaning and
the extinguishing of fires are of the greatest importance and must
receive most careful consideration. Again, the so-called system of
water-carriage in the disposal of domestic and manufacturing wastes,
constituting the field of sewage-disposal, depends wholly upon the
efficiency of the water-supply.
=147. Amount of Public Water-supply.=—The first question confronting
an engineer in the design of public water-supply is the amount which
should be provided, usually stated on the basis of an estimated
quantity per head of population. This is not in all cases completely
rational, but it is by far the best basis available. If the
water-supply is designed for a small city or town previously supplied
by wells or other individual sources, the first year’s consumption
will be low per head of population for the reason that many people
will retain their own sources instead of taking a share of the public
supply. As time elapses that portion of population decreases quite
rapidly in numbers, and in a comparatively few years practically the
whole population will use the public supply. In communities, therefore,
where public systems have long existed and it is desired either to
add to the old supply or to install new ones, the only safe basis of
estimate is the entire population.
=148. Increase of Daily Consumption and the Division of that
Consumption.=—The amount of water required per head of population might
naturally be assumed identical with the past consumption, but that
would frequently be incorrect. It is one of the most prominent features
of the history of public water-supplies in this country that the
consumption per head of population has increased with great rapidity
from the early years of the installation of the different systems,
for reasons both legitimate and illegitimate. The daily average
consumption of water from the Cochituate Works of the Boston supply
increased from 42 gallons per head of population in 1850 to 107 gallons
in 1893, and in the Mystic Works of the same supply the increase was
from 27 gallons in 1865 to 89 gallons in 1894. Again, the daily average
consumption in Chicago rose from 43 gallons per head per day in 1860
to 147 gallons in 1893, while in Philadelphia during the same period
the increase was from 36 gallons per head per day to 150 gallons. In
Cambridge, Mass., the increase in daily average consumption per head
of population was from 44 gallons in 1870 to 70 gallons in 1894. These
instances are sufficient to show that, under existing conditions, the
daily consumption was increased at a rapid rate in the cities named,
and they have been selected as fairly representative of the whole
field. Civil engineers have made extended studies in connection with
this question in a great number of cities, for it bears upon one of the
most important lines of public works. It is absolutely essential to
the health and business prosperity of every city that the water-supply
should be abundant, safe, and adapted to the industrial and commercial
pursuits of its population. It is imperative, therefore, that the
division of the daily supply should be carefully analyzed. For this
purpose the water-supply of a city may be, and frequently is, divided
into four parts:
(1) That used for domestic purposes;
(2) That used for commercial and industrial purposes;
(3) That used for public purposes;
(4) That part of the supply which is wasted.
1. That portion of the supply consumed for domestic purposes includes
not only the water used in private residences, but in those branches
of consumption which may be considered of a household character found
in hotels, clubs, stores, markets, laundries, and stables, or for
any other residential service. As might be expected, this branch of
consumption varies largely from one city to another. The results
of one of the most interesting and suggestive studies ever made in
connection with this subject are given by Mr. Dexter Brackett, M. Am.
Soc. C. E., in the Transactions of the American Society of Civil
Engineers for 1895. In Boston the purely domestic consumption varied
in different houses and apartments from 59 gallons per head per day
in costly apartments down to 16.6 gallons per head per day in the
poorest class of apartment. In Brookline, one of the finest suburbs of
Boston, the quantity was 44.3 gallons per day. In some other cities
of Massachusetts, as Newton, Fall River, and Worcester, this class of
consumption varied from 6.6 gallons to 26.5 gallons per day, the latter
quantity being found at Newton in some of the best residences, and the
former at houses also in Newton having but one faucet each. In Yonkers,
N. Y., where the system was metered, the amount was 21.4 gallons per
head of population per day, while in portions of London, England, it
varied from 18.6 to 25.5 gallons per head per day. The average of these
figures gives a result of 18.2 gallons per head per day, which, in
round numbers, may be put at 20 gallons.
2. It is obvious that the rate of consumption for commercial and
industrial purposes in any city must vary far more than that for
domestic purposes, for the reason that some cities may be essentially
residential in character while others may be essentially manufacturing.
At the same time, it is to be remembered that many manufacturing
establishments may have their own water-supply. The city of Fall River,
Mass., is eminently a manufacturing city, yet Mr. Brackett found that
the manufacturing demand on the public water-supply amounted to 2
gallons only per inhabitant per day, as the manufacturers draw the
most of their supply from the river, but that where the manufacturers
depend upon the public supply for all their water the amount rises to
a value between 20 and 30 gallons per inhabitant. In Boston in 1892
the water consumed for all manufacturing and industrial purposes,
including railroads, gas-works, elevators, breweries, etc., amounted
to 9.24 gallons per head of population per day, while in Yonkers in
1897 the total consumption for commercial purposes was 27.4 gallons per
head per day. In the city of New York, as nearly as can be estimated,
the consumption for commercial purposes is probably not far from 25
gallons per inhabitant per day. Reviewing all these results, it may
be stated that the water consumption for commercial and industrial
purposes will generally range from 10 to 30 gallons per inhabitant per
day.
3. The consumption of water for public purposes is a smaller amount
than either of the two preceding. It covers such uses as public
buildings, schools, street-sprinkling, sewer-flushing, fountains,
fires, and other miscellaneous objects, more or less similar to those
just named. The total use of this character was 3.75 gallons per
inhabitant per day for Boston in 1892, and 5.57 gallons per inhabitant
per day for Fall River in 1899. A few other cities give the following
results: Minneapolis in 1897, 5 gallons; Indianapolis, 3 gallons;
Rochester, N. Y., 3 gallons; Newton, Mass., 4 gallons; Madison, Wis.,
10 gallons. In Paris it is estimated that not far from 2.5 gallons per
head of population per day are used. It is probable, therefore, that an
amount of 5 gallons per day per inhabitant will cover this particular
line of consumption.
4. A substantial portion of the water-supply of every city fails to
serve any useful purpose, for the reason that it runs to waste either
by intention or by neglect. The sources of this waste are defective
plumbing, including leaky faucets and cocks; deliberate omission to
close faucets and cocks, constituting wilful waste; defective or broken
mains, including leaky joints; and waste to prevent freezing.
=149. Waste of Public Water.=—All these wastes except the last are
inexcusable. There is no difficulty in detecting defective plumbing,
and its existence is generally known to the householder; but if the
wasted water is not measured and paid for, it is far too frequently
considered more economical to continue the waste than to pay for
the plumber’s services. In a multitude of cases cocks are left open
indefinitely for all sorts of insignificant reasons; in closets,
under the erroneous impression that the continuous running of the
stream will materially aid in a more effective cleansing of soil-
and sewer-pipes, failing completely to appreciate that a far more
powerful stream is required for that purpose; sometimes in sinks, for
refrigerating purposes, and in many other inexcusably wrong ways.
These sources of wilful waste lead to large losses and constitute
one of the most unsatisfactory phases of administration of a public
water system. Such losses result in a vicious waste of public money.
The amount of water flowing from leaky joints and from leaks in pipes
and mains is necessarily indeterminate because it escapes without
evidence at the surface except in rare cases. In every instance where
examinations have been made and a careful record kept of the amount
of water supplied to a city, it has been found that the aggregate of
the measured amounts consumed fail nearly to equal the total supply.
There are probable errors both in the measurement of the quantities
supplied and in the quantities consumed, but the large discrepancy
cannot be accounted for in this manner. In many cases consumed water
has even been carefully measured by meters, as at Yonkers, New York,
Newton, Milton, and Fall River, Mass., Madison, Wis., and at other
places, but yet the discrepancy appears to be nearly as wide as ever.
Again, in 1893 observations were carefully made on the consumption of
the water received by the Mystic supply of the Boston system at _all_
hours of the twenty-four. Obviously between 1 and 4 A.M. the useful
consumption should be nearly nothing, but, on the contrary, it was
found to be nearly 60 per cent of the average hourly consumption for
the entire twenty-four hours. The waste at Buffalo, N. Y., in 1894 was
estimated at 70 per cent of the total supply. Similar observations
in other places have given practically the same results. It has also
been found that, in a number of instances, where old watercourses have
been completely obliterated by considerable depths of filling required
by the adopted grades of city streets and lots, and excavations for
buildings have subsequently been opened practically the full volume of
the former streams are flowing along the original but filled channel.
This result has been observed under a practically impervious paved
city surface. It is difficult to imagine the source of such a supply
except from defective pipe systems or sewers. A flow of a least 100,000
gallons per day from a broken pipe which found its way into a sewer has
also been discovered without surface evidence. These and many other
results of experience conclusively demonstrate that much water flows
to waste unobserved from leaky joints and defective or broken pipes.
Inasmuch as cast-iron water-pipes are produced in lengths which net 12
feet as laid, there will be at least 440 joints per mile. Furthermore,
as leaky joints and broken pipes are as likely to occur at one place
as another, it seems reasonable to estimate leakage through them as
proportionate to the length of the pipe-line in a system; and that
conventional law is frequently assumed. New pipe-lines have sometimes
shown a leakage of 500 to 1200 gallons per mile of line per day. Civil
engineers have sometimes specified the maximum permissible leakage of
a new pipe-line at 60 to 80 gallons per mile of line per day for each
inch in diameter of pipe, thus permitting 600 to 800 gallons to escape
from a 10-inch pipe. In 1888 the late Mr. Chas. B. Brush reported a
leakage of about 6400 gallons per mile per day from a practically new
24-inch cast-iron main, 11 miles long, of the Hackensack Water Company,
the pressure being 110 pounds per square inch. Tests of water-pipes in
German and Dutch cities have been reported as showing less waste than
300 gallons per mile per day, but such low results, unless for very low
pressures and short lines, may reasonably be doubted. Obviously losses
of this character will probably increase with the age of the pipe. By
a very ingenious procedure based upon his own experience, Mr. Emil
Kuichling of Rochester, N. Y., reaches the conclusion that a reasonable
allowance for the waste from leaky joints and defective pipes is 2500
to 3000 gallons per mile of cast-iron pipe-line per day. If, as is
frequently the case, the population per mile of pipe ranges from 300 to
1000, the preceding allowance amounts to 3 to 10 gallons per head of
population per day. The loss or waste due to running cocks or faucets
to prevent freezing cannot be estimated with sufficient accuracy to
receive a definite valuation, but it must be considered an element of
the total item of waste.
=150. Analysis of Reasonable Daily Supply per Head of Population.=—It
has repeatedly been found that the losses or wastes set forth in the
preceding statements amount apparently to quantities varying from 30 to
50 per cent of the total supply; or, to put it a little differently,
the water unaccounted for in even the best systems now constructed
apparently may reach one third to one half of the total supply. This
is an exceedingly wasteful and unbusinesslike showing. It is probable
that the statement is, to some extent at least, an exaggeration. It
is practically certain that either the amount supplied or the amounts
consumed, or both, are never measured with the greatest accuracy, and
that the errors are such as generally swell the apparent quantity
wasted. After making judicious use of the data thus afforded by
experience, it is probable that the following tabular statement given
by Messrs. Turneaure and Russell represents limits within which should
be found the daily average supply of water in a well-constructed and
well-administered system.
+---------------------------+--------------------------------+
| | Gallons per Head per Day. |
| Use. +----------+----------+----------+
| | Minimum. | Average. | Maximum. |
+---------------------------+----------+----------+----------+
| Domestic | 15 | 25 | 40 |
| Industrial and commercial | 5 | 20 | 35 |
| Public | 3 | 5 | 10 |
| Waste | 15 | 25 | 30 |
+---------------------------+----------+----------+----------+
| Total | 38 | 75 | 115 |
+---------------------------+----------+----------+----------+
The values given in the preceding table are reasonable and sufficient
to supply the legitimate needs of any community, but, as will be shown
in the succeeding table, there are cities in this country whose average
consumption is more than twice the maximum rate given above.
=151. Actual Daily Consumption in Cities of the United States.=—The
following table exhibits the average daily consumption of water
throughout the entire year for the cities given, as determined for the
years indicated in the table.
The city of Buffalo shows a daily consumption of 271 gallons per
inhabitant, and Allegheny, Pa., 247 gallons per inhabitant. There are a
considerable number showing an average daily consumption per inhabitant
of 160 gallons or more. All such high averages exhibit extravagant use
of water, or otherwise inefficient administration of the water-supply.
The reduction of such high rates of consumption is one of the most
difficult problems confronting the administration of public works. The
use of the meter has proved most efficient in preventing wastes or
other extravagant consumption, as in that case every consumer pays a
prescribed rate for the amount which he takes.
TABLE I.
-------------------+-----------+----------+-------+-----------
| | | |Consumption
| | | Per | per
|Population.|Population|Cent of| Inhabitant
| | per Tap. | Taps | Daily,
| | |Metered| Gallons.
-------------------+-----------+----------+-------+-----------
| 1890. | 1890. | 1890. | 1890.
-------------------+-----------+----------+-------+-----------
New York | 1,515,301 | 13.9 | 20.2 | 79
Chicago | 1,099,850 | 7.1 | 2.5 | 140
Philadelphia | 1,046,964 | 6.1 | 0.3 | 132
Brooklyn | 838,547 | 8.7 | 2.5 | 72
St. Louis | 451,770 | 11.8 | 8.2 | 72
Boston | 448,477 | 6.6 | 5.0 | 80
Cincinnati | 305,891 | 8.5 | 4.1 | 112
San Francisco | 298,997 | 9.9 | 41.4 | 61
Cleveland | 270,055 | 8.7 | 5.8 | 103
Buffalo | 255,664 | 6.3 | 0.2 | 186
New Orleans | 242,039 | 54.0 | 0.4 | 37
Washington | 230,392 | 6.5 | 0.3 | 158
Montreal | 216,000 | 5.3 | 1.7 | 67
Detroit | 205,876 | 5.1 | 2.1 | 161
Milwaukee | 204,468 | 11.1 | 31.9 | 110
Toronto | 181,000 | 4.0 | 4.1 | 100
Minneapolis | 164,738 | 16.5 | 6.3 | 75
Louisville | 161,129 | 11.9 | 5.9 | 74
Rochester | 133,896 | 5.4 | 11.4 | 66
St. Paul | 133,156 | 12.7 | 4.2 | 60
Providence | 132,146 | 9.4 | 62.4 | 48
Indianapolis | 105,436 | 35.6 | 7.6 | 71
Allegheny | 105,287 | 7.0 | 0 | 238
Columbus | 88,150 | 11.5 | 6.4 | 78
Worcester | 84,655 | 8.9 | 89.4 | 59
Toledo | 81,434 | 18.6 | 9.4 | 72
Lowell | 77,696 | 9.2 | 22.9 | 66
Nashville | 76,168 | 14.9 | 0.8 | 146
Fall River | 74,398 | 14.9 | 74.6 | 29
Atlanta | 65,533 | 20.0 | 89.6 | 36
Memphis | 64,495 | 11.9 | 3.7 | 124
Quebec | 63,000 | 10.4 | 0 | 160
Dayton, O. | 61,220 | 20.0 | 3.8 | 47
Camden, N. J. | 58,313 | . . . | . . . | 131
Des Moines, Ia. | 50,093 | 20.0 | 60.0 | 55
Ottawa, Ont. | 44,000 | 4.2 | 0 | 130
Yonkers, N. Y. | 32,033 | 12.0 | 82.4 | 68
Newton, Mass. | 24,379 | 5.5 | 67.4 | 40
Madison, Wis. | 13,426 | 11.0 | 31.0 | 40
Albany, N. Y. | 98,000 | . . . | 0.4 | 162
New Bedford, Mass. | 55,000 | . . . | . . . | 99
Springfield, Mass. | 49,299 | . . . | . . . | 87
Holyoke, Mass. | 40,000 | . . . | . . . | 77
-------------------+-----------+----------+-------+-----------
-------------------+-------+-----------+-----------
| |Consumption|Consumption
| Per | per | per
|Cent of| Inhabitant| Inhabitant
| Taps | Daily, | Daily,
|Metered| Gallons. |Gallons.
-------------------+-------+-----------+-----------
| 1895. | 1895. | 1900.
-------------------+-------+-----------+-----------
New York | 27.0 | 100 | 115
Chicago | 2.8 | 139 | 190
Philadelphia | 0.74 | 162 | 229
Brooklyn | 1.9 | 89 | . . .
St. Louis | 7.4 | 98 | 111
Boston | 5.2 | 100 | 143
Cincinnati | 6.5 | 35 | 121
San Francisco | 28.0 | 63 | 73
Cleveland | 4.5 | 142 | 175
Buffalo | 0.85 | 271 | 262
New Orleans | . . . | 35 | 48
Washington | 1.5 | 200 | 174
Montreal | 1.6 | 83 | . . .
Detroit | 8.2 | 152 | 156
Milwaukee | 51.0 | 101 | 84
Toronto | 3.7 | 100 |
Minneapolis | 16.0 | 88 | 93
Louisville | 6.6 | 97 | . . .
Rochester | 18.0 | 71 | 83
St. Paul | 1.7 | 60 | 51
Providence | 74.0 | 57 | 54
Indianapolis | 7.1 | 74 | 79
Allegheny | 7.1 | 247 | . . .
Columbus | 9.3 | 127 | 183
Worcester | 90.0 | 66 | 67
Toledo | 35.0 | 70 | 59
Lowell | 33.0 | 82 | 83
Nashville | 24.0 | 139 | 140
Fall River | 82.0 | 35 | 35
Atlanta | 99.0 | 42 | 61
Memphis | 4.6 | 100 | 98
Quebec | 0 | 170 | . . .
Dayton, O. | 24.0 | 50 | 62
Camden, N. J. | 0 | 200 | 185
Des Moines, Ia. | 42.6 | 43 | 48
Ottawa, Ont. | 0 | 0 | . . .
Yonkers, N. Y. | 99.8 | 100 | 76
Newton, Mass. | 77.3 | 65 | 62
Madison, Wis. | 61.0 | 52 | 44
Albany, N. Y. | 12.3 | . . . | 192
New Bedford, Mass. | 15.4 | . . . | 101
Springfield, Mass. | 31.9 | . . . | 88
Holyoke, Mass. | 5.82 | . . . | [4]103
-------------------+-------+-----------+------------
[4] Estimated.
=152. Actual Daily Consumption in Foreign Cities.=—It has been for
a long time a well recognized fact that the daily use of water in
American municipalities is far greater per inhabitant than in European
cities. It is difficult to explain the marked difference, but it is
probably due in large part to the more extravagant general habits of
the American people. Examinations in a number of cases have shown that
the actual domestic use of water, at least in some of the American
cities, is not very different from that found in corresponding foreign
cities. Table II exhibits the consumption of water in European cities,
as compiled from various sources and given by Turneaure and Russell.
TABLE II.
---------------------------------------+-------------+-----------
| |Consumption
| Estimated | per Capita
City. | Population. | Daily,
| | Gallons.
---------------------------------------+-------------+-----------
England, 1896-97:[5] | |
London | 5,700,000 | 42
Manchester | 849,093 | 40
Liverpool | 790,000 | 34
Birmingham | 680,140 | 28
Bradford | 436,260 | 31
Leeds | 420,000 | 43
Sheffield | 415,000 | 21
Nottingham | 272,781 | 24
Brighton | 165,000 | 43
Plymouth | 98,575 | 59
Germany, 1890 (Lueger): | |
Berlin | 1,427,200 | 18
Breslau | 330,000 | 20
Cologne | 281,700 | 34
Dresden | 276,500 | 21
Düsseldorf | 144,600 | 25
Stuttgart | 139,800 | 26
Dortmund | 89,700 | 78
Wiesbaden | 62,000 | 20
France, 1892 (Bechmann): | |
Paris | 2,500,000 | 53
Marseilles | 406,919 | 202
Lyons | 401,930 | 31
Bordeaux | 252,654 | 58
Toulouse | 148,220 | 26
Nantes | 125,000 | 13
Rouen | 107,000 | 32
Brest | 70,778 | 3
Grenoble | 60,855 | 264
Other countries, 1892-96 (Bechmann): | |
Naples | 481,500 | 53
Rome | 437,419 | 264
Florence | 192,000 | 21
Venice | 130,000 | 11
Zurich | 80,000 | 60
Geneva | 70,000 | 61
Amsterdam | 515,000 | 20
Rotterdam | 240,000 | 53
Brussels | 489,500 | 20
Vienna | 1,365,000 | 20
St. Petersburg | 960,000 | 40
Bombay | 810,000 | 61
Sidney | 423,600 | 38
Buenos Ayres | 680,000 | 34
---------------------------------------+-------------+-----------
[5] Compiled, except the figures for London, by Hazen. _Engineering
News_, 1899, XLI. p. 111.
These foreign averages, with three exceptions, represent reasonable
quantities of water used, and they have been confirmed as reasonable by
many special investigations made in this country.
=153. Variations in Rate of Daily Consumption.=—The preceding
observations are all based upon an average total consumption found by
dividing the total annual consumption by the number of days in the
year. This is obviously sufficient in a determination of the total
supply needed, but it is not sufficient in those matters which involve
a rate of supply during the different hours of the day, or the amount
of the supply for the summer months as compared with those of the
winter. As a general rule the greatest supply will be required during
the hot summer months when lawn- and street-sprinkling is most active.
It appears from observations made in a considerable number of the
large cities of the United States that the maximum monthly average
consumption may run from about 110 to nearly 140 per cent of the
monthly average throughout the year. As an approximate value only, it
may be assumed for ordinary purposes that the maximum monthly demand
will be 125 per cent of the average.
The daily rate taken throughout the year is considerably more variable
than the monthly. There are days in some portions of the year when
consumption by hotels and industrial activities is at a minimum. On the
other hand, there are other days when those activities are at a maximum
and the total draft will be correspondingly high. Experience has
shown that the maximum total draft may vary from about 115 to nearly
200 per cent of the average. It is permissible, therefore, to take
approximately for general purposes the maximum total daily consumption
as 150 per cent of the average. Manifestly any total consumption will
have an hourly rate which may vary greatly from the early morning
hours, when the draft should be almost nothing, to the forenoon hours
on certain days of the week, when the draft is a maximum. These
variations have frequently been investigated, and it has been shown
that the maximum rate per hour of a maximum day may sometimes rise
higher than 300 per cent of the average hourly rate for the year. These
considerations obviously attain their greatest importance in connection
with the capacity of the plant, either power or gravity, from which the
city directly draws its supply. The hourly capacity of the pumps or
steam-plant furnishing the supply need not necessarily be equal to the
maximum, since storage-reservoirs may be and usually are used; but the
capacity of the pipe system leading from such storage-reservoirs must
be equal to the maximum hourly rate required.
=154. Supply of Fire-streams.=—The draft on a water-supply for
fire-extinguishing purposes may have an important influence upon
the hourly rate of consumption. These observations are particularly
pertinent in connection with the water-supply of small cities where the
draft of fire-engines may be considered a large percentage of the total
hourly consumption. It is obviously impossible to assign precisely the
number of fire-streams which may be required simultaneously in a city
having a given population, but experiences of a considerable number of
civil engineers furnish reasonable bases on which such estimates may be
made. Table III exhibits such estimates as made by the civil engineers
indicated. It is given by Mr. Emil Kuichling in the Transactions of the
American Society of Civil Engineers for December, 1897. Probably no
more reasonable estimate can be now presented.
TABLE III.
TABLE EXHIBITING ESTIMATED NUMBER OF FIRE-STREAMS REQUIRED
SIMULTANEOUSLY IN AMERICAN CITIES OF VARIOUS MAGNITUDES.
---------------+-------------------------------------------------
| Number of Fire-streams Required Simultaneously.
Population of +-----------+-----------+-----------+-------------
Community. | 1 | 2 | 3 | 4
| Freeman. | Shedd. | Fanning. | Kuichling.
---------------+-----------+-----------+-----------+-------------
1,000 | 2 to 3 | .. | .. | 3
4,000 | .. | .. | 7 | 6
5,000 | 4 to 8 | 5 | .. | 6
10,000 | 6 to 12 | 7 | 10 | 9
20,000 | 8 to 15 | 10 | .. | 12
40,000 | 12 to 18 | 14 | .. | 18
50,000 | .. | .. | 14 | 20
60,000 | 15 to 22 | 17 | .. | 22
100,000 | 20 to 30 | 22 | 18 | 23
150,000 | .. | .. | 25 | 34
180,000 | .. | 30 | .. | 38
200,000 | 30 to 50 | .. | .. | 40
250,000 | .. | .. | .. | 44
300,000 | .. | .. | .. | 48
---------------+-----------+-----------+-----------+-------------
The discharge of each fire-stream will of course vary with its diameter
and the pressure at the fire-engine, but as an average it is reasonable
to assume that each stream will discharge 250 gallons per minute. The
quantity of water required, therefore, to supply the estimated number
of streams given in Table III is found by simply multiplying the number
of those streams by 250, to ascertain the total number of gallons
consumed per minute. If _x_ is the number of thousand inhabitants in
any city, and if _y_ represents the required number of streams, then
Mr. Kuichling deduces the following formulæ for _y_ by the use of the
preceding tables, i.e., these formulæ express the results given in the
preceding table as nearly as simple forms of formulæ permit.
___
{ _y_ min. = 1.7√_x_ + 0.033_x_,}
{ }
For Freeman’s data: { _x_ } (1)
{ _y_ max. = --- + 10. }
{ 5 }
____ ___
For Shedd’s data: _y_ = √5_x_ = 2.24 √_x_. (2)
_x_
For Fanning’s data: _y_ = --- + 9. (3)
10
___
For the author’s data: _y_ = 2.8 √_x_. (4)
While for the average ordinary consumption of water, expressed in
gallons per head and day, _q_, Mr. Coffin’s formula, as given in his
paper previously cited, may be taken
_q_ = 40_x_⁰˙¹⁴. (5)
By combining equation (5) with equation (4), remembering that the
maximum rate of consumption is usually about 1.5 times the average,
the total draft in gallons per minute upon the discharging system at
the time of a conflagration will become as follows:
___ 3 40 × 1000
_Q_ = 250(2.8 √_x_) + ---- ---------- _x_¹˙¹⁴
2 1440
( ___ _x_¹˙¹⁴)
= 250(2.8√_x_ + --------) (6)
( 6 )
This maximum rate of consumption during a conflagration does not affect
the total supply of a large city like New York, Boston, or Chicago, but
it may become of relatively great importance in a small city or town.
In a large city this draft may and frequently does tax the capacity of
a small district of the discharging system. In designing such systems,
therefore, even for large cities, it is necessary to insure all
districts against a small local supply when a large one may pressingly
be needed.
CHAPTER XV.
=155. Waste of Water, Particularly in the City of New York.=—The
quantity of water involved in designing a water-supply for cities and
towns is much larger than that which is actually needed. The experience
of civil engineers in many cities, both in this country and in Europe,
shows conclusively that the portion of water actually wasted or running
away without serving any purpose will usually run from 30 to 50 per
cent of the total amount brought to the distributing system. In the
city of New York there is strong reason to believe that the wastage
is not less than two thirds of the total quantity supplied. It is
frequently assumed that both the quantities supplied and the quantities
uselessly wasted in New York are larger than in other places. As a
matter of fact those quantities are actually smaller than in some other
large cities. While the supply per inhabitant in New York City is much
larger than should be required, the use of water by its citizens is not
extravagant when gauged by the criterion of use in other large cities.
This question was most carefully and exhaustively investigated in 1899
and the early part of 1900 by Mr. John R. Freeman of Boston, acting for
the comptroller of the city of New York.
The usual wastes of a water-supply system may be distributed under six
principal heads. First, leaky house-plumbing; second, and “possibly
first in order of magnitude,” leaky service-pipes connecting the
house pipe system with street-mains; third, leaving water-cocks open
unnecessarily; fourth, leaky joints in street-mains or pipes; fifth,
possibly pervious beds and banks of distributing-reservoirs; sixth,
stealing or “unlawful diversion” of water through surreptitious
connections.
The sixth item is probably an extremely small one in New York, although
instances of that kind of waste have been found. It is an old wastage
known as far back in time as the ancient Roman water-supply. The second
and third items probably constitute the bulk of the wastage in this
city.
=156. Division of Daily Consumption in the City of New York.=—In the
course of his search for the various sources of consumption, Mr.
Freeman concluded from his examinations and from the use of the various
means placed at his command for measuring the daily consumption between
December 2nd and December 5th, 1899, and December 8th and December
15th, 1899, that the average daily consumption could be divided as
follows:
Gallons per Inhabitant
per Day.
Probable average amount really used 40
Assumed incurable waste 10}
Curable waste, probably 65} 75
----
Daily uniform rate of delivery by
Croton Aqueduct 115
In his investigations Mr. Freeman had the elevation of water in the
Central Park reservoir carefully observed every six minutes throughout
the twenty-four hours. At the same time the uniform flow through the
new Croton Aqueduct was known as accurately as the flow through such
a conduit can be gauged at the present time. Knowing, therefore, the
concurrent variation of volume in the Central Park reservoir supplied
by the new Croton Aqueduct and the rate of flow in that aqueduct, the
consumption of water per twenty-four hours would be known with the same
degree of accuracy with which the flow in the aqueduct is measured. It
was found by these means that the actual consumption between the hours
of 2 and 4 A.M. was at the rate of 94 gallons per inhabitant per day,
although the actual use at that time was as near zero as it is possible
to approach during the whole twenty-four hours. Nearly all of that rate
of consumption represents waste.
Summing up the whole matter in the light of his investigations, Mr.
Freeman made the following as his nearest estimate to the actual
consumption of the daily supply of water of New York City:
Gallons per
Inhabitant
ACTUAL USE: per Day.
Domestic (average) 12 - 20
Manufacturing and commercial 20 - 30
City buildings, etc. 2 - 4
Fires, street flushing and sprinkling 0.4 - 0.7
-----------
Total 34 - 55
INCURABLE WASTE (probabilities):
Leaks in mains 1 - 2
Leaks in old and abandoned service-pipes 1 - 2
Poor plumbing, all taps metered and closely inspected 2 - 3
Careless and wilful wastes 1 - 2
Under-registry of meters 1 - 1
-------
Total incurable waste and under-registry 6 -10
========
Minimum use and waste 40 -65
NEEDLESS WASTE:
Leaks in street-mains (a guess) 15- 10
Leaks in service-pipes between houses and street-mains 15- 10
(a guess)
Defective plumbing (a guess) 25- 15
Careless and wilful opening of cocks (a guess) 17- 14
To prevent freezing in winter and for cooling in summer 3 - 1
--------
Total needless waste 75- 50
========
Total consumption 115-115
=157. Daily Domestic Consumption.=—The quantity assigned in the
preceding statement to domestic use is confirmed by the abundant
experience in other cities where services are carefully metered, as in
Fall River, Lawrence, and Worcester, Mass., and in Woonsocket, R. I.,
where measurements by meters show that the domestic consumption has
varied from 11.2 to 16.3 gallons per inhabitant per day. Furthermore,
annual reports of the former Department of Public Works and the
present Department of Water-supply for the City of New York show that
during the years 1890 to 1898 such meters as have been used in the
territory supplied by the Croton and the Bronx aqueducts indicate a
daily consumption varying from 13.8 to 24.2 gallons per inhabitant
per day. The same character of confirmatory evidence can be applied
to the quantities assigned to manufacturing and commercial uses, city
buildings and fires, street flushing and sprinkling.
=158. Incurable and Curable Wastes.=—The items composing incurable
waste, unfortunately, cannot be so definitely treated. It is perfectly
well known, however, among civil engineers, that a large amount of
leakage takes place from corporation cocks, which are those inserted
in the street-mains to form the connection between the latter and the
house service-pipes. Again, many of these service-pipes are abandoned
and insufficiently closed, or not closed at all, leaving constantly
running streams whose continuous subsurface discharges escape detection
and frequently find their way into sewers. Water-pipes which have been
laid many years frequently become so deeply corroded as to afford
many leaks and sometimes cracks. Doubtless there are many portions of
a great distributing system, like that in New York City, which need
replacing and afford many large leaks, but undiscoverable from the
surface. Many lead joints of street-mains also become leaky with age,
while others are leaky when first laid in spite of inspection during
construction. Just how much these items of waste would aggregate it
is impossible accurately to state, but from careful observations made
in other places 5 to 10 gallons per day per head of population seems
reasonable. A three-year-old cast-iron fire-protection pipe 5.57 miles
long and mainly 16 inches in diameter, under an average pressure of 114
pounds per square inch, was tested in Providence in 1900 and showed a
leakage at lead joints only of 446 gallons per mile per twenty-four
hours, which was equivalent to .22 gallon per foot of lead joint per
twenty-four hours. Further tests in 1900 of seven lines of new pipe
laid by the Metropolitan Board of Boston, and tested under pressures
varying from 50 to 150 pounds per square inch by Mr. F. P. Stearns,
chief engineer, and Mr. Dexter Brackett, engineer of Distribution
Department, and having an aggregate length of 51.4 miles with diameters
ranging from 16 to 48 inches, gave an average leakage per lineal foot
of pipe in gallons per twenty-four hours ranging from .6 to 3.7 gallons
(average 2.47 gallons), equivalent to an average leakage of 3 gallons
per twenty-four hours per lineal foot of lead joint. The possible rates
of leakage from street-mains are to be applied to a total length of
pipe-lines of 833 miles for the boroughs of Manhattan and the Bronx.
The borough of Brooklyn has somewhat over 600 miles of street-mains,
but they are not to be considered in connection with the Croton and
Bronx water-supply.
All these considerations either confirm or make reasonable the
estimates of the various items of actual use and waste set forth by Mr.
Freeman.
=159. Needless and Incurable Waste in City of New York.=—Concisely
summing up his conclusions, it may be stated that in the year 1899 the
average consumption per inhabitant of the boroughs of Manhattan and the
Bronx was 115 gallons; of these 115 gallons the needless average waste
may be 65 gallons, while the incurable or necessary waste may probably
be taken at 10 gallons per inhabitant per day. It is further probable
that the total underground leakage in New York City is to be placed
somewhere between 20 to 35 gallons per inhabitant per day.
=160. Increase in Population.=—The total volume of daily supply to
any community is determined by the population; but the population is
as a whole constantly increasing. It becomes necessary, therefore, to
estimate the capacity of a water-supply system in view of the future
population of the city to be supplied. No definite rule can be set
as to the future period for which the capacity of any desired system
is to be estimated. It may be stated that no shorter period of time
than probably ten years should be considered, indeed it is frequently
prudent to provide for a period of not less than twenty years, and it
may sometimes be necessary or advisable to consider a possible source
of supply for even fifty years. Provision must be made not only for the
present population, but for the increase during those periods of time,
or at least for the possible development that may be needed.
The increase in population of cities will obviously vary for different
locations with the character of the occupations followed and with
the development of such important factors of industrial life as
railroad connections, facilities for marine commerce, the capacity for
development of the surrounding country, and other influences which aid
in the increase of commerce and industrial activity and the growth
of population. It has been observed, as a matter of experience, that
large cities generally reach a point where their subsequent increase of
population is represented by a practically constant percentage, the
value of that percentage depending upon local considerations. In 1893,
when it was desired to estimate the future population of London for as
much as forty years, it was found that the increase for the ten years
from 1881 to 1891 was 18.2 per cent, with an average of about 20 per
cent for several previous decades. It could, therefore, be reasonably
estimated for the city of London that its population at the end of any
ten-year period would be 18.2 per cent greater than its population
at the beginning of that period. In Appendix 1 of the report of the
Massachusetts State Board of Health upon the Metropolitan water-supply
for the city of Boston made in 1895, the increases for the two ten-year
periods 1870-1880 and 1880-1890 were 6 per cent and 9.6 per cent
respectively for the city proper, but for the population within a
ten-mile radius from the centre of the city they were 28.7 per cent
and 33.7 per cent respectively. The corresponding percentages for the
cities of New York, Philadelphia, and Chicago for the same periods are
as shown in the following tabular statement:
------------+-----------------------------+------------------------
| Population. |Percentages of Increase.
------------+---------+---------+---------+---------+--------------
| 1870. | 1880. | 1890. |1870-80. | 1880-90.
------------+---------+---------+---------+---------+--------------
New York |1,626,119|2,131,051|2,821,802| 31 | 32
Philadelphia| 726,247| 921,458|1,162,577| 27 | 26
Chicago | 310,996| 550,618|1,075,158| 77 | 95[6]
------------+---------+---------+---------+---------+--------------
[6] Includes added territory.
Obviously every estimate of this kind must be made upon the merits
of the case under consideration. The probable increase of population
for any particular city is sometimes estimated by considering the
circumstances of growth of some other city of practically the
same size, and if possible with the same commercial industries or
residential environment, or making suitable allowances for variations
in these respects. Since it is imperative to secure as accurate
estimates as practicable, both methods or other suitable methods should
be employed, in order that the results may be confirmed or modified by
comparison. In every case the supply system should be designed to meet
reasonable estimated requirements for the longest practicable future
period, preferably not less than twenty years.
=161. Sources of Public Water-supplies.=—One of the most important
features of a proposed water-supply is its source, since not only
the potable qualities are largely affected by it, but frequently
the amount also. The two general classes into which potable waters
are divided in respect to their sources are surface-waters and
ground-waters. Surface-waters include rain-water collected as it falls,
water from rivers or smaller streams, and water from natural lakes;
they are collected in reservoirs and lakes or impounding reservoirs.
Ground-waters are those collected from springs, from ordinary or
shallow wells, from deep or artesian wells, and from horizontal
galleries, like those sometimes constructed near and parallel to
subsurface streams or in subsurface bodies of water, affording
opportunity for filtration from the latter through sand or other open
materials to them.
The quality of water will obviously be affected by the kind of material
through which it percolates or flows. Surface-waters, flowing over
the surface of the ground or percolating but a short distance below
the surface, naturally have contact with vegetable matter, unless
they are collected in a country where the soil is sandy and where the
vegetation is scarce. If such waters flow through swamps or over beds
of peat or other similar vegetable mould or soil, they may become so
impregnated with organic matter or so deeply colored by it that they
are not available for potable purposes. Ground-waters, on the other
hand, possess the advantage of having flowed through comparatively
great depths of sand or other earthy material essentially free of
organic matter. They may, however, in some locations, carry prejudicial
amounts of objectionable salts in solution, rendering them unfit for
use. As a rule, ground-waters are apt to be of better quality than
surface-waters, but they do not generally stand storage in reservoirs
as well as surface potable waters. It is advisable to store them in
covered reservoirs from which the light is excluded, rather than in
open reservoirs. They are sometimes impregnated with salts of iron to
such an extent as to make it necessary to resort to suitable processes
for their removal, and they are also occasionally found so hard as to
require the employment of methods of softening them.
Both sources of supply are much used in the United States. Table IV
shows the percentages of the various classes of supplies as found in
this country during the year 1897; the total number of supplies having
been at that time nearly 4000.
TABLE IV.
WATER-SUPPLIES OF THE UNITED STATES.
Source. Per Cent of Total.
{Rivers 25
Surface-waters: {Lakes 7
{Impounding reservoirs 6
{Combinations .5
----- 38.5
{Shallow wells 26
{Artesian wells 10
Ground-waters: {Spring 15
{Galleries and tunnels 1
{Combinations 2
----- 54
Surface- and {Rivers and ground-waters 6
ground-waters: {Lakes and ground-waters 1
{Impounding reservoirs and ground-waters .5
---- 7.5
-------
Total 100.0
It will be observed that a little more than one half of the supplies
are from ground-waters. The practice in connection with European public
water-supplies is different in that a considerably larger percentage of
the total is taken from ground-waters.
The original source of essentially all the water available for public
water-supplies is the rainfall. It becomes of the greatest necessity,
therefore, to secure all possible information regarding rainfall
wherever it may be necessary to construct a public water-supply. Civil
engineers and other observers have for many years maintained continuous
records of rainfall observations at various points throughout the
country, but it is within only a comparatively short time that the
number of those points has been large. Through the extension of the
work of the Weather Bureau, points of rainfall observation are now
scattered quite generally throughout all States of the Union. The
oldest observations are naturally found in connection with stations
located in the Eastern States, where the rainfall is more uniformly
distributed than in many other portions of the country. Obviously
rainfall records become of the greatest importance in those localities
like the semiarid regions of the far West where long periods of no rain
occur.
=162. Rain-gauges and their Records.=—The instrument used for the
collection of rain in order to determine the amount falling in a given
time is the rain-gauge, which may be fitted with such appliances as to
give a continuous record of the rate of rainfall. It has been found
that the location of the rain-gauge has a very important influence
upon the amount of rain which it collects. It should be placed where
wind currents around high structures in its vicinity cannot affect its
record. The top of a large flat-roofed building is a good location in
a city, although the elevation above the surface of the ground, as is
well known, affects the quantity of water collected by the gauge. The
collection will be greater at a low elevation than at a high one, in
consequence of the greater wind currents at the higher point, it being
well known that less rain will be collected where there is the most
wind, other things being equal.
[Illustration: Ordinary Rain-gauge.]
=163. Elements of Annual and Monthly Rainfall.=—In consequence of
the great variations in the rate of rainfall, not only for different
portions of the country, but at different times during the same
storm, it becomes necessary to determine various quantities such as
the maximum, minimum, and mean annual rainfall, the actual monthly
rainfall for different months of the year, and the maximum and minimum
monthly rainfall for as long a period as possible. The minimum monthly
rainfall and the minimum annual rainfall are of special importance in
connection with public water-supply and water-power questions, since
those minima will, in connection with the area of a given watershed,
determine the greatest amount of water which can be made available for
use. In entering upon the consideration of such questions, therefore,
civil engineers must inform themselves with the greatest detail as
to the characteristics of the monthly and the annual rainfall of the
locality in which their works are to be located.
[Illustration: MONTHLY VARIATIONS IN RAINFALL.
FIG. 1.]
TABLE V.
GENERAL RAINFALL STATISTICS FOR THE UNITED STATES.
----------------------------+---------+---------+---------
| | Per Cent|
| | of | Per Cent
| Mean | Summer | Driest
Station. | Yearly | and | Year to
|Rainfall.| Autumn | Mean
| | Rain to| Year.
| | Mean |
| | Yearly. |
----------------------------+---------+---------+---------
North Atlantic: | | |
Boston | 45.4 | 50 | 60
New York | 44.7 | 52 | 62
Philadelphia | 42.3 | 52 | 70
Washington | 42.9 | 51 | 69
South Atlantic: | | |
Wilmington | 53.7 | 61 | 75
Charleston | 49.1 | 61 | 48
Augusta | 48.0 | 50 | 81
Jacksonville | 54.1 | 65 | 74
Key West | 38.2 | 70 | 54
Gulf and Lower Mississippi: | | |
Montgomery | 52.5 | 42 | 76
Mobile | 62.6 | 51 | 68
New Orleans | 60.3 | 52 | 64
Galveston | 47.7 | 58 | 50
Nashville | 50.2 | 46 | 67
Ohio Valley: | | |
Pittsburg | 36.6 | 53 | 70
Cincinnati | 42.1 | 50 | 60
Indianapolis | 42.2 | 51 | 59
Cairo | 42.6 | 47 | 62
Louisville | 47.2 | 48 | 74
Lake Region: | | |
Detroit | 32.5 | 56 | 65
Cleveland | 36.6 | 54 | 71
Duluth | 30.7 | 63 | 65
Upper Mississippi Valley: | | |
St. Louis | 40.8 | 52 | 55
Chicago | 34.0 | 54 | 66
Milwaukee | 31.0 | 55 | 66
Madison | 33.2 | 58 | 39
The Plains: | | |
Omaha | 31.4 | 63 | 57
North Platte | 18.1 | 61 | 56
Denver | 14.3 | 48 | 59
Cheyenne | 12.7 | 55 | 39
The Plateau: | | |
Tucson | 11.7 | 65 | 44
Santa Fé | 14.6 | 69 | 53
Salt Lake City | 18.8 | 39 | 55
Walla Walla | 15.4 | 38 | 46
Pacific Coast: | | |
Astoria | 77.0 | 33 | 64
Portland | 46.2 | 31 | 67
Sacramento | 19.9 | 16 | 42
San Francisco | 23.4 | 17 | 51
Los Angeles | 17.2 | 15 | 33
San Diego | 9.7 | 18 | 30
----------------------------+---------+---------+---------
| | |
| | |
| Per Cent| Per Cent| Number
Station. | Two | Three | of
| Driest | Driest | Years’
| Years. | Years. | Record.
| | |
| | |
----------------------------+---------+---------+---------
North Atlantic: | | |
Boston | 70 | 80 | 79
New York | 77 | 80 | 61
Philadelphia | 75 | 80 | 72
Washington | 71 | 74 | 45
South Atlantic: | | |
Wilmington | 80 | 81 | 26
Charleston | 55 | 62 | 89
Augusta | 88 | 87 | 27
Jacksonville | 77 | 83 | 27
Key West | 61 | 73 | 49
Gulf and Lower Mississippi: | | |
Montgomery | 80 | 83 | 24
Mobile | 75 | 78 | 26
New Orleans | 75 | 77 | 26
Galveston | 65 | 72 | 26
Nashville | 73 | 83 | 32
Ohio Valley: | | |
Pittsburg | 78 | 85 | 54
Cincinnati | 72 | 71 | 62
Indianapolis | 76 | 82 | 27
Cairo | 75 | 81 | 25
Louisville | 81 | 85 | 25
Lake Region: | | |
Detroit | 72 | 79 | 46
Cleveland | 74 | 81 | 41
Duluth | 81 | 88 | 26
Upper Mississippi Valley: | | |
St. Louis | 65 | 75 | 60
Chicago | 80 | 86 | 30
Milwaukee | 74 | 73 | 53
Madison | 58 | 68 | 28
The Plains: | | |
Omaha | 63 | 70 | 27
North Platte | 67 | 72 | 22
Denver | 71 | 77 | 27
Cheyenne | 62 | 75 | 27
The Plateau: | | |
Tucson | 79 | 80 | 19
Santa Fé | 63 | 66 | 37
Salt Lake City | 64 | 74 | 29
Walla Walla | 81 | 86 | 27
Pacific Coast: | | |
Astoria | 68 | 77 | 34
Portland | 76 | 79 | 27
Sacramento | 67 | 84 | 47
San Francisco | 73 | 78 | 47
Los Angeles | 48 | 59 | 24
San Diego | 54 | 61 | 47
----------------------------+---------+---------+---------
The diagram Fig. 1 and Table V are constructed from data given in
the bulletins of the Weather Bureau and exhibit some of the general
features of the rainfall for different points throughout this country.
The heavy lines of the diagram show the average monthly precipitation
at the points indicated, for periods of a considerable number of
years, as shown in the table. It will be observed that the rainfall is
comparatively uniform in the North Atlantic States but quite variable
on the Pacific coast, as well as in the Mississippi and Missouri
valleys and west of those valleys.
=164. Hourly or Less Rates of Rainfall.=—Although not often of great
importance in connection with public water-supply systems, it is
sometimes necessary to possess data regarding maximum hourly (or less)
rates of precipitation in connection with sewer or drainage work.
The earlier records give exaggerated reports of maximum rates of
rainfall, although that rate varies rapidly with the time. Throughout
a rain-storm the rate of precipitation is constantly varying and
the maximum rate seldom if ever extends over a period equal to a
half-hour; usually it lasts but a few minutes only. In this country
an average rate of 1 inch per hour, extending throughout one hour,
is phenomenal, although that rare amount is sometimes exceeded. A
maximum rate of about 4 inches per hour, lasting 15 to 30 minutes, is,
roughly speaking, about as high as any precipitation of which we have
reliable records. The waste ways or other provisions for the discharge
of surplus or flood-waters of the Metropolitan Water-supply of Boston
are designed to afford relief for a total precipitation of 6 inches
in twenty-four hours. It is safe to state that an excess of that
accommodation will probably never be required.
=165. Extent of Heavy Rain-storms.=—In all engineering questions
necessitating the consideration of these great rain-storms it is
necessary to remember that their extent is frequently much greater
than the areas of watersheds usually contemplated in connection with
water-supply work. The late Mr. James B. Francis found in the great
storm of October, 1869, which had its maximum intensity in Connecticut,
that the area over which 6 inches or more of rain fell exceeded 24,000
square miles, and that the area over which a depth of 10 inches or more
fell was 519 miles. Again, in the New England storm of February, 1886,
6 inches or more of rain fell over an area of at least 3000 square
miles. Storm records show that as much as 8 or 10 inches in depth have
fallen over areas ranging from 1800 to 500 square miles, respectively,
in a single storm.
=166. Provision for Low Rainfall Years.=—The capacity of any public
water-supply must evidently be sufficient to meet not only the general
exigencies of the year of lowest rainfall, but also the conditions
resulting from the driest periods of that year. It is customary among
civil engineers to consider months as the smaller units of a dry year.
It is necessary, therefore, to examine not only the annual rainfalls
but the monthly rates of precipitation during critical years, i.e.,
usually during dry years.
It is impossible to determine absolutely the year of least rainfall
which may be expected, but evidently the longer the period over which
observations have extended the nearer that end will be attained. It
is sometimes assumed that the lowest annual rainfall likely to be
expected in a long period of years is 80 per cent of the average annual
rainfall for the same period. Or, it is sometimes assumed that the
average rainfall for the lowest two or three consecutive years will be
80 per cent of the average for the entire period, and that the year
of minimum rainfall may be expected to yield two thirds of the annual
average precipitation. Such features will necessarily vary with the
location of the district considered. Conclusions which may be true for
the New England or northern Atlantic States probably will not hold for
the south Atlantic and Gulf States. Data for such conclusions must
be obtained from the rainfall of the locality considered. Table VI
exhibits the comparative monthly rainfall which J. T. Fanning suggests
may be used approximately for the average Atlantic coast districts.
TABLE VI.
+-----------+-----------+------------+-----------+
| | Mean | | Probable |
| | Monthly | Respective | Depth in |
| | Rainfall, | Ratios. | Inches of |
| | Inches. | | Actual |
| | | | Rainfall. |
+-----------+-----------+------------+-----------+
| January | 4 × 1.65 = 6.6 |
| February | 4 × 1.50 = 6.0 |
| March | 4 × 1.65 = 6.6 |
| April | 4 × 1.45 = 5.8 |
| May | 4 × .85 = 3.4 |
| June | 4 × .75 = 3.0 |
| July | 4 × .35 = 1.4 |
| August | 4 × .25 = 1.0 |
| September | 4 × .30 = 1.2 |
| October | 4 × .45 = 1.8 |
| November | 4 × 1.20 = 4.8 |
| December | 4 × 1.60 = 6.4 |
+-----------+------------------------------------+
If the average monthly rainfall throughout the year were one inch, the
values of the ratios would show the actual monthly precipitation. In
general the table would be used by dividing the total yearly rainfall
by 12, and then multiplying that monthly average by the proper ratio
taken from the table opposite the month required. Such tables should
only be used for approximate purposes and when actual rainfall records
are not available for the district considered.
=167. Available Portion of Rainfall or Run-off of Watersheds.=—If the
public water-supply is to be drawn from a stream where the desired
rainfall records exist, it is necessary to know what portion of the
rainfall, either in the driest or in other years, may be available.
This is one of the departments of the hydraulics of streams for which
much data yet remain to be secured. The watersheds or areas drained
by some streams, like the Sudbury River of the Boston, and the Croton
of the New York water-supply, have, however, been studied with
sufficient care to give reliable data. The amount of water flowing in
a stream from any watershed for a given period, as a year, is called
the annual “run-off” of the watershed, and it is usually expressed
as a certain percentage of the total rainfall on the area drained.
For certain purposes it is sometimes more convenient to express the
run-off from the watershed as the number of cubic feet of water per
second per square mile of area. Table VII, taken from Turneaure and
Russell, exhibits run-off data for a considerable number of streams in
connection with both average and minimum rainfalls.
TABLE VII.
STATISTICS OF THE FLOW OF STREAMS.
-----------------+---------+-------+----------------+-----------------
| Area | | Average Yearly.|Year of Minimum
Stream. | Drained,| | | Flow.
| Square | Years.+-----+-----+----+-----+------+----
| Miles. | |Rain,|Flow,|Per |Rain,|Flow,|Per
| | |Inch.|Inch.|Cent|Inch.|Inch.|Cent
-----------------+---------+-------+-----+-----+----+-----+-----+----
Sudbury | 75.2 |1875-97|45.77|22.22|48.6|32.78|11.19|34.1
Cochituate | 18.87|1863-96|47.08|20.33|43.2|31.20| 9.76|31.3
Mystic | 26.9 |1878-96|43.79|19.96|45.6|31.22| 9.32|29.8
Connecticut |10,234 |1871-85|44.69|25.25|56.5|40.02|18.25|45.6
Croton | 338 |1870-94|48.38|24.57|50.8|38.52|14.54|37.8
Upper Hudson | 4,500 |1888-96|39.70|23.36|59.0|33.49|17.46|52.2
Genesee | 1,060 |1894-96|39.82|12.95|32.5|31.00| 6.67|21.5
Passaic | 822 |1877-93|47.08|25.44|54.0|35.64|15.23|42.7
Upper Mississippi| 3,265 |1885-99|26.57|4.90 |18.4|22.86| 1.62| 7.1
=================+=========+=======+=====+=====+====+=====+=====+====
| | | Average for |Average for June
| Area | |December to May.| to November.
Stream. | Drained,| | |
| Square | Years.+-----+-----+----+-----+-----+----
| Miles. | |Rain,|Flow,|Per |Rain,|Flow,| Per
| | |Inch.|Inch.|Cent|Inch.|Inch.|Cent
-----------------+---------+-------+-----+-----+----+-----+-----+----
Sudbury | 75.2 |1875-97|22.98|17.52|76.0|22.61| 4.70|20.8
Cochituate | 18.87|1863-96|22.97|14.87|64.7|24.10| 5.46|22.7
Mystic | 26.9 |1878-96|22.11|15.12|68.4|21.66| 4.84|22.4
Connecticut |10,234 |1871-85|20.13|17.95|89.1|24.56| 7.30|29.7
Croton | 338 |1870-94|23.39|17.81|76.1|24.99| 6.76|27.0
Upper Hudson | 4,500 |1888-96|18.20|16.23|89.0|21.50| 7.13|33.0
Genesee | 1,060 |1894-96|19.58|10.20|52.2|20.24| 2.75|13.6
Passaic | 822 |1877-93|22.47|18.22|81.1|24.39| 7.19|29.5
-----------------+---------+-------+-----+-----+----+-----+-----+----
The information to be drawn from this table is sufficient to give clear
and general relations between the recorded precipitation and run-off.
The percentage of run-off is seen to vary quite widely, but as a rule
it is materially less for the year of minimum flow than for the average
year. That feature of the table is an expression of the general law,
other things being equal, that the smaller the precipitation the less
will be the percentage of run-off. A number of influences act to
produce that result. During a year of great precipitation the earth is
more nearly saturated the greater part of the time, and hence when rain
falls less of it will percolate into the ground and more of it will run
off. Again, if the ground is absolutely dry, a certain amount of rain
would have to fall before any run-off would take place. The area and
shape of a watershed will also affect to some extent the flow of the
stream which drains it. A larger run-off would reasonably be expected
from a long narrow watershed than from one more nearly circular in
outline. The greater the massing of the watershed, so to speak, the
more opportunity there is for the water to be held by the ground and
the less would be the run-off.
TABLE VIII.
AVERAGE YIELD OF SUDBURY WATERSHED, 1875-1899, INCLUSIVE,
VARIOUSLY EXPRESSED.
(Area of watershed, 75.2 square miles.)
---------+--------------------+-------------------------------
| Per Square Mile. | Rainfall.
Month. +--------------------+----------+----------+---------
|Cubic Feet |Million |Collected,|Per Cent | Total,
|per Second.|Gallons |Inches. |Collected.| Inches.
| |per Day.| | |
---------+-----------+--------+----------+----------+---------
January | 1.937| 1.252| 2.233| 51.6| 4.33
February | 2.904| 1.877| 3.050| 71.7| 4.26
March | 4.489| 2.901| 5.175| 117.4| 4.41
April | 3.124| 2.019| 3.485| 107.5| 3.24
May | 1.680| 1.086| 1.936| 58.1| 3.33
June | .735| .475| .821| 28.0| 2.93
July | .305| .197| .352| 9.3| 3.77
August | .478| .309| .551| 13.3| 4.16
September| .376| .243| .419| 13.0| 3.23
October | .829| .536| .956| 21.9| 4.37
November | 1.474| .953| 1.645| 39.0| 4.22
December | 1.612| 1.042| 1.859| 51.9| 3.58
+-----------+--------+----------+----------+--------
Year | 1.655| 1.070| 22.482| 49.1| 45.83
---------+-----------+--------+----------+----------+--------
=168. Run-off of Sudbury Watershed.=—Table VIII has been given by
Mr. Charles W. Sherman, as representing the average yield of the
Sudbury watershed for the period 1875 to 1899, inclusive, expressed in
several different ways. The average rainfall was 45.83 inches, and the
percentage which represents the run-off is 49.1 per cent of the total.
The average monthly run-off varies from .305 cubic foot (for July) to
4.489 cubic feet (for March) per second per square mile. As a general
rule it may be stated that the average run-off from the drainage areas
of New England streams amounts very closely to 1,000,000 gallons per
square mile per day. The area of the Sudbury watershed is 75.2 square
miles, with 6.5 per cent of that total area occupied by the surface of
lakes or reservoirs. As will presently be seen, the amount of exposed
water surface in any watershed has an appreciable influence upon its
run-off.
=169. Run-off of Croton Watershed.=—The total area of the Croton
watershed, from which New York City draws its supply, i.e., the area
up-stream from the new Croton Dam, is 360.4 square miles, of which 16.1
square miles, or 4.47 per cent, of its total area is water surface. Mr.
John R. Freeman found in the investigations covered by his report to
the comptroller of the city of New York in 1900 that the average annual
rainfall on that area for the thirty-two years beginning 1868 and
ending 1899 was 48.07 inches, and that the average run-off for the same
period was 47.7 per cent of the total average rainfall, equivalent to a
depth of 22.93 inches.
[Illustration: Aqueducts near Jerome Park Reservoir, New York City.]
Table IX gives the main elements of the rainfall and run-off for the
Croton watershed during the thirty-two year period, for the averages
just given.
TABLE IX.
RAINFALL ON CROTON WATERSHED IN TOTAL INCHES—1868-1898.
NATURAL FLOW OF CROTON RIVER AT OLD CROTON DAM,
IN EQUIVALENT INCHES.
PERCENTAGE OF RUN-OFF TO RAINFALL FOR EACH YEAR.
-----------+---------+---------+-----------
Year. | Total | Total |
|Rainfall.| Run-off.| Per Cent.
-----------+---------+---------+-----------
1868 | 50.33 | 33.33 | 66.22
1869 | 48.36 | 23.61 | 48.82
1870 | 44.63 | 19.20 | 43.02
1871 | 48.94 | 19.46 | 39.76
1872 | 40.74 | 16.92 | 41.53
1873 | 43.87 | 25.02 | 57.03
1874 | 42.37 | 25.10 | 59.24
1875 | 43.66 | 24.77 | 56.73
1876 | 40.68 | 21.09 | 51.84
1877 | 48.23 | 20.22 | 41.92
1878 | 55.70 | 27.17 | 48.78
1879 | 47.04 | 19.65 | 41.77
1880 | 36.92 | 12.63 | 34.21
1881 | 46.69 | 19.25 | 41.23
1882 | 52.35 | 24.28 | 46.38
1883 | 42.70 | 13.33 | 31.22
1884 | 51.28 | 24.08 | 46.96
1885 | 43.67 | 17.71 | 40.55
1886 | 47.74 | 20.10 | 42.10
1887 | 57.29 | 26.61 | 46.45
1888 | 60.69 | 35.27 | 58.12
1889 | 55.70 | 31.39 | 56.36
1890 | 54.05 | 25.95 | 48.01
1891 | 47.20 | 23.48 | 49.75
1892 | 44.28 | 17.68 | 39.93
1893 | 54.87 | 29.05 | 52.94
1894 | 47.33 | 20.56 | 43.44
1895 | 40.58 | 15.95 | 39.31
1896 | 45.85 | 23.26 | 50.73
1897 | 53.12 | 25.59 | 48.17
1898 | 57.40 | 29.72 | 51.77
1899 | 44.67 | 22.28 | 49.88
Average for| 48.07 | 22.93 | 47.70
32 years. | | |
-----------+---------+---------+-----------
The table shows that the least annual rainfall was 36.92 inches for
1880, and that the run-off represented a depth of 12.63 inches only, or
34.21 per cent of the total annual precipitation. As a rule the same
feature of a low percentage of run-off will be found belonging to the
years of low rainfall, although there are many irregularities in the
results. On the other hand, the high percentages of run-off are for the
years 1868, 1888, and 1889, and they will generally be found belonging
to years of relatively great precipitation. A low percentage of run-off
will also be lower if the year to which it belongs follows a dry year
or a dry cycle of two or three years. Similarly the high percentages
of run-off will, as a rule, be higher if they follow years of high
precipitation; that is, if they belong to a cycle of relatively great
rainfall.
=170. Evaporation from Reservoirs.=—If it is contemplated to build
reservoirs on a watershed the capacity of which is being estimated
on the basis of either the driest year or the driest two- or three
year cycle, it is necessary to make a deduction from the rainfall
for the evaporation which will take place from the surface of the
proposed reservoir. In order that that deduction may be made as a
proper allowance for added water surface in a drainage area, it is
necessary that the amount of evaporation be determined for the district
considered. The rate of evaporation is dependent upon the area of water
surface, upon the wind, and upon the temperature both of the water and
air above it. Numerous evaporation observations have been made both in
this and other countries, and extensive evaporation tables have been
prepared by the Weather Bureau, from which a reasonable estimate of the
monthly evaporation for all months in the year may be made for almost
any point in the United States. Particularly available observations
have been made by Mr. Desmond Fitzgerald of Boston on the Chestnut
Hill reservoirs of the Boston Water-supply, and by Mr. Emil Kuichling,
engineer of the Rochester Water-works, on the Mount Hope reservoir of
the Rochester supply. Table X exhibits the results of the observations
of both these civil engineers.
[Illustration: Aqueduct Division Wall of Jerome Park Reservoir, New
York City.]
As would be anticipated, the period from May to September, both
inclusive, shows by far the greatest evaporation of the whole year,
while December, January, and February are the months of least
evaporation. The total annual evaporation at Boston was 39.2 inches and
34.54 inches at Rochester.
TABLE X.
MEAN MONTHLY EVAPORATIONS.
----------------+-------------------------+-------------------------
| |
| Chestnut Hill Reservoir,| Mount Hope Reservoir,
Month. | Boston, Mass. | Rochester, N. Y.
| |
----------------+------------+------------+------------+------------
| | Per Cent | | Per Cent
|Evaporation,| of Yearly |Evaporation,| of Yearly
| Inches. |Evaporation.| Inches. |Evaporation.
----------------+------------+------------+------------+------------
January | 0.96 | 2.4 | 0.52 | 1.5
February | 1.05 | 2.7 | 0.54 | 1.6
March | 1.70 | 4.3 | 1.33 | 3.9
April | 2.97 | 7.6 | 2.62 | 7.6
May | 4.46 | 11.4 | 3.93 | 11.4
June | 5.54 | 14.2 | 4.94 | 14.3
July | 5.98 | 15.2 | 5.47 | 15.8
August | 5.50 | 14.0 | 5.30 | 15.4
September | 4.12 | 10.4 | 4.15 | 12.0
October | 3.16 | 8.1 | 3.16 | 9.1
November | 2.25 | 5.7 | 1.45 | 4.2
December | 1.51 | 3.9 | 1.13 | 3.2
+------------+------------+------------+------------
Total for year | 39.20 | | 34.54 |
+------------+------------+------------+------------
Mean temperature| 48°.6 | 47°.8
----------------+-------------------------+-------------------------
A reference to data of the Weather Bureau will show that annual
evaporation as high as 100 inches, or even more, may be expected on
the plateaux of Arizona and New Mexico. Other portions of the arid
country in the western part of the United States will indicate annual
evaporations running anywhere from 50 to 90 inches per year, while on
the north Pacific coast it will fall as low as 18 to 40 inches.
=171. Evaporation from the Earth’s Surface.=—Data are lacking for
anything like a reasonably accurate estimate of evaporation from the
earth’s surface. It is well known that the loss of water from that
source is considerable in soils like those of swamps, particularly
when exposed to the warm sun, but no reliable estimate can be obtained
for the exact amount. Nor is this necessary for the usual water-supply
problems, since it is included in the difference between the total
rainfall of any district and the observed run-off in the streams.
Indeed evaporation from reservoirs is similarly included for reservoirs
existing when the run-off observations are made.
CHAPTER XVI.