before deciding upon the source for a supply of water.--Having fixed
upon the Croton River as a stream possessing the requisite advantages
for a supply, questions naturally arose as to the manner in which it
should be conveyed to the city. The distance being about forty miles,
over a country extremely broken and uneven, and following a direction,
for a portion of this distance, parallel with the Hudson River,
encountering the streams which empty into it and form deep valleys in
their courses. It will be interesting to notice the different plans
which were suggested for forming a channel-way to conduct the water.
The following modes were presented:--a plain channel formed of earth,
like the ordinary construction of a canal feeder:--an open channel,
protected against the action of the current by masonry:--an arched
culvert or conduit, composed essentially of masonry; and iron pipes.
In deciding which of these modes should be adopted, it was necessary
to make a comparison among them as to their efficiency for conducting
the water in purity, and in the quantity required, their permanency as
structures, and their cost.
The disadvantages attendant upon an open canal were, that by filtration
through the banks there would be a heavy loss of water;--the difficulty
of preserving the water from receiving the wash of the country, and
preventing injurious matter from being thrown into it and rendering
it impure, and the impurities which might be contracted by passing
through different earths. Evaporation would also occasion a serious
loss of water. The banks would be liable to failure in seasons of
long-continued rains, and the city depending upon this for a supply,
would be cut off, except there should be sufficient in the reservoirs
to furnish a supply during the period of repairs. The canal could
never be subjected to a _thorough_ repair, because of the necessity
of keeping it in a condition for furnishing water constantly during
the whole year, so that all repairs would be done under great
disadvantages, and the channel would be yearly growing worse until its
failure might become a public calamity. In regard to the open channel
having the sides protected by masonry, the objections were found to be
such as would apply equally to every species of open channel; namely,
that it would be exposed in many situations to receive the wash of
the country; that it would be unprotected from the frost, and liable
to be interrupted thereby, and lastly, that there would be a loss by
evaporation. It was supposed that these objections might be obviated by
certain precautions; for example, the wash could be avoided by making
sufficient side drains; and the interruption liable to occur from frost
and snow, and the evaporation, to a certain extent, could be prevented
by closing the channel entirely with a roof over the top. The close
channel or culvert, composed essentially of masonry seemed to possess
all the requisite advantages for conducting the water in a pure state
and keeping it beyond the influence of frost or any interruption which
would be liable to occur to an open channel. In point of stability this
plan had a decided preference over either of the other plans proposed,
and the only objection offered was the cost of the work constructed in
this way. To avoid too great expense it was proposed to make use of a
mixed construction, using the close channel or culvert in situations
where deep excavations occurred and it would be desirable to fill in
the earth again to the natural form, also where the line of Aqueduct
intersected villages, and using the open channel with slope walls for
the residue of the distance.
In regard to iron pipes for conducting the water, it was found that
a sufficient number of them to give the same sectional area as would
be adopted by either of the other plans would be more expensive, and
considering the great distance and the undulating surface over which
they would extend, other disadvantages were presented which added
to the objections, and the plan was considered inexpedient. Could a
line be graded so as to give a regular inclination from the Fountain
Reservoir to one at the city, then the expense of laying iron pipes
for conducting the proposed quantity of water, would be greater than
for constructing a channel-way of masonry; and when laid, the pipes
were thought to be less durable. Should the pipes follow the natural
undulations of the ground, there would be so much resistance offered
to the flow of water that the discharge would be diminished in a very
great degree.
The close channel or conduit of masonry was adopted as the plan best
calculated to answer all the purposes of conducting the water to the
city.
_Sources of the Croton River._
The sources of the Croton River are principally in the county of
Putnam, at a distance of fifty miles from the city of New-York; they
are mostly springs which in that elevated and uneven country have
formed many ponds and lakes never-failing in their supply. There are
about twenty of these lakes which constitute the sources of the Croton
River, and the aggregate of their surface areas is about three thousand
eight hundred acres.
From these sources to the mouth of the Croton at the head of Tappan
Bay in the Hudson, the distance is about twenty-five miles. The
country bordering upon the Croton is generally elevated and uneven,
not sustaining a dense population and cleared sufficiently to prevent
injury to the water from decayed vegetable matter. The river has a
rapid descent and flows over a bed of gravel and masses of broken rock.
From these advantages there is good reason to suppose that the water
will receive very little impurity from the wash of the country through
which it flows, and there is no doubt that the sources furnish that
which is peculiarly adapted to all the purposes of a large city.
The water is of such uncommon purity that in earlier days the native
Indian gave a name to the river which signified “_clear water_.”[5]
_Flow of Water in the Croton River, Capacity of the Fountain Reservoir,
&c._
The medium flow of water in the Croton, where the fountain reservoir
is formed, exceeds fifty millions of gallons in twenty-four hours, and
the minimum flow, after a long-continued drought, is about twenty-seven
millions of gallons in twenty-four hours.
The dam on the Croton River is about 38 feet above the level which was
the surface of the natural flow of water at that place, and sets the
water back about six miles, forming the Fountain Reservoir which covers
an area of about four hundred acres. The country forming the valley of
the River was such as to give bold shores to this reservoir generally,
and in cases where there was a gentle slope or a level of the ground
near the surface of water, excavations were made so that the water
should not be of less depth than four and a half feet.
The great length of this Reservoir is favourable for the purity of the
water which enters the Aqueduct: spread over this large surface, it
will have an opportunity to settle and part with some of the impurities
which it receives, during rainy seasons, from the wash of the country
through which it flows.
The available capacity of this Reservoir, down to the level where the
water would cease to flow off in the Aqueduct, has been estimated at
six hundred millions of gallons.
Could we suppose that the Croton River will ever in any season of
drought, fail to furnish a supply greater than would be carried off
from this Reservoir and the Reservoirs at the city by evaporation,
we have still a supply of water which would be sufficient for one
million of inhabitants during the space of thirty days (estimating the
amount necessary for each inhabitant to be twenty gallons for every
twenty-four hours.)
But we may assume the number of inhabitants at present to be one third
of a million, and therefore we have a sufficient store of water in
this Fountain Reservoir to supply them for the space of ninety days,
in the emergency before supposed. In addition to the quantity in the
Fountain Reservoir, we have sufficient in the Reservoirs at the city
to supply one third of a million of inhabitants for about twenty-five
days, at the rate of supply before mentioned. Thus we find, should such
a limit as we have supposed ever happen to the supply from the River,
the season of drought cannot certainly be supposed to continue during
the length of time (about four months) that would be required for the
present population of the city to exhaust the quantity in store when
all the Reservoirs are full.
The minimum flow of water in the river where the dam is constructed,
has been stated to be twenty-seven millions of gallons for every
twenty-four hours. This would be a sufficient supply for one million
of inhabitants, and should the population of the city increase to one
million and a half, this supply, together with the quantity in store,
will probably be sufficient during any season of drought. There is,
therefore, no fear in regard to the supply for the present, and should
the time arrive when the city will require more than the present
facilities afford during low stages of the river, other streams may
be found which can be turned into the upper branches of the Croton,
or into the Aqueduct along its course. Other Reservoirs may also be
constructed farther up the Croton to draw from in seasons of drought.
These suggestions would only be useful to provide a supply during the
low stages of the river, for at other seasons the flow of water in the
Croton would be equal to the full capacity of the Aqueduct.[6]
_General Design of the Channel-way and Reservoirs._
A description of the general design and purpose of the channel-way in
connection with the Reservoirs will serve to give a clear understanding
of the operation of the work. Having ascertained the elevation in
the city at which it would be desirable to use the water, it was
only necessary then, to find a point on the Croton River where a dam
could be constructed that would turn the water into a channel having
a gradual descent to the required elevation at the city. So that it
may easily be conceived, it is only diverting the water into another
channel where it will flow on unobstructed. The manner in which water
is conducted from its natural channel, for the purpose of propelling
the machinery of manufacturing establishments, by a race-way or other
channel, is a simple illustration of the operation of this great work.
At the place where it was determined to build the dam across the
Croton River, the surface of the natural flow of water was about 38
feet below the elevation required as a head for the water to flow into
the Aqueduct leading to the city. By going farther up the river the
dam would have been of less height, and a point might have been found
where it would be only necessary to build a dam to turn the water, and
not form a pond of much extent above it, but for such purpose it would
have been necessary to go above where some important tributaries enter
the river, and would have required a considerable extension of the
Aqueduct. It was perhaps desirable to form this Fountain Reservoir, so
that it would afford a supply of water to draw from, should there at
any future time, in a season of drought, be more required for the use
of the city than would be flowing in the river.
No essential change occurs in the form of the channel-way from the
Fountain Reservoir on the Croton, to the Receiving Reservoir on the
island of New-York; a distance of thirty-eight miles, except in
crossing Harlem River to reach the island, and in passing a deep valley
on the island, where iron pipes are used instead of the channel-way of
masonry to provide for the pressure consequent upon a depression from
the regular plane.
At these points the iron pipes descend and rise again, so that when
the water is flowing in the channel-way they will be constantly full.
Thus it will be perceived that the channel-way of masonry will never
be filled entirely, so as to occasion a pressure on all its interior
surface.
The surface of the Fountain Reservoir is 166⅙ feet above the level
of mean tide at the city of New-York; and the difference of level
between that and the surface of the Receiving Reservoir on the island
of New-York, (a distance of thirty-eight miles) is 47⅙ feet, leaving
the surface of this reservoir 119 feet above the level of mean tide.
From the Receiving Reservoir the water is conducted (a distance of two
miles) in iron pipes to the Distributing Reservoir, where the surface
of the water is 115 feet above the level of mean tide. This last is the
height to which the water may generally be made available in the city.
GENERAL CONSTRUCTION OF THE AQUEDUCT.
Plate I. is a section of the Aqueduct showing the form of the masonry
used in earth excavations. The foundation is formed with concrete; the
side walls of stone; the bottom and sides of the interior being faced
with brick, and the top covered with an arch of brick.
In forming the concrete a mortar is made by mixing three parts of
sand with one of hydraulic lime, and then mixing about three parts of
stone, broken to a size allowing them to pass through a ring an inch
and a half in diameter. Having thoroughly mingled the broken stone
and mortar, the concrete is placed in its proper position and form,
and brought into a compact state by using a _pounder_; and is then
suffered to remain until it set, or become indurated, before any work
is commenced upon it. The object should be to mix as many stones or
pebbles as will thoroughly bed in the mortar, allowing none of them to
come in contact, but all to be enveloped in mortar. This forms a body
which becomes indurated and makes a foundation under the whole length
of the Aqueduct like one continuous stone. It attains a degree of
hardness which gives it the appearance of the conglomerate bearing the
name of _Pudding-stone_, and is an article of the greatest importance
in forming foundations for walls of great weight; superseding in many
instances, where the soil is soft, the use of piles or other timber
foundation.
Though we have evidence that concrete was used by the Ancient Romans in
the foundations of some of their structures and even in the formation
of their roads--such as the Appian-Way, and though we find it used
in the foundations of the feudal castles of the Norman Barons of
England, still it has not been introduced into the general practice of
architecture until quite a modern date, and even at the present time
is not widely appreciated in this country as a material of so much
importance in foundations.
The side walls are laid up in a character of workmanship styled
“_rough-hammered work_;” the stone required to be of sound and durable
quality and laid in a manner to render the work water-tight. Though
attention is given in some degree to insure a proper bond to the wall,
yet the point more particularly attended to, is to make it compact
and impervious to water. The bonding of the wall is not by any means
disregarded, in all situations where it is required, yet the position
of the work generally, where it is in excavation below the natural
surface of the ground, renders such precaution of less importance
than that of making it compact. The mortar used in these side walls
is formed by mixing clean sharp sand with hydraulic lime, using the
proportions of three parts of the sand to one of the lime; and these
are thoroughly mixed and incorporated before they are wet; when this
mixture is wet and thoroughly worked, it is used immediately and always
kept properly tempered so as to render it plastic, and to prevent any
disposition to become hardened before it is in the wall. After the
side walls are finished and the concrete between them has received its
proper form, a coating of plastering, about three eighths of an inch
in thickness, is put on over the surface of the concrete and on the
face of the walls before the interior facing of brick is commenced.
The proportions of this plastering are two parts of sand to one of the
hydraulic lime.
The bricks used in this work are generally of quite a different
character from those used in ordinary house-building; being harder
burnt and of a superior quality of material. They are required to be
burnt to such a degree of hardness that they present a cherry red, or
brownish color, and give a clear ringing sound when struck; and when
broken, must present a compact and uniform texture. All bricks brought
upon the work which are soft and of a pale color, such as are usually
denominated _salmon brick_, are rejected. Those which are used, possess
nearly the hardness and durability of ordinary building stone, and are
calculated to resist the action of the water, to which they will be
exposed.
The advantage of using brick is, that a smooth channel offering little
resistance to the flow of water can be formed with less expense than
with stone, and greater security can be obtained against any leakage;
for besides the coat of plastering which covers the face of the walls
and the top of the concrete, there is also a mortar joint between
this plastering and the brick work. The bricks being of good form and
easily handled, can be more expeditiously and closely laid than the
face of a wall of stone, and afford a smooth and uniform face to the
wall with less expense. They are required to be bedded full and flush
with mortar, so that on lifting one from its position in the work,
no imperfections be discovered, but the impress of the brick be found
distinct throughout.
The proportions of the mortar for the brick work, are two parts of sand
to one of hydraulic lime.
The inverted arch of brick, as well as the brick facing on the sides,
is four inches thick, and the roofing arch of brick is eight inches
thick.
After the masonry is finished the excavation which was done to receive
it, is filled up around it, and over the top of the roofing arch
generally to the height of 3 to 4 feet, and in some instances of deep
excavation, up to the natural surface. If the natural surface be not
of sufficient height for the top of the earth covering, the earth is
raised to the requisite height with proper width on the top and slopes
on the sides for protection to the Aqueduct masonry.
[Illustration: I
_F. B. Tower._ _Gimber._
]
[Illustration: II
_F. B. Tower._ _Gimber._
]
[Illustration: III
_F. B. Tower._ _Gimber._
]
[Illustration: IV
_F. B. Tower._ _Gimber._
]
Plate II. is a section of the Aqueduct in open cutting in rock.
After the rock has been excavated to the required depth and width,
the bottom is levelled up with concrete to the proper height and form
for the inverted arch of brick, which is laid in the manner before
described for earth excavation. The side walls of stone and brick are
bonded together by headers of brick entering the stone walls as shown
in the drawing, and the walls of stone are built closely against the
sides of the rock and forming a junction with it. On the exterior
of the roofing arch a heavy spandrel of stone masonry (of the same
character as the stone walls beneath it) is built, filling the space
between the arch and the rock. After the masonry is finished, the rock
cut above it is filled with earth to the same height above the
roofing arch as mentioned for earth excavation.
Plate III. is a section of the Aqueduct in tunnel cutting in rock.
The width of the tunnel excavation in rock is the same as that of open
excavation in rock; and the manner of building the masonry to form
the channel-way is the same, with the exception that the rock roof of
the tunnel serves as the roof of the channel-way, where it is sound,
but in cases where the rock is soft and liable to fall, a brick arch
is built over the channel-way, and the space between its extrados, or
outer surface, and the rock roof is filled with earth closely rammed
in. In some instances where the tunnel perforated rock which was at
first quite hard, the roofing has by exposure to the air, become soft
and insecure, so as to render it necessary to turn an arch for its
support. This is attended with inconvenience and some difficulty after
the channel-way has been completed and closed through the tunnel.
Plate IV. is a section of the Aqueduct in tunnel cutting in earth.
When the earth is dry and compact, the excavation for the bottom and
sides is made of a proper form to receive the masonry, which is built
closely against it: the top is excavated sufficiently high to give room
to turn the arch, and the space above is afterwards filled with earth
closely rammed in. Where the earth is wet and there is difficulty in
making it stand, the excavation is made larger, and props of timber
and plank are used to support the top and sides until the masonry
be completed; and the whole space exterior to the masonry is then
compactly filled with earth.
Plate V. is a section of the Aqueduct showing the manner of
constructing it across valleys, or where the natural surface of the
ground falls below the plane of grade.
In such cases the Aqueduct is supported upon a foundation wall of
stone laid dry, and formed by using large stones laid in positions
to give proper bond, and to allow small broken stone to be closely
packed in, filling up all the interstices so as to form a compact and
uniform mass. The wall is generally allowed to stand some months after
it is completed, before the masonry of the Aqueduct is commenced upon
it, lest by this weight being placed upon it before it has found its
bearing, it should settle and cause cracks in the masonry. That such
settlement should in some instances occur, even after the Aqueduct
is completed, is not surprising, for passing over so many different
elevations, and encountering such numerous transitions from a hard
soil, or from rock, to valleys of alluvial deposit, it would be beyond
human powers of foresight and vigilance to prevent it.
[Illustration: V
_F. B. Tower._ _Gimber._
]
To render the Aqueduct more secure in such positions, the concrete
foundation has an increased thickness, and in preparing it a greater
proportion of hydraulic lime is used; the proportion being two and
a half parts of sand to one of lime. The dimensions of the stone
side walls and the spandrel backing of the roofing arch, are also
increased; and the proportion of hydraulic lime to the sand in the
mortar for these is increased. Another precaution has been taken to
render the work secure, by plastering the interior of the Aqueduct over
these foundation walls. The embankment adjacent to foundation walls
has various slopes according to circumstances, and is generally
protected with a dry stone wall on the face, and is carried up of
sufficient width to insure the requisite covering over the Aqueduct
masonry.
Along side hills an excavation is made for the Aqueduct into the hill,
and a protection wall of stone built on the lower side so as to support
a covering of earth over the masonry; great care being taken to obtain
a deep and firm footing for this wall in order to render the work
secure. In such a position the Aqueduct is perhaps less secure than
in those before described. Where the soil is wet from springs, and
the formation clay, there is danger of slides; and in rainy seasons
there is danger from the torrents which gather on the hill sides and
come down with destructive force: the earth covering is liable to be
carried away, and the Aqueduct itself to be undermined. Great care has,
however, been used in such cases to form strong paved channels for the
passage of the water over the top of the Aqueduct, or by culverts to
pass it underneath.
WASTE-WEIRS.
At suitable places on the line of the Aqueduct, waste-weirs are
constructed to discharge surplus water. They are constructed in one
side of the channel-way, in such manner as to allow the water to flow
off when it rises above a given level, and arrangements are also made
at these places to close the channel-way entirely, by means of stop
planks, and to discharge the whole of the water through waste-gates;
so that the water might be running from the Fountain Reservoir through
a portion of the Aqueduct and discharging from these waste-weirs
while the remainder of the channel-way, or portions of it, would be
drained so as to admit of inspection or repairs. There are six of these
waste-weirs constructed for the Aqueduct.
VENTILATORS.
For the purpose of ventilation hollow cylinders of stone are erected
over the top of the Aqueduct and rising about 14 feet above the surface
of the ground, or earth covering. These occur every mile, and every
third one is constructed with a door to afford an entrance to the
Aqueduct.
Those allowing an entrance have an interior diameter of 4 feet, and the
others have an interior diameter of 2 feet; each, however, slightly
diminishing towards the top. An iron grating covers the top to prevent
any thing being thrown in.