Part 6

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.

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.

Plate VI. is a view of an entrance ventilator; this stands on one side of the Aqueduct, where the masonry of the side wall is enlarged for its base; we can descend from the door and gain an entrance to the channel-way by an opening in the side of the roofing arch. The sill of the door is about 12 feet above the bottom of the channel-way.

Those not intended for an entrance stand directly over the top of the Aqueduct and are groined into the roofing arch.

Besides these Ventilators, there are openings 2 feet square in the top of the roofing arch, every quarter of a mile: they are covered with a flag stone and the place is marked by a small stone monument projecting above the surface of the ground. These may be useful to obtain entrance to the Aqueduct, or to afford increased ventilation should it ever become necessary.

CULVERTS.

Where streams intersect the line of Aqueduct, culverts are built to allow them to pass under it. They are simply a stone channel-way built under the Aqueduct of such form and dimensions as will allow the stream to pursue its natural direction without causing injury to the work. The foundation of these culverts is formed by laying down concrete, upon which an inverted arch of cut stone is laid forming the bottom of the water-way: side walls of stone are built and surmounted by an arch of stone. The span, or width of water way, of the culverts built, varies from 1½ foot to 25 feet. Those of 1½ foot span have a square form for the water-way, and are constructed by making a foundation of concrete, upon which a flooring of well dressed stone is laid forming the bottom of the water-way, and from this, side walls are built and covered by a course of thick stone flagging well dressed and closely fitted. At each end of the culvert a deep wall is built underneath so as to prevent the water from doing injury by undermining it. Buttresses and wing walls are built at each end of the culvert to guide the water to and from the channel-way, and a parapet wall is built over the top of the channel-way at each end to sustain the embankment of earth over the culvert. These wing walls and parapets have various forms; sometimes the parapet is built across the top of the culvert, and the wing walls built at right angles to it, and sloping down to the buttresses, and sometimes the wing walls and parapet form one continuous wall of a semi-circular form, the top sloping up from the buttresses in a plane parallel with the slope of the embankment covering the Aqueduct above. These culverts are permanently constructed, and in preparing the plans for them much skill has been displayed in adapting the form and size which the circumstances required, and much taste displayed in the design for their construction.

Plate VII. is an isometrical drawing of one of the culverts with rectangular wings and parapets; the body of the culvert is cut in two in the drawing, showing that it may be of any length, according to the width of the embankment through which it is constructed. The length is generally arranged so that the slope of the embankment may intersect the rear of the top of the parapet and pursue a direction down, parallel with the slope of the top of the wing walls.

_Gate Chamber at the Head of the Aqueduct and Grade of the Water-way of the Aqueduct._

Plate VIII. is a longitudinal section through the _tunnel_ and _gate chamber_ at the head of the Aqueduct showing its connection with the _Fountain Reservoir_. This gate chamber is not in any way connected with the dam itself, but stands some distance from it, and the water reaches it by means of the tunnel which leaves the Reservoir above the dam and passes through the solid rock of the hill against which the masonry of the dam is built, a distance of over 200 feet. This tunnel descends into the Reservoir, so that the centre of it at the mouth is about 12 feet below the surface of the water; any floating substance cannot enter it, and during the winter season when the water is frozen over no obstruction can take place to the flow into the Aqueduct, and during the summer season the water will be drawn from a level where it is cooler than at the surface.

The gate chamber has two ranges, or sets of gates; one called _regulating gates_, and the other _guard gates_: the regulating gates are made of gun metal, and work in frames of the same material which are fitted to stone jambs and lintels: the guard gates are made of cast iron, and work in cast iron frames also attached to stone jambs and lintels. The gates are all managed by means of wrought iron rods attached to them, having a screw formed on the upper part on which a brass nut works, being set in a cast iron socket-cap.

The bottom of the water way, of the Aqueduct, where it leaves the gate chamber is 11.40 feet below the surface of the Fountain Reservoir, and 154.77 feet above the level of mean tide at the city of New-York. The following table shows the length of the Aqueduct as it is divided into different planes of descent, from the gate chamber at the Croton dam to the gate chamber at the Receiving Reservoir on the Island of New-York. Commencing at the south side of the gate chamber at the Croton dam,

and the ft. miles, descent The 1st plane of Aqueduct extends 26099.72 = 4.943, 2.94 ft. The 2d plane of Aqueduct extends 148121.25 = 28.053, 30.69 ft. Length of pipes across Har. River, 1377.33 = 0.261. Diff. of level betw’n extremes of pipes 2.29 ft. The 3d plane of Aqueduct extends 10733.14 = 2.033, 2.25 ft. Length of pipes across Manhat. valley, 4105.09 = 0.777. Diff. of level betw’n extremes of pipes 3.86 ft. The 4th plane of Aqueduct extends 10680.89 = 2.023 1.60 ft. --------- ------ --------- 201117.42 = 38.090 43.63 ft.

Making the whole distance from the gate chamber at the Croton dam to the gate chamber at the Receiving Reservoir 201117.42 feet, or 38.09 miles, and the whole descent 43.63 feet.

The descent on the first plane is about 7⅛ inches per mile.

The descent on the second and third plane is about 13¼ inches per mile.

The descent on the fourth plane is about 9½ inches per mile.

In crossing Harlem River there is a fall of 2 feet more than there would have been had the Aqueduct continued across with its regular inclination: this _extra_ fall will afford an opportunity to adjust the number and capacity of the pipes (which descend below the level of the Aqueduct and rise again) to discharge the full quantity of water as freely as the Aqueduct, or channel-way of masonry, would have done had it continued its regular inclination across the valley.

In crossing Manhattan Valley there is an _extra_ fall of 3 feet for the same reasons as before stated for that at Harlem River. In both cases, by using the pipes, there is a loss of the head of water for the City Reservoirs, equal to the amount of this _extra_ fall; but this small loss of head was not considered of such importance as to induce the building of structures across these valleys up to the plane of Aqueduct grade.

The bottom of the water-way of the Aqueduct at the gate chamber where it enters the Receiving Reservoir, is 7.86 feet below the level of top water line in the Reservoir, thus when the Reservoir is full the water will rise to within 7¼ inches of the top of the interior of the Aqueduct at that place, and the height from top water to the top of the interior will increase, going northward according to the inclination of the plane of Aqueduct grade, until it reach the surface level of the flow of water in the Aqueduct.

The height of the interior of the Aqueduct is 8 feet 5½ inches, and the greatest width is 7 feet 5 inches. The sectional area of the interior is 53.34 square feet. On the _first plane_, the Aqueduct is larger; being 2.05 feet higher at the gate chamber, 2.31 feet higher at 2244. feet from the chamber, and then diminishing, to the head of the second plane, where it assumes the size above mentioned and continues of that size throughout the remainder except in tunnels, where it assumes the forms before described. Where the Aqueduct on the _first plane_ is larger, the width across the interior at the spring line of the roofing arch is the same as the general width, but the increase takes place only in the height of the side walls, and the slope of the inner face of the walls being the same, the width across at the spring line of the inverted arch will be less according to the increased height of walls. The original design was to continue the inclination which the _second plane_ has, up to the _Fountain Reservoir_; but it was considered desirable to draw from this Reservoir at a lower level, and the head of the Aqueduct was depressed for that purpose, and a less inclination adopted for the length of the _first plane_. The roofing arch was left on the same inclination as was originally designed, except for the distance of 2244. feet from the gate chamber, where it was built on a level.

The curves which are used to change the direction of the line of the Aqueduct are generally formed with a radius of 500 feet; some have a radius of 1000 feet, and in a few instances larger ones are adopted, but the majority of them are of 500 feet radius.

The velocity of the water in the Aqueduct has been ascertained to be about one mile and a half an hour when it is 2 feet deep; this was determined by floating _billets_ of wood from the Croton Dam to Harlem River and noting the time of their passage. Such an experiment would express the surface velocity and would give a greater velocity than it would be proper to attribute to the _whole body_ of water in the Aqueduct; but the depth of water in the Aqueduct will be probably 4 feet as soon as it is brought into general use, and then there will be a corresponding increase in the velocity of the _body_ of water. This velocity of a _mile and a half an hour_ may be taken in general terms as the _velocity of the water in the Aqueduct_.

DESCRIPTION OF THE LINE OF AQUEDUCT.

The dam, built to form the Fountain Reservoir, is about six miles above the mouth of the Croton River. The reservoir forms a beautiful sheet of water in the lap of the hills in the wild region of the Croton, and has received the name of the “Croton Lake.”

Pine’s Bridge over the Croton River, which is mentioned in the early history of the country, occupied a position which is now about the middle of this Reservoir, and there is at that place a bridge over the Reservoir resting upon piers and abutments.

The hills which bound the Croton Valley where the Reservoir is formed are so bold as to confine it within narrow limits: for about two miles above the dam the average width is about one eighth of a mile; at this distance from the dam the valley opens so that for the length of two miles more the width is about a quarter of a mile; here the valley contracts again and diminishes the width until the flow line reaches the natural width of the River at the head of the lake. The country immediately contiguous to the shore has been cleared up, and all that would be liable to impart any impurity to the water has been removed. This gives a pleasing aspect to the lake, showing where the hand of art has swept along the shores leaving a clean margin. Retiring from the water are the richly cultivated slopes with the neat farm houses overlooking the lake, or the hills crowned with forest trees, while at intervals a valley or ravine opens and empties in its tributary stream.

Plate IX. is a view taken above the dam showing the position of the entrance to the tunnel which leads from the Reservoir to the gate chamber at the head of the Aqueduct. The entablature which is seen on the left against the rock, is built directly over the mouth of the tunnel, and from this the tunnel extends through the rock to the gate house, which is seen on the right of the picture and some distance from the dam. The structure which is seen in the centre of the picture and on the ridge of the dam is a gate house over a culvert which extends through the body of the dam; this culvert is 30 feet below the surface of water when the Reservoir is full, and has gates which are operated by means of rods which rise to the interior of the house. During low stages of the River the water which is not drawn off by the Aqueduct may pass through this culvert and allow none to pass over the dam.

The entrance to the tunnel is protected by a screen of timber work.

Plate X. is a representation of the entablature over the mouth of the tunnel, showing the inscriptions upon it, relating to the date of the commencement of the dam and its completion, the persons who had contracts for building it, and those having charge of the work during the time.

Plate XI. is a view taken from a point below the dam and shows the relative positions of the dam and the gate chamber at the head of the Aqueduct.

The original channel of the River where the dam is built, was about 120 feet wide; the average depth of water at this place was about 4 feet; and the greatest depth 10 feet.

The left bank of the river arose abruptly with rock, the channel was gravelly, and on the right bank a sandy table land about 3 feet above the ordinary surface of water extended about 80 feet; then a sandy hill arose on a slope of about forty-five degrees.

In making the plan for a dam at this place it was determined to fill the main channel and the table land on the right bank with an embankment of earth; and on the left bank where rock was found, to build a body of masonry against the slope to the requisite height for the surface of the Reservoir and connect it with the embankment in the channel; this masonry formed the overfall for the water, and the rock in the side of the hill adjacent to it was excavated down to the level of the overfall, thereby extending it into the hill, making the space for the water to pass over partly of masonry and partly of rock. The embankment extended with a slope on the upstream side giving it a broad base, and the lower or downstream side was faced with a heavy wall of stone. There was a timber pier constructed in the embankment extending across the channel and faced with plank on the upstream side. The overfall was made of such length as was thought sufficient to pass all the water of the river during its highest stages, and with the view of adapting it to such purpose, examinations were made to find the highest marks of floods on the banks of the river; and those who were engaged in determining these marks were guided also by the observations of the inhabitants of the vicinity who had long known the river in its various stages. High freshets were witnessed during the construction of the work, for in the course of two years that the work was going up, all the various changes and freshets of rainy seasons were experienced, and those in charge of it did not neglect to note the quantity of water flowing on such occasions.

With such opportunities to become acquainted with the changes of the stream they could not fail to know the quantity of water flowing at periods of the highest freshets, and knowing it, to adapt an overfall of sufficient capacity for its discharge. For this purpose it was thought ample provision was made; yet at the time when the work was nearly completed such a flood occurred as could not have been anticipated from previous knowledge of the River; the water filling the entire passage at the overfall, flowed over the top of the embankment where it was not supposed it could ever reach. The lower slope of this embankment was covered with a wall not calculated to resist the action of the water and it gave way; the water broke through the embankment and rushed along the valley with most disastrous consequences. The breach occurred at an early hour in the morning; and many persons were suddenly aroused from their sleep to escape before the approaching waters. Dwelling-houses and mills were carried away and three lives were lost. Two of those who were drowned had taken refuge in the tops of trees, but these being swept away they were drowned; while others who were not able to reach the main land, but had also taken refuge in trees, were saved. The change wrought by the flood, in the appearance of the country, was truly wonderful and the destruction was complete. Night had closed over that valley where all was happiness and quiet, but day opened upon a scene of desolation. The fertile fields were torn up and covered with masses of stone and gravel, and the flood left marks of its fury far up on the hill sides.

At the commencement of the rain which caused this flood, the ground was covered with snow to the depth of eighteen inches: the weather became warm and the powerful rain storm continued incessantly for forty-eight hours. Notwithstanding the immense volume discharged at the overfall of the dam, the water was rising, during the night previous to this disaster, at the rate of fourteen inches per hour over the Reservoir, covering an area of four hundred acres.

It occurred on the 8th of January, 1841.

In repairing the breach it was decided to build an extension of solid hydraulic masonry in the place of the portion of embankment which was carried away.

The gate house and wing wall, which is seen on the ridge of the dam, shows where the masonry of the original structure connected with the embankment which extended across the river. The whole length of the overfall is 251 feet. Access to the house over the culvert, is gained by a foot bridge which is seen in the picture. The masonry of the original structure has a rock foundation, and the extension of the overfall which is seen on the left of the house extending across to the embankment has an artificial foundation of concrete.

The masonry of the dam is about 8 feet thick at the top and 65 feet at the base; it is built in a vertical form on the upstream side, with occasional offsets, and the lower face has a curved form such as to pass the water over without giving it a direct fall upon the apron at the foot; this apron is formed of timber, stone, and concrete; and extends some distance from the toe of the masonry, giving security at the point where the water has the greatest action. A secondary dam has been built at a distance of 300 feet from the masonry in order to form a basin of water setting back over the apron at the toe of the main dam so as to break the force of the water falling upon it. This secondary dam is formed of round timber, brush wood, and gravel; it may be seen in the picture directly under the bridge which extends across below the main structure.