OF PLANS PROPOSED FOR FURNISHING THE CITY WITH WATER, AND OF THE PLAN ADOPTED.
In the course of examinations which were made to determine sources whence water could be obtained, questions of deep importance presented themselves in regard to the source to be relied upon for a supply, also in reference to the plan which should be adopted for conducting the water to the city.
It was of so much importance to the city that the supply should be such as not only to answer the present purposes, but be adequate to the future increased demands, and that the quality of the water should be unquestionable, that it became necessary to extend the examinations over every watered district in the vicinity, in order to judge of the comparative merits of different sources. The Engineers who were employed, traversed the country, gauged the streams, reported their supply, the quality of the water, and plans which might be adopted for conveying it to the city. It was a field for the exercise of the talent and research of the Engineer: in resorting to a distant stream for a supply, any plan which he might propose for conveying the water, would encounter obstacles requiring skill and ingenuity to overcome. He would find it necessary to build up the valleys, pierce through the hills, and span the waters of the arms of the sea which embrace the city and make it an island. Structures would be required, which, in their design, would find no parallel among the public works of this country, and in forming plans for them he might study with advantage, the works constructed for similar purposes by the Ancient Romans.
The examinations embraced all the sources from which a supply of water might be obtained in the neighboring counties of Westchester and Putnam; giving a comparison of the different streams in regard to their elevation, their capacity, and the quality of the water. It was decided that the Croton River would supply a sufficient quantity of water at all seasons of the year; at an elevation precluding the use of steam or any other extraneous power, and that the quality of the water was unexceptionable. Other streams were found which would furnish water equally pure, but too limited in quantity at certain seasons of the year, and not at a sufficient elevation.
In addition to the information furnished by the Engineers employed, the Water Commissioners received communications from other sources suggesting plans for supplying the city with water.
It was suggested that water might be obtained from the Passaic Falls, at a distance of about eighteen miles from the city, in New-Jersey. The objections to this project were, that it would be going into another state, that an Aqueduct bridge over the Hudson River would obstruct its navigation, and iron pipes laid across the bed of the river would be exposed to injury from the anchors of the shipping. Another plan was proposed which contemplated a permanent dam across the Hudson River extending from the city to the Jersey shore. This dam was proposed to be built about 2 feet above the level of high tide, thereby keeping all the salt water below; and above the dam would be the fresh water for supplying the city, which must be pumped up into a reservoir by means of water-wheels, which would be operated by the overfall of water when the tide was low, but when the tide was up within 2 feet of the top of the dam there would not be sufficient fall to propel the wheels. Locks were to be inserted in the dam, of a sufficient number to accommodate the vessels on the river. The river, at the place where it was proposed to locate the dam, is over a mile in width, and in the channel the depth below the surface to proper foundation for such a structure, would probably be 50 feet. The difference of tides is about 5 feet, which added to the height of dam above high tides, would give 7 feet of the top of the dam exposed to the pressure of the water on the up stream side when the tide is low.
It was suggested that the hydraulic power here obtained, could be used for manufacturing purposes, except that portion of it which would be required for elevating the water to the reservoir. This plan of supplying the city with water was objected to, because it could not be accomplished except by an Act of the Legislature of New-Jersey as well as that of New-York, and it was also questionable whether such obstructions could be placed in navigable rivers without interfering with the powers of Congress to regulate the commerce of the nation. It was feared that in locking vessels through, the salt water would become mingled with the fresh above the dam where a supply would be taken for the city, to such a degree, that it would render it unfit for domestic use. The quantity of land that would be overflowed by the water set back by the dam, presented another objection. The space of time that the tide would be sufficiently low to allow the wheels to work in pumping water into the reservoir, would be entirely too short to insure a supply. This objection was offered by Frederick Graff, Esq., the superintendent of the Philadelphia Water Works, who stated that although the dam on the Schuylkill River is raised 6 feet 6 inches above the highest tides, the delay in pumping, occasioned by the tides, averages seven hours out of the twenty-four; and in full moon tides, from eight to nine hours.
The projector of this plan set forth many advantages which he thought would arise from the construction of the dam, but the obstruction to the navigation of the river, the destruction of the shad fishery, and various objections besides those already mentioned, induced the Water Commissioners to reject the idea of building a dam across the Hudson.
We have now gone over most of the preliminary steps which were taken 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.