Imágenes de páginas

nine feet of water, where the figure 3 indicates the present depth. The vessels were placed checker-wise, in such a manner as to impede naviga. tion, while interfering least with the discharge of the water. The effect, nevertheless, was the formation of a shoal in a short time, and the scouring out of two channels, one on each side of the obstructions, through which twelve and fourteen feet can now be carried at low water. The increased water-way thus given to the ebb-tide caused it to abandon the old nine-foot channel on the less direct course to deep water. We have here the total obstruction of a channel which was of considerable importance to the southward trade by new conditions introduced at a point four miles distant from wbere the effect was produced, and we are warned how carefully all the conditions of the hydraulic system of a harbor must be investigated before undertaking to make any change in its natural conditions, lest totally unlooked for results be produced at points not taken into consideration.

NEW YORK HARBOR. Approaching now more closely to the consideration of the tidal con. ditions in New York Harbor, we will examine the progress of the tidewave through Long Island Sound from the eastward to its meeting with that entering New York Bay at Sandy Hook.


[merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small]

We see from the annexed diagram that about seven and a half hours after the transit of the moon high water has advanced just within Block Island with an elevation of two feet, and at the same time has just passed Sandy Hook with an elevation of four and a half feet. Travers.

ing the sound at a rate indicated by the Roman figures, with increasing heights indicated by the Arabic numerals, it reaches Sand's Point eleven and a half hours after the transit of the moon with a height of seven and seven-tenths feet. The observed time of transmission from the Race to Sand's Point is two hours one minute, and the time computed from the depths, according to the law developed by Airy, is two hours fourteen minutes—a very good approximation, when we consider the irregularities in the configuration of the sound, which could not be taken into account. Advancing still farther, the height somewhat declines in consequence of the changes of direction in the channel and its shallowness. At Hell Gate this tide-wave is met by that which had entered at Sandy Hook, and advanced more slowly, owing to the narrowness and intricacies of the channel, especially in the East River.

These two tides, which meet and overlap each other at Hell Gate, differing from each other in times and heights, cause contrasts of waterelevations between the sound and harbor which call into existence the violent currents that traverse the East River. The conditions of the tidal circulation through Hell Gate are such, that if there were a partition across it, the water would sometimes stand nearly five feet higber, and at other times five feet lower, on one side than on the other. In the actual case of the superposition or compounding of the two tides the difference of level existing at any time is of course much less; but the difference of one foot is often observed within the space of 100 feet in the most contracted portion of Hell Gate, off Hallett's Point Referring now more particularly to the diagram representing New York Bay and Harbor, it is important to observe that the entrance from Long Island Sound is a natural depression or arm of the sea, which is not changed by the forces now in operation. The tidal currents which flow through it do not change the channel, but are obliged to follow it in its tortuous course. The Sandy Hook entrance, on the contrary, is characterized by a cordon of sands, extending from Sandy Hook to Coney Island, intersected by channels, which are maintained against the action of the sea, that tends to fill them up, by the scour of the ebbtide from the tidal basin of New York Harbor.

Unlike Hell Gate passage, where permanence is the leading characteristic, the bar and channels of Sandy Hook have undergone continual changes within the brief period of our bistory. The advance of Sandy Hook upon the main ship-channel is among the notable and important instances of the effect of tidal currents. Within a century it has increased a mile and a quarter. In the place where the beacon on the end of the Hook dow stands were forty feet of water fifteen years before it was built. The cause of this growth is a remarkably northwardly current along both shores of the Hook, running both during the flood and the ebb tides with varying rates, and resulting from those tides directly and indirectly.

The best water over the bar is about two miles east of Sandy Hook

light, in a direct line with the swash channel, which is the second opening-shown on the sketch-above the Hook; the shoal lying botween the main or Hook channel and the swash channel being known as Flynn's Knoll. The greatest depth over the bar is twenty-two feet at mean low water; and very nearly the same depth can now be carried through the swash channel, which formerly was three feet shallower,

[merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][ocr errors][ocr errors][ocr errors][merged small][ocr errors][merged small][ocr errors]

but has deepened since the cross-section between the Hook and Flynn's Knoll has been diminished by one-third its area by the growth of the Hook. This relative change in the capacity of the channels has not, however, affected the depth on the outer bar, which, according to the principles above laid down, is dependent mainly upon the area of the tidal basin within.

The depth of twenty-two feet at mean low water, which is now maintained at the entrance through the sands constantly thrown up by the

waves of the sea, may be considered as depending upon the following elements:

1st. The large basin between Sandy Hook ard Staten Island, including Raritan Bay, which furnishes more than one-half of the whole ebbscour;

2d. What is called the Upper Bay, including the Jersey flats and Newark Bay;

3d. The North River, perhaps as far as Dobbs' Ferry, maintaining the head of the ebb-current, although not directly taking part in the outflow; and,

4th. A portion of the sound tide, which flows in through Hell Gate.

The proportion of the three first divisions in producing the depth of channel may be approximately estimated by a comparison of the areas and distances from the bar. In order to maintain the depth which we now have, it is important that the area of the tidal basin should not be encroached upon. In proportion as that is diminished the depth of the channels will decrease.

The flats, just bare at low water, but covered at high tide, form as important a part as any other portion, for it is obvious that it is only the volume of water contained between the planes of low and high water-tbe“ tide-prism"—that does the work in scouring the channels. The water on the flats is especially useful by retarding the outflow, thus allowing a greater difference of level to be reached between the basin and the ocean.

When we yield to the demands of commerce any portion of the tidal territory to be used for its wharves and docks, we must do so with full cognizance of the sacrifice we are about to make in the depth of water over the bar; and in order to form any well-founded judgment in regard to the effect of such encroachments, it is necessary to be in possession of the fullest knowledge of all the physical facts involved in the problem, and no measure of encroachment should be determined upon except in pursuance of the advice of scientific experts.

A proposition frequently mooted by men of enterprise, and resisted by those interested in the welfare of the city of New York, is the occupation of the Jersey flats from Paulus Hook to Robbins Reef for docks and wharves. Without expressing any opinion as to the relative value of the gain of accommodation for shipping and the loss of depth in the channel, I venture to say that the withdrawal of that area from the domain of the tide would occasion a loss of not less than one foot in the depth of the bar off Sandy Hook, and certainly not more than two feet.

The part which the fourth division in our classification of the basin of New York, that of the East River and Hell Gate passage, plays in the outflow of the ebb-tide through the Sandy Hook channels, depends less upon the area involved than upon the difference in point of time and height of tide in Hell Gate, already adverted to. The westerly current, usually called the ebb-stream, since it falls in with the ebh. stream of New York Harbor, taking place when the sound-tide is highest, starts from a level of three and a half feet higher than the easterly, and thus a much larger amount of water flows out through the Sandy Hook channels than through the narrows at Throg's Neck. It is apparent, then, that this portion of the ebb-stream, re-enforcing as it does the of the harbor proper at the most favorable times, performs a most important part in maintaining the channels through the Sandy Hook bar. It may be estimated that the closing of Hell Gate would cause the loss of certainly not less than three feet in the depth of those channels.

From wbat has been said with regard to the meeting of the tides in Hell Gate, it will be seen that the violent currents experienced in that locality are due to causes beyond our coutrol. The daugers to navigation arising from these currents, however, by their setting vessels upon the rocks and reefs, may, in a great measure, be done away with by the removal of the obstructions, in which work considerable progress bas already been made. The removal of the reef at Hallett's Point, the work upon which is now in progress, will doubtless, in a great degree, do away with the eddies and under-currents produced by the sharp turn which the channel now takes at that point. It is not improbable that the successful removal of those obstructions will yet cause the sound entrance to be used in preference to the other by the fleets plying hetween European ports and the great commercial metropolis of America.

NOTE.—The reader who wishes to enter upon the mathematical treatment of the subject of tides is referred to Airy's treatise on tides and waves, and to the memoirs of Whewell and Lubbock, in the Philosophical Transactions of the Royal Society; and for investigations of the laws of the tides on our own coasts, to the papers on that subject by Bache and others in the annual reports of the Coast Survey. Among the latter, the lecturer is particularly indebte 1 to the “Report on the tides and currents of Hell Gate," by Henry Mitchell, 1867, in which the complicated problem of the tidal circulation of New York Harbor is treated with great ability and success.

« AnteriorContinuar »