Volume 3 — THE PHYSICAL SURVEY OF NEW YORK HARBOR AND ITS APPROACHES.

No coast offers more admirable opportunities for the study, on a large scale, of the effects of winds, waves, and currents, tidal and others, on the movable matters which line the ocean shores, than that from New York southward. Besides the peculiar local actions, there are general ones, which are changing, slowly or rapidly, the whole of the sandy coast line. While here the pebbles of the ancient drift are being assorted by size and shape, and rolled into ridges and heaps, by the action of the waves, there heaps and ridges of wet sand are formed by the waves and travel under their motion, and the dry sand is forced along by the winds, covering up meadows and woods, and changing the ocean shore line; and in other or the same localities, sub-currents, setting in a nearly constant general direction, roll onward the movable materials of the bottom of the sea, or tidal currents roll them forward and backward, giving the general direction of the resulting motion.

The reports of the Government and State engineers and commissioners, public and private, who have studied the improvements of different localities, have given us glimpses of the local, and even of the general actions; but most commonly there has been a want of means or such preliminary experiments as were necessary fully to develop the actions, and which, like the stitch which saves nine, would often have saved the costly experiment on the full scale of construction. Remarkable instances of complete modes of investigation occur in the examination of the Mississippi River by Captain A.A. Humphreys and Lieutenant Abbot, of the Topographical Engineers, and by the commission of which General Totten, Prof. Bache, and Admiral Davis were members. As most familiar to me, from having taken an official part in the experiments and observations made, I propose to notice the Physical Surveys of New York and Boston, indicating the chief agents which are at work in destroying and building up, so as to produce the present condition of these important ports.

In connection with the surveys made a few years since by direction of the Commissioners on Harbor Encroachments, there was undertaken, as an incidental inquiry, an investigation into the physical conditions under which the shoals and beaches in and about New York harbor had submitted to those changes of position and area which the repeated surveys revealed. It was at the request of these Commissioners that Professor Bache, the Superintendent of the Coast Survey, gave his personal attention to this subject. He drew up a comprehensive scheme for a series of observations upon all the natural agencies at work, and, for the execution of the project, selected one of his assistants, whose experience had already been considerable in similar studies.

The investigation was commenced in the lower harbor early in the spring of 1856. Records were kept of tides, currents, winds, and waves, and the most careful notes were made on the immediate effects of these working agents as observable in the movements of the sands.

A glance at a general coast chart discovers at once a marked contrast between two different sections of our seaboard: to the eastward of us, the principal harbors of New England are rockbound, with elevated back countries; while to the southward, in the region of alluvial drift, which extends all along the coast of the Middle and Southern States, the harbors have flat and sandy shores. The harbor and neighborhood of New York, holding an intermediate position between these diverse sections, exhibit a singular combination of the leading physical features of both, and present to the hydrographer a field for research that is quite without a parallel.

We recognize in the Bar of New York simply a submerged portion of that sandy cordon which skirts the coast from Montauk Point to Florida; and although, in the ordinary sense, the lower entrance to the harbor is not an inlet, it may nevertheless be regarded as belonging to the same class.

This sandy cordon, which may be said to be the principal characteristic of our coast, is an exceedingly interesting feature; it appears to have been formed by the action of the sea, which has disintegrated the borders of shallow flats, bearing away the light vegetable moulds, but suffering the coarse quartz sand to remain rolled up into ridges. In many places the dry winds have caught up these sands, when laid bare at low water, and elevated them into dunes or galls.

The distance of the sand ridge from the mainland is observed to vary with the slope of the adjacent country. It is the motion of translation which a wave acquires on reaching shallow water, that gives it such great capacity for the transportation of material.

This translative action, as it is technically called, commences ordinarily in about three fathoms water, and is most violent in six or eight feet depths, within which the sea breaks. It is just within the breaker that the windrows of sand are observed to form on exposed flats.

This disposition of the sea to cast up well defined boundaries of sand along its margin, is so great and persistent, that the inland waters are dammed up and suffered only to escape into the ocean by narrow avenues, where their rapid currents maintain a supremacy of power—albeit with unceasing contest.

Wherever, along our coast, the waves drive obliquely upon the beach, a movement of the sand takes place, and the inlets are consequently continually shifting.

The Long Island inlets are moving westward, and Sandy Hook advances to the northward, because the sea rolls in along the axis of the great bay between Long Island and New Jersey, and necessarily sweeps along the beaches, instead of taking the direction of a normal to the shore line.

The movement of Sandy Hook to the northward is, however, a problem not so easily disposed of as we might conceive from the above considerations; for although, in the most general sense, its existence must be regarded as the work of the waves, there are other agents influencing materially its form and its rate of progress. The currents control, to a very great extent, the final disposition of the sands worn away or kept in motion by the waves.

Professor Bache's investigations in the neighborhood of Sandy Hook have been published, and we should not especially refer to them here, except that the recent physical changes reported by Colonel Delafield to the Engineer Department, have reawakened an interest in the matter.

The measurements of the Coast Survey, made in 1856 and 1857, showed that the Hook was being washed away on the east and west shores, but was extending slowly to the northwest, where it already encroached on the main ship channel. This order of things has continued up to the present time, and is now in progress.

The able Superintendent of the fortifications at Sandy Hook has evinced considerable alarm lest the new fort shall fall a prey to the encroachments, or be separated from the main body of the beach by slue-ways. The Coast Surrey has been notified of the matter, and the assistant to whom I have already referred has visited the Hook, and made an informal report, which agrees essentially with the statements of Colonel Delafield. A complete and reliable report can only be made upon actual surveys; and we trust these will be executed, and the Government placed in possession of the whole truth.

We understand that Colonel Delafield has already, upon a small scale, made some very successful experiments of curvilineal dikes, constructed with caissons of concrete; and we have no doubt that, with adequate means at his disposal, this ingenious engineer could avert the dangers which threaten, not only the fort, but the noble harbor of New York.

To return to the Physical Survey, and to speak as briefly as we may upon so extended a subject, we hold that it is possible, by a patient collection of facts and figures, to determine the natural scheme of the harbor—we had almost said the formula of its development.

It is ascertained that the group of shoals which form the Bar—composed, as they are, for the most part, of loose and shifting sands—are not accidental accumulations, modified by violent storms and freshets, but that they are orderly arrangements, made by the currents, to whose unceasing activities are due the form and preservation of each bank and channel. The peculiar contours of the shoals given by our most ancient charts are still developed by recent surveys, although alterations in magnitude have taken place. The order of the physical forces is unchanged, but their work is still progressing.

Now, since these currents have determinable laws, regulating their periods, durations, velocities, and directions, it was only necessary to compile observations, in order to reduce this study to a simple consideration of the composition of forces.

'The process by which sand is swept along by currents upon the bottom of the sea, is not unlike the motion of dunes upon the land; a ridge of sand is propagated in the direction of the current by the continual rolling of the particles from the rear to the front. This movement is exceedingly slow when compared with that of the current which induces it, and for this reason a shoal, though traversed by violent tidal currents, may, as a whole, remain stationary when the alternate drifts are equal and opposite; for in this case, though the sand upon the surface is drifted to and fro, it undergoes no more ultimate change of position than it would if the forces which acted upon it were simultaneous and in equilibrium.'[9]

Of course, so simple a case as that in which the ebb and flood forces are equal and opposite, is rarely presented; for at most of the stations on the Bar the direction of the flow varies from hour to hour, going quite round the circle in a half-tidal day: the velocities and directions also vary with the depth. These circumstances complicate the computation a little, but the problem is still simple and direct. Everything depends upon the faithfulness of the observations.

The physical diagrams which have been plotted from the results of these studies may be regarded as decided successes, for they show in most cases that the shoals lie in the foci or in the equilibrium points of the observed forces.

The current stations occupied cover a district embracing not only the immediate vicinity of the shoals, but extending many miles from them in different directions; for it was deemed necessary that each elementary force should be separately studied before it reached its working point. It has been ascertained that among the causes of the different shoal formations there exists a mutual relation and dependence, so that they may be regarded as a single physical system. It will be seen from this consideration, that any artificial disturbance of the conditions, at a single point, may interrupt the operations of nature in other localities more or less remote, or cause general changes in the hydrogaphy of the harbor.

It is not simply the superficial drift of the tidal and other currents that these observations comprehend; but, with the use of apparatus suitably arranged, the movements at all depths have been determined, with the exact amount of power exerted by streams coursing along the bed of the sea. The necessity for this minuteness of examination has been fully shown in some of the curious discoveries that have been made.

In several parts of our harbor, systems of counter-currents have been detected, occupying strata of water at different depths, and these present, in their motions, striking contrasts of directions, velocities, and epochs. The most remarkable exhibition of these sub-currents was observed in the neighborhood of the city, in the channel between Governor's and Bedloe's Islands. In this locality, during the last quarter of the ebb, floating objects drift southward toward the sea, while the heavier material upon the bottom is borne northward toward the city piers. While, upon the surface, the ebb exceeds the flood both in velocity and duration, the motions of the lowest water-stratum are subject to the reverse conditions: it therefore follows that the heavier deposits from the city drainage cannot be swept away through this the main avenue to the sea. This contrast of motion between the upper and lower drifts was observed in greater or less degree throughout the entire distance from the Bar to a point in the Hudson River off Fort Washington. These results appear to us of the highest importance, since they would seem to indicate that the scouring action of the currents will not be sufficient to prevent the accumulation of certain classes of deposits in the upper harbor—as the ashes from the steamers, and the like.

The course of the land waters in their progress seaward was followed nearly sixty miles beyond the Bar, where currents of considerable velocity were still observed. At the station farthest seaward, where the sounding is thirty fathoms, the observations at different depths disclosed some very remarkable peculiarities. It was perceived that the moving stratum was not always of the same depth; the whole body of the sea moving steadily forward at one time, while at another no motion could be detected below a superficial stream.

The land waters, to which allusion has been made, augment the ebb current to such a degree, that a general eastwardly preponderance was observed in the drift along the south shore of Long Island; and this preponderance, increasing steadily from station to station at each remove, was found, at a point twenty-five miles east of Fire Island Light, to outlive the tidal currents and maintain itself as a constant coastwise stream.

One very curious discovery was made with regard to this stream along shore. It was ascertained that during easterly gales a portion of the water, crowded up into the bight of the coast, escapes seaward by a sub-current. Shells, carefully marked, were deposited in the sea during fine weather, and, after an easterly gale, were picked up on the shore of Fire Island, four miles eastward of the place of deposit. There was no evidence that these shells travelled any distance during still weather.

We do not despair of the possibility of artificial improvements at the Long Island inlets.

At present the great inland basins on the southern portion of Long Island communicate with the sea only by narrow passes obstructed by bars and shoals; yet, in spite of the dangers which are always presented, large fleets of market vessels pass out daily through the inlets, laden with farm produce and shell fish. It requires no thought to perceive that if these inlets were made safe and permanent by suitable marine constructions, and were furnished with the proper buoys and beacons, there would spring up in their neighborhood great commercial enterprises.

While, in the case of the lower harbor and its approaches, it was the design of the observations to detect in the movement of the waters the causes of alterations in the physical geography, the same kind of studies, undertaken afterward in Hell Gate, had for their object the reverse inquiry, viz., to ascertain to what degree and in what manner the form of the rocky channel influenced the tides and currents, in order that some prediction might be made of the consequences likely to follow the removal of obstructions from the waterways. The propagations of the tide wave meet at Hell Gate, so that here the observations, when plotted, exhibit compound curves, in which the portion due to the wave from Sandy Hook is easily distinguishable from that due to the wave from Long Island Sound. The Sandy Hook tide wave differs so widely in height and time from that of Long Island Sound, that there is over three feet difference of level between the harbor and the Sound at certain stages of the tides; and at these times the currents rush through the Gate, vainly endeavoring to restore the inequalities.

The problem of referring a current to a tidal head is a very difficult one. The current, for instance, which renders Hell Gate so dangerous, is not at any time so great as a permanent head, equal to the difference of the tides observed, would engender. The currents are so very slow in their movements, compared with the undulations of the tide wave, that it cannot be ascertained as yet, what are the magnitudes of such elements as inertia and friction, and how they are to be corrected for, so as to predict the time and velocity of the current from observations of the vertical rise and fall.

It is due to the officers of the Coast Survey to state that their services to the Harbor Commissioners were rendered gratuitously; the work offered to them only an opportunity for research.

This Physical Survey must, at the outset, have held out small inducements to patient labor—the field was so large and ill defined, and had been so long the region of mere speculation; but the few simple and useful generalizations it has now grasped should, hereafter, prove the stepping stones to larger inductions, valuable alike to physical science and commercial interests.

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