Popular Science Monthly/Volume 86/June 1915/Wave Work on the New Jersey Coast
|WAVE WORK ON THE NEW JERSEY COAST|
WARREN S. SMITH
DURING the winter of 1913–1914 three unusually severe storms with violent on-shore winds visited the coast of New Jersey. The first of these, known as "the Christmas storm," attained its maximum strength early on the morning of December 26, and was accompanied by winds which attained a velocity of 123 miles an hour. Professor W. M. Wilson gives the following brief description of this storm in the Monthly Weather Review for December, 1913.
The second, or "New-Year's storm," was even more disastrous than the first, partly because the coast had been left in an unprotected condition by the preceding attack. On January 4 the fury of the second storm reached its maximum. Driven by a terrific gale whose extreme velocity reached 120 miles per hour, the waves broke upon the beach with a thunderous roar. Bulkheads which had been destroyed or weakened during the earlier storm afforded no protection for the unconsolidated sand of the beach, and every wave seemed to sweep a little of it out to sea. At Seabright groups of dejected men, soaked to the skin by driving rain and salt spray, stood helplessly by the shore and watched the waves remove the land from under their houses, the houses tip over into the sea, and the waves pound them to kindling wood in the space of a few moments. Others labored to place wooden rollers under some of the smaller cottages and drag them farther back upon the beach before the waves demolished them: while still others hurriedly removed their furniture in wagons, abandoning the buildings to their fate.
A few men stood under the corner of a partially undermined summer residence and struggled in vain to erect a plank bulwark which would stop the advancing sea, while the surging waters nearly swept them from their feet. Crowds of spectators waded knee-deep through the foam blown up from the surf zone by the winds, and watched the water of the sea leap high above three-story dwellings where waves were breaking against some resisting bulkhead. Many observed with anxiety the attack of the sea upon the beach under the large Octagon Hotel, and shortly before noon saw the great building collapse and become, in a short time, a tangled mass of splintered wood rising and falling with the waves. Occasional waves sent water across the narrow bar, the streets of Seabright were inundated, and the unusually high tide which accompanied the storm kept many houses on the lower, back part of the beach surrounded by water for hours. Fortunately, the waters were quiet in the protected bay, and the damage here was slight compared with that on the exposed outer shore, where for a couple of hours a large building collapsed on an average every fifteen minutes. North of Seabright the waves broke through the breakwater which protects the railroad, swept part of the tracks out to sea, and buried other portions under masses of heavy stones.
On February 14 and 15 occurred the third and least destructive miles per hour. At this time the waves completed the destruction of the Seabright Beach Clubhouse and certain other structures badly damaged by the earlier storms. In the unprotected state of the shore some further damage has been subsequently accomplished by normal wave erosion during comparatively calm weather, at least one valuable summer residence being demolished in this manner.
The portion of the New Jersey coast which suffered most from the storm waves lies south of Sandy Hook and north of Long Branch. (See Fig. 1.) At the latter place the waves of the sea are attacking the mainland of New Jersey and have cut a marine cliff some 20 feet in height in the seaward edge of the coastal plain. The débris eroded from this cliff has been carried northward by longshore currents and built into a narrow bar which has extended across the mouth of Shrewsbury and Navesink Rivers, and out into the Bay of New York to form the Sandy Hook spit. Monmouth Beach, Seabright and Highland Beach are small towns built upon the bar, and are therefore but a few feet above high-tide level. Formerly the sea broke against the mainland just back of Seabright and at Navesink Highlands; and old marine cliffs, now grass covered, may be seen at these points. But the bar now lies a short distance in front of these old cliffs, and protects them from erosion. Shrewsbury and Navesink Rivers are really bays of the ocean formed by a sinking of the land or a rising of the ocean level which permitted the seawater to flood pre-existing river valleys. They are "drowned valleys" but not "rivers" in the true sense of the term.
In order fully to appreciate the effects of storms upon the New Jersey coast, it is necessary to keep in mind some of the conditions affecting wave energy. The destructive power of a wave depends in part upon its size, and this in turn partly upon the water depth. Waves usually break and dissipate their energy when they come into water of a depth equal to the wave height. Hence, the deeper the water immediately at the shore the larger the waves which can attack it, and the greater the damage they will effect at that point. It follows from this that the rise of the tide must increase the destructive power of storm waves on the coast, not only because it brings the zone of wave activity farther in upon the shore, but also because the deepening of the water as the tide rises against the steeper upper part of the shore profile permits larger and more powerful waves to break against the shore cliffs. In all the recent storms the chief damage to the New Jersey coast occurred at the high-tide periods, and the citizens worked feverishly during low water to prepare for the violent wave attack which they knew would ensue at the next high tide.
On-shore winds increase the destructive power of the waves in a variety of ways. First of all, they raise the water level by blowing the surface of the sea along the coast faster than the water escapes seaward
along the bottom as undertow. In the second place, on-shore winds, by driving the surface of the sea landward, insure a vigorous undertow seaward; this undertow carries the beach material out to deep water, thereby aiding beach destruction. Vigorous on-shore winds drive in large waves, whereas off-shore winds, no matter how violent, can not form large waves in the immediate vicinity of the shore. During the recent storms on-shore winds raised the high-tide level on the New Jersey coast from one to several feet above its normal elevation, much beach sand was sucked out to sea by the resulting undertow, and large waves were driven upon the shore with terrific violence.
Other things being equal, the greatest damage will occur where the land exposed to wave erosion is lowest. Waves may expend their energy in two ways: in eroding the land or sea-bottom, or in transporting débris. If the land is high, the waves break at the base of a high cliff which sheds much débris into the water as its base is undermined. This débris must be removed by the waves if effective erosion is to continue, as otherwise the cliff would soon be protected by the accumulated waste. Removal of the débris requires much of the waves' energy, and leaves them less competent to wear back the cliff. If the land is low, the low cliff sheds but a small amount of waste upon the shore, the waves quickly dispose of it and energetically continue their landward advance. It is true that the effect of cliff height may be more than offset by other factors, among which the form of the adjacent sea bottom is important. Of still greater importance for such a region as the one in question is the effect of artificial sea defences, such as breakwaters, bulkheads and similar devices. At Seabright and adjacent towns the greater damage was suffered where the bar was lowest, or where the defenses were weakest. One of the lowest places on the bar was occupied by the Octagon Hotel, which was completely destroyed (Fig. 2). For a great distance along this low region the shore was cut back from 100 to 150 feet, and the damage to buildings was greater than elsewhere. Both north and south of Low Moor station are unusually high portions of the bar, and here the advance of the sea was not so great, even where the defences were battered down and the feet of the shore front was cut away in this region, while the sea gained nothing from the land where the bulkheads remained intact. On the other hand, there are plenty of instances where unusually weak defences failed to prevent fairly extensive erosion of comparatively high areas, and where strong defences saved low areas from attack.
Variations in the character of the material composing a coast necessarily influence the wave erosion. In the Seabright district there is not enough of such variation in the material of the bar to be of any importance. The bar first formed some distance seaward of its present position, and has been pushed landward by the waves. A salt marsh formed back of the bar, and the sands of the latter have been driven in over the surface of the marsh deposits. Hence, the wave-cut cliff on the seaward edge of the bar shows at the base a layer of somewhat indurated black sand, mud and peat, projecting as a little terrace where recently exposed. Above this the yellow-brown beach sands constitute the rest of the cliff, which stands nearly or quite vertical where recently cut into. As these conditions appear to be essentially uniform along the length of the bar under discussion, there is little difference in the rate of erosion at different points which could be attributed to variations in resistance of materials.
An examination of the Seabright shore indicates that the greatest damage to bulkheads resulted from the direct impact of the waves, whereas the buildings suffered most from the undermining of the ground upon which they stood. When it is remembered that ocean waves strike a vertical face with a force of from a few hundred pounds to more than six thousand pounds per square foot, their enormous destructive power may readily be appreciated. Solid blocks of granite have been shattered by wave impact upon the coast of Holland, and it is therefore not surprising that the wooden bulkheads of the Jersey coast should yield to the attack of the sea wherever they were not reinforced by parallel rows of piling with heavy stone filling, or otherwise rendered especially strong. Fig. 7 shows one of the weaker bulkheads in the early stages of destruction.
Most of the bulkheads were surmounted by a broad boardwalk which served to shed falling wave crests back into the sea, and thus protected the cliff from erosion. The force generated by masses of water falling from the great height to which they are projected when a storm wave strikes a vertical wall, may be sufficient to crush such a boardwalk, even if supported by heavy timbers. During a severe gale at Buffalo, New York, many large timbers, 12 × 12 inches in thickness, 12 feet long, and 10 feet between supports, were broken like match sticks by the impact of falling water which had been hurled from 75 to 125 feet into the air by breaking waves. There are several localities in the Seabright district where the demolishing of the bulkheads had been hastened in this manner.
Many of the bulkheads are protected by rows of piling set some distance out in the sea to break the force of the oncoming waves. Even where the sea attack was powerless to break these pilings or to tear them from their positions, the waves passed between the pilings and still retained sufficient force to destroy the bulkheads which presented a more continuous surface to their impact. Fig. 6 shows such a series of protecting piling, which remained largely intact while the bulkhead immediately in front of the house was battered down and the house itself destroyed. Fig. 2 shows a series of pilings surmounted by undamaged bath-houses, back of which the shore has been so badly eroded that the superjacent houses have collapsed.
As a rule, the houses were not damaged as much by direct wave impact as by the undermining of the beach upon which they stood. It is of course true that a building from under which most of the support was already removed by the sapping action of the waves, often received its "death blow" from some extra-large wave: and a building which was once tipped over into the edge of the sea as a result of being undermined was soon pounded to pieces by the waves. The Octagon Hotel
(Fig. 2) located on an unusually low part of the bar, is said to have collapsed bodily under the blow of one very large wave; while the house shown in Fig. 5 was consumed more gradually after undermining had allowed it to tip over. Indeed, the real damage was usually accomplished independently of direct impact upon the structures themselves. Even where the beach was low and flat, as near the Octagon Hotel in the town of Seabright, the foundations were sapped from under dwellings, allowing them to tip over toward the sea; seldom if ever were these houses crushed in the first instance by the direct impact of the waves. This is clearly shown in the case of the small houses of Fig. 3, which were but a short distance from the hotel, and it is probable that the hotel itself was first weakened by the undermining process. In some cases the houses collapsed piecemeal as the sea advanced under them; or were crushed by the fall when they tipped over into the sea.
Where houses were built on pilings driven into the beach sand the removal of the sand left the buildings precariously supported on the pilings alone until shaken down by the moderate waves of some later storm. The successive stages of this process are well illustrated in Figs. 6-9, which represent four photographs of the same house. In Fig. 6 a small wave has passed through the outlying line of protecting piling and is breaking against the bulkhead built to preserve the house from destruction. Fig. 7 shows that the bulkhead has been breached and that the corner of the house is beginning to be undermined. The next figure represents a still later stage, when the sand has been removed from under most of the house, leaving it supported by the pilings alone. In this condition it fell an easy prey to the smaller waves of later storms, and Fig. 9 shows the final wreck of the building.
One reason for the destructiveness of the undermining action as compared with the direct wave attack is to be found in the fact that lines of piling and bulkheads were together able to break the force of the waves to a large extent, but could not prevent the water of each wave from washing against the foot of the low cliff, removing part of the sand, and carrying it back to sea. In many places the lines of piling, and even the bulkheads, are still in a state of partial preservation, while the cliff back of them is badly eroded and the superjacent houses completely destroyed. The fact that the houses were at a higher level than the cliffed beach was, of course, another factor which rendered direct wave impact less destructive than undermining.
Some have supposed that the active erosion of the New Jersey coast by storm waves is the natural consequence of the gradual subsidence of that coast which has been inferred by some geologists. The evidence for and against the theory of subsidence has been considered by the senior author in various publications, and need not be repeated here. Suffice it to say that the loss of land during the recent storms represents exceptionally rapid erosion of a purely temporary character, due to an unusual disturbance of the profile of equilibrium by exceptional storms. Most of the material removed from the bar was carried seaward to perfect the proper shore profile of equilibrium demanded by the storms. Part of it, at least, should be carried back to the beach as new conditions demand a new profile. In fact, this process is already in operation, and the shore has been built so far forward in places as to obscure much of the erosive effect accomplished a few months ago. Nothing in the nature of the erosion or of the deposition indicates any change in the relative level of land and sea.
If the land were sinking at the rate of one or two feet per century, the problem of maintaining sea defenses against the ravages of the ocean in the Seabright district would be more serious than it is. The tendency of storm waves to cut into the land would be more marked than at present, and the tendency of marine forces to repair the damage by deposition during calm weather would be less evident. Even as it is the problem is sufficiently serious. Along with the alternate erosion and accretion in the shore zone, due to the varying effect of the marine forces, there is a slow loss of land resulting from the action of currents in transporting some of the eroded debris to deep, quiet waters farther out to sea. This loss can only be arrested by superior methods of artificial protection.
That the damage of which the sea is capable justifies the expenditure of large sums of money in improved sea defences is abundantly proved by the recent storms. The actual value of property completely destroyed in the single town of Seabright was enormous and the suffering of its citizens can not be estimated in money. Many who were unable to bear any loss saw the savings of a lifetime swept out to sea by the merciless waves. It would be difficult to estimate the depreciation in the value of property along this part of the coast alone, a depreciation shared by lands not actually eroded because of the apparent magnitude of the dangers to which they are subjected.
So long as the defence of the land is in a large number of hands and every landowner is practically free to do as little or as much as he pleases toward preventing the sea from gaining access to his property, many must suffer from the failure of a few to take proper precautions against marine erosion. As soon as the sea finds a point of weakness in the defences, it rapidly widens the breach and attacks adjoining property on either side. In some places where the bulkheads in front of one man's property resisted the direct attack, the property was badly damaged by erosion from one or both sides after the sea had entered neighboring lots. Some method of government supervision of marine defences would seem to be the only satisfactory solution of this serious problem.
- The substance of this article appears as part of Bulletin 12 of the Geological Survey of New Jersey.