Wave breaking

“ A study of breaking waves shows us that the cause of their great power to effect damage to coast structures, such as piers, harbour works, and shipping in harbours, is really due to the forward motion of the water as the wave is breaking. Every cubic foot of water weighs 631⁄2 lbs., so that a cubic yard of water weighs about three-quarters of a ton. If this water is moving with a speed of many feet per second in a forward direction, the energy of motion stored up in it is tremendous, and fully sufficient to account for the destructive power of storm waves on a coast. ”

“ When a large surge, in the act of breaking, just as it curls over, is hurled against the face either of a wall or of a vertical rock, the sound of the blow is not a crash nor a roar; it is a report as loud as, and in every respect similar to, that of a great gun, and the wave is dashed back from the rock with force scarcely diminished, but reversed in direction,—it now recedes from the shore, and at the instant that it encounters the following breaker, the result is the vertical bound of both which is here rendered by Turner. ”

“ As each wave slows, the next wave behind it, in deeper water, tends to catch up. As the wavelength decreases, the height generally becomes greater. The lower part of a wave, being nearest the bottom, is slowed more than the top. This may cause the wave to become unstable, the faster-moving top falling forward or breaking. Such a wave is called a breaker, and a series of breakers is surf. Swell passing over a shoal but not breaking undergoes a decrease in wavelength and speed, and an increase in height, which may be sudden and dramatic, depending on the steepness of the seafloor’s slope. ”

“ But in a gale, the silent machinery of a sailing-ship would catch not only the power, but the wild and exulting voice of the world’s soul. Whether she ran with her tall spars swinging, or breasted it with her tall spars lying over, there was always that wild song, deep like a chant, for a bass to the shrill pipe of the wind played on the sea-tops, with a punctuating crash, now and then, of a breaking wave. At times the weird effects of that invisible orchestra would get upon a man’s nerves till he wished himself deaf. ”

“ I saw the worst assault at night on a seaside parade where the sea smote on the doors of England with the hammers of earthquake, and a white smoke went up into the black heavens. There one could thoroughly realise what an awful thing a wave really is. I talk like other people about the rushing swiftness of a wave. But the horrible thing about a wave is its hideous slowness. ”

“ If the liquid space is free, the vibrations are gradually extinguished; if they meet an obstacle, there is a shock. If the obstacle is a shore, they form a tidal-wave and raise large billows, while a few miles away from the shore the sea is quiet. When the obstacle is a shoal or a contrary vibration, heavy waves are raised on the surface of the sea; they seem to start from the bottom, and put ships in great danger. The waves produced in both cases are waves of transmission, as also are those which beat on reefs in pleasant weather, and those which prolong the swell after storms. ”

“ High, high she rises, till half her length is in the air over the crest of the wave, and then down she sinks; then the crash comes. Waves dash over her, her masts fall, her boats are wrenched from her sides, and the next minute we see her, a tangled mass of wreck and cordage, firmly embedded on the pitiless rocks. ”

“ Up came a second wave. The lulling of the wind gave the waves a chance to gather force and height. This one curled fairly over my head and, looking up and over my shoulder at the great, green, foam-streaked wall of water, I thought my last minute above the surface had come!
It broke. I can remember nothing at all of the ensuing few moments. I only know that I was smothered, drowned, completely overwhelmed by the deluge of water that came inboard. The force of it burst open the slide of the hatch and barrels of water flooded into the cabin. The Wavecrest settled. ”

“ It may be added that floating wave-breaks, such as would be constituted by a large number of spars or planks left to drift, afford a perfect amelioration of the agitation of the waves. Hitherto engineers have applied their efforts only against the larger waves. Why not attack the evil in its origin? Why not take up the ripples and the wavelets, and oppose them with floating ripple-breaks? ”

“ As soon as the shock is over, the body of water thus forced out to sea returns as a huge wave, and, on approaching a sloping shore, rears up like a wall, and breaks with overwhelming force. Sometimes, however, its volume, height, and velocity are so great that it comes ashore bodily, and breaks far inland, causing even greater destruction to life and property. ”

“ In addition to seismic sea waves, earthquakes below the surface of the sea may produce a longitudinal pressure wave that travels upward at the speed of sound. When a ship encounters such a wave, it is felt as a sudden shock which may be so severe that the crew thinks the vessel has struck bottom. ”

“ These invasions of the sea, or tidal-waves, are often more dreaded by the people, who have had experience of them, than the shocks on land. The huge wave* are also propagated in the ocean to a very great distance from the center of disturbance. ”

“ Each drop as it strikes the water becomes a centre of disturbance, from which a series of ring-ripples expand outwards. Gravity and inertia are the agents by which this wave-motion is produced, and a rough experiment will suffice to show that the rate of propagation does not amount to a foot a second. A series of slight mechanical shocks is experienced by a body plunged in the water, as the wavelets reach it in succession. But a finer motion is at the same time set up and propagated. ”

“ One, who in a storm watches the breaking-up of a wreck or the tearing down of a sea-wall, is impressed by the immense energy of the waves. Of course, when it is pointed out that in the absence of wind no such results can be produced, he recognizes the truth that the sea is in itself powerless, and that the power enabling it to destroy vessels and piers is given by the currents of air which roughen its surface. ”

“ When the water begins to increase, it comes on with a prodigious noise as if it were an earthquake. In its first great approach it makes three great waves. The first wave washes over the bark from stem to stem: The second is not so strong; at the third they raise the anchor and resume their voyage up the river, rowing with such swiftness that they seem to fly for the space of six hours, while the flood lasts. ”

“ It is a mixture of fog and storm; and even in our own day we cannot well account for the phenomenon. Hence many disasters.
We try to explain all things by the action of wind and wave; yet in the air there is a force which is not the wind, and in the waters a force which is not the wave. That force, both in the air and in the water, is effluvium. Air and water are two nearly identical liquid masses, entering into the composition of each other by condensation and dilatation, so that to breathe is to drink. Effluvium alone is fluid. The wind and the wave are only impulses ”

“ IT is impossible for the most careless spectator to look at a steam-vessel making her way along a lake, a boy’s boat skimming across a pond, or even a duck paddling on a stream, without noticing that the moving body is accompanied in all cases by a trail of waves or ripples, which diverge from it and extend behind. In the case of a steamer there is an additional irregular wave-motion of the water caused by the paddle-wheels or screw, which churn it up, and leave a line of rough water in the steamer’s wake. ”

“ Reduced to a single float, the breakwater might remain under the waves too long, but, owing to the two others, it rights itself, warps around, and always presents the spur of its sharp roof to the wave. ”

“ By some unexplored tidal law, parties would seem to move through successive ebb and flow toward a final culmination of mischievous extreme, each refluent wave returning with heavier mass, until the accumulated weight of madness and folly overtopples, breaks, and dissolves in noisy foam. ”

“ The motion of the particles of the water in a deep-sea wave resembles that of the white knobs in the model described. Those who swim will recall to mind their sensations as a sea wave surges over them. The wave [6] lifts up the swimmer, then pushes him a little forward, then lets him down, and, lastly, drags him back. It is this dragging-back action which is so dangerous to persons who cannot swim, when they are bathing on a steep coast where strong waves are rolling in towards the shore. ”

“ All eyes are fixed upon the object, as trembling and tossing amid the boiling white waves it survives the roughest waters. One breaker past and it will have reached the object of its mission. But being partly filled with water and striking a sunken rock, that next wave sends it hurling to the bottom. ”

“ But there is another reason why falling water makes a sound, and often even a loud roaring noise in the cataract and in the breaking waves of the sea. You do not only hear the water dashing against the rocky ledges or on the beach, you also hear the bursting of innumerable little bladders of air which are contained in the water. As each of these bladders is dashed on the ground, it explodes and sends sound-waves to your ear. Listen to the sea some day when the waves are high and stormy, and you cannot fail to be struck by the irregular bursts of sound. ”

“ The needles of ice which form in cold weather on the surface of the water are excellent natural wavelet-breaks. Generally, every cause hindering the formation of wavelets appeases the agitation of the waves. Thus a rain, every drop of which breaks a ripple, calms the sea to a certain extent. Sailors know this. Billows have been fought against with ordinary wave-breaks. Wavelets may be destroyed by employing light bodies, ripples with dust, microscopic ripples with an infinitely fine powder. A liquid will serve the end admirably. Oil is the best of all agents for the purpose. ”

“ It has already been shown that when a single wave-disturbance is made upon water it gradually develops itself into a group of waves. The single wave when created causes a disturbance on water which extends both forwards and backwards. As the wave moves forward the wave-disturbance is always growing in front and dying away behind, and the wave-group therefore moves forward, but the centre or limits of the group move with only half the velocity of a single wave. ”

“ By the initial impulse of explosion or lifting of the ground, a ‘great sea wave’ is generated, which travels 175shorewards with a velocity dependent upon its size and the depth of the ocean. At the same instant, a ‘sound wave’ may be produced in the air, which travels at a quicker rate than the ‘great sea wave.’ A third wave which is produced, is an ‘earth wave,’ which will reach the shore with a velocity dependent on the intensity of the impulse and the elasticity of the rocks through which it is propagated. ”

“ Now this [139] motion increases with the onward career of the gale. “Vires acquirit eundo.” And when the mass of the fluid over which the wind is blowing has once been agitated, and the equilibrium of its upper layers been thoroughly disturbed, if the current of air not only continue, but wax stronger and stronger, there scarcely seems any limit to the momentum which the rolling wave may thus acquire. The arithmetic of a few facts upon this subject will, however, give us some idea of what that momentum must be on certain occasions. ”

“ The huge billows that roll over the sea in storms are seldom much more than fifteen feet high, and they pass over the surface without disturbing the water beyond the depth of a few feet. Every wave has a top, a middle part, and a bottom, or lower part. When a wave coming in from the sea approaches the shore, the bottom of the wave strikes the land first. The sand catches and holds it back and makes it go slower. The top of the wave, not feeling this friction against the ground, rushes forward, leaving the lower part behind. ”

“ The entire frame of the breakwater, the double apparatus crushed and mingled confusedly, came in a whirl of foam, rushing upon the stone barricade like chaos upon a mountain, where it stopped. Here the fragments lay together, a mass of beams penetrable by the waves, but still breaking their force. The conquered barrier struggled nobly against destruction. The waves had shattered it, and in their turn were shattered against it. Though overthrown, it still remained in some degree effective. The rock which barred its passage, an immovable obstacle held it fast. ”

“ But while they lived, always submerged, always beneath the wave. Beneath the wave they might have powerful roots, stronger than iron; they might be tenacious and dangerous in their soft waving within the flood. ”

“ In depths beyond the reach of wave motion, and apart from suspension across a submarine gully, which will sooner or later result in a rupture of the cable, the most frequent cause of interruption is seismic or other shifting of the ocean bed, while in shallower waters and near the shore the dragging of anchors or fishing trawls has been mostly responsible. ”

“ Suddenly, on the night of February 5th, while some of the people were sleeping in the boat, and others were resting on a low plain near the sea, in the neighborhood, another shock occurred, a great mass was torn from a neighboring mountain and hurled with a crash on the plain, and immediately afterwards a wave, twenty feet or more in height, rolled over the level plain, sweeping away the people. ”

“ As a wave progresses over the surface, wave-energy is continually being imparted to portions of the water in front, and it is transferred away from others in the rear. In the very act of setting a fresh particle of water in oscillation, the portions already vibrating must diminish their own motion. They may hand on the whole of their energy or only a part of it to their neighbours. This distinction is a very important one, and it determines whether a single act of disturbance shall create a solitary wave or wave-train in a medium. ”