Tidal flat

“ If the Straits were wide enough to admit the progress of the tidal wave there would be a rise and fall in the ports of the Mediterranean, but as they are not, only the tidal current rushes in and out twice every day. ”

“ Rufus went on as if he had not spoken. "There's danger all along that beach as far as the Spear Point. Adam will tell you the same. When it's a spring tide there's times when there's such a swell that it's round the Point and over the pool like a tidal wave. You'll hear the bell-buoy tolling when there's a swell like that. We call it the Death Current hereabouts, because there's nothing could live in it, and the bell always tolls. And once it comes up like that the way to the cliff-path is under water in less than thirty seconds. And the quicksand is the only chance left." ”

“ The range of the tide at times of new moon and full moon 'is greater than the average,' as 'high water then rises higher and low water falls lower than usual.' The tides at such times are called 'spring tides.' When the moon is in its first and third quarters, 'the tide does not rise as high nor fall as low as on the average.' At such times the tides are known as 'neap tides.' 'Tidal Datum Plane,' U.S.C.oast and Geodetic Survey, Special Publication No. ”

“ In considering the effect of the natural forces still in operation during the many thousands of years during which they have been at work, it is necessary to bear in mind that the level of the sea has all along remained the same, except so far as it is varied by the rise and fall of the tide, and by the exceptional exaggerations of tidal movement caused by the wind. ”

“ In such cases, however, where the rivers flow into tidal seas, it is important to place the jetties sufficiently apart to avoid any loss of tidal influx, since the tidal flow assists the fresh-water discharge in keeping the outlet open; whereas, with rivers flowing into tideless seas, a moderate restriction of the width between the jetties increases the scour. ”

“ When the tide is in, the breakers dash against these rocks even in calm weather with a force that sends the spray very many feet high, and any one then attempting to get round the point does so at the imminent peril of his life; the river also becomes swollen by the tidal inflow, and although a ferryman lives on the opposite bank, he wisely forbears from venturing to cross in his canoe until the tide has well run out again. ”

“ Lying funnel-shaped toward the great tidal movement from east to west it gathers from the incoming tide a great deal of water that does not belong to it, and then gradually compressing it between narrowing shores, piles it up in places sixty feet in height, and this gives rise to many peculiarities. ”

“ It did not check his progress; he was never one to be easily daunted. It was contrary to his very nature to seek shelter in a storm. He went swinging on to the very edge of the quay, and there stood facing the violence of the waves, the fierce turmoil of striving elements.
The tide was extraordinarily high—such a tide as he believed he had never seen before in summer. He stood in the pouring rain and looked first one way, then the other, with a quick birdlike scrutiny, but as far as his eyes could pierce he saw only an empty desolation of waters. ”

“ This augmentation Augmentation of Height in Estuaries. of the height of the tide-wave is due to the concentration of the energy of motion of a large mass of a large mass of water into a narrow space. At oceanic ports the tidal phenomena are much less marked, the range of tide being usually only 2 or 3 ft., and the interval from high to low-water sensibly equal to that from low to high-water The changes from spring to neap tide and the relation of the time of high-water to the moon's transit are, however, the same both on the open coast and in rivers. ”

“ The tidal information becomes important and must be considered in navigation or in anchoring in waters where the available depth at low water approximates the draft of the vessel. In the general use of coast charts it is also important to observe the effect of the stage of tide on the appearance of many features. Rocks rising some feet above low water may be entirely submerged at high water. In some areas the aspect may be radically changed between high and low water by the baring of extensive shoals or reefs. ”

“ The in-shore tidal waters having to encounter the shelving shores, islands, sunken rocks, and projecting points of land, which lie in their course up the Firth , acquire lateral as well as perpendicular motion, and being thus checked in their progress, are brought sooner to a maximum state than the off-shore stream, which flows in deeper water, and comparatively free of obstruction, till it reaches its ultimate limits; though it gradually diminishes in breadth, till the stream of the new tide gaining strength becomes general ”

“ The tidal attraction is primarily due to the natural effect of gravity, whereby the attraction between two bodies is in direct proportion to the product of their respective masses and in inverse proportion to the square of their distance apart; but as the tide-producing effect of the sun and moon is a differential attraction, and not a direct one, their relative effect is inversely as the cube of their distances. ”

“ Though we have not yet put the tides into harness, yet tides are not idle. Work they will do, whether useful or not. In some places the tidal currents are scouring out river-channels; in others they are moving sand-banks. From a scientific point of view the work done by the tides is of unspeakable importance. To realize the importance, let us ask the question, Whence is this energy derived with which the tides do their work? The answer seems a very obvious one. If the tides are caused by the moon, the energy they possess must also be derived from the moon. ”

“ This rise and fall of the waters along the coast, accompanied frequently by strong currents, are produced directly by the arrival in the shoal water of a pulsation of the ocean, which becomes a true onward-moving gravity wave as it nears the land. In the open sea the amplitude of the tidal undulations is but two or three feet, and their rate of travel in general 700 to 800 miles per hour. ”

“ And, seeing that we are writing more particularly for the young naturalists of our own island, we must devote a little space to the study of the movements of the tidal wave round Great Britain, in order that we may understand the great diversity in the time of high tide on any one day on different parts of the coast, and see how the time of high tide for one part may be calculated from that of any other locality. ”

“ Hence, as the tides may be regarded as a machine doing work, we have to ascertain the origin of that energy which they are continually expending. It is at this point that we first begin to feel the difficulties inherent in the theory of tidal evolution. ”

“ These tremendous tides cause peculiar phenomena; they make the rivers seem to actually run up-hill at times, while the tidal "bore" or wall of water, which is the advance of the flood, moves up the streams and across the extensive mudflats with the speed of a railway train, often catching the unsuspecting who may be wandering over them. The elaborate wharves made for boat-landings are 278 built up like three-story houses, with different floor-levels, so as to enable the vessels to get alongside at all stages of the tide. ”

“ The subject of the tidal wave as experienced in England requires careful treatment, as many text-books leave the impression on the minds of children that the tidal wave in the North Sea travels from east to west, and that the shores of the Baltic are experiencing low tide when the eastern coast of England is having a high tide. ”

“ A tidal wave that propagates up a relatively shallow and sloping estuary or river in a solitary wave. The leading edge presents an abrupt rise in level, frequently with continuous breaking and often immediately followed by several large undulations. An uncommon phenomenon, the tidal bore is usually associated with very large ranges in tide as well as wedge-shaped and rapidly shoaling entrances. ”

“ Along the south coast from Land's End to Beachy Head the average velocity is 40 miles per hour, the rate reducing as the wave approaches Dover, in the vicinity of which the tidal waves from the two different directions meet, one arriving approximately twelve hours later than the other, thus forming tides which are a result of the amalgamation of the two waves. On the ebb tide the direction of the waves is reversed. ”

“ The semi-synchronous oscillation of the Southern Ocean raises the magnitude of oceanic tides somewhat above these directly generated values.
Oceanic tides are true waves, not currents. Coast tides are currents. The momentum of the water, when the tidal wave breaks upon a continent and rushes up channels, raises coast tides to a much greater height—in some places up to 50 or 60 feet, or even more. ”

“ There are another ten thousand who show in varying degrees some tiny, almost imperceptible, superiority of standing a long spell without air under water. Raising of the land or the set of winds gives a season of abnormally low high tides. The animals just on the upper edge of the tidal belt die out for lack of their regular tidal supply of water, except some few who can, having the slight differential advantage apparent among them, stand the strain of living so long in the air. ”

“ Not so limited, however, as they would be without the tides. Consider for a moment the denudation import of the tides: how does the existence of tidal rise and fall affect the geological problem?
The scouring action of the tidal currents themselves is not to be despised. It is the tidal ebb and flow which keeps open channel in the Mersey, for instance. But few places are so favourably situated as Liverpool in this respect, and the direct scouring action of the tides in general is not very great. ”

“ Stand on the bank of an estuary or river up and down which a great tidal current ebbs and flows; you [42] will see the water copiously charged with sediment which the tide is bearing along. Engineers are well aware of the potency of the tide as a vehicle for transporting stupendous quantities of sand or mud. A sand-bank impedes the navigation of a river ”

“ Tide is the vertical rise and fall of the water, and tidal current is the horizontal flow. The tide rises and falls, the tidal current floods and ebbs. The navigator is concerned with the amount and time of the tide, as it affects access to shallow ports. The navigator is concerned with the time, speed, and direction of the tidal current, as it will affect his ship’s position, speed, and course. ”

“ But in the present condition of the globe, where the oceans are separated by continents and broken by islands, the progress of the tidal waves is obstructed, deflected, and wholly stopped in a great variety of ways and places, so that the hours, amount, and behavior of the tides are exceedingly varied in different regions, and are often very puzzling, forming one of the most difficult matters with which the practical navigator has to deal. ”

“ The result will be that the particles will travel a little farther out from the shore each time it is swung to and fro in the alternating movement of the tide.
The effect of tidal movement in nurturing marine life is very great. It aids the animals fixed on the bottoms of the deep seas to obtain their provision of food and their share of oxygen by drawing the water by their bodies. All regions which are visited by strong tides commonly have in the shallows near the shores a thick growth of seaweed which furnishes an ample provision of food for the fishes and other forms of animal life. ”

“ At low tide for miles out one sees only a vast plain, moist, strewn with shells, and intersected here and there with deep hollows and shifting sandbanks; but when the tide changes, and long before the waters appear in sight, are heard tremendous sounds, crash after crash, thunder after thunder, of the advancing tide, which comes in leaping like a huge monster, thirty to forty feet high, and breaks with terrific violence against the shore, carrying everything before it. ”

“ It sometimes happens that an unusually high tide occurs. In the port of Dublin, for instance, we have had from time to time exceptionally high water in the Liffey, which has flooded the basement stories of low-lying dwellings. The cause of such extraordinary phenomena is not to be attributed to any unusual development of the strength of the moon's tide-producing capacity, it is rather to be explained in a manner which the tide- predicting engine renders easy to understand. ”

“ On we swept like a tide that advances upon a coast, encompasses each salient rock, island and projection, and evading it by embracing it, rises still further into the bays and harbors, and brings the full tide at last to its most remote limits. So columns and stairways, halls, and wings, and arms, of buildings successively were surged round, and the vast complex pushed its way to the great Hall of Attention. ”

“ Out in mid-ocean, for instance, an island like St. Helena is washed by a tide only about three feet in range; an enclosed sea like the Caspian is subject to no appreciable tides whatever, while the Mediterranean, notwithstanding its connection with the Atlantic, is still only subject to very inconsiderable tides, varying from one foot to a few feet. The statement that water always finds its own level must be received, like many another proposition in nature, with a considerable degree of qualification. ”