Tidal bore

“ It is this appreciation of dynamic greatness which overwhelms an observer of the tidal bore as it sweeps in from Hangchow Bay and rushes past Haining, a solid wall of water from two and a half to three miles wide, perhaps ten, twelve or even twenty feet high, with a speed of ten to twenty miles an hour, according to the intensity of the tide. ”

“ 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. ”

“ BORE, a high tidal wave rushing up a narrow estuary or tidal river. The bore of the Severn is produced by a tide that rises 18 ft. in an hour and a half. This body of water becomes compressed in the narrowing funnel-shaped estuary, and heaped up into an advancing wave extending from bank to bank. ”

“ 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. ”

“ The rise and fall of the tide allows sea water to percolate landward and the fresh water to percolate seaward; but the friction is so great as to obliterate most of the tidal wave. Thus the sea at high tide is kept out, the fresh water behind the barrier remaining at a level slightly above that of mean tide, and just above that level we may find a wet soil on which trees can grow. But, and here is the important point, a protected land-surface behind such a barrier can never lie below the level of mean tide ”

“ The flood tide comes up the deep and narrow estuary 125from the outer channel with such swiftness, and is so laterally compressed that a gradual rise is impossible and the water comes surging up as a great and formidable wave, like a wall, from five to six feet in height. At such times when westerly gales or spring tides prevail, the Bore easily rises to nine feet in height. It is always an impressive spectacle, seen from the river bank ”

“ There cannot be a doubt that in this epoch of the earth's history, so long as the price of coal is only a few shillings a ton, the tide-mill, even though we seem to get its power without current expense, is vastly more expensive than a steam-engine. Indeed, Sir William Thomson remarks, that wherever a suitable tidal basin could be found, it would be nearly as easy to reclaim the land altogether from the sea. ”

“ 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. ”

“ The failure of groins, erected with pile and plank, appear to arise from their being placed in a wrong situation, from their not extending far enough into the sea, from the piles not being driven sufficiently into the beach, and from their sudden elevation, present an abrupt surface for the tidal wave to play upon, which during heavy gales of wind upon spring tides, cannot withstand its powerful effect, should the materials lying adjacent to or between them be removed. ”

“ 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. ”

“ The wave is not noticeable at sea, but becomes apparent along the coasts, particularly in funnelshaped estuaries. In certain river mouths, or estuaries of particular configuration, the incoming wave of high water overtakes the preceding low tide, resulting in a steep, breaking wave which progresses upstream in a surge called a bore. ”

“ The tides were then far more violent.
Halving the distance would make them eight times as high; quartering it would increase them sixty-four-fold. A most powerful geological denuding agent. Trade winds and storms were also more violent.
If ever the moon were close to the earth, it would have to revolve round it in about three hours. If the earth rotated on its axis in three hours, when fluid or pasty, it would be unstable, and begin to separate a portion of itself as a kind of bud, which might then get detached and gradually [Pg 380] pushed away by the violent tidal action. ”

“ Placing a stick into the ground, in a few moments a heap of sand would accumulate on the windward side. These sand storms fill up all the mouths of the Alt, and pile the sand up in big banks. If there was no correcting force these sand storms would quickly fill up the shallow shores and destroy their capacity to impound the tidal water which assists the scouring power of the main stream; but at high water with a westerly gale the waves churn up these deposits of sand, and the ebb tide carries them out to sea. ”

“ At the upper end of the Bristol Channel the range of the tide is double the range in the sea outside the channel. The Bay of Fundy is another place where a similar phenomenon occurs. When a river or estuary is shallow and the range of the tide is great, so that its rise is rapid, the flood tide in some cases advances in the form of a wave or “bore,” causing a sudden rise in the water-level and a sudden reversal of the flow of the stream. A bore is most pronounced at spring tides. ”

“ 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. ”

“ On the days we observed there were each time a baker's dozen of them, but sometimes as many as thirty junks may be seen utilizing tidal energy for ascending the river at a speed exceeding that of an ordinary steam vessel of equal size. As soon as they could steer a little, they made for the shelters behind the buttresses, where they allowed themselves to be stranded by the falling tide. ”

“ The stream, however, is found not to be constant, but to cease regularly when the tide has fallen to a certain distance, and not to flow again till the tide returns to the same level, its force increasing regularly as the tide advances; and it may be observed to be propelled with much force, even after the bore is overflowed by the tide. ”

“ Were it not for the inertia of the ocean and the resistance offered by the irregular continents, high tide would always exist exactly under the moon, and we should have high water at any place just at the time when the moon is in the south and crossing the meridian of that place. But while the inertia of the water tends to make all tides late, the irregular distribution of the land breaks up the tidal wave into so many wave-crests and greatly retards their progress. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 ”

“ The great aim, on the contrary, of all tidal river improvement should be to facilitate to the utmost the flow of the flood-tide up a river, to remove all obstructions from the channel so as to render the scouring efficiency of the flood and ebb tides as great as possible, and by making the tidal flow extend as far up the river as possible to reduce to a minimum the period of slack tide when deposit takes place. ”

“ Tidal water is, indeed, unlimited, in volume; but, unlike the drainage waters which must be discharged into the sea, it only flows up rivers where there is a channel and space available for its reception. Consequently, it is possible to exclude the tide by injudicious works, such as the sluices which were erected long ago across the fen rivers to secure the low-lying lands from the inroads of the sea; the tidal influx is also liable to be reduced by accretion in an estuary resulting from training works. ”

“ In the Ouse the tide rises and falls, so it is necessary to stop the mouths of the great drains going into it, which is done by a structure of timber or brick-work like the gates of a dock, which close at high tides to keep out the sea-water, and open at low tides to let out the land water. Either from insufficient workmanship or bad locality, or unusual pressure, or from burrowing animals, the land water, or sea water, or both, made a flank movement, and burst up the whole sluice, so that the tide, instead of being confined to the river, obtained free way up the drain also. ”

“ 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 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 ”

“ Rivers discharging in bays demand open way to the ocean. Strong tidal currents also are able to keep open channels scoured by their ebb and flow. In such cases the most that land waste can do is to build spits and shoals, narrowing and shoaling the channel as much as possible. Incomplete bay bars sometimes have their points recurved by currents setting at right angles to the stream of shore drift and are then classified as hooks (Fig. 142) .
Fig. 143. Cross Section of Sand Reef sr, and Lagoon; sl, Sea Level
Sand reefs. ”

“ Indeed, they must grow monstrously bigger out to sea, for there must be ten sea miles of open water sown with them as thick as a country place with houses, some standing thirty feet above the tides, some covered, but all perilous to ships; so that on a clear, westerly blowing day, I have counted, from the top of Aros, the great rollers breaking white and heavy over as many as six-and-forty buried reefs. But it is nearer in shore that the danger is worst; for the tide, here running like a mill race, makes a long belt of broken water—a Roost we call it—at the tail of the land. ”