THE HIGHEST OCEAN WAVES.

Oamaru Mail, Volume XXXIX, Issue 10903, 21 October 1911, Page 3 (Supplement)

THE HIGHEST OCEAN WAVES.

| 11. -inurem-nts 01" the size of waves [ iia'. 1 uow !> made systematically for ! :•>?»!>• year.-. litit they relate- chiefly to ill" tile open sea. whore tile d-p:t! 01 the wat'-r in so great that .the i:i i ;ii <i! the sea bottom exercises no A li'w months. ago ti:e North German Lloyd liner lJrandenburg came into New York harbor with ii"r crow's nest. above the water line, stove in. and bearing many other marks oi' the damage wrought by a monster nave thai broke over her bows about 1000 miles fast of Sandy Hook. The officers estimated the height of the wave at 65 feet. This height, aeoerding to the Scientific American, is exceptional, but not unprecedented, for it in list he remembered that the breaking of a wave against an obstacle throws the water to a far greatcx height than the unbroken wave could attain. TFiibrokeri waves due to the wind may ill extreme cases reach a height from trough t« crest of 40 to 50 feet. Much higher waves occasionally occur as a result of carthnuakes or seaquakes. "Solitary" waves of -this character have sometimes been encountered in otherwise tranquil weather, taking vessels l>v surprise and not infrequently sending them to the bottom. : According to Vaughan Cornish, who has probably devoted more attention to this subject than any other contemporary man of science, the average height of the waves encountered in a scrverc storm at sea is 20 feet, but the ordinary maximum height of the waves in the same storm will attain 30 ,feet. In a storm of very exceptional Violence the average height may reach 30 feet, and the maximum height 45 feet. This is regarded as about the limit of the height of waves due. to wind only. Cornish finds that in the open sea the height of a wave in feet is about one- . half the velocity of the wind in miles per hour. I

So much for the waves on the high seas- These waves, though they may race along at th e speed of itn express train, do not carry the surface water far with them; each particle of water describes a local circular orbit during the transit of the wave, so that wliat advances is rather the form than the sub-? stance.

The case is quite different when waves break upon a shore, where the-shoaling water produces "waves of translation." These waves ,-ire relatively short and stegp and break when they enter water the depth of which is equal to or a little exceeds their height from trough to crest. The.v approach the shore in a direction nearly at right angles to the general shore line, whatever the direction of the wind. is explained by the fact that if the wave is at first directed at an acute angle to the shore when it reaches shallow water the side of the wave nearest the shore is first retarded, so that the wave tends to swing around until it faces the shore. In planning harbor construction and the protection of coasts it is customary tp consider the amount of exposure to which the coast is subject, i.e., the extent of open sea in a straight line at right angles to the shore. This is exiled technically the "fetch." The relation of the fetch to the possible height of the waves was announced by Stevenson in 1852. According to his formula the height of waves in a gale in feet is one and one-half times the square root of the length of the fetch "in nautical miles.

The force of a great wave breaking against a sea wall or other construction is so terrific as to tax the strength of the best-planned work of the engineer. A marine dynamometer for measuring the force of impact of such waves was devised by Stevenson over half a century ago, and modifications of this instrument have since been introduced -b,y several investigators. According— to Stevenson the maximum force of an Atlantic wave is three tons per square foot. French engineers rind that the force of the waves on the breakwater at Cherbourg may attain three and alialf tons per square foot. Some interesting c-xnmiJles of the height to which breaking waves may be thrown and the work they may do in moving heavy objects are given by Wheeler iu his "Practical Manual of Tides and Waves."

Stevenson records a case in which water was thrown to a height of lOGft at the boll Rock light. At the Alderney breakwater it is said that water has been thrown upward 200 ft. At Peterhead, where the "fetch" is 300 miles, waves of 30ft in height and from 500 to 600 ft in length have been recorded ; the water has struck the breakwater with such force as to be thrown upward 120 ft aud blocks of concrete weighing 40 tons have been displaced at levels of 17 to 36ft below low water.

At Wick two stones weighing 8 and 10 tons each were thrown over the parapet of the breakwater, the top of which wa s 21i't above high water; while blocks of concrete weighing respectively 1350 and 2500 tons were displaced, though there is some doubt whether the latter movement was due entirely to wave action.

At the Bishop Rock lighthouse, which is exposed to the full force of the Atlantic waves, an iron column weighing over three tons was thrown up 20ft and landed on top of a rock. At the harbor works of Bilbao in 1894 a solid block of the breakwater weighing 1700 tons was overturned from its place and dropped into the water. At Ymuiden breakwater -a block of concrete weighing 20 tons, placed outside the harbor walls, was lifted by a wave to a height of 12ft vertically and landed oil top of the pier, which was sft above lugh water. The above cases illustrate the sheer force of the individual wave as an engine of destruction, but the imagination of mankind is more impressed by the widespread effects wrought by the great storm waves that sometimes inundate low-lying coasts. These waves are often 5 miscalled "tidal waves," the only justification of the latter name being the fact that their effects are most pronounced when the waves propagated outward from a storm area happen to coincide with the occurrence of flood tide on the coast affected.

The precise mode of origin of the storm wave has been the subject of much discussion, and even now is not full}* understood. Such waves attend every severe cyclonic storm at .sea, and as they travel much faster than the storm (i'.e., the storm as a whole, not the wind revolving about the storm centre) they often occur on a coast when the weather is otherwise serene, and thus serve as a valuable prognostic of the storm's approach in case the coast happens to lie in the stbrm track. It is well known that the barometric pressure is much lower at the centre than at the periphery of a storm—tlio difference sometimes amounting to-two inches or more—and this difference of pressure must disturb -the equilibrium of the water, causing it to become heaped up at the storm centre. This bulging of the water would amount, theoretically, to about one foot for each inch oS barometric depression. However, although this process doubtless contributes to the production of the wave the violent winds at the' vortex of-the-storm are probably a miich more important factor. •'

The - mechanism of storm waves was studied bv a board appointed by the chief -of the United States Weather Bureau for this purpose 'in 1901, and the reader is referred to the report of the "board, published in the Monthly Weather Review of October, 1901, for further information on the subject. " We are concerned here chiefly with the effects of storm waves (whic-h the ♦newspapers and the public call "tidal waves" as persistently, and with as much reason, as they call tornadoes "cyclones"). These are most severe when the wave moves toward a lowlying coastal region, having a converging shore line: tins converg'Vice producing, the same effect as 'll ui a tidal "bore.''

Tho iiiost disastrous slitiii waves have occurred alonjr the coast oi >he Hay oi on the extensive flats lyinn; a'louc the months of the the Mt'sina. etc. The storm wave oi October 7111. 1737. is said to have risen tOft in the Hurdi. swf"pin.L£ away Oil'i souls. In M;.v. 1787. at Corin-

mi. !>"ir the mouth of the f !odavery. such a wave is said to have taken toll of I'll.lKlO lives. The Calcutta cyclone of October oth, 18tM, caused the inundation of the flotjj on both sides of the Hucdi estuary, with a loss of about 45,000 human lives and the destruction

jof 100.000 head of cattle. The greatest I disaster of recent times in this nuiehaffiicted region was the Baekergunge hurricane of the night of October 31Xovember 1, 1876, which cost the lives of over 100.(KJO persons. In this storm the water rose from 30 to 40ft in less than hall an hour. The islands of the Pacific are also subject to visitations of this character 011 a huge scale in connection with tropical hurricanes. The latest of these was the storm of March, 1910, which was especially remarkable for the vast area that it covered, its track extending .some 2500 miles from Fiji to New Caledonia, Norfolk Island, and the North Island of New Zealand. Statistics or the loss of life and property in this storm are 1 not yet available.

The American seaboard has repeatedly suffered from the effects of storm waves, in the Galveston hurricane of September, 1000, a series of waves invaded the city, 60!KJ lives were lost, and the destruction of property amounted to £6,000,000. The damage was due to wind a s well as water, but chieHv to the latter.