NATURAL PHENOMENA 



ports and should take measures to prevent damage to the ship or 

 property. 



Tide Bores— The tide bore may be defined as a sudden rise of 

 tide which rolls up certain rivers and bays in the form of a breaker. 

 Bores form a hazard to shipping and generally occur in a shallow 

 estuary where the range of tide is high. The high water moves 

 inward at greater speed than the preceeding low water because of 

 the greater depths at high water. When the high water overtakes 

 the low water an abrupt front is formed. 



An example of this phenomenon occurs in the Seine River, France, 

 where it is known locally as a "mascaret". This wave takes the form 

 of a fairly abrupt front the slope of which varies between 1/100 and 

 1/150, and the height from 3 to W2 feet. This front travels upstream 

 with a velocity of 22 to 25 feet per second and is followed by wave 

 undulations of 2 to 3 feet high and a few score yards in length. This 

 phenomenon is dependent on numerous factors and can be consider- 

 ably lessened or even entirely suppressed by prevailing atmospheric 

 conditions or hydrologic conditions of the river. 



The impact of the mascaret on a ship at her mooring results in a 

 more or less alternating force; generally, the maximum influence of 

 this force takes place at the passage of the wave front and is directed 

 upstream attaining a value nearly equal to the product of the dis- 

 placement of the vessel by the slope of the wave (i.e. approx. 1/100 

 to 1/150 of the vessel's displacement). The following undulating and 

 downstream forces, however, produce both upstream and downstream 

 forces comparable to that produced by the initial wave front. 



Thus the vessel experiences violent alternating forces attaining 

 scores of tons with a period of some 10 to 20 seconds. 



Moorings should therefore be such as to introduce opposing 

 reactions to the movement of the vessel before any appreciable 

 speed is obtained resulting in a considerable amount of kinetic 

 energy. Mooring lines should also have the ability to stretch to allow 

 for the lifting of the vessel as the tide rises. 



Traditional mooring lines (wire hawsers, hemp ropes, etc.) are 

 not suitable since they are not sufficiently elastic to be hove taut 

 to limit the movement of the vessel at low tide and to subsequently 

 undergo a rapid change of height. Furthermore, as it is not possible 

 to balance the tension of each mooring line there is the risk of 

 breaking one after the other when subjected to the forces generated 

 by the mascaret. 



The Port of Rouen Authority recommends employment of nylon 

 springs shackled to customary mooring lines, particularly on wire 

 hawsers. These nylon springs should be 26 to 32 feet in length and 3 to 

 4 inches in diameter. The lines are hove as taut as possible at low 

 water. When conditions allow, both anchors should be laid out with 

 slightly taut chains to increase the ship's resistance to the wave 

 front. Ships should not use engines as this increases the downstream 

 force. The nylon springs should not be in contact with bollards or 

 bitts but should end in metal thimbles. 



Glacial Wave.— lAtay a. Bay (58°38'N., 137°39'W.), on the south- 

 east coast of Alaska, is a deep indentation with a narrow entrance. 

 The shores are backed by high peaks from which glaciers descend to 

 the water. This bay, on several occasions, has been subjected to 

 waves and water movements of exceptional amplitude. 



Discovered in 1786, by La Perouse, the bay then appeared to 

 offer sheltered anchorage, but that explorer lost two small craft with 

 crews sent to reconnoiter the area. The craft were lost from being 

 thrown against the reefs by the strong tidal currents which attain 

 rates of up to 12 knots at the entrance. 



On the evening of July 9, 1958 an earthquake in Alaska caused 

 an enormous mass of ice to break off Lituya Glacier and to fall into 

 the bay. The resulting wave swept the shores to a considerable 

 height. The few witnesses reported that the cataclysm caused the 

 water to attain heights of 492 feet (150 meters) at certain points on 

 the coast, more than twice that height southward of Lituya Glacier, 

 and 164 feet (50 meters) on Cenetaph Island within the bay. 



Such heights had remained an enigma for oceanographers; but 

 several observations made by the U. S. Geological Survey and the 

 U. S. Coast and Geodetic Survey, and taking into consideration 

 similar phenomena, permitted the idea that such heights were not 

 exaggerated. 



The complete disappearance of two Indian villages, mentioned 

 by La Perouse, were reported in 1853 or 1854 by hunters. A wave 

 of great amplitude was reported in 1936; and prior to 1958 a geologist 



noticed that the trees around the bay shore grew according to the 

 age of the trees with marked delineations or bands on the hillsides 

 to a height of 393 feet (120 meters). The most recent boundary and 

 age group corresponded with the wave of 1936, and the second age 

 group corresponded to the winter of 1853-54. 



Returning to the bay after July 9, 1958 the geologist discovered 

 that the mountainsides he had previously observed as completely 

 covered with trees, and the slopes descending to the shore were bare 

 to heights of 1,640 feet (500 meters). He noted also that the rock 

 fissures were clear of accumulated debris. 



The observations were confirmed by aerial photographs taken 

 after the quake. The photos indicated that some great masses of 

 ice had fallen into the bay in addition to large avalanches. Photo- 

 grammetric measurements also indicated that the sea wave of 1958 

 exceeded the 1,280 foot (360 meter) level. 



HISTORY OF A DISAPPEARING ISLAND 



Falcon Island (Fonua F'ou). — This volcanic feature, located at 

 20°19'S., 175°25'W., in the Tonga Islands, has a history of appearance 

 and submergence and has changed in size and form on several known 

 occasions. 



In 1865, the H.M.S. Falcon, from which it derives its name, ob- 

 served it as a breaking reef. The H.M.S. Sappho observed smoke 

 rising from the reef in 1877. 





In 1889, the H.M.S. Egeria, a surveying ship, surveyed the area 

 and found an island IVi miles long north to south, one mile wide and 

 wedge-shaped with a 153 foot-high hill at the southern part. The 

 island was formed of loose volcanic ash and cinders which were 

 constantly slipping down, as the sea action undermined it. It was 

 estimated that two-thirds of the island had worked away since 1885. 

 No growing coral was found. It soon dispersed entirely. The Egeria 

 was able to anchor off all but the east side. 



In April 1894, the area was observed at a distance of 2 miles and 

 the only visible relief was a low streak of black rock. In December 

 of the same year volcanic action and a new crater was observed to 

 have formed with the resultant island 3 miles long and 1V4 miles 

 wide. It was 50 feet high with a hot surface. It was reported in 

 July 1898, that Falcon Island had disappeared completely and that in 

 its place was a shoal about 100 yards in extent with the sea breaking 

 heavily on it. Discolored water was also observed about 1 mile 

 southward of the breakers. Again in April 1900, it appeared above 

 water and was estimated to be about 9 feet high at its northern end. 

 The SS Cormoran could find no above-water features in the area in 

 1913. 



The location of Falcon Island was examined by the H.M.S. 

 Veronica in 1921. A very heavy swell, discolored water, rips, and a 

 continuous breaker was observed at the southwest part of the area. 

 The breaker was found to be caused by a rock about 15 yards in 

 diameter, mottled green, covering and uncovering. The bottom was 

 visible for some distance from the rock. It was estimated that in 

 calm weather the rock would have had a depth of about 3 feet over it. 



In October 1927, Falcon Island erupted again. It was examined 

 by the H.M.S. Laburnum which cruised around it and found the 



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