Broken dykes, such as this one near Rotterdam, 

 caused areas of Ho/land to be flooded 

 during the storm surge of 1953. This tidal 

 storm occurred when high winds, reinforced by 

 spring tides, caused a piling up of water 

 at the southern end of the North Sea. 



which flows into China's Bay of Hangchow. There, at high tide, 

 the bore surges upriver as a wave up to eleven feet high with a 

 speed up to twelve knots or more. Within a period of ten minutes 

 the river rises nine feet, and about one and three-quarter million 

 tons of water flow upstream per minute. The Chinese boatmen 

 take advantage of the great rush of water following the bore by- 

 allowing it to carry their junks up the river. 



Since the tides depend on the motions of the Moon and Sun, 

 normally we can predict them rather accurately - the times of high 

 and low water to within a few minutes, and the heights of the water 

 itself to within a few inches. But from time to time our predictions 

 are thrown off by meteorological disturbances. The water may be 

 several feet higher or lower than we predicted, and it may come 

 earUer or later than we expected. 



These disturbances are known as storm surges, and they can be 

 disastrous over a large area. The most destructive surge occurring 

 in northwest Europe in recent years was the one of January 3 1 and 

 February i, 1953, when strong northerly winds caused a piUng up 

 of water in the southern part of the North Sea. Because the peak 

 of the surge came very near the time of high water of a spring tide, 

 this storm tide inundated the coasts of Holland and eastern England 

 at levels up to ten feet higher than were predicted; and high waves 

 generated by the gale added to the destructive force of the water. 

 In Holland more than eighteen hundred people were killed, while 

 England lost more than three hundred, and in both countries there 

 was extensive damage to property. This disaster made it obvious 

 that some sort of flood warning service should be set up in Britain. 

 Such a service is now operated jointly by the Admiralty and the 

 Meteorological Office, and by methods based on a study of previous 

 surges we can forecast the probable height of a surge about twelve 

 hours before it strikes. 



The surges born of hurricanes in the Gulf of Mexico and along 

 the Atlantic coast of the United States, and those brought on by 

 typhoons in the China seas, have a special character. The wind piles 

 up the water in the storm region, as it did in the 1953 North Sea 

 surge, but at the same time the low pressure area of the storm 

 causes a temporary rise in sea level. And a third effect - resonance - 

 may build up the surge to even greater heights if the cyclone or 

 hurricane happens to be moving at almost the same speed as that 

 at which a tidal wave, once set up, would travel freely. 



Destructive surges of still different character can be produced 

 by submarine earthquakes. These surges are popularly known as 

 "tidal waves," although they have nothing to do with the tides. 

 Scientists prefer to call them by their Japanese name, tsunamis. 

 When an earthquake jars the ocean floor, the resulting disturbance 

 in sea level sets up a wave that fans out for hundreds or thousands 

 of miles in all directions at a speed of about five hundred miles an 

 hour. In the deep ocean the height of a tsunami is only one or two 

 feet, and a ship overtaken by one may not even detect it. But on 

 entering shallow water, although the wave's speed is greatly reduc- 

 ed, its height may build up to thirty or forty feet before it over- 

 whelms the shore and rushes inland. These waves do not occur 

 singly but in a long train, and in deep water a hundred miles or more 

 may separate them from crest to crest. On reaching the shore the 

 third or fourth wave in the train is usually the highest and most 



