89. What makes a very slight swell (wave) become much higher 

 when it breaks on the shore as surf? 



Until a wave approaches the shore, its height is usually about one- 

 twentieth its length (distance from crest to crest); thus, if the crests are 

 20 feet apart, the wave height would be 1 foot. 



When the water depth equals half the wave length, bottonn friction 

 begins to slow down the speed of advance. With a wave length of 20 

 feet, this would take place when the water depth is 10 feet. 



As the wave slows, the back of the wave crowds the front, piling the 

 water higher. The lower part of a wave, being nearest the bottom, is 

 slowed more than the top; as a result, the top begins to curl over. When 

 the wave height reaches three-fourths the water depth, the wave topples 

 over as a breaker. 



Bowdltch, Nathaniel 



American Practical Navigator, U. S. Naval Oceanographic Office, 



1958. 

 Deacon, G. E. R. (Ed.) 



Seas, Maps, and l\/len, Doubleday and Company, 1962. 

 Engel, Leonard and Editors of LIFE 



The Sea, Life Nature Library, Time, Inc., 1961. 



107 



