108 



BREAKERS AND SURF 



In general, waves that are less than 10 to 15 times as long out in. 

 deep water as they are high, are only about as high when they break 

 as they were offshore. But relatively longer waves may increase 

 considerably in height. At South Beach, Marthas Vineyard, for 

 example, the ratio at the 30-foot contour, and at the time of breaking, 

 between the measured heights for waves of different degrees of steep- 

 ness has ranged between 1 : 1 and 1 : 2.2, while ratios of 1 : 1.1 to 1 : 1.9 

 have been recorded at La Jolla, California, between waves offshore 

 and the breakers caused by them. And the increase in height may be 

 considerably greater yet in the cases of very long swells; in fact, an 

 old swell that is relatively so low out over deep water as to be hardly 

 perceptible, but very long, may mount to such a height during the last 

 few yards of its advance as to cause a dangerous surf, even in calm 

 weather. And it is during calm or moderate weather that the height 

 of surf is chiefly of importance in landing operations. 



It is much easier to measure the periods of waves than to measure 

 their lengths. The alteration in height on a gently sloping bottom is 

 therefore summarized in table 31 for waves of representative sizes, 

 according to their periods. 



Table 31. — Heights (to the nearest foot) attained by ivaves, of different initial heights 

 and periods, at various depths on a gently sloping bottom. Blank spaees indicate 

 that the waves would, in all likelihood, have broken in deeper water, or that waves of 

 the stated shapes could not exist. This table is derived from figure 21 and is based 

 upon theoretical studies, substantiated by measurements of waves taken at the Scripps 

 Institution of Oceanography and at the Woods Hole Oceanographic Institution 



It appears, from published observations, that the height of the 

 surf may even be several times the preceding height of the waves in 



