CAUSES OF SURF 105 



being that they run a little faster than might be expected of lower waves 

 of the same length. In extreme cases their velocities may increase in 

 this way by as much as 10 percent, but usually the effect is much less 

 than this. 



It is commonly stated that the periods of breakers on the shore are 

 the same as those of the waves out in deeper water, since the reduction 

 in the velocities and in lengths of the latter, over the shoaling bottom, 

 are in at least approximately the same ratio. Any other condition 

 would require either the development of entirely new waves or an 

 accumulation of waves, or the complete disappearance of some of the 

 waves during the last part of their advance shoreward. 



On the other hand, it has recently been reported, from observations 

 taken with a special wave meter, working semiautomatically, that the 

 periods of waves advancing over a shoaling bottom usually increase — 

 and by a maximum of 25 percent — but with some of the readings 

 showing a small decrease. 33 



Similarly, the average periods of the waves a short distance offshore 

 were reported shorter by about one-half 'second than those of the 

 breakers 3-1 percent of the time, btit larger 22 percent of the time, dur- 

 ing a series of 38 observations off the island of Martha's Vineyard. 

 And the periods of small breakers, watched by us on another recent 

 occasion, averaged 3 to 1 seconds when the waves half a mile out from 

 the land had averaged only about 25 feet long shortly previous, which 

 corresponds to an average period there of only about 2.25 seconds. 



We believe, however, that an explanation for these discrepancies 

 can be found in the conditions under which observations are likely to 

 be made. When there is an old swell, with younger and shorter waves 

 running on top of it, as is often the case, the breakers from the larger 

 waves may swamp out or obscure the smaller ones before these reach 

 the point where they would break. In such cases, the periods would 

 average longer for the breakers than for the waves farther out. On 

 the other hand, when the sea is irregular and confused, some of the 

 waves are likely to be overlooked offshore, even if they do not differ- 

 greatly in size; but since they, too, would develop nonetheless into 

 definite breakers, the periods for the latter would be shorter than the 

 apparent periods of the waves farther out. The individual periods 

 of any given run of breakers always differ considerably from one to 

 the next, as do those of the waves offshore, because of the variation in 

 length among the latter. The following tabulation of observations, 

 made recently at South Beach, Martha's Vineyard, may serve as an 

 indication of monthly variations for the northeastern coast of the 

 United States. 



83 Ivanov, A. A. 1938. On the propagation of waves near the coastline. (In Russian.) 

 English summary. Izv. Akad. nauk. ser. geogr. geofiz. pp. 490-491. 



