SURF AROUND BAYS 



159 



give the approximate height of breaker, for waves that come in ob- 

 liquely, provided always that the bottom is smooth, with a similar 

 slope off all parts of the beach. Calculations of this sort are not as 

 simple as they sound if precision is sought, because they involve an 

 exact knowledge of the initial ratio between the heights of waves 

 offshore and their lengths— or between their heights and their periods, 

 which last can be translated into length. But it is not difficult to make 

 rough estimates of heights and periods of waves, if the weather is 

 moderate; and estimates of the height of the surf are not likely to be 

 helpful, except in moderate weather. 



Table 37. — Percentage decrease in height between deep water and the breaker zone, 

 for waves of different initial degrees of steepness approaching a straight shore line 

 (with straight and parallel bottom contours) at different angles. It is assujned that 

 the waves break where the depth of water is 1.3 times the breaker heights 



As a general rule, the decrease in the height of surf due to refrac- 

 tion is negligible for waves that come in at angles smaller than, say, 

 30°, no matter what their steepness may be offshore. But when the 

 waves are coming in at an angle greater than, say, 60°, the decrease in 

 their heights may make landing possible at a place where this wouid 

 not be so otherwise. 



Waves advancing in a uniform direction are refracted to the same 

 degree all along a coast that is straight, if the bottom contours are 

 parallel with the shore line (fig. 46) ; hence the decrease in their 

 heights (if any) from this cause will be as great at one point along the 

 shore as it is at another. But if the coast is strongly curved, the 

 waves are refracted much more at one place than at another, so that 

 the breakers may differ considerably, in their heights, from place to 

 place. The local differences in the character of the surf that results 

 are most conveniently discussed (a) around the shores of bays and 

 beaches, and (b) around headlands and in the shelter of these. 



SURF AROUND THE SHORES OF BAYS 



Bays can be classified, roughly, as short and broad, or as long and 

 narrow. 



Wave crests that are parallel to the general trend of the shore line 

 are not refracted at all along the central sector of a short, broad beach, 



