Chapter 8 



DIRECTION AND HEIGHT OF BREAKERS IN RELATION TO THE 

 SHAPE OF THE COAST 



It is much the easiest, and the safest, to bring a boat in end on, if 

 the breakers are large enough to be troublesome, lest she be swamped. 

 Hence, it may be important to know the angle at which the breakers 

 are striking the shore. 



THE REFRACTION OF WAVES 



The angle at which breakers strike the shore is governed by the 

 general direction of advance of the waves offshore in relation to the 

 contour of the bottom and to the shape of the coast. Observers not 

 accustomed to surf are often astonished to find that the breakers may 

 be striking the beach at only a small angle, even at times when the line 

 of advance of the waves of swells out at sea is parallel to the general 

 trend of the coast. This is because the inshore ends of the waves are 

 delayed in their advance by the shoaling bottom, as explained on 

 pages 56 and 103, while their crests farther out are moving more 

 rapidly; consequently their inshore ends are bent around or "re- 

 fracted," as the alteration is commonly termed. It is often easy to 

 observe this refraction, if one looks out over the water of some cove, 

 pond, or river, when a smart breeze is blowing parallel to the shore, 

 and it is shown clearly on photographs taken from airplanes (fig. 45). 



The amount that a wave approaching the coast is refracted can be 

 calculated, provided that the angle is known that it makes with the 

 coast while it is still in deep water, that either its length, its period, or 

 its velocity is known there, and that the shape of the bottom is also 

 known. The relationship for straight beaches with parallel bottom 

 contours is summarized in table 35: 16 



In general, short waves are refracted less than long ones, unless they 

 advance into very shallow water indeed, because they are slowed less, 

 as described on page 104. Old swells are thus bent around much more 

 than younger storm waves are. Table 35 also illustrates another in- 

 teresting point namely, that the surf often breaks on the beach much 

 more obliquely than one might gather from a cursory reading of the 

 literature on waves. A high wave, too, of a given length, suffers less 



49 The basic formula is : 8ine a = — ; where a o is the angle offshore, a is the angle 

 at any given point inshore, and Co and are the corresponding velocities. 



155 



