EFFECTS OF TROUGHS AND RIDGES 175 



SUBMARINE TROUGHS AND RIDGES AS AFFECTING SURF 



It has been assumed in the foregoing discussion that the submarine 

 contours are more or less parallel to the coast, out to the depth beyond 

 which the waves are not refracted by the bottom. And this is usually 

 the case, at least within broad limits. But the refraction of waves by 

 the slopes of submarine troughs, or of ridges or spits that run out from 



55 



— I - 



55' 



Figure 57. — Chart of Arno Atoll, Marshall Group, western tropical Pacific. 



Soundings in fathoms. 



the shore line may also be enough to affect the height of the local surf 

 considerably off some coasts. 



In the first of these cases, the wave crests are delayed in their prog- 

 ress along the two slopes of the trough, so that they are refracted 

 around, much as they would be if running into a long narrow bay, 

 whereby their heights may be decreased to such an extent that the 

 surf may be appreciably lower along the sectors of the coast that face 

 the trough than it is on either hand. And the local fishermen on the 

 coast of California, in particular, have long known that a small boat 



