176 BREAKER DIRECTION AND HEIGHT 



can often lie comfortably at anchor at the head of a submarine canyon, 

 such as the one oif La Jolla, at times when the swell is too heavy for 

 this in the shallower water on either side of it. The surf immediately 

 on the beach may also differ in height accordingly from place to place, 

 or it may even be interrupted altogether, at the head of such a trough. 

 The effect of a submarine ridge is, of course, the reverse of that of a 

 submarine trough, because waves coming in at right angles to it ad- 

 vance more rapidly along its deeper flanks than along its shallower 

 axis, so that they are refracted toward the latter. If the water on 

 the bar is shoal enough, and if the waves are large enough, they may 

 break upon it, as well as at the extreme tip of the promontory from 

 which it projects. In such cases the surf may be enough lower on the 

 sides of the promontory for a boat to come in in moderate weather. 

 But where the water over the bar is so deep that the waves do not 

 break there, its presence may not only focus them upon the promon- 

 tory, but the wave trains that run in along its two flanks may produce 

 a cross sea, as they are bent the one toward the other, so that the surf 

 may be heavier opposite the base of the ridge than it is along the 

 neighboring coast on either hand. 



FORECASTING BREAKERS AND SURF 



Since surf conditions depend directly on the waves coming toward 

 a beach and on the configuration of the bottom off the beach, the state 

 of the surf can be predicated from wind data by predicting the sea and 

 swell as discussed previously, and applying knowledge of the behavior 

 of waves entering shoaling water. The relation between surf and 

 swell has already been discussed on previous pages. A detailed treat- 

 ment of the prediction of surf conditions has been published by the 

 Hydrographic Office as Breakers and surf ; principles in forecasting. 



Knowledge of the effect of bottom on waves can be applied to show 

 which parts of a section of coast will have the lightest surf under any 

 given conditions of sea and swell. Thus, wartime operations may be 

 planned far in advance and yet take advantage of the most probable 

 conditions. At the time of the landing itself, surf predictions have 

 been found to be very useful. For instance, just before the landing 

 at Gela, Sicily, the seas were very rough in the Straits of Malta, and 

 there was some question among those in command as to whether the 

 beaches would be approachable or not. Consideration of the meteoro- 

 logical situation indicated not only a quick subsidence of the seas, but 

 also that the landing beaches would be somewhat in the lee of the is- 

 land. On this basis, it was decided to make no alteration in plan, and 

 the landing which followed caught the Italians napping as they had 

 gone off the alert during this period of unusually high seas. 



