146 



CHARACTER OF SURF 



same way b}' tidal currents, during the periods when these are run- 

 ning strong along the coast, is proverbial; the surf breaks on the 

 shores there with great force during stormy weather, as soon as the 

 current slackens (Stevenson, 1874, pp. 61-62). 



Rip currents. — The water that is carried landward by the surf is re- 

 turned seaward, in many localities, by local currents flowing offshore, 

 or "rip currents." a name which must not be confused with ""tide rips" 

 because they have nothing to do with the tide. They have been studied 

 along the coast of California chiefly, 41 but they are to be expected 

 along any open coast where heavy waves break commonly. They are 



Figure 42. — Aerial photograph showing a rip current off Coronado Beach, Cali- 

 fornia. (Official U. S. Navy photograph.) 



not known to extend more than half a mile or so out from the beach, 

 and they are usually less than 100 feet across at their inshore ends, 

 but they widen more or less at their offshore ends. (fig. 42). They 

 are temporary phenomena only accompanying high breakers and dy- 

 ing out when the surf flattens, but their velocities may be considerable 

 in their narrow inshore necks. One, for example, that was recently 

 observed on the outer coast of Cape Cod was flowing at an estimated 

 velocity of at least 1.5 knots. Waves running in against a rip cur- 

 rent are steepened as they are by any contrary current of equal veloci- 

 ty, so that they may break much farther out than would otherwise 

 happen, and in a very choppy and irregular fashion, much as in any 

 ordinary tide rip (fig. 32). A current of this sort may thus cause a 

 more or less definite gap in the lines of surf, if the latter is a regular 



tl For an excellent account of rip currents, see Shepard, F. P., K. O. Emery, and E. C. 

 LaFond, 1941, Kip currents; a process of geological importance. J. geol., vol. 49, 

 pp. 337-369. 



