Waves 



121 



rip currents (Shepard, Emery, and LaFond, 

 1941) which flow seaward more or less per- 

 pendicular to sandy shores (Fig. 110). At 

 their landward end these rip currents are 

 supplied by lateral feeder channels which 

 gather water from one or both sides. Where 

 the rip current passes through the breaker 

 zone it is narrow and reaches its greatest 

 speed, as much as 1 meter/sec. Speeds 

 within and landward of the breaker zone are 

 more or less uniform from surface to the 

 bottom, excepting for minor reduction by 

 bottom friction, so that channels are com- 

 monly cut several feet deep in the sand along 

 the route of the rip currents (Fig. 31). Fairly 

 high speeds may continue as far as 1000 feet 

 beyond the breakers where the water forms a 

 large bulbous head which may be sheared 

 away by the general coastal current and 

 moved parallel to the shore. 



The rest of the water piled up landward of 

 the breakers escapes by a seaward movement 

 at the bottom, but the velocity is low, mostly 

 less than about 1 per cent of the orbital 

 velocity under the wave crest (Munk, \949a; 

 Shepard and Inman, 195 \b). Where rip cur- 



_^^^^^^^^^^H^^^ ■ ' ^^^^^^^^^ ' ' ^^^^^^^^^1 



Figure 107. Effects of refraction of waves on various kinds 

 of shores. Dash Unes indicate bottom contours; narrow 

 lines, wave crests; wide lines, orthogonals; black areas, 

 land. Note that orthogonals converge on a projecting 

 point inshore of a submarine ridge, leading to concen- 

 tration of wave energy and maximum erosive force. 

 Orthogonals diverge within a bay inshore of a submarine 

 canyon, leading to diffusion of wave energy and minimum 

 erosion or even deposition of sediments. 



WAVE PRESSURE 





^ 



HORIZONTAL VELOCITY 



'di 



TIME IN SECONDS- 



. I I 



SERIES NO. 14 

 DEPTH 7.3 FT. 



Figure 108. Orbital current meter record made 0.8 foot above sand bottom at a depth of 8.5 feet just seaward of breaker 

 zone at Scripps Pier. Note that the maximum horizontal water velocity is onshore under the wave crest. In this par- 

 ticular zone the maximum vertical water velocity is upward and is also nearly under the wave crest owing to steepness 

 of the wave fronts. From Inman and Nasu (1956). 



