4.432 Diffuse Return Flow . Wind- and wave-induced water drift, pres- 

 sure gradients at the bottom due to setup, density differences due to 

 suspended sediment and temperature, and other mechanisms produce patterns 

 of motion in the surf zone that vary from highly organized rip currents 



to broad diffuse flows that require continued observation to detect. Dif- 

 fuse return flows may be visible in aerial photos as fronts of turbid 

 water moving seaward from the surf zone. Such flows may be seen in the 

 photos reproduced in Sonu (1972, p. 3239). 



4.433 Rip Currents . Most noticeable of the exchange mechanisms between 

 offshore and surf zone are rip currents. (See Figure 4-14, and Figure 

 A-7, Appendix A.) Rip currents are concentrated jets that carry water 

 seaward through the breaker zone. They appear most noticeable when long, 

 high waves produce wave setup on the beach. In addition to the classi- 

 cal rip currents, there are other localized currents directed seaward 

 from the shore. Some are due to concentrated flows down gullies in the 

 beach face, and others can be attributed to interacting waves and edge 

 wave phenomena. (Inman, Tait, and Nordstrom, 1971, p. 3493.) The origin 

 of rip currents is discussed by Arthur (1962), and Sonu (1972). 



Three-dimensional circulation in the surf is documented by Shepard and 

 Inman (1950), and this complex flow needs to be considered, especially in 

 evaluating the results of laboratory tests for coastal engineering purposes. 

 However, at present, there is no proven way to predict the conditions that 

 produce rip currents or the spacing between rips. In addition, data are 

 lacking that would indicate quantitatively how important rip currents are 

 as sediment transporting agents. 



4.44 LONGSHORE CURRENTS 



4.441 Velocity and Flow Rate . Longshore currents flow parallel to the 

 shoreline, and are restricted mainly between the zone of breaking waves 

 and the shoreline. Most longshore currents are generated by the long- 

 shore component of motion in waves that obliquely approach the shoreline. 



Longshore currents typically have mean values of 1 foot per second 

 or less. Figure 4-15 shows a histogram of 5,591 longshore current veloc- 

 ities measured at 36 sites in California during 1968. Despite frequent 

 reports of exceptional longshore current speeds, most data agree with 

 Figure 4-15 in showing that speeds above 3 feet per second are unusual. 

 A compilation of 352 longshore current observations, most of which appear 

 to be biased toward conditions producing high speed, showed that the maxi- 

 mum observed speed was 5.5 feet per second, and that the highest observa- 

 tions were reported to have been wind-aided. (Calvin and Nelson, 1967.) 

 Although longshore currents generally have low speeds, they are important 

 in littoral processes because they flow along the shore for extended peri- 

 ods of time, transporting sediment set in motion by the breaking waves. 



The most important variable in determing the longshore current veloc- 

 ity is the angle between the wave crest and the shoreline. However, the 

 volume rate of flow of the current and the longshore transport rate depend 



4-45 



