act on, the hydrography of the surf and nearshore zones. Figure 4-21 shows 

 the nearshore current system measured for particular wave conditions on the 

 southern California coast. 



At first observation, there appears to be an extensive exchange of water 

 between the nearshore and the surf zone. However, the breaking wave itself is 

 formed largely of water that has been withdrawn from the surf zone after 

 breaking (Galvin, 1967). This water then reenters the surf zone as part of 

 the new breaking vave, so that only a limited amount of water is actually 

 transferred offshore. This inference is supported by the calculations of 

 Longuet-Higgins (1970a, p. 6788), which show that little mixing is needed to 

 account for observed velocity distributions. Most of the exchange mechanisms 

 indicated act with speeds much slower than the breaking vave speed, which may 

 be taken as an estimate of the maximum water particle speed in the littoral 

 zone indicated by equation (4-19). 



b. Diffuse Return Flow . Wind- and wave-induced water drift, pressure 

 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. Diffuse 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). 



c. Rip Currents . Most noticeable of the exchange mechanisms between 

 offshore and the surf zone are rip currents (see Fig. 4-22 and Fig. A-7, App. 

 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 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, there is presently 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. Longshore Currents . 



a. 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 longshore component 

 of motion in waves that obliquely approach the shoreline. 



4-50 



