steepness (height-to-length ratio) decreases. (Galvin, 1967.) Of the 

 four breaker types, spilling breakers most closely resemble the solitary 

 waves whose speed is described by Equation 4-13. (Galvin, 1972.) Spill- 

 ing breakers differ little in fluid motion from unbroken waves (Divoky, 

 LeMehaute, and Lin, 1970), and thus tend to be less effective in trans- 

 porting sediment than plunging or collapsing breakers. 



The most intense local fluid motions are produced by plunging break- 

 ers. As the wave moves into shallower depths, the front face begins to 

 steepen. When the wave reaches a mean depth about equal to its height, 

 it breaks by curling over at the crest. The crest of the wave acts as a 

 free-falling jet that scours a trough into the bottom. At the same time, 

 just seaward of the trough, the longshore bar is formed, in part by sedi- 

 ment scoured from the trough and in part by sediment transported in rip- 

 ples moving from the offshore. 



The effect of the tide on nearshore currents is not discussed, but 

 tide-generated currents may be superimposed on wave-generated nearshore 

 currents, especially near estuaries. In addition, the changing elevation 

 of the water level as the tide rises and falls may change the area and 

 the shape of the profile through the surf zone, and thus alter the near- 

 shore currents. 



4.43 ONSHORE-OFFSHORE CURRENTS 



4.431 Onshore-Offshore Exchange . Field and laboratory data indicate 

 that water in the nearshore zone is divided by the breaker line into 

 two distinct water masses between which there is only a limited exchange 

 of water. 



The mechanisms for the exchange are: mass transport velocity in 

 shoaling waves, wind-induced surface drift, wave-induced setup, currents 

 induced by irregularities on the bottom, rip currents, and density cur- 

 rents. The resulting flows are significantly influenced by, and act on, 

 the hydrography of the surf and nearshore zones. Figure 4-13 shows the 

 nearshore current system measured for particular wave conditions on the 

 southern California coast. 



At first observation, there appears to be 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 wave itself is formed largely of water that has been withdrawn from the 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 wave, so that only a limited amount 

 of water is actually transferred offshore. This inference is supported 

 by the calculations of Longuet-Higgins (1970, 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-wave speed, which may be taken as an estimate of the 

 maximum water particle speed in the littoral zone indicated by Equation 

 4.13. 



4-43 



