2. Fluid Motion in Breaking Waves . 



During most of the wave cycle In shallow water, the particle velocity 

 is approximately horizontal and constant over the depth, although right at 

 breaking there is significant vertical velocity as the water is drawn up into 

 the crest of the breaker. The maximum particle velocity under a breaking wave 

 is approximated by solitary wave theory (eq. 2-66) to be 



Ut, _, = C =\/g (H + d) (4-19) 



D max \o V / 



where (H + d) is the distance measured from crest of the breaker to the 

 bottom. 



Fluid motions at breaking cause most of the sediment transport in the 

 littoral zone, because the bottom velocities and turbulence at breaking 

 suspend more bottom sediment. This suspended sediment can then be transported 

 by currents in the surf zone whose velocities are normally too low to move 

 sediment at rest on the bottom. 



The mode of breaking may vary significantly from spilling to plunging to 

 collapsing to surging, as the beach slope increases or the wave 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 (4-19) (Galvin, 1972). Spilling breakers differ little 

 in fluid motion from unbroken waves (Divoky, LeMehaute, and Lin, 1970) and 

 generate less bottom turbulence and thus tend to be less effective in 

 transporting sediment than plunging or collapsing breakers. 



The most intense local fluid motions are produced by plunging breakers. 

 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-failing 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 sediment scoured from the 

 trough and in part by sediment transported in ripples moving from the 

 offshore. 



The effect of the tide on neairshore currents is not discussed here, 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 nearshore currents. 



3. Onshore-Offshore Currents . 



a. Onshore-Offshore Exchange . Field and laboratory data indicate that 

 vrater 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: (1) mass transport velocity in 

 shoaling waves, (2) wind-induced surface drift, (3) wave-induced setup, (4) 

 currents induced by irregularities on the bottom, (5) rip currents, and (6) 

 density cuErents. The resulting flows are significantly influenced by, and 



4-4 9 



