the subject of Section 4.4, include longshore currents and mass-transport 

 currents in the on shore -offshore direction. It is possible to have ripple 

 forms moving shoreward while residual currents above the ripples carry 

 suspended sediment clouds in a net offshore direction. Information on the 

 transport of sediment above ripples is given in Bijker (1970), Kennedy and 

 Locher (1972), and Mogridge and Kamphuis (1972). 



b. Surf Zone. The stress of the water on the bottom due to turbu- 

 lence and wave- induced velocity gradients moves sediment in the surf zone 

 with each passing breaker crest. This sediment motion is both bedload 

 and suspended- load transport. Sediment in motion oscillates back and 

 forth with each passing wave, and moves alongshore with the longshore 

 current. On the beach face, the landward termination of the surf zone, 

 the broken wave advances up the slope as a bore of gradually decreasing 

 height, and then drains seaward in a gradually thinning sheet of water. 

 Frequently, the draining return flows in gullies and carries sediment to 

 the base of the beach face. 



In the surf zone, ripples cause significant sediment suspension, but 

 here there are additional eddies caused by the breaking wave. These eddies 

 have more energy and are larger than the ripple eddies. The greater energy 

 suspends more sand in the surf zone than offshore. The greater eddy size 

 mixes the suspended sand over a larger vertical distance. Since the size 

 is about equal to the local depth, significant quantities of sand are sus- 

 pended over most of the depth in the surf zone. 



Since breaking waves suspend the sediment, the amount suspended is 

 partly determined by breaker type. Data from Fairchild (1972, Figure 5), 

 show that spilling breakers usually produce noticeably lower suspended 

 sediment concentrations than do plunging breakers. See Fairchild (1972) 

 and Watts (1953) for field data; Fairchild (1956 and 1959) for lab data. 

 Typical suspended concentrations of fine sand range between 20 parts per 

 million and 2 parts per thousand by weight in the surf zone, and are about 

 the same near the ripple crests in the offshore zone. 



Studies of suspended sediment concentrations in the surf zone by 

 Watts (1953) and Fairchild (1972) indicate that sediment in suspension in 

 the surf zone may form a significant portion of the material in longshore 

 transport. However, present understanding of sediment suspension, and 

 the practical difficulty of obtaining and processing sufficient suspended 

 sediment samples have limited this approach to predicting longshore trans- 

 port. 



4.513 Profiles . Profiles are two-dimensional vertical sections showing 

 how elevation varies with distance. Coastal profiles (See Figs. 4-1 and 

 4-18) are usually measured perpendicular to the shoreline, and may be 

 shelf profiles, nearshore profiles, or beach profiles. Changes on near- 

 shore and beach profiles are interrelated, and are highly important in 

 the interpretation of littoral processes. The measurement and analysis 

 of combined beach and nearshore profiles is a major part of most engineer- 

 ing studies of littoral processes. 



4-54 



