reach a depth v^ere the vater motion near the bottom begins to affect the 

 sediment on the bottom. At first, only low-density material (such as seaweed 

 and other organic matter) moves. This material oscillates back, and forth with 

 the waves, often in ripplelike ridges parallel to the wave crests. For a 

 given vvave condition, as the depth decreases, water motion immediately above 

 the sediment bed increases until it exerts enough shear to move sand 

 particles. The sand then forms ripples with crests parallel to the vave 

 crests. These ripples are typically uniform and periodic, and sand moves from 

 one side of the crest to the other with the passage of each vave. 



As depth decreases to a value several times the wave height, the velocity 

 distribution with time changes from approximately sinusoidal to a distribution 

 that has (a) a high shoreward component associated with the brief passage of 

 the vave crest and (b) lower seaward velocities associated vdth the longer 

 time interval occupied by the passage of the trough. As the shoreward v«ater 

 velocity associated vdth the passing crest decreases and begins to reverse 

 direction over a ripple, a cloud of sand erupts upward from the lee (landward) 

 side of the ripple crest. This cloud of sand drifts seaward with the seaward 

 flow under the trough. At these shallow depths, the distance traveled by the 

 cloud of suspended sediment is two or more ripple wavelengths, so that the 

 sand concentration at a point above the ripples usually exhibits at least two 

 maximums during the passage of the wave trough. These maximums are the 

 suspension clouds shed by the tvo nearest upstream ripples. The approach of 

 the next vave crest reverses the direction of the sand remaining suspended in 

 the cloud. The landvjard flow also drags material shore vard as bedload. 



For the nearshore profile to be in equilibrium with no net erosion or 

 accretion, the average rate at which sand is carried avay from a point on the 

 bottom must be balanced by the average rate at which sand is added. Any net 

 change will be determined by the net residual currents near the bottom which 

 transport sediment set in motion by the vaves. These currents, the subject of 

 Section IV, include longshore currents and mass-transport currents in the 

 onshore-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). 



(2) Surf Zone . The stress of the vater on the bottom due to 

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

 VTith each passing breaker crest. This sediment motion is both bedload and 

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

 each passing vave and moves alongshore with the longshore current. On the 

 beach face — the landvard 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 vave. 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 



4-59 



