4.512 Zones of Transport . Littoral transport occurs in two modes: hed- 

 toad transport J the motion of grains rolled over the bottom by the shear 

 of water moving above the sediment bed; and suspended-load transport j the 

 transport of grains by currents after the grains have been lifted from 

 the bed by turbulence. 



Both modes of transport are usually present at the same time, but it 

 is hard to distinguish where bedload transport ends and suspended-load 

 transport begins. It is more useful to identify two zones of transport 

 based on the type of fluid motion initiating sediment motion: the off- 

 shore zone where transport is initiated by wave-induced motion over rip- 

 ples, and the surf zone where transport is initiated by the passing break- 

 er. In either zone, net sediment transport is the product of two pro- 

 cesses: the periodic wave-induced fluid motion that initiates sediment 

 motion, and the superimposed currents (usually weak) which transport the 

 sediment set in motion. 



a. Offshore Zone . Waves traveling toward shallow water eventually 

 reach a depth where the water motion near the bottom begins to affect the 

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

 weed and other organic matter) moves. This material oscillates back and 

 forth with the waves, often in ripple-like ridges parallel to the wave 

 crests. For a given wave 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 wave 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 wave. 



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

 ity distribution with time changes from approximately sinusoidal to a dis- 

 tribution that has a high shoreward component associated with the brief 

 passage of the wave crest, and lower seaward velocities associated with 

 the longer time interval occupied by the passage of the trough. As the 

 shoreward water velocity associated with 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 pass- 

 age of the wave trough. These maximums are the suspension clouds shed by 

 the two nearest upstream ripples. The approach of the next wave crest 

 reverses the direction of the sand remaining suspended in the cloud. The 

 landward flow also drags material shoreward as bedload. 



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

 accretion, the average rate at which sand is carried away 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 waves. These currents, 



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