perpendicular to the shoreline; longshore transport has an average net 

 direction parallel to the shoreline. The instantaneous motion of sedimentary 

 particles typically has both an onshore-offshore and a longshore component. 

 Onshore-offshore transport is usually the most significant type of transport 

 in the offshore zone, except in regions of strong tidal currents. Both 

 longshore and onshore-offshore transport are significant in the surf zone. 



Engineering problems involving littoral transport generally require 

 answers to one or more of the following questions: 



(1) What are the longshore transport conditions at the site? (Needed for 

 the design of groins, jetties, navigation channels, and inlets.) 



(2) What is the trend of shoreline migration over short and long time 

 intervals? (Needed for design of coastal structures, including navigation 

 channels.) 



(3) How far seaward is sand actively moving? (Needed in the design of 

 sewage outfalls and water intakes.) 



(4) What is the direction and rate of onshore-offshore sediment motion? 

 (Needed for sediment budget studies and beach-fill design.) 



(5) What is the average shape and the expected range of shapes for a 

 given beach profile? (Needed for design of groins, beach fills, navigation 

 structures, and flood protection.) 



(6) What effect will a postulated structure or project have on adjacent 

 beaches and on littoral transport? (Needed for design of all coastal works.) 



This section presents recommended methods for ansvering these and related 

 questions. The section indicates accepted practice based on field observa- 

 tions and research results. Chapter 4, Section V, 2 deals with onshore- 

 offshore transport, presenting material pertinent to answering questions (2) 

 through (6). Section V deals with longshore transport, presenting material 

 pertinent to questions (1), (2), and (6). 



b. Zones of Transport . Littoral transport occurs in two modes: bedload 

 transport, the motion of grains rolled over the bottom by the shear of water 

 moving above the sediment bed and suspended-load transport, the transport of 

 grains by currents after the grains have been lifted from the bed by turbu- 

 lence. 



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: (1) the offshore zone where 

 transport is initiated by wave-induced motion over ripples and (2) the surf 

 zone where transport is initiated primarily by the passing breaker. In either 

 zone, net sediment transport is the product of two processes: the periodic 

 wave-induced fluid motion that initiates sediment motion and the superimposed 

 currents (usually weak) which transport the sediment set in motion. 



(1) Offshore Zone . Waves traveling toward shallow water eventually 



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