erosion, accompanied by longshore transport which has carried sand along 

 the coast to supply the barriers and spits extending across the bays. 

 The primary agent producing this erosion and transport is the action of 

 waves impinging on the shore. 



Littoral transport is the movement of sedimentary material in the 

 littoral zone by waves and currents. The littoral zone extends from the 

 shoreline to just beyond the most seaward breakers. 



Littoral transport is classified as onshore-offshore transport or as 

 longshore transport. Onshore-offshore transport has an average net direc- 

 tion perpendicular to the shoreline; longshore transport has an average 

 net direction parallel to the shoreline. The instantaneous motion of 

 sedimentary particles has both an onshore-offshore and a longshore com- 

 ponent. 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: 



(a) What are the longshore transport conditions at the site? 

 (Needed for the design of groins, jetties, navigation channels, and 

 inlets.) 



Cb) What is the trend of shoreline migration over short and long 

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

 navigation channels.) 



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

 of sewage outfalls and water intakes.) 



(d) What is the direction and rate of onshore-offshore sediment 

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



(e) 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.) 



(f) 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 answering these and 

 related questions. The section indicates accepted practice based on 

 field observations and research results. Section 4.52 deals with onshore- 

 offshore transport, presenting material pertinent to answering questions 

 (b) through (f). Section 4.53 deals with longshore transport, presenting 

 material pertinent to questions (a) , (b) , and (f) . 



4-52 



