of the ice-foot fomations which help to protect shorelines from winter 

 storms (Bajorunas and Duane, 1967; Marsh, et al, 1973). At harbor 

 entrances, bars must often be dredged periodically to maintain navigable 

 depths in entrance channels. 



The longshore bar pattern is a useful parameter for delimiting the 

 effects of coastal structures. Where the bar pattern is locally altered 

 to be significantly different from the pattern in adjoining or similar 

 areas, it is either because one of the fundamental variables in bar 

 formation (sand supply or wave exposure) is locally modified or some 

 destructive process is locally active. This effect can be seen on off- 

 shore bars in the Great Lakes and has also been observed along the Gulf 

 Coast. 



VX. SEDIMENT ENTRAINMENT AM) ALONGSHORE TRANSPORT 



1. General . 



Sediment movement in the presence of fluid motion over a mobilebed 

 is an extremely complex problem. Material can move completely within 

 the fluid (suspension), at the fluid-bottom interface (bedload) , or 

 within the sediment (creep). Motion can be unidirectional, as in a river 

 bed; bidirectional, by oscillating flow such as wave- induced motion; or 

 random, as in well developed turbulent flow. 



2. Bed Movement in Unidirectional Flow . 



As the fluid flow velocity over a flat surface of loose grains is 

 increased, a condition is eventually reached where a few grains first 

 begin to move due to forces exerted on them by the flow. Since 1753 

 when Brahms investigated the problem, researchers have attempted to 

 establish the conditions of incipient motion (RaudJcivi, 1967). Shields 

 (1936) first presented a graph of an entrainment function versus a 

 particle's Reynolds number; he also indicated that the type of bed form 

 which will develop is determined by flow velocity conditions and can be 

 predicted from his graph. 



3. Bed Movement in the Presence of Waves . 



Waves traveling toward shallow water eventually reach a depth where 

 the water motion near the bottom begins to agitate the sediment. At first, 

 only low density material moves, e.g., seaweed and other organic matter. 

 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, the water velocity 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 offshore 

 sand ripples are typically uniform and periodic, and the sand moves from 

 one side of the ripple crest to the other with the passage of each wave. 



17 



