TRANSECTS 



DEPTH 

 (M) 



BERM 



BREAKER 



UPPER 

 SHORE 

 FACE 



20 



20 



LlLtLk 



10 



12 



14 



LOWER 

 SHORE 

 FACE 



40 2 40 

 DIAMETER 



75°30' 



75°00' 



74°30' 



(from Swift, 1976) 



Figure 4-28. Distribution of grain sizes along transects of the Virginia- 

 North Carolina coast. 



b. Initiation of Sand Motion . Extensive laboratory results indicate two 

 separate criteria for motion initiation by oscillatory flow over a level bed 

 of sediment with d^Q between 0.1 and 2.0 millimeters (Hallermeier , 1980). 

 In field conditions, the appropriate threshold flow velocity for sand motion 

 is 



max, 



X-d) 



= [8 (y /y -1) g d 



,0.5 



50^ 



(4-23) 



where u is peak fluid velocity at the sediment bed. 



For waves that are not mean breaking, measured maximum near-bottom velocities 

 can be adequately determined using small-amplitude wave theory (Thornton and 

 Kraphol, 1974; Grace, 1976). That (Ch. 2, Sec. 11,3) provides equation (2-13) 

 which can be rewritten as 



moLX 



(-cO 



H 



smh [-;— J 



(4-24) 



This expression is plotted as a function of water depth for common field 

 values of wave period in Figure 4-29 . 



4-67 



