vertical position of the mass center was relatively constant since the 

 equilibrium bar volume was attained rapidly, and horizontal movement of the 

 bar was limited. However, the overall trend for bars formed on erosional 

 beach profiles was for the vertical distance to the bar mass center to 

 increase with time. 



264. As expected, bars appeared to be initiated at the same location 

 along the profile for the same wave conditions and initial beach slope, 

 irrespective of beach grain size. However, the bar center of mass at later 

 times was usually located farther offshore and in deeper water for finer 

 grain- sized beaches. 



265. The locations of both the bar crest and the bar center of mass 

 were used as reference points to calculate the speed of bar migration. 

 Evolution of bar speed had the same characteristic features for both refer- 

 ences. Only bars formed on erosional profiles were included in analysis of 

 migration speed, since bars on accretionary profiles were almost stationary 

 (see Figure 22). Furthermore, if an inner bar grew together with the main 

 breakpoint bar, only the seaward portion of the bar conglomerate was consid- 

 ered to eliminate spurious instantaneous shoreward displacements of the center 

 of mass resulting from coalescence of the bars. Figure 23(a-d) displays speed 

 of bar migration. Positive speeds of bar migration indicate movement directed 

 offshore. The main trend was similar for all cases and independent of 

 definition (reference point) , exhibiting a high initial speed of bar migration 

 which slowed as the profile approached the equilibrium shape. 



266. Case with a simulated tide . Case 911 from the CE experiment, 

 which had a cyclical variation in water level, showed cyclical onshore and 

 offshore bar movement, i.e., negative bar speeds as the water level dropped. 

 The main purpose of Case 911 was to demonstrate that a variation in water 

 level would produce a more gently sloping bar*. A negative speed of bar 

 migration also occurred if bar shape changed considerably during a run, 

 particularly if the location of the crest were used as the reference point. 

 For example. Case 100 showed a negative bar speed after about 20 hr , as the 



Personal Communication, Thorndike Saville, Former Technical Director, 

 Coastal Engineering Research Center, Ft. Belvoir, VA. 



96 



