velocity near the bed. In contrast, in suspended mode, the upward transport 

 of eddy momentvun keeps sediment particles in the water column and available 

 for transport by any current. Figure 12b illustrates the improvement obtained 

 by using H^/D for classifying beach profile change produced in small-scale 

 experiments. Although the points denoting bar and berm profile response 

 overlap to some extent, the delineation obtained is somewhat better than in 

 Figure 12a. 



233. The effect of wave period in scaling is also emphasized through 

 the significant difference between small-scale and prototype -size experiments 

 in classifying beach response using the dimensionless fall speed. 



Form and Movement of Bars 



Bar genesis 



234. Several theories have been advanced to explain the formation of 

 longshore bars. Since a wide variety of bed forms has been classified as a 

 bar-like feature by various authors, various mechanisms may presumably prevail 

 in the formation process. Here, bar generation by depth-limited breaking 

 waves is investigated, the "classical" viewpoint of bar genesis. As waves 

 break near shore, energy is dissipated producing a turbulent fluid environment 

 where sediment is entrained and maintained in suspension. Depending on the 

 vertical profile of both the cross -shore fluid velocity field and the sediment 

 concentration, the sediment will experience net onshore or offshore movement, 

 resulting in a berm or bar profile. Sediment transported in the offshore 

 direction will drop out of the water column to be deposited where the turbu- 

 lence begins to decrease, somewhat seaward of the plunge point, where breaking 

 waves undergo maximum energy dissipation (Miller 1976, Skjelbreia 1987). A 

 berm is formed as material is transported onshore and deposited on the 

 foreshore, for which the force of gravity and properties of the uprush bore 

 determine the berm height (Bagnold 1940, Sunamura 1975). In the field, long- 

 period (infragravity) wave motions, if present, may also influence and perhaps 

 dominate foreshore development, since the energy of these waves is not depth 

 limited, as is the case for short-period waves. However, no direct evidence 



77 



