diagnostic bed forms. Their presence reveals certain characteristics 

 of the flow that formed them. Interpretations must be made cautiously, 

 however, as the bed forms need not reflect present conditions because 

 they may persist long after the fluid motion that formed them has ceased. 

 Bed forms are common in sedimentary rocks, and are used as a means of 

 interpreting the environment in which the rocks were originally formed. 

 A single bed form may not characterize general flow conditions. In the 

 nearshore zone a variety of flows are superimposed. Depending on the 

 situation, the flows may be ranked as to their effect in transporting 

 sediment. Each flow, in interaction with the bed, may have produced a 

 distinctive bed form. For example, uprush and backwash produce a planar 

 swash zone, but the waves producing the swash may also ripple the bottom 

 seaward of the plunge point; these same waves drive longshore currents 

 that may form lunate ripples atop the first bar, transverse to the first- 

 mentioned ripples (Clifton, Hunter, and Phillips, 1971). Simultaneously, 

 adjacent flows may be strongly influenced by stream discharge and, farther 

 offshore, by wind drift or density currents. The total flow system is 

 thus composed of many vector fields each associated with a distinct bed 

 form. Tlierefore, to specify the system even with regard to transport 

 direction, much less flow conditions, a thorough statistical analysis of 

 bed forms is required. 



One of the simplest methods of classifying bed forms considers only 

 their wavelength: ripples have wavelengths less than 2 feet; megaripples 

 between 2 and 20 feet; and sand waves greater than 20 feet (Coastal 

 Research Group, 1969). Within this classification, each type of bed form 

 is further subdivided into a number of subtypes according to crest 

 linearity and continuity. The size that a bed form actually assumes, 

 within these defining limits, is often a fimction of shear velocity, 

 grain size, overall slope and roughness of the bed, and flow depth. A 

 change in any of these fundamental variables initiates maximum rates of 

 morphologic change, after which the rate of change of the bed form (or 

 alternatively the bottom) decreases unless or until flow conditions again 

 change. When the rate of change becomes negligible, the bed form (or 

 bottom) is said to be in equilibrium with the flow. Bed forms in 

 equilibrium can continue to migrate; the migration, if not balanced by 

 some other mechanism, changes the shape of the bottom. Therefore the 

 mechanism of migrating bed forms, e.g., ripples moving ashore, can 

 function as a method for adjusting a bottom that is not in equilibrium 

 with flow conditions (perhaps due to a change in lake level) . 



Linear shoals with wavelengths of hundreds of feet are common features 

 on the Atlantic Inner Continental Shelf from Long Island to southern 

 Florida (Duane, et al, 1972). These linear shoals are distinct from tidal 

 sand waves such as those described in the North Sea (Stride, 1972), 

 Georgia (Oertel and Howard, 1972), and Virginia (Ludwick, 1972). 



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