Swift, et al. (1971) and Duane, et al . (1972) hypothesized that shoals 

 originate along the shoreface and as the shoreline retreats landward in 

 adjustment to rising sea level, the shoals eventually become segmented 

 and isolated on the inner shelf. Evidence gathered in this study sup- 

 ports that interpretation. 



Because of their shallow depths and proximity to the littoral zone, 

 shoals on the shoreface are the most active. They are more vulnerable 

 to wave and current action, and a source of sediment is readily avail- 

 able from the eroding shoreface. Their internal configuration of clean, 

 medium-grained sand unconformably overlying relict coastal deposits is 

 similar to that of the isolated linear shoals of the inner shelf. Orien- 

 tation with respect to the shoreline is similar for both shoreface shoals 

 and isolated shoals; side slopes and surface configuration are also simi- 

 lar for both shoal types . 



The mechanism causing initial formation of shoals is unknown, but 

 probably some irregularity along the shoreline (ebb tidal delta, ancestral 

 stream delta) forms the nucleus for further accretion by impounding of 

 nearshore sediments. There does appear to be some association of shore- 

 face shoals in the Delmarva coast with existing or historical inlets (Fig. 

 18) which may imply that tidal currents play a role in the initial devel- 

 opment of a ridge. The shoreface of Ocean City and Assateague Island has 

 many perturbations which may be incipient shoreface shoals, but such fea- 

 tures are difficult to document. 



Regardless of how they are first initiated, the process by which 

 shoreface shoals subsequently evolve remains to be completely understood. 

 Duane, et al. (1972) proposed that storm-generated coastal currents were 

 the significant process responsible for the growth and development of 

 shoreface shoals. According to them, "Wind-drift currents tend to develop 

 speeds of one-twentieth to one-fiftieth that of surface and wind speeds, 

 and a northeaster blowing at 40 knots per hour could generate a surface 

 current of 1 or 2 knots (1.7 to 3.4 feet per second) which, during several 

 days of downward momentum transfer, might extend some part of that veloc- 

 ity to the inner shelf floor." 



Detailed studies of bathymetric changes, current measurements, and 

 textural patterns on shoreface shoals (Moody, 1964; Swift, et al . , 1971) 

 suggest the effect of storm-generated, wave-drift currents on shoreface 

 shoals is significant. Moody, for example, noted that large-scale migra- 

 tion of ridges off Bethany Beach, Delaware, occurred during the major 

 storm of March 1962. The inferred transport along the bottom was at 

 right angles to the direction of surface, wave-driven currents. Because 

 of the Ekman effect, the subsurface flow is thought to be helical in 

 nature, but this is an unproven assumption. The presumed helical flow 

 in ridge troughs is thought to be generated by wind stress on a moving 

 viscous medium that produces Langmuir-type circulation. As the cross- 

 sectional area decreases toward the trough head, the competence of the 

 bottom currents increases, thereby causing headward erosion of the trough 



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