Discussion

Inner Shelf Dunes: The location and orientation of crescentic, barchan-like dunes on the inner shelf suggests they have formed in response to localized sediment surpluses on this sediment-starved shelf sector. Local sediment source areas consist primarily of the hardbottoms. However, additional sources probably include outcropping Quaternary sediments such as those found on the shore-oblique ridges (see Figure 1), or possibly even beach nourishment material from the upper shoreface.

The bedforms appear to be oriented transverse to the probable mean direction of storm-induced inner shelf flows (to the south and obliquely offshore). Combined stresses due to waves and currents should also play a role in controlling the transport in this setting. It is likely that dunes are mobilized during maximum storm events like hurricanes for which we have no direct measurements. The mobility of these bedforms poses an interesting question. Based on velocity measurements on the northern California shelf, Cacchione and others (1994) computed bed stresses and potential storm-induced bedload transport for broad, crescentic dunes of similar dimensions to those off Wrightsville Beach. Their findings suggest that it would take over three years, given the local California storm climate, for the bedforms to migrate one wavelength. They concluded that this furnished an explanation for the observed stability of the features as recorded by multiple sidescan-sonar observations over a four-year period.