58 



transport paths, but the flood-tidal delta materials remain virtually undisturbed 

 in the lee of the restored barrier. 



Many barrier inlets are important navigation channels, affording access to 

 back-barrier lagoon complexes from the open sea. For this reason, it has 

 become common practice to stabilize inlets and thus prevent migration and 

 reduce shoaling of main channels with sediment. The chief means of stabili- 

 zation has been the building of jetties and structural stabilization of the banks 

 of the inlet throat. Since the jetties partly or wholly block alongshore 

 transport, they may effectively trap some sediments that would formerly have 

 migrated across the ebb-tidal shoals and reached the downdrift shore. Thus, 

 jetties may create serious sand starvation in downdrift areas. 



In recent years, various methods have been adopted to artificially bypass 

 sand in inlet areas to prevent its loss into the inlet-associated shoals or its 

 accretion on the updrift side of jetties. All require a detailed knowledge of 

 local sediment transport and rates and pathways to materially affect inlet navi- 

 gation, yet emulate the natural sediment transport processes. 



Shelf Shoals 



Large ridge-like shoals with a relief of up to 10 m or more that extend for 

 tens of kilometers are common features of the continental shelf. They are 

 especially well-developed and numerous in the Middle Atlantic Bight region, 

 where extensive shoal fields occupy much of the shelf area. Similar shoals 

 have been described on the shelf off Argentina and in the North Sea off 

 Germany (Swift et al. 1978). Other shoals occur on the Mississippi River 

 delta plain (Penland et al. 1989). 



Shoals have attracted considerable attention because of questions regarding 

 their origin and development and because most that have been investigated 

 contain large amounts of clean fine- to coarse-grained sand and gravel poten- 

 tially useful for beach fill or construction aggregate (Anders and Hansen 

 1990). Only a small number have been investigated in any detail as yet, using 

 methods such as seismic reflection, profiling, and coring (Duane et al. 1972; 

 Ludwick 1975; Coleman, Berquist, and Hobbs 1988; Meisburger and Duane 

 1971; Meisburger and Field 1975; Meisburger and Williams 1980; 1982). 

 Much remains to be learned about the origin, nature, sediment characteristics, 

 and variability of shelf shoals. 



Linear shoals on the Atlantic Shelf exhibit at least 3 m of relief between 

 the crest and the surrounding surface. Two types are recognized: shoreface- 

 connected and isolated (Duane et al. 1972) (Figure 22). Shoreface-connected 

 shoals show a seaward excursion of the 10-m depth contour, and isolated 

 shoals occur farther seaward on the shelf floor. Isolated linear shoals are 

 thought to have originated as shoreface-connected shoals during the Holocene 



Chapter 3 Variable Coastal Feature's 



