a. Shelf Profiles . The shelf profile is typically a smooth, concave- 

 up curve showing depth to increase seaward at a rate that decreases with 

 distance from shore. (bottom profile in Figure 4-18.) The smoothness of 

 the profile may be interrupted by other superposed geomorphic features, 

 such as linear shoals. (Duane, et al., 1972.) Data for shelf profiles 

 are usually obtained from charts of the National Ocean Survey (formerly, 

 U.S. Coast and Geodetic Survey). 



The measurable influence of the shelf profile on littoral processes 

 is largely its effect on waves. To an unknown degree, the shelf may also 

 serve as a source or sink for beach sand. Geologic studies show that 

 much of the outer edge of a typical shelf profile is underlain by rela- 

 tively coarse sediment, indicating a winnowing of fine sizes. (Dietz, 

 1963; Milliman, 1972; and Duane, et al . , 1972.) Landward from this resi- 

 dual sediment, sediment often becomes finer before grading into the rela- 

 tively coarser beach sands. 



b. Nearshore Profiles . The nearshore profile extends seaward from 

 the beach to depths of about 30 feet. Prominent features of most near- 

 shore profiles are longshore bars; see middle profile of Figure 4-18 and 

 Section 4.525. In combination with beach profiles, repetitive nearshore 

 profiles are used in coastal engineering to estimate erosion and accre- 

 tion along the shore, particularly the behavior of beach fill, groins, 

 and other coastal engineering structures. Data from nearshore profiles 

 must be used cautiously. (see Section 4.514.) Under favorable condi- 

 tions nearshore profiles have been used in measuring longshore transport 

 rates. (Caldwell, 1956.) 



c. Beach Profiles . Beach profiles extend from the foredunes, cliffs, 

 or mainland out to mean low water. Terminology applicable to features of 

 the beach profile is in Appendix A (especially Figures A-1 and A-2). The 

 backshore extends seaward to the foreshore, and consists of one or more 

 berms at elevations above the reach of all but storm waves. Berm sur- 

 faces are nearly flat and often slope landward at a slight downward angle. 

 (See Figure 4-1.) Berms are often bounded on the seaward side by a break 

 in slope known as the berm crest. 



The foreshore is that part of the beach extending from the highest 

 elevation reached by waves at normal high tide seaward to the ordinary 

 low water line. The foreshore is usually the steepest part of the beach 

 profile. The boundary between the backshore and the foreshore may be 

 the crest of the most seaward berm, if a berm is well developed. The 

 seaward edge of the foreshore is often marked by an abrupt step at low 

 tide level. 



Seaward from the foreshore, there is usually a low-tide terrace which 

 is a nearly horizontal surface at about mean low tide level. (Shepard, 

 1950; and Hayes, 1971.) The low-tide terrace is commonly covered with sand 

 ripples and other minor bed forms, and may contain a large bar-and-trough 

 system, which is a landward-migrating sandbar (generally parallel to the 

 shore) common in the nearshore following storms. Seaward from the low-tide 



4-56 



