to the local depth, significant quantities of sand are suspended over most of 

 the depth in the surf zone. 



Since breaking waves suspend the sediment, the amount suspended is partly 

 determined by breaker type. Data from Fairchild (1972, Fig. 5) show that 

 spilling breakers usually produce noticeably lower suspended sediment con- 

 centrations than do plunging breakers (see Fairchild (1972) and Watts (1953a) 

 for field data; Fairchild (1956, 1959) for lab data). Typical suspended 

 concentrations of fine sand range between 20 parts per million and 2 parts per 

 thousand by weight in the surf zone and are about the same near the ripple 

 crests in the offshore zone. 



Studies of suspended sediment concentrations in the surf zone by Watts 

 (1953a) and Fairchild (1972) indicate that sediment in suspension in the surf 

 zone may form a significant portion of the material in longshore transport. 

 However, present understanding of sediment suspension and the practical 

 difficulty of obtaining and processing sufficient suspended sediment samples 

 have limited this approach to predicting longshore transport. 



c. Profiles. Profiles are two-dimensional vertical sections showing how 



elevation varies with distance. Coastal profiles (Figs. 4-1 and 4-26) are 

 usually measured perpendicular to the shoreline and may be shelf profiles, 

 nearshore profiles, or beach profiles. Changes on nearshore and beach 

 profiles are interrelated and are highly important in the interpretation of 

 littoral processes. The measurement and analysis of combined beach and 

 nearshore profiles are a major part of most engineering studies of littoral 

 processes. 



(1) 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 4-26). 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 Service (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 relatively coarse 

 sediment, indicating a winnowing of fine sizes (Dietz, 1963; Milliman, 1972; 

 Duane, et al., 1972). Landward from this residual sediment, sediment often 

 becomes finer before grading into the relatively coarser beach sands. 



(2) Nearshore Profiles . The nearshore profile extends seaward from 

 the beach to depths of about 9 meters (30 feet). Prominent features of most 

 nearshore profiles are longshore bars (see middle profile of Figure 4-26 and 

 Section V,2). In combination with beach profiles, repetitive nearshore 

 profiles are used in coastal engineering to estimate erosion and accretion 

 along the shore, particularly the behavior of beach fill, groins, and other 

 coastal engineering structures. Data from nearshore profiles must be used 

 cautiously (see Sec. V, 1). Under favorable conditions nearshore profiles have 

 been used in measuring longshore transport rates (Caldv«ll, 1956). 



4-60 



