The treatment of suspended-load transport collinear with waves has 

 received Increased Investigation (Nakato et al., 1977; MacDonald, 1977; 

 Nielsen, 1979) . This research has established Important temporal and spatial 

 gradients of suspended-sediment concentration In relatively simple oscillatory 

 flows. Prediction of suspended-load transport requires several empirically 

 determined coefficients, which at present cannot be simply related to wave and 

 sediment characteristics. A further difficulty Is that field data have shown 

 that breaker type controls suspended-sediment concentration In the surf zone 

 (Kama, 1979), but this effect has not been thoroughly Investigated under 

 controlled conditions. 



Despite the lack of recommendable prediction procedures for transport 

 rates, useful guidance can be provided concerning aspects of onshore-offshore 

 transport Important In coastal engineering. 



a. Sediment Effects. Properties of Individual particles Important In 

 sediment transport Include size, shape, and composition. Collections of 

 particles have the additional properties of size distribution, permeability, 

 and porosity. These properties Influence the fluid forces necessary to 

 initiate and maintain sediment movement. For usual nearshore sediment, size 

 is the only particle property which varies greatly. Grain size changes 

 sediment motion conditions, sediment fall velocity, and hydraulic roughness of 

 the grain bed. The hydraulic roughness affects flow energy dissipation, which 

 also results directly from bed permeability ( Bretschneider and Reid, 1954; 

 Bretschneider, 1954) . Bed permeability, depending on sediment size and 

 sorting, can cause a net onshore sand transport from far offshore (Lofquist, 

 1975) and Influences wave runup at the shoreline (see Ch. 7; Savage, 1958). 

 Sediment size clearly figures in beach swash processes (Everts, 1973; 

 Sallenger, 1981). Thus, grain size figures in a variety of processes from the 

 landward to the seaward limit of hydrodynamic sediment transport. 



Some data indicate that differential transport according to sediment size 

 occurs near the shore. A gross Indication of a size effect is the appearance 

 of coarse sediment in zones of maximum vave energy dissipation and the depo- 

 sition of fine sediment in areas sheltered from wave action (e.g.. King, 1972, 

 pp. 302, 307, 426). Regular variation in sediment size is common over ripples 

 (Inman, 1957) and large longshore bars (Saylor and Hands, 1970). Regular 

 sediment-size variations on a more extensive scale have been documented across 

 some nearshore profiles (e.g., Duane, 1970a; Swift, 1976). Figure 4-28 

 displays surface sediment sizes from three transects of a historically eroding 

 coast, with well-sorted sand becoming progressively finer seaward to a water 

 depth of about 10 meters, there abutting coarser, less well-sorted sand. 



This common seaward-fining of active nearshore sands demonstrates a 

 sedlment-slze effect in onshore-offshore transport, but the process respon- 

 sible for this is a controversial subject. The effect appears consistent with 

 the "neutral line" concept (Cornaglla, 1889), which Incorporates qualitative 

 consideration of bedload sediment movements in terms of wave energy, bottom 

 slope, and sediment characteristics; hovever, recent discussions of 

 Cornaglla' s concept emphasize its limitations and those of further laboratory- 

 based quantltlve developments (Zenkovich, 1967b, Sec. 9; Komar, 1976, Ch. 11; 

 Bowen, 1980; Hallermeier, 1981b). The seaward fining of nearshore sands has 

 also been ascribed to suspended-load transport by rip currents (Swift, 1976). 



4-66 



