For coastal engineering, the accuracy of shelf profiles is usually 

 less critical than the accuracy of beach and nearshore profiles. Gener- 

 ally, observed depth changes between successive surveys of the shelf do 

 not exceed the error inherent in the measurement. However, soundings 

 separated by decades suggest that the linear shoals superposed on the 

 profile do show small but real shifts in position. (Moody, 1964, p. 143.) 

 Charts giving depths on the continental shelves may include soundings 

 that differ by decades in date. 



Plotted profiles usually use vertical exaggeration or distorted 

 scales to bring out characteristic features. This exaggeration may lead 

 to a false impression of the actual slopes. As plotted, the three pro- 

 files in Figure 4-18 have roughly the same shape, but this sameness has 

 been obtained by vertical exaggerations of 2x, lOx, and 50x. 



Sand level changes in the beach and nearshore zone may be measured 

 quite accurately from pipes imbedded in the sand. (Inman and Rusnak, 1956; 

 Urban and Galvin, 1969; and Gonzales, 1970.) 



4.52 ONSHORE -OFF SHORE TRANSPORT 



4,521 Sediment Effects . Properties of individual particles which have 

 been considered important in littoral transport include: size, shape, 

 immersed specific gravity, and durability. Collections of particles 

 have the additional properties of size distribution, permeability, and 

 porosity. These properties determine the forces necessary to initiate 

 and maintain sediment motion. 



For typical beach sediment, size is the only property that varies 

 greatly. However, quantitative evaluation of the size effect is usually 

 lacking. A gross indication of a size effect is the accumulation of 

 coarse sediment in zones of maximum wave energy dissipation, and deposi- 

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

 1972, pp. 302, 307, 426.) Sorting by size is common over ripples (Inman, 

 1957) and large longshore bars (Saylor and Hands, 1970). Field work on 

 size effects in littoral transport does not permit definite conclusions. 

 (King, 1972, p. 483; Inman, Komar, and Bowen, 1969; Castanho, 1970; Ingle, 

 1966, Figure 112; Yasso, 1962; and Zenkovich, 1967a.) 



The shape of most littoral materials is approximately spherical; 

 departures from spherical are usually too slight to affect littoral 

 transport. 



Immersed specific gravity (specific gravity of sediment minus spec- 

 ific gravity of fluid) is theoretically an important physical property of 

 the sediment particle. (Bagnold, 1963.) However, the variation in immersed 

 specific gravity for typical littoral materials in water is small since 

 most beach sediments are quartz (immersed specific gravity = 1.65), and 

 most of the remainder are calcium carbonate (immersed specific gravity 

 = 1.9). Thus, little variation in littoral transport is expected from 

 variation in immersed specific gravity. 



4-60 



