where (()„, is the sediment size in phi units; i.e., finer than 84 percent by 



v«ight, of the sample. If the sediment size in the sample actually has a 



lognormal distribution, then a, is the standard deviation of the sediment in 



9 

 phi units. For perfectly sorted sediment, a = . For typical well-sorted 



sediments, a, » 0.5 . 



9 



^ 



The degree by which the phi-size distribution departs from symmetry is 

 measured by the skewness (Inman, 1952) as 



M - M 

 ()) d^ 



a. = 



(j) a<|) 



(4-5) 



vAiere M, is the mean, M i ^ is the median, and o, the standard deviation in 



(j) a(j) (j) 



phi units. For a perfectly symmetric distribution, the mean equals the median 

 and the skewness is zero. 



Presently, median grain size is the most commonly reported size charac- 

 teristic, and there are only limited results available to demonstrate the 

 usefulness of other size distribution parameters in coastal engineering 

 design. Howaver, the standard deviation equation (4-4) is an important 

 consideration in beach-fill design (see Hobson, 1977; Ch. 5, Sec. 111,3). 



Extensive literature is available on the potential implications of a, , 



9 

 a , and other measures of the size distribution (Inman, 1957; Folk and Ward, 



1957; McCammon, 1962; Folk, 1965, 1966; Griffiths, 1967). For example, the 

 conditions under which nearshore sediment has been transported and deposited 

 might be inferred from consideration of size measures (e.g., Charlesworth, 

 1968). 



b. Composition and Other Properties . Littoral material varies in 

 composition, shape, and other properties. In considering littoral processes, 

 composition normally is not an important variable because the dominant 

 littoral material is quartz sand, which is mechanically durable and chemically 

 inert. However, littoral material may include carbonates (shell, coral, and 

 algal material), heavy and light minerals (Ch. 4, Sec. 11,2), organics (peat), 

 and clays and silts. Table 4-2 includes the specific gravities of common sand- 

 size littoral materials. 



The shape of littoral material ranges from nearly spherical to nearly 

 disklike (shells and shell fragments). Littoral sands are commonly rounded, 

 but usual departures from sphericity have appreciable effects on sediment 

 setting, sieve analyses, and motion initiation (Ch. 4, Sec. II,l,c; Sec. II, 

 7, a; and Sec. V,2,b). Sediment grain shapes have been applied to the inter- 

 pretation of nearshore processes (Bradley, 1958; Van Nieuwenhuise et al., 

 1978). 



Sediment color has been used to distinguish littoral from continental 

 shelf sands (Chapman, 1981). Tracing of sediment transport has utilized the 

 natural radioactivity of certain littoral materials (Kamel, 1962; Kamel and 

 Johnson, 1962). 



4-17 



