sediment in phi units. For perfectly sorted sediment, 0^ = 0. For 

 typical well-sorted sediments, o, =» 0.5. 



The degree by which the phi size distribution departs from symmetry 

 is measured by the skewness (Inman, 1952), as 



^<t> " ^d4> (4-5) 



* a<l> 



where M^ is the mean, I^cf) is the median, and a^ is the standard 

 deviation in phi units. For a perfectly symmetric distribution, the mean 

 equals the median, and the skewness is zero. 



Extensive literature is available on the definition, use, and impli- 

 cation of a, a, and other measures of the size distribution. (Inman, 

 1952; Folk and Ward, 1957; McCammon, 1962; Folk, 1965, 1966; and 

 Griffiths, 1967), but despite a long history of investigation and a 

 considerable background of data, applications of size distribution infor- 

 mation "are still largely empirical, qualitative, and open to alternative 

 interpretations." (Griffiths, 1967, p. 104.) 



Currently, median grain size is the most commonly reported sand size 

 characteristic, probably because there are only limited data to show the 

 usefulness of other size distribution parameters in coastal engineering 

 design. However, the standard deviation (Equation 4-4) must also be 

 given as a parameter for use in beach fill design. (See Section 5,3; 

 Krumbein and James, 1965; Vallianos, 1970; Berg and Duane, 1968.) 



4.212 Composition . In addition to classification by size, littoral 

 material may be classified by composition. For shore protection pur- 

 poses, composition normally is not an important factor, since the domi- 

 nant littoral materials are quartz sands which are durable and chemically 

 inert for periods longer than typical project lifetimes. However, sedi- 

 ment composition is useful when the material departs from this expected 

 condition. Other than quartz, littoral material may be composed of car- 

 bonates (usually shell, coral, and algal material), organics (most often 

 peat), and clays and silts (marsh and tidal flat deposits). 



4.213 Other Characteristics . In addition to size and composition, sedi- 

 ments have a number of other properties by which they may be classified. 

 Table 4-2 lists some density-related properties. Radioactive properties 

 of naturally occurring thorium minerals have been used as tracers in beach 

 sands. (Kamel, 1962; Kamel and Johnson, 1962, p. 324.) Other properties 

 more directly related to soil mechanics studies are found in soil mechanics 

 manuals. 



4.22 SAND AND GRAVEL 



By definition the word sand refers to a size class of material, but 

 sand also implies the particular composition, usually quartz (silica). 



4-18 



