Durability (resistance to abrasion, crushing, and solution) is usu- 

 ally not a factor within the lifetime of an engineering project. (Kuenen, 

 1956; Rusnak, Stockman, and Hofmann, 1966; and Thiel, 1940.) Possible 

 exceptions may include basaltic sands on Hawaiian beaches (Moberly, 1968), 

 some fragile carbonate sands which may be crushed to finer sizes when sub- 

 ject to traffic, (Duane and Meisburger, 1969, p. 44), and carbonate sands 

 which may be soluble imder some conditions. (Bricker, 1971.) In general, 

 recent information lends further support to the conclusion of Mason (1942) 

 that, "On sandy beaches the loss of material ascribable to abrasion . . . 

 occurs at rates so low as to be of no practical importance in shore pro- 

 tection problems." 



Size distribution and its relation to sediment sorting may be impor- 

 tant for design of beach fills. (See Sections 5.3 and 6.3.) Permeability 

 and porosity affect energy dissipation (Bretschneider and Reid, 1954; Bret- 

 schneider, 1954) and wave runup. (See Section 7.21; and Savage, 1958.) 



Sediment properties are physically most important in determining fall 

 velocity and the hydraulic roughness of the sediment boundary. Fall velo- 

 city effects are important in onshore-offshore transport. Hydraulic rough- 

 ness effects have been insufficiently studied, but they appear to affect 

 initiation of sediment transport and energy dissipation. This may be par- 

 ticularly true for flow of the swash on the plane surface of the foreshore. 

 (Everts, 1972.) 



4.522 Initiation of Sediment Motion . Considerable hydraulic and coastal 

 engineering research has been devoted to the initiation of sediment motion 

 under moving water. From this research has come general agreement (Graf, 

 1971, Chapter 6; Hjulstrom, 1939; and Everts, 1972) that the initiation of 

 motion on a level bed of fine or medium sand requires less shear (lower 

 velocities) than the initiation of motion on a level bed of silt or gravel; 

 (Figure 4-7 for size classes). It is also generally agreed that critical 

 entraining velocities for sand are usually less than 1 foot per second. 



Velocities of wave-induced water motion in the offshore zone can be 

 estimated fairly well from the equation of small -amplitude theory. (See 

 Chapter 2.) This theory leads to Equation 2-13 which can be transformed 

 into a dimensionless expression for velocity at the sand surface (z = -d) 



max ( j-> TT , ^ 



y '^> , (4-19) 



H 



sinh 2^<y^ 



which is plotted in dimensionless form in Figure 4-20 and for common values 

 of wave period in Figure 4-21. Figures 4-20 and 4-21 give maximum bottom 

 particle velocity, ^imax{-d) > ^^ ^ function of depth, wave period, and 

 local wave height. This prediction by linear theory for the offshore zone, 

 and the related results from solitary-wave theory for the zone near break- 

 ing, agree fairly well with measurement in the field. (Inman and Nasu, 

 1956; and Cook and Gorsline, 1972, Figures 5 and 6.). 



4-61 



