Thus, the immersed rate is equal to flux rate multiplied by a term 

 which is assumed to be constant. 



2. Purpose . 



There is some confusion concerning the relative validity of equations 

 (2) and (5) as predictors of longshore transport. Authoritative publica- 

 tions have urged the use of 1^ in equation (5) , and this has created 

 the impression that the SPM equation (2) is distinct from, and inferior 

 to, the immersed weight rate of computing longshore transport. 



This report shows that, based on present knowledge, there is at this 

 time (1978) no practical difference between equations (2) and (5) . How- 

 ever, an immersed weight prediction could be important if significant 

 variations in a' or y' s exist on the shore. This possibility suggests 

 that measurements of a' and Yg should be included in field programs 

 to measure longshore transport rate. 



II. UNIT WEIGHT OF SAND 



The applicability of an immersed weight transport prediction depends 

 on a knowledge of the unit weight of sand. The discussion in this sec- 

 tion concerns the unit weight of dry sand, but it is easily extended to 

 cover sand immersed in seawater. 



The unit weight of sand, y , is dependent on two variables. The 

 first variable is the specific gravity of the sand grains, given by 



G = WYw (6) 



where y s is the weight density of the material making up the sand grain, 

 and y w is the weight density of distilled water. Most sand grains are 

 quartz with a specific gravity of 2.65. However, on some beaches sand 

 grains may be composed of calcium carbonate with a specific gravity, when 

 a pure solid, from 2 to 11 percent higher than quartz (G of 2.71 for pure 

 calcite to 2.94 for pure aragonite) . (Carbonate sands may also be effec- 

 tively lighter than quartz when grains are made of porous shell material.) 



The second variable is the amount of space taken up by voids in the 

 sand deposit. This can be described by several terms. The usual soil 

 mechanics parameter is the void ratio, e, defined as 



Volume of voids/volume of solids . (7) 



