Net longshore transport rates along ocean beaches range from near zero to 

 765,000 cubic meters (1 million cubic yards) per year, but are typically 

 76,500 to 382,00 cubic meters (100,000 to 500,000 cubic yards) per year. Such 

 quantities, if removed from a 16- to 32-k.ilometer (10- to 20-mile) stretch of 

 beach year after year, would result in severe erosion problems. The fact that 

 many beaches have high rates of longshore transport without unusually severe 

 erosion suggests that an equilibrium condition exists on these beaches, in 

 which the material removed is balanced by the material supplied. 



II. LITTORAL MATERIALS 



Littoral materials are the solid materials (mainly sedimentary) in the 

 littoral zone on which the waves, wind, and currents act. 



1. Classification . 



The characteristics of the littoral materials are usually primary input to 

 any coastal engineering design. Median grain size is the most frequently used 

 descriptive characteristic. 



a. Size and Size Parameters . Littoral materials are classified by grain 

 size into clay, silt, sand, gravel, cobble, and boulder. Several size clas- 

 sifications exist, of which two, the Unified Soil Classification (based on the 

 Casagrande Classification) and the Wentworth classification, are most commonly 

 used in coastal engineering (see Fig. 4-7). The Unified Soil Classification 

 is the principal classification used by engineers. The Wentworth clas- 

 sification is the basis of a classification widely used by geologists, but is 

 becoming more widely used by engineers designing beach fills. 



For most shore protection design problems, typical littoral materials are 

 sands with sizes between 0.1 and 1.0 millimeters. According to the Wentworth 

 classification, sand size is in the range between 0.0625 and 2.0 millimeters; 

 according to the Unified Soil Classification, it is between 0.074 and 4.76 

 millimeters. Within these sand size ranges, engineers commonly distinguish 

 size classes by median grain size measured in millimeters. 



Samples of typical beach sediment usually have a few relatively large 

 particles covering a wide range of diameters and many small particles within a 

 small range of diameters. Thus, to distinguish one sample from another, it is 

 necessary to consider the small differences (in absolute magnitude) among the 

 finer sizes more than the same differences among the larger sizes. For this 

 reason, all sediment size classifications exaggerate absolute differences in 

 the finer sizes compared to absolute differences in the coarser sizes. 



As shown in Figure 4-7, limits of the size classes differ. The Unified 

 Soil Classification boundaries correspond to U.S. Standard Sieve sizes. The 

 Wentworth classification varies as powers of 2 millimeters; i.e., the size 

 classes have limits, in millimeters, determined by the relation 2^ , where 

 n is any positive or negative whole number, including zero. For example, the 

 limits on sand size in the Wentworth scale are 0.0625 and 2 millimeters, which 

 correspond to 2 and 2 millimeters. 



4-12 



