This technique assumes that both composite native and borrow material 

 distributions are nearly lognormal. This assumption is correct for the 

 composite grain-size distribution of most natural beaches and many borrow 

 materials. Pronounced bimodality or skewness might be encountered in poten- 

 tial borrow sources that contain multiple layers of coarse and fine material, 

 such as clay-sand depositional sequences, or in borrow zones that crosscut 

 flood plain deposits associated with ancient river channels. 



The four possible combinations that result from a comparison of the 

 composite grain-size distribution of native material and borrow material are 

 listed in Table 5-1 and indicated as quadrants in Figure 5-3. 



The engineering application of the techniques discussed above requires 

 that basic sediment-size data be collected in both the potential borrow area 

 and the native beach area. An estimation of the composite grain-size charac- 

 teristics of native material should follow the guidelines in Hobson (1977). 

 The determination of the composite distribution of the borrow zone material 

 depends on the variation of materials and their individual properties. If the 

 textural properties of the potential borrow material exhibit considerable 

 variation in both area and depth, extensive coring may be required to obtain 

 reliable estimates of the composite distribution of properties. Since 

 detailed guidelines have not been established for evaluating borrow deposits, 

 it is recommended that core sampling be carried out as a two-phase program — 

 the first phase inventories the general borrow region and the second phase 

 samples in detail those areas with the greatest potential. 



(2) Renourishment Factor . James (1975) provides a second approach to 

 the planning and design of nourishment projects. This approach, which relates 

 to the long-term maintenance of a project, asks the basic question of how 

 often renourishment will be required if a particular borrow source is selected 

 that is texturally different from the native beach sand. With this approach, 

 different sediment sizes will have different residence times within the 

 dynamic beach system. Coarse particles will generally pass more slowly 

 through the system than finer sizes. This approach also requires accurate 

 composites of native and borrow sediment textures. 



To determine periodic renourishment requirements, James (1975) defines a 

 renourishment factor, Rj , which is the ratio of the rate at which borrow 

 material will erode to the rate at which natural beach material is eroding. 

 The renourishment factor is given as 



^J = ^ 



(|)b ^ 



(jjn 



2 



'<j)b 



(j)n 



(5-3) 



where A is a winnowing function. The A parameter is dimensionless and 

 represents the scaled difference between the phi means of noneroding and 

 actively eroding native beach sediments. James (1975) estimates values of A 

 ranging between 0.5 and 1.5 for a few cases where appropriate textural data 

 were available and recommends A = 1 for the common situation where the tex- 

 tural properties of noneroding native sediments are unknown. Equation (5-3) 

 is plotted in Figure 5-4 for A = 1 . Figure 5-3 should be used for 



5-12 



