The distribution of grain sizes naturally present on a stable beach repre- 

 sents a state of dynamic equilibrium between the supply and the loss of mate- 

 rial of each size. Coarser particles generally have a lower supply rate and 

 a lower loss rate; fine particles are usually more abundant but are rapidly 

 moved alongshore and offshore. Where fill is to be placed on a natural beach 

 that has been relatively stable (i.e., exhibiting a steady rate of change or 

 dynamic stability, or only slowly receding) the size characteristics of the 

 native material can be used to evaluate the suitability of potential borrow 

 material. Borrow material with the same grain-size distribution as the native 

 material is most suitable for fill; material slightly coarser is usually 

 suitable. If such borrow material is available, the volume required for fill 

 may be determined directly from the project dimensions, assuming that only 

 insignificant amounts will be lost through sorting and selective transport and 

 that the sorting is not significantly different from the native material. In 

 cases where these conditions do not apply, an additional volume of fill may be 

 required as determined by an overfill factor. 



(1) Overfill Factor . Unfortunately it is often difficult to find 

 economical sources of borrow material with the desired grain-size distribu- 

 tion. When the potential borrow material is finer than the native material, 

 large losses of the beach-fill material often take place immediately following 

 placement. Currently, there is no proven method for computing the amount 

 of overfill required to satisfy project dimensions. Krumbein's (1957) study 

 provides a quantitative basis for comparison on the material characteristics 

 considered to have the greatest effect on this relationship. Subsequent work 

 by Krumbein and James (1965), James (1974), Dean (1974), and James (1975) 

 developed criteria to indicate probable behavior of the borrow material on the 

 beach. The use of the overfill criteria developed by James (1975) will give 

 the best results in the majority of cases. It should be stressed, however, 

 that these techniques have not been fully tested in the field and should be 

 used only as a general indication of possible beach-fill behavior. 



The procedures require that enough core samples be taken from the borrow 

 area to adequately describe the composite textural properties throughout the 

 entire volume of the borrow pit (see Hobson, 1977). Textural analyses of both 

 borrow and native beach samples can be obtained using either settling or siev- 

 ing grain-size analysis techniques. The composite grain-size distributions 

 are then used to evaluate borrow sediment suitability. 



Almost any offshore borrow source near the shore will include some 

 suitable size material. Since the source will control cost to a major degree, 

 an evaluation of the proportional volume of borrow material with the desired 

 characteristics is important in economic design. The overfill criteria 

 developed by James (1975), presented graphically in Figure 5-3, give a 

 solution for the overfill factor, R. , where 



R. = the estimated number of cubic meters of fill material required 

 to produce 1 cubic meter of beach material when the beach is 

 in a condition compatible with the native material, 



o, = the standard deviation and is a measure of sorting (see Ch. 4, 

 Sec. II) where 



5-10 



