5,332 Selection of Borrow Material . After the characteristics of the 

 native sand and the longshore-transport processes in the area are deter- 

 mined, the next step is to select borrow material for beach fill and for 

 periodic nourishraent„ When sand is mechanically deposited on the beach, 

 waves imiriediately start a sorting and winnowing action on the surface 

 layer of the fill, moving finer particles seaward, and leaving coarser 

 material at or shoreward of the plunge point. This sorting continues 

 until a layer of coarser particles compatible with the wave climate 

 armors the beach and makes the slope temporarily stable for normal wave 

 conditions. However, if this armor is disturbed by a storm, the under- 

 lying material is again subjected to the sorting process. 



Because of these processes, beach fill with organic material or 

 large amounts of the finer sand fractions may be used since natural pro- 

 cesses will clean the fill material. This has been confirmed with fills 

 containing foreign matter at Anaheim Bay, California, and Palm Beach, 

 Florida. Material finer than that exposed on the natural beach face, if 

 exposed on the surface during a storm, will move to a depth compatible 

 with its size to form nearshore slopes flatter than normal slopes before 

 placement. Fill coarser than the sand on the natural beach will remain 

 on the foreshore, and may be expected to produce a steeper beach. The 

 relationship between grain size and slope is discussed in Section 4.526. 

 If borrow sand is very coarse, it will probably be stable under normal 

 conditions, but it may make the beach less desirable for recreation. If 

 the borrow material is much finer than the native beach material, a large 

 amount will move offshore and be lost. 



The distribution of grain sizes naturally present on a stable beach 

 represents a state of dynamic equilibrium between the supply and loss of 

 material 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. I'/here fill is to be placed on 

 a natural beach that has been stable or only slowly receding, the size 

 characteristics of the native material can be used to evaluate the suit- 

 ability of potential borrow material. A borrow material with the same 

 grain size distribution as the native material, or one slightly coarser, 

 will usually be suitable for fill. If such borrow material is available, 

 the volume required for fill can be determined directly from the project 

 dimensions, assuming that only insignificant amounts will be lost through 

 sorting and selective transport. 



Unfortunately it is often difficult to find economic sources of 

 borrow material with the desired properties. When the potential borrow 

 material is finer than the native material, large losses of the borrow 

 material often take place immediately following its emplacement. Cur- 

 rently there is no proven method for computing the amount of overfill 

 required to satisfy project dimensions once the fill material has under- 

 gone this initial sorting action and attained a stable configuration. 

 Studies by Krumbein (1957) provide a quantitative basis for comparison 

 on the material characteristics considered to have the greatest effect 

 on this relationship. His subsequent work with James (Krumbein 5 James, 



5-9 



