The two straigh-t-line normal distributions (Fig. 10) are identified with 

 sediments deposited during storm (A) and nonstorm (B) conditions. Mixing these 

 populations in the ratios of 9A: 91B and 30A:70B provides very close approxi- 

 mations to the observed cored trap and cored beach composites, respectively. 

 The ratios themselves reflect the graphically determined inflection points of 

 the cored composites which is a somewhat arbitrary determination method. Never- 

 theless, repeating the technique using these composites, as well as the beach 

 and trap grab composites, range composites, and transect grain-size composites 

 still results in the identification of two normal sediment populations with 

 mean and sorting values quite close to those shown as storm and nonstorm in 

 Figure 10. The conclusions drawn here are that (a) the coarse component A 

 (storm?) makes up about three times as much of the beach sediments as it does 

 the cored trap sediments, and that (b) high energy transport conditions, 

 intense enough to transport coarse population A, were required for only about 

 10 percent of the trap fill. These results also support the earlier conclu- 

 sion that coring is probably not the best method for assessing native beach 

 sediment texture because the data collected are easily biased by the storm 

 history of the beach. 



VI. BEACH-FILL CONSIDERATIONS 



1. Beach-Fill Models . 



The Channel Islands sand trap is used to interrupt longshore transport in 

 order to maintain the entrance channel to the harbor, and the accumulated sands 

 are periodically bypassed downcoast as beach nourishment. The data presented 

 in Table 5 can be used as one method to assess the impact on beach-fill design 

 by the textural modifications caused by the trap structure and trap-filling 

 process. In this case, the native beach is the updrift beach rather than the 

 downdrift beach that normally would receive the bypassed sand. The downcoast 

 beach is similar in many ways but it has been nourished frequently and adequate 

 textural data for describing its "native sediments" were unavailable for this 

 analysis. 



The effect of the coarse, cored native composite is immediately evident in 

 Table 5. This cored sand was both coarser and more poorly sorted than the 

 borrow and this combination resulted in a predicted overfill requirement of 150 

 percent. An even higher overfill estimate would result if the cored native 

 were compared with the even finer grab-sampled fill. Overfill requirements of 

 10 and 5 percent are estimated when the surface-sampled native beach sands are 

 compared to grab and core trap fill, respectively. These estimates are more in 

 keeping with known processes of trap filling during which the offshore break- 

 water reduced transport energies in the trap, resulting in a fill that is 

 slightly finer than the native beach sediments. Again, Table 5 demonstrates 

 the possible pitfalls or bias produced by core sampling beach sediments. 



2. Dredging and Sediment Bypassing . 



The maps of trap-fill sediment texture (Figs. 5 and 6) provide a way to 

 evaluate different dredge and bypass operational plans. In a sense, the trapped 

 sediments can be likened to an ore body which is periodically mined (dredged) 

 and whose richness, as measured by texture, varies within the trap. These 

 variations in texture in the Channel Islands trap are very much like those for 

 the native beach and are oriented essentially shore-normal with sediments 



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