the same size as the native profile sands from core 25. The inshore relation- 

 ships are also similar with core 5 containing alternating coarse and fine 

 layers and graded layers which again reflect both storm and nonstorm deposi- 

 tional influences. The main differences between the shallow-water cores is 

 that the trapped sediments are generally finer grained than those cored from 

 the native beach (core 23) which, for these particular data, suggests that the 

 trap fill contains less storm-deposited material than found in the native 

 profile envelope. 



One method for estimating the importance of storms for filling the sand 

 trap is to identify storm and nonstorm grain-size distributions and to deter- 

 mine the contribution of each in making up the cored beach and trap-fill 

 sediments. This can be done following the polymodal interpretive methods 

 proposed by Sinclair (1973) and Ashley (1978), who assume that nonnormal grain- 

 size distributions can result from the mixing of normally distributed grain- 

 size populations which are themselves associated with specific depositional 

 environments. A graphical procedure has been developed for identifying compo- 

 nent normal distributions from the analysis of nonnormal cumulative grain-size 

 plots and for determining the proportions of the normal components needed to 

 accurately reproduce the distributions of interest (see Ashley, 1978, for a 

 complete discussion of this interpretive approach). Figure 10 shows the results 

 of this technique applied to the cored Channel Islands composites. 



-e- 



<L 





1 



1 1 1 1 1 1 



1 1 





s* _ 









o 



Cored Trap 



2.37 



0.93 



1 



- 





+ 



Cored Beach 



1.83 



1.20 - 









A 



Storm 



0.30 



0.45 





- Ck.^ 



+^^^^^^^^ 

















^^^^^ B 



Nonstorm 



2.40 



0.86 







1 



- 



s 







.4^ 



- 





- 





1 \X 

 1 \;^ 

 1 ^ 



X 







^^ 



2 









<n + 











- 





/9IB 

 70 B 





V 





- 



3 







< \ 





^^^"\^ 





— 





- 





1 O 

 1 rD 





^^^ 



^^.^^ 



- 



4 





1 



Tl . 



1 1 t 



r 1 1 



X 



"^""■"^+0 - 



50 90 



Weight Coarser ( pet ) 



99 



Figure 10. Polymodal cumulative probability curve for cored trap 

 and beach sediments shown with partitioned lognormal 

 subpopulations (A and B) and inflection points (arrow) 

 showing position of subpopulation overlap (after Ashley. 

 1978). 



30 



