2. Sediment Textures . 



a. Composites . Composites are single representative grain-size distribu- 

 tions for a particular sediment population and are obtained by averaging the 

 grain-size distributions of a series of samples collected to represent each 

 population (Hobson, 1977). At Channel Islands, sampling was accomplished both 

 by coring and grab methods for the native beach and sand trap sediment popu- 

 lations. Figure 7(A) shows size frequency plots for these four composites 

 along with their phi mean and sorting values. 



In general, sands found within the trap are slightly finer and slightly 

 better sorted than native beach sands, as would be expected, since energy con- 

 ditions within the sheltered trap area should be less than on the exposed native 

 coastline. The unexpected relationships found on this plot are (1) the simi- 

 larity of both trap composites to the grab-sampled native composite and (2) the 

 great dissimilarity between these three distributions and the cored native 

 beach composite. 



(1) Similarity . Both composite distributions for trapped sediments 

 are essentially the same suggesting that either sampling technique is adequate 

 for describing these sediments and that their texture was accurately described. 

 Also, the similarity of fill to the grab-sampled native composite would suggest 

 that trap-filling processes (waves and currents) were sufficient to transport 

 most native beach sediment sizes into the sheltered trap basin. In other words, 

 the similarity of these three composites supports the idea that the Channel 

 Islands structures do essentially trap all longshore transport; thus, the sedi- 

 ment volumes trapped, if accurately surveyed, can be used directly to evaluate 

 various longshore transport models. 



(2) Dissimilarity . The cored native beach composite is quite dis- 

 similar to the others in Figure 7(A). These sands are coarser by about 0.1 

 millimeter and are much more poorly sorted. Also, the distribution is bimodal, 

 possibly trimodal, with a fine mode at about 2.5 phi, about the same as the 

 mean values for the other three composites, and coarse modes at about 1.0 phi 

 and possibly near about 0.0 phi as well. The presence of these coarse sizes 

 creates several interpretive paradoxes including: (a) Can the profile 914 

 data actually be considered representative of native beach conditions? (b) 

 Are these coarse, cored sediments being deposited in the trap or is the beach 

 actually "coarsening" and perhaps enlarging updrift of the trap? (c) Is 

 coring the "active profile envelope" a useful approach generally for character- 

 izing native beach sediments? 



b. Native Profile . Profile 914 lies about 400 meters upcoast from the 

 northern end of the offshore breakwater, and waves and currents occurring at 

 this location are assumed to be unaffected by the structure except possibly 

 during rare periods of current reversal. This assumption is based on diffrac- 

 tion calculations of waves passing a single breakwater (U.S. Army, Corps of 

 Engineers, Coastal Engineering Research Center, 1977). For the most part, 

 currents are driven steadily southward by waves from the west with intensities 

 consistent for this coast. The rare current reversals are caused by waves from 

 the southwest (Fig. 1, 215°) but at profile 914, the breakwater-refracted 

 waves still produce a weak, southward longshore current capable of moving some 

 sand into the trap (Bruno, et al., 1981). Although these periods of "reversals" 

 might cause winnowing of finer sands from the sediments at profile 914, the 



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