amount of sand involved is negligible in terms of the total sediment volume 

 transported; thus, winnowing is considered an inadequate explanation to account 

 for the coarse texture of the cored native beach sediments. 



c. Coarse-Grained Trap Fill . Similarities of all but the cored native 

 composites in Figure 7(A) suggest that all native sand sizes are being carried 

 into the trap. Figure 7(B) confirms this interpretation. This figure shows 

 grain-size composites for sediments cored along the four shore-parallel tran- 

 sects within the trap (solid lines) compared with the cored native profile 

 composite (dashline) . Each transect composite is unimodal, the average grain 

 size generally decreases toward seaward transects, and the total range of sizes 

 for these composites encompasses those cored from the beach. Although not 

 shown, composite grain-size distributions for each core location along a spe- 

 cific transect are similar, indicating that although the intensity of sedimen- 

 tation processes decreased offshore, the intensities alongshore were essentially 

 the same from the updrift to downdrift end of the trap (Figs. 5 and 6). 



From these relationships it can be inferred that sediment transport proc- 

 esses were at least periodically able to move the coarsest native beach sedi- 

 ments into the trap, but that the amount of coarse fill is much less than the 

 amount core-sampled at profile 914. If the cored native composite is an 

 accurate sample of native beach sediments, then perhaps selective sorting is 

 occurring and only finer sands are reaching the trap. If so, a coarsening of 

 native sediment through time might be expected at profile 914 or perhaps, a 

 buildup through time of coarse (untransportable?) sand should be observed close 

 to the mouth of the trap. 



Textural composites of sediments collected along profile 914 after each 

 study phase (not shown) are almost identical with phi means ranging between 

 2.24 and 2.36 phi (0.21 and 0.19 millimeter) and sorting values between 0.75 

 and 1.00. The coarsest and most poorly sorted 2.24 and 1.00 phi) were for 

 phase V samples, but these differences are too small to be considered a "trend" 

 of beach coarsening through time. 



Figure 8 shows shoreline shifts between surveys for profile 914 and for 

 three other profiles between it and the sand trap entrance. The general "saw- 

 tooth" patterns for the profiles show multiple erosion-accretion sequences 

 which tend to dispel the idea of overall updrift beach accretion. Also, maxi- 

 mum extensions and retreats of the shoreline occurred at different times for 

 each profile and somewhat sequentially as well. For example, shoreward- 

 retreating maximums observed for the period covered by the first seven surveys 

 appear, in order, at profiles 914, 822, 670, and 762, while the corresponding 

 order of seaward maximums are at 762, 914, 822, and 670. These staggered 

 patterns seem to suggest that sand moved through the area in the form of 

 "waves" or pulses that tended to swell and shrink the shoreline with their 

 passage. This conceptual model of sediment pulses is also supported by the 

 varying rates of trap filling discussed in a previous section. Figure 8 also 

 reveals that there are times when either general accretion or erosion dominated 

 the area (e.g., accretion at the time of surveys 6, 8, and 12 and erosion for 

 surveys 4, 9, and 11). These trends of dominant accretion or erosion generally 

 alternate and are consistent with the model of longshore transport occurring 

 as pulses in the area. Finally, although the last survey of this study does 

 document a general buildup of sand updrift of the trap, this buildup is inter- 

 preted as the next pulse of sediment transport to affect the area rather than 

 as accumulation of sediments too coarse to be transported into the trap. 



26 



