accumulated during rapid pulses of influx, was buried quickly, and therefore 

 became unavailable for modification by waves and currents entering the trap 

 from the south. 



In conclusion, the textural distribution of trap-fill sediments is similar 

 to that found for the unprotected updrift beach indicating that at least 

 during periods of rapid sedimentation, current and wave energies in the trap 

 were sufficient to redistribute most sizes of littoral drift sediments. Also, 

 although patterns of composite mean and sorting values in the trap are similar 

 for core versus grab sediment samples, the grab samples are slightly finer and 

 better sorted than the core samples. These slight differences seem mainly due 

 to minor postdepositional sorting rearrangements of surface sediment layers. 



V. SAMPLING AND SEDIMENT TEXTURE 

 1. Sampling . 



Core sampling and surface grab sampling techniques were employed to charac- 

 terize both native beach and trap-fill sediments. The two methods were chosen 

 to determine if textural differences existed between the composite grain-size 

 distributions obtained, and if there were differences, to evaluate their 

 effects upon the predicted performance of trapped sediments as beach fill. In 

 addition, data obtained using the two sampling methods would be used with other 

 similar data to evaluate sampling techniques for characterizing beach sediments. 



As described earlier, surface samples were collected at 20 trap sites and 

 at 8 sites along "native" profile line 914 at the end of each fill monitoring 

 phase of the sediment transport study, and cores were obtained at these same 

 sites after completion of the study. Differences between minimum and maximum 

 elevations as surveyed during the study period (Table 3) were used to deter- 

 mine the core length needed at each site to sample sediments involved in active 

 transport (Table 2, active thickness). It should be noted that these elevation 

 differences may reflect dissimilar interpretations for trap versus native beach 

 process-response relationships. 



Although some erosion and sediment redistribution occurred in the trap 

 during the study, minimum surveyed elevations generally were close to bottom 

 elevations as dredged at the start of the study. The trap then generally 

 "filled like a bucket" with sediments entering from the north and filling the 

 dredged depressions to the final maximum elevations surveyed. Local redistri- 

 bution within the trap caused only slight variations in this simple filling 

 process. 



This "bucket" analogy, however, cannot be applied to the beach. Here, the 

 lowest elevation simply measures the deepest scour or erosion surveyed and the 

 highest elevation, the greatest deposition at a location. Beach scour gener- 

 ally occurs during storms and scoured sections generally contain coarse lag 

 deposits that grade upward into finer sands that are deposited as the beach is 

 rebuilt by smaller poststorm waves. At Channel Islands, erosion was not the 

 greatest at the beginning of the study nor did it necessarily affect the entire 

 profile equally. Also, elevations were higher at some locations during the 

 study than when the profile was cored. Therefore, the sediments contained with- 

 in the active profile at any particular time are most likely to be a mixture of 

 one or more coarse storm-lag layers overlain and interlayered with finer sands 

 associated with nonstorm longshore transport. 



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