IV. TEXTURAL CONSIDERATIONS 



Figures 5 and 6 provide phi mean and phi sorting values, respectively, for 

 all sediments that accumulated in the trap during this study. Two maps of each 

 variable are shown — one of composite grain-size parameters as determined by 

 core sampling at the end of the study period and the other of surface grab 

 samplings following the five study phases. In general, the patterns for each 

 variable are similar, which suggests that either sampling technique is adequate 

 for describing the trap-fill sediments. The sampling technique is discussed 

 further in Section V. 



In general, higher coastal energy conditions correlate with coarser grain 

 sizes and more poorly sorted (well-graded) sediments. For example, coarse, 

 poorly sorted sediments are usually found near the highly active plunge zone of 

 a beach and both grain size and sorting decrease landward toward the foreshore 

 and offshore as breaking and nonbreaking wave energy decreases, respectively. 

 Longshore current velocities are generally greatest slightly landward and sea- 

 ward of the plunge zone; as indicated, these areas of relatively high energy 

 correlate with relatively coarser and more poorly sorted sediments. Mean grain 

 size can be considered to reflect the severity of processes (waves and currents) 

 and sorting the range of energies affecting a particular locality. Thus the 

 plunge zone waves move the coarsest sands while finer materials are deposited 

 between waves and at times when the plunge zone is shifted by tidal change; 

 variations in wave and current energies at the bottom are much smaller offshore, 

 resulting in finer, better sorted deposits. For this study, phi mean and 

 sorting values are used to describe sediment texture. Phi mean values are 

 based on a logarithmic transformation of grain size in millimeters and increase 

 for finer sediments and decrease for the coarser; phi sorting values are 

 dimensionless and describe the range of size grades for a particular grain-size 

 distribution. Hobson (1979) gives further discussions of the phi grade scale. 



Although the trap area is sheltered by the offshore breakwater, the tex- 

 tural patterns of sediments within it are similar to those characterizing an 

 open coastline. The sand is coarsest and most poorly sorted in the nearshore 

 zone. It becomes finer and better sorted offshore (Figs. 5 and 6). A cross 

 section of phi mean and sorting along any range line within the trap is nearly 

 identical to that found along native profile 900. The pattern of mean values 

 (Fig. 5, A and B) suggests that the coarser material moved both alongshore, as 

 far as the northern jetty, and into the central part of the trap (Fig. 5, A). 

 This "bulge" of coarse sediments on the map is matched by a similar pattern of 

 poor sorting (Fig. 6, A) and appears to represent the redistribution of near- 

 shore transport path sediments discussed in the previous section. Patterns of 

 sorting values generally imitate those for phi means although contour positions 

 shift for core versus grab samples. For this study, poor sorting coincides with 

 the coarser sands and better sorting with fine sediments. The best sorted sedi- 

 ments occupy an offshore position in the southern end of the trap, as would be 

 expected, but the position of these well-sorted sands (S()) < 0.60) lies more 

 shoreward for grab samples than for core (Fig. 5, A versus B) . The zone of the 

 best sorted grab samples lies immediately to the lee of the northern harbor 

 entrance jetty with poorer sorting toward the offshore breakwater, suggesting 

 that waves and currents from the south may enter the trap between the break- 

 water and jetties and produce locally higher energy conditions which redis- 

 tribute surface sediments. For core samples, the zone of best sorting is in 

 the southwest corner of the trap, which may reflect the fact that most sediment 



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