III. FILLING HISTORY 



The five phases of trap filling (I to V), which vary in length, were estab- 

 lished using the dates that hydrographic surveys and surface sand samples were 

 simultaneously collected. Phase V, the most recent phase (Table 3), was ini- 

 tially planned as tv70 phases but storms and equipment failures prevented sand 

 sample collection during the June 1977 hydrosurvey. Finally, these phases are 

 arbitrary divisions of trap-filling history and do not correspond to recogniza- 

 ble episodes of sedimentation. 



Figures 2, 3, and 4, respectively, show cross sections of the cored trap- 

 fill sediments, cored native beach sediments, and maps of relief and thickness 

 changes that occurred in the trap as it filled. Cross sections are keyed to a 

 mean lower low water (MLLW) elevation datum and are shown along four shore- 

 parallel transects through the trap area (Figs. 2 and 3). Core numbers corre- 

 spond to those shown in Figure 1 with base elevations for the trap bottom as 

 dredged prior to monitoring. The presence or absence of a particular sedimen- 

 tation phase (deposition or erosion) was determined by comparing sequential 

 elevation changes for a location. 



Three maps are used to show the patterns of infilling for each filling 

 phase monitored (Fig. 4). These maps show (a) bottom elevations surveyed; (b) 

 erosion-deposition patterns for a particular phase; and (c) cumulative erosion- 

 deposition patterns within the trap. In addition. Figure 4 contains a core 

 location key map, a map showing the original trap topography (labeled "prefill 

 elevations"), and an additional sequence of three maps showing trap conditions 

 monitored midway through phase V (labeled "phase V-1") which were surveyed on 

 7 June 1977 but not sand sampled. Phase V-2 (Fig. 4) shows the final trap 

 conditions for which sand sample data are available. Finally, elevations and 

 elevation differences shown on the maps are plotted at cored locations. 



The trap filled in about 1.5 years and in general, three simultaneously 

 occurring filling trends were observed: (a) filling occurred in pulses from 

 updrift to downdrift trap locations; (b) nearshore locations filled before 

 offshore locations; and (c) the rate of filling accelerated during the study 

 period. 



Phase I (31 days) filling occurred mainly along the updrift (north) shore- 

 line of the trap (Fig. 2, a) and extended that shoreline about 25 meters sea- 

 ward. Small thicknesses (about 0.33 meter) of phase I sediments were also 

 found at core sites 2 and 18 on opposite ends of transect 260 but not in other 

 intermediate cores, indicating the possibility of (a) deposition followed by 

 erosion at those localities, (b) sediment redistribution along the southern 

 margin of the trap (a reasonable explanation considering the erosional pattern 

 in that area. Fig. 4, phase I), or (c) the apparent lack of phase I sediments 

 may reflect survey errors which can be as great as these thicknesses (Bruno 

 and Gable, 1976). 



During phase II (83 days) the nearshore zone continued filling deeper 

 (southward) into the trap and offshore as well. Along transect 170, exposed 

 beach sites (cores 9 and 13) were raised more than 1 meter above MLLW while 

 more than 2 meters of fill was deposited at sites 1 and 5. Other accumulations 

 of less than 1-meter thicknesses occurred more generally throughout the trap, 

 and it appears that phase I materials served as a transport path which allowed 



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