The north-to-south decrease in mean grain size confirms earlier findings 

 by Swift, et al. (1971) and Shideler (1973). A downdrift decrease in sand 

 size has been noted at other localities along the east coast (e.g., Ramsey and 

 Galvin, 1977). The coarse sand along the northern section of the study area 

 is characterized by a bimodal-size distribution. The northward-coarsening 

 trend does not continue northward of the study area (Goldsmith, Sturm, and 

 Thomas, 1977), but appears to be localized between Caffey's Inlet and the 

 vicinity of Duck. Swift, et al. (1971) attributed this coarse anomaly to a 

 local source of gravel which is excavated from the former Albemarle River 

 channel. 



b. Nearshore Sediments . In August 1979 scuba divers collected a set of 

 35 short-core sediment samples on three shore-normal transects — along the pier 

 centerline and along parallel lines 75 meters both north and south of the pier 

 centerline. The results of the settling tube (RSA) analysis of these samples 

 are plotted as box plots in Figure 51. Each sample is plotted relative to its 

 distance (in meters) from the FRF base line, along the shore-normal transect. 

 Values of the 10th, 16th, 25th, 50th (median), 75th, 84th, and 90th percent- 

 iles of the cumulative size distribution are also plotted for each sample. 

 Sample depths, as determined by lead-line soundings and corrected to MSL 

 elevations, are plotted for each transect. The statistics are summarized in 

 Table 11. 



According to Folk's (1965) classification, the bottom material is gener- 

 ally moderately well sorted, medium to fine sand. Median grain size ranges 

 from 0.28 to 0.12 millimeter (1.85 to 3.11 phi) with sorting values ranging 

 between 0.74 and 0.40 millimeter (0.44 and 1.31 phi) (Table 11). A zone of 

 sandy silt is encountered at 13- to 15-meter (45 to 49 feet) depths. No 

 gravel was directly observed, although one sample (Table 11, transect 1,13) 

 taken 43 meters (140 feet) directly seaward of the pier end did contain a 

 secondary mode in the 1.4- to 1.0-millimeter (-0.5 to phi) size fraction 

 (very coarse sand). 



The bottom was generally observed to be rippled, except in the surf zone 

 where ripples were wiped out by surging breakers. Ripples were generally 

 shore parallel with wavelengths ranging from 4 to 12 centimeters (1.5 to 5 

 inches) and heights from 1 to 4 centimeters (0.4 to 1.5 inches). At a 2.9- 

 raeter water depth megaripples were observed to be the primary bed form with 

 smaller ripples superimposed. Megaripple wavelength was 2 meters (6.5 feet); 

 height was 15 centimeters (6 inches). 



c. Subbottom Sediments. Field (1973) summarized the results of a 



subbottom geophysical survey conducted at the site in 1972-73. His analysis 

 of four nearshore vibracores and five drill holes (Figs. 52 and 53) showed 

 that the beach is underlain by more than 15 meters of sand at the shoreline, 

 thinning to about 1.5 meters at the 12-meter contour. Sediments vary from 

 coarse sand with gravel layers to dense, poorly graded (well-sorted), fine 

 sand. Alternating silts, clays, and silty sands are common below this sand 

 prism. Geophysical records show a nearly horizontal reflector (layer) at -12 

 meters MSL nearshore that appears to intersect the bottom and become exposed 

 at about -14 meters MSL. The depth of this major reflector was found to cor- 

 relate with the change from sand with gravel layers to silts and clays noted 

 in the core logs (Fig. 53). The surface samples and visual observations 

 discussed above confirm an outcrop of the silt layer at -13 to -15 meters 

 MSL. Detailed core logs and geophysical records are on file at CERC. 



75 



