SUMMARY OF SEDIMENT SAMPLING AND BED-FORM DESCRIPTION IN THE NEARSHORE 

 REGION OF THE FIELD RESEARCH FACILITY 



by 



Allan E. DeWall 



On 7-9 August 1979, a set of short core samples and visual bed-form descriptions were 

 collected along four nearshore range lines in the vicinity of the CERC pier at Duck, North 

 Carolina (Fig. C-1). Range I was along the pier centerline, range II was 76 meters north, 

 range III was 76 meters south, and range V was 305 meters south of the pier. A fifth line — 

 range IV, located 305 meters north of the pier — was planned but time did not allow for 

 sampling. 



Using profile data collected in September 1978, stations were preselected at 2-meter- 

 depth increments and at breaks in slope extending from a depth of approximately 3 meters to 

 a maximum depth of 15.8 meters which occurred at the Waverider buoy 3.3 kilometers offshore. 

 Samples and bed-form descriptions were collected by a team of twj divers working from a 

 Zodiac inflatable boat. Positioning was accomplished using a Motorola "Mini-Ranger," coupled 

 to a Hewlett-Packard Mini-Computer and flatbed plotter. This positioning system was put 

 together and operated by Frank Musialowski, CERC Geotechnical Engineering Branch. The real- 

 time plotting capability allowed for the immediate reduction and display of sampling position 

 as well as a great deal of flexibility in modifying the sampling plan. 



Samples were collected using a hand-held piston corer 3.2 centimeters in diameter and 

 approximately 40 centimeters long. The core was extruded directly into a prelabeled sample 

 bag and no attempt was made to differentiate laminations within the core. While one diver 

 collected the sample, the second diver recorded conditions on the bottom. This description 

 Included sediment type; presence of bed forms; ripple height, wavelength, and orientation; 

 and degree of bioactlvity. 



Visibility on the bottom ranged from to 3 meters. A thennocline was encountered at 

 approximately 7.5 meters below the surface. Water temperature on the surface was approx- 

 imately 27° Celsius (80° Fahrenheit) and dropped to an estimated 18° Celsius (65° Fahrenheit) 

 below the thermocllne. 



A listing of samples Including location, depth, time, and bedform description is in Table 

 C-1. Table C-2 lists the RSA results for size analyses. Samples containing a significant 

 amount of silt-sized material (finer than 0.625 millimeter) cannot be reliably analyzed with 

 the RSA; therefore, results from these should be interpreted with caution. The samples could 

 be wet-sieved to determine the percentage of silt. Techniques are available to analyze size 

 distribution within the silt fraction, if required. Samples that have a significant silt 

 fraction include the following: 1-4, 1-5, II-l, II-2, III-l, III-2, III-3, III-4, III-6, and 

 V-2. All size data are representative of composite samples of the 40-centimeter (approxi- 

 mate) core, with the exception of samples V-4, V-6, and V-8. Part of the core sampler was 

 lost, so the last three samples represent only the top 2 or 3 centimeters of the bottom. 



Bottom sediment ranged from medium sand in the vicinity of the pier to sandy silt and mud 

 at the 12- to 14-meter depth. Very fine sand occurs farther offshore at the Waverider loca- 

 tion (15.8-meter depth). No gravel was observed. 



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

 were wiped out by the surge of passing breakers. Ripples were generally shore-parallel with 

 wavelengths ranging from 4 to 12 centimeters and heights of 1 to 4 centimeters. At station 

 III-IO (2.9-meter depth) megarlpples were the primary bed form with smaller ripples super- 

 imposed. Megaripple wavelength was 2 meters and height was 15 centimeters. It is possible 

 that megarlpples occurred at other stations but were not detected due to poorer visibility. 

 Attempts were made to photograph these features but were not successful due to flooding of 

 the underwater camera. 



For further information, please contact Allan E. DeWall at (202) 325-7380. 



91 



