The most important characteristic of the sediment in suspension is 

 fall velocity. In general, fall velocity depends on size, shape, specific 

 gravity, and water viscosity. Since most of the sediment was rounded quartz 

 sand grains, the shape and specific gravity do not significantly vary, so 

 the most important sediment characteristic is size. To initiate transport 

 of a sediment particle, local water velocities must exceed the threshold 

 velocity of the given particle size. The threshold velocity for the range 

 of particles sizes reported for the Ventnor data (0.12 to 0.15 millimeter, 

 median diameter) is about 0.8 foot per second (Ranee and Warren, 1968; 

 Komar and Miller, 1973; U.S. Army, Corps of Engineers, Coastal Engineering 

 Research Center, 1975). Considering the range of wave periods and heights 

 reported here as 5 to 10 seconds and up to 4 feet in height, maxiinum hori- 

 zontal velocities up to 8 feet per second (more than 10 times threshold 

 velocity) may be expected before the waves break in a 5.5-foot water depth 

 (Inman, 1956). 



6. Wave Data . 



In the Ventnor data, significant wave heights and periods were reduced 

 from strip-chart recordings obtained from a wave gage system. The system 

 consisted of a pressure transducer, recorder, amplifier, and about 800 feet 

 (244 meters) of a two-wire cable. The pressure transducer was anchored 

 near the bottom, off the end of the pier at station 1014 in a 15-foot (4.6 

 meters) water depth. This station was about 600 feet (183 meters) seaward 

 of the midrange of the sampling stations where water depths averaged only 

 2 to 5 feet (0.6 to 1.5 meters). The wave gage system produced a strip- 

 chart recording of the wave characteristics during each sample collection, 

 except during the last 2 days of the fieldwork when the recorder failed. 

 Visual estimates of wave breaker height were then made on a spot-check 

 basis. 



In the Nags Head data, wave records were obtained from a CERC staff 

 gage located on Jennette's Pier. The gage produced 20-minute programed 

 recordings on a paper strip chart and continuous recordings on magnetic 

 tape. The magnetic-tape records were analyzed by the CERC wave spectrum 

 analyzer which gives a wave energy spectrum over a range of wave periods, 

 a linear average, a square average, and peak wave heights. The signifi- 

 cant wave height, Hg, was obtained from the peak wave height, using the 

 formula, SH^/PH^ = 0.67, where SH^ is the significant height on a paper 

 strip-chart recording and PH^ is the peak height from, the analysis by 

 the spectrum analyzer of the magnetic-tape record. Values of significant 

 wave height thus obtained for Nags Head are compiled in Appendix A. 



III. DATA ANALYSIS 



This section discusses the suspended- sediment concentrations obtained 

 from the piers at Nags Head and Ventnor. The approach is empirical, and 

 aims at separating relations between the independent variables of equation 

 (1) and the dependent variable, concentration, using data in Appendixes A 

 to D. The discussion includes sediment-size data, the relation between 

 concentration and the position of the sample, the relation between concen- 

 tration and the wave conditions, and causes of observed scatter. 



22 



