nozzle elevations. In general, there is no direct correlation between 

 depth and concentration, a fact which is indicated by the data on Figure 

 17. 



The elevation of the nozzle above the bottom, E, will have an impor- 

 tant effect on the measured concentration, C. This is evident in Figure 

 16 where the concentration at the breaker line goes from about 2 parts per 

 thousand for E less than 0.28 foot, to about 0.5 parts per thousand for 

 E between 0.53 and 0.63 foot, to about 0.3 parts per thousand for E 

 between 0.85 and 1.05 feet. 



Figure 18 shows all data obtained at Ventnor in a plot of E versus C. 

 The trend of increasing C with decreasing E is apparent, but the scatter 

 is large, evidently due to the effect of other variables. Superimposed on 

 Figure 18 are two sets of data from short-time intervals when these other 

 variables are assumed not to vary. Over these shorter time periods, the 

 expected trend shows less scatter. 



Figures 19 and 20 illustrate C versus E plots from Ventnor and Nags 

 Head, respectively. Figure 19 shows the logarithmic decrease in concentra- 

 tion with elevation above bottom which has previously been found in both 

 laboratory and field studies. In comparing the three plots on Figure 19, 

 the highest concentrations (C = 2.4 parts per thousand) are achieved in the 

 one set of data where E was less than 0.4 foot. The fact that the linear 

 relation on a log plot breaks down near the bottom has been noted in lab- 

 oratory tests (MacDonald, 1977), and is illustrated by the data on Figure 

 20 which suggest that C increases more rapidly near the bottom. 



4, Wave Conditions . 



There are three variables of interest to this study describing the 

 waves in the surf zone--wave height, wave period, and breaker type. The 

 distribution of wave heights and periods during the data collection is 

 shown in histogram form on Figures 21 and 22. These data suggest that the 

 most commonly occurring wave height was a little more than 1 foot at both 

 sites, but the extreme heights were greater at Nags Head (Fig. 21). The 

 dominant wave periods were 5 to 8 seconds at Ventnor and 7 to 12 seconds 

 at Nags Head (Fig. 22). 



In general, suspended-sediment concentration does not vary greatly 

 with wave height when all other variables are fairly constant. Figures 

 23 and 24 show no definite correlation between wave height and concentra- 

 tion at Ventnor and Nags Head. The lack of correlation between Hg and C 

 is similar to the lack of correlation between d and C (Fig. 17). However, 

 the ratio, Hg/d, of these two variables does have a positive correlation 

 with C (Fig. 25). (See Fig. 8 in Fairchild (1973) for additional plots.) 

 Possibly, Hg/d is a measure of sampling position with respect to the 

 breaker position, and thus is related to the correlation between C and S 

 (Fig. 16). 



34 



