1 . Concentration . 



Figure 9 shows the distribution of suspended-sediment concentration 

 (in parts per thousand) for this study. The median concentration of all 

 samples is about 0.15 parts per thousand for Nags Head and slightly greater 

 for Ventnor. The maximum measured concentration reached 4 parts per 

 thousand in the Nags Head samples and 2.6 parts per thousand in the Ventnor 

 samples. Some of the extreme Nags Head samples were possibly due to the 

 nozzle being without the footplate (Fig. 6) during that part of the study. 

 Without the footplate the nozzle may have been closer to the bottom at times 

 and sucked more sediment. 



2. Sediment Size . 



Sediment-size data are summarized in the table, in Figures 10, 11, and 

 12, and in Appendix D. These data indicate that the median size of 

 suspended-sediment samples was typically about 0.13 millimeter at Ventnor 

 and about 0.18 millimeter at Nags Head. Contemporary beach samples at 

 Ventnor had a median size of about 0.20 millimeter (App. D, Fig. D-4) ; 

 contemporary bottom samples at Nags Head were between 0.23 and 0.35 milli- 

 meter at stations where suspended- sediment size was only about 0.16 to 

 0.22 millimeter (see Table). Because no bottom samples were collected 

 contemporaneously with the suspended sediments at Ventnor in 1965, a few 

 bottom and suspended-sediment samples were collected later in March 1971 

 at that locality (Schweppe, 1971). Bottom samples at Nags Head were 

 collected by a grab sampler operated from the pier deck, and at Ventnor 

 by a diver scooping sand into a plastic sample bucket. 



The table compares the median size of bottom and suspended- sediment 

 samples. The median size of bottom or beach sediments is, in all cases, 

 greater than the median size of the suspended sediment at that time. For 

 the data shown, the median size of bottom samples at Nags Head was about 

 63 percent greater than the suspended samples; the median size of the 

 beach samples in May 1965 at Ventnor was about 54 percent greater than the 

 size of the suspended samples. However, the few data from Ventnor in March 

 1971 indicate a percent difference of only 22 percent, apparently because 

 the suspended- sediment samples were much coarser in March 1971 than in May 

 1965. The reason for this is not known, but possible explanations include 

 effects of depth changes, water tem.perature, or wave conditions (Schweppe, 

 1971). 



The circles in Figures 10, 11, and 12, show that coarser sediment does 

 get suspended, and the coarsest 5 percent of the sample is often more than 

 twice the median size in millimeter. Also, these data show surprisingly 

 little variation in median size with increase in water depth (Fig. 10), 

 nozzle elevation (Fig. 11), or distance from the breaker (Fig. 12). There 

 does appear to be a slight tendency for size to decrease with increasing 

 water depth and nozzle elevations. 



One restriction on attempting to establish relations between the median 

 size of the suspended sediment and the other independent variables of equa- 

 tion (1) is that median-size variation in the available sand is usually 



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