As a result, sand fluxes measured with one nozzle (the C nozzle), observed to 

 have no effect on the movement of sand near the mouth and with a near -unity 

 on-bed hydraulic efficiency (0.96), were assumed to represent the ambient sand 

 flux. Values of sand-trapping efficiency for the near-bed position were 

 calculated for the 60 to 66 cm/sec range of flow speeds which produced the 

 flat-bed sand transport condition most like the surf zone. At lower flow 

 speeds, large sand ripples formed and moved in the direction of flow. This 

 type of bedform is unique to the surf zone. Because sand transport occurred 

 for the most part in a 5 -cm-high layer above the bed, nozzles located off the 

 bed collected small amounts of sand. Values of sand- trapping efficiency for 

 the off -bed nozzle position were taken to be equal to the hydraulic efficien- 

 cies for a similar midflow speed. 



128. Main conclusions from this study are as follows: 



a. The streamer trap nozzle previously used in the DUCK85 field 

 data collection project performed well in an off-bed position, 

 with an off -bed sand- trapping efficiency equal to 0.92 

 (standard deviation 0.02). Significant scour occurred at the 

 bottom of the nozzle when it was located near the bed, and the 

 on-bed sand- trapping efficiency was calculated as 0.13 

 (maximum error equal to 0.50). 



b. The SUPERDUCK nozzle, used in a field data collection project 

 conducted at Duck, North Carolina, in 1986, had an off -bed sand- 

 trapping efficiency equal to 1.02 (standard deviation 0.01). 

 Behavior of the nozzle near the bed was judged to be fair, with 

 scour occurring approximately half the testing period. The on- 

 bed sand -trapping efficiency was calculated as 0.68 (maximum 

 error equal to 0.51). 



c. The C-type nozzle was observed to have no effect on sand move- 

 ment in the region upflow of the nozzle. Small bedforms moving 

 in the direction of flow were observed to enter unobstructed 

 into the nozzle mouth. The off -bed sand -trapping efficiency was 

 calculated as 0.93 (standard deviation 0.02), and the on-bed 

 sand- trapping efficiency was, by definition, 1.00. 



d. The pressure-difference H-S sampler had values of hydraulic 

 efficiency ranging from 1.20 at a midflow position to 1.48 

 when located at the bed. These values are slightly smaller 

 than values quoted in the literature (1.54 (Druffel et al . 

 1976)); however, testing flow speeds were lower in the 

 present experiment program, and efficiency was observed to 

 increase with flow speed. The sampler in the sand- trapping 

 phase of the experiment distorted the normal patterns of 

 sand movement in the tank. The H-S sampler increased the 



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