during the test until sand began passing under the nozzle near the end of the 

 test. The SUPERDUCK bottom nozzle occasionally scoured around its outer 

 edges, and sand would either pass around or under the outer edges. Sometimes 

 scour would occur at the SUPERDUCK nozzle during the entire 5-min test; at 

 other times it would be intermittent and disappear after a few seconds. The 

 H-S sampler effectively behaved as a vacuum cleaner, creating turbulence and 

 large-scale longitudinal eddies such that even sand to the rear of the sampler 

 moved upstream and into the nozzle at the higher flow speeds. The H-S sampler 

 thus dug into the bed and buried itself. Qualitatively, the C nozzle appeared 

 to function optimally ("perfectly"). The moving sheet of sand and occasional 

 small bedforms were observed to continuously enter unhindered into the nozzle. 

 Nozzles with both long and short, straight and curved, bottom lips tended to 

 enhance scour rather than reduce it. 

 Basin efficiency 



77. The efficiency of the catch basins was qualitatively evaluated 

 relative to characteristics of an ideal pit sampler. An ideally functioning 

 pit sampler comprised of a large number of independent basins aligned in the 

 direction of flow is expected to function as follows: 



a. The quantity of sand collected in each basin would decrease in 

 the downflow direction such that the farthest downflow basin 

 would not collect any sand. 



b. The quantity of sand collected in the downflow basins would 

 increase as the midflow speed increased. 



c. The sizes of grains collected in the downflow basins would 

 increase as midflow speed increased, as the flow could entrain 

 and transport larger -sized sediment. 



78. In the experiment, the average percentages of sand collected in 

 Basins 1 (upflow) , 2 (middle), and 3 (downflow) were 97.7, 1.7, and 0.6 per- 

 cent, respectively. The quantity of sand in each basin thus decreased signif- 

 icantly in the downflow direction. Figure 24 presents the percent of the 

 total amount of sand collected in the second and third basins as a function of 

 midflow speed. There is much scatter in the data; however, a trend for an 

 increase in the quantity of sand collected in Basins 2 and 3 can be observed 



53 



