trapping apparatus, is a pressure -dif f erence device 

 developed in 1971 for use in measuring bed-load transport 

 on natural streams carrying coarse sediments (Helley and 

 Smith 1971). The sampler is similar in design to the 

 Arnhem or Dutch sampler (Figure A9), and consists of a 

 removable mesh bag attached to an expanding nozzle (Figure 

 A10). Helley and Smith (1971) hydraulically calibrated the 

 sampler in a 60-ft (18.3-m) recirculating flume and noted 

 that speeds in the sampler at 4 inches (10.2 cm) in front 

 of the sampler were consistently higher than ambient 

 speeds , and that they tended to increase as the ambient 

 flow speed increased. Ambient bed-load transport rates 

 were calculated by subtracting measured suspended transport 

 rates from total load transport rates. A concentration 

 sampler was used to measure the suspended material , and a 

 splitter 1/2 in. (1.3 cm) in width was used to measure the 

 total load. The splitter is a device at the end of a flume 

 that retains the water -sediment mixture as it traverses the 

 width of the flume. The quantities of sediment collected by 

 the sampler, determined in a recirculating flume, were 

 found to vary considerably, with the variation in samples 

 equal in most cases to the mean value of collected sedi- 

 ment. However, the bed -load transport rates inferred from 

 collected quantities of sediment were in reasonable 

 agreement predictions from the Meyer -Peter and Muller bed- 

 load equation for coarse sediments (0.4 to 30 mm). For 

 sand- sized material, the studies suggested that over- 

 registration as high as 50 percent might occur. The 

 sampler was very stable in high flows, but it tended to 

 scoop up bed- load material as it was lowered to sampling 

 position. An extensive hydraulic calibration of the 

 sampler was conducted by Druffel et al . (1976) for various 

 entrance-to-exit ratio versions of the device. Druffel et 

 al. (1976) calculated hydraulic efficiencies from 1.06 to 

 1.54 as the exit-entrance area ratios increased from 1.00 

 to 2.62. They recommended and continued testing the 

 version with the hydraulic efficiency equal to 1.54. 

 Filling of the sampler bag up to 40 percent did not 

 decrease efficiency; however, particles close to the 

 diameter of the mesh collection bag (0.2 mm) did decrease 

 efficiency, presumably because of mesh clogging. The 

 sediment -trapping efficiency of the sampler was evaluated 

 in the field using the East Fork River, Wyoming, conveyor 

 belt sampler to measure the ambient bed-load transport rate 

 (Figure All)(Emmett 1980). A large data set with a 1,000- 

 fold range in measured transport rates was obtained. 

 Emmett (1980) concluded that the Helley-Smith sediment- 

 trapping efficiency for material 0.5 to 16 mm in diameter 

 is 100 percent. For material sized from 0.25 to 0.5 mm, 

 the sampler was 175 percent efficient; the comparably 



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