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<br /> <br /> <br />Figure 6. Helley~Smith bedload samplers having standard <br />nozzles. A, 3- by 3~jn. entrance; B, 6- by 6-in. entrance. Both <br />nozzles have an area ratio of 3.22. <br /> <br />of full (8 to 9 hours) and partial (less than 8 hours) days <br />when the flume was aClually operaling; conseculive day <br />numbers do not necessarily represent consecutive calendar <br />days. <br />Samplers being calibraled were slored on overhead <br />tracks on a mobile sampling platform (fig. 9). The platform <br />supported a high~speed chain hoist that could be moved to <br />pick up and relurn samplers from slorage and to raise and <br />lower them at the downstream edge of the platform. A <br />quick~release conneclor allowed samplers 10 be altached and <br />detached rapidly from the hoisl, and a quick-release tether <br />(fig. 6A) prevented downstream drift during sampler place- <br />ment and "scooping" during sampler retrieval. This arrange- <br />ment facilitated sampling sequentially with a group of sam- <br />plers (usually six) during a run so that the same hydraulic <br />conditions and true bedload transport rates applied to all <br />samplers in the group. That is, the samplers were used, one <br />after the other, to obtain one or more samples from the cross <br />section; then. after all samplers had been used. the sequence <br />was repeated. Repetitions continued until the desired num. <br />ber of samples had been collected with each sampler. In runs <br />1-4 with the 6.5-mm bed material, each sampler was used <br /> <br />10 collect three individual samples at lateral stations coinci- <br />dent with the centerlines of pans 2, 4, and 6; in all other runs <br />with all bed materials, a single sample was collected with <br />each sampler at center.channel, in line with the center of <br />pan 4. <br />Individual samples and wet -sieved portions of sam- <br />ples were weighed in tared buckets under water to eliminate <br />the necessity of drying samples. The submerged net weights <br />of both samples and weigh.pan accumulations were cor. <br />rected later to dry weights by multiplying by the ratio 2.651 <br />1.65. The weighing apparatus, which consisted of a boltom- <br />loading balance, a weighted table with a load-dampening <br />release mechanism, and an immersion tank, is shown in <br />figure lOA. <br />To calibrate the tested bedload samplers, transport <br />rates, determined from the dry weight of individual samples <br />collected in a known time and adjusted to a unit-width basis, <br />were compared with true transport rates (dry.weight basis) <br />determined from the calibration facility. Details of the cali- <br />bration procedures are given by Hubbell and others (1985); <br />the procedure involves comparison of rate distributions, <br /> <br /> <br /> <br />Figure 7. Helley-Smith bedload samplers having modified <br />nozzles. A, 3. by 3.in. entrance; 8, 12. by 6-in. entrance. Both <br />nozzles have an area ratio of 1.1. <br /> <br />6 laboratory Data on Coarse-Sediment Transport for Bedload-Sampler Calibrations <br />