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<br />l <br /> <br /> <br />Figure 10. A, Sample.weighing apparatus. Tared buckets con. <br />taining samples were suspended for weighing beneath the bal- <br />ance in the immersion tank. Handle of the load-dampening <br />mechanism (right side of tablel rotates to support or release <br />positions. 8, Measurement cart. Cart supported equipment for <br />profiling bed-surface elevation longitudinally and for measur- <br />ing velocity gradients vertically. <br /> <br />frequency distribution of sampled transport rates. The distri- <br />butions demonstrate the typical extreme variations in sam- <br />pled rates observed in all runs. Sampled rates. sampling <br /> <br />times, and other information for all samples collected in all <br />runs are available on magnetic media (see Availability of <br />Data, inside back cover) in data files named in the far right <br />column of table 3. <br /> <br />Measured Bedload-Transport Rates <br /> <br />The true bedload-transport rate (bedload discharge) <br />was determined directly as the time rate of change of weight <br />in a weigh pan. Ordinarily, even though cumulative weights <br />were recorded every 6 seconds, rates were averaged over <br />periods, called composite periods, that were long enough to <br />minimize the effects of force fluctuations on the weigh pans <br />caused by turbulence in the flow above the slot and by <br />pressure waves created by periodic dumping of the pans. <br />The duration of the composite (averaging) period for a run <br />was selected to correspond closely to the sampling time <br />(time to collect an individual sample) used during the run. <br />For each run with each bed material, the number of com- <br />posite periods for which rates are available, the duration of <br />the composite period, the mean transport rate measured at <br />each weigh pan for the specified size range, and the data-file <br />name are listed in table 4. Individual composite.period rates <br />for all weigh pans, runs, and bed materials are available on <br />magnetic media (see Availability of Data, inside back <br />cover) in data files named in the far right column of table 4. <br />The most striking feature of the measured transport <br />rates, as well as of the sampled rates, is their extreme and <br />generally cyclic variation throughout time. Typical temporal <br />records of measured transport rates during two different runs <br />with the 6.5-mm bed material (fig. 19) exemplify these <br />characteristics; similar variations were observed in all runs <br />with all bed materials. The characteristics also are demon- <br />strated indirectly in figure 20, which shows rates measured <br />at weigh pan 4 in the same format and for the same runs as <br />were used to present sampled rates in figures 11-18. <br />Mean bedload-transport rates measured in the cross <br />section and measured at weigh pan 4 alone during the runs <br />with each different bed material are presented in table 5, <br />together with average values of pertinent hydraulic vari- <br />ables. <br /> <br />Particle-Size Distributions <br /> <br />Each of the uniform bed materials used in the experi. <br />ments was prepared by sieving about 400 tons of <br />commercial-grade sediments obtained from natural fluvial <br />deposits. The sieving technique consisted of continually <br />depositing unsieved material at the upper end of a 3- by 6- ft <br />sieve screen that was mounted in a sloping position and <br />vibrated at 60 Hertz. Particles retained on the screen were <br />directed into one pile, and particles passing through the <br />screen were directed to another pile. When two separations <br />were required, the process had to be repeated. <br /> <br />Hydraulic and Sedimentologic Data 9 <br />