A field comparison of three pressure-difference bedload samplers

Abstract

The movement of bedload over a cross-section is often sampled using a “pressure-difference bedload sampler”, such as the Helley–Smith. Whereas several types are in use, no one device has gained universal acceptance as the standard for use in all types of streams. Moreover, evidence suggests that similar devices may collect substantially different amounts of bedload because of only slight modifications in design. In this study, sample weights collected by three types of pressure-difference samplers are compared to determine whether differences are statistically significant or whether sampler performance is so irregular and overlapping that one might regard them as being the same. The results confirm that the weights of samples collected by the devices are significantly different. Generally, the US BLH 84 collected less material, the Sheetmetal Helley–Smith collected more material, and the Original Helley–Smith was intermediate; these tendencies were consistent at two sites where bedload was measured. The implication of these results is that measured transport rates will vary depending on the sampler used and, therefore, they are not directly comparable without some mode of calibration. To place this finding in a larger context, sediment rating curves, determined from weights of samples and measurements of flow, were integrated over available flow records and used to estimate annual yield. Three estimates of annual yield, one for each device, were then compared with measures of annual accumulation from a weir pond below one of the collection sites. The results indicate that despite differences between the devices, data obtained with pressure-difference samplers estimated annual accumulations of sediment reasonably well. Predicted accumulations were within 40–50% of the measured yield for two samplers whereas the third sampler predicted within 80%.