A field comparison of three pressure-difference bedload samplers

The movement of bedload over a cross-section is often sampled using a "pres sure-difference bedload sampler", such as the Helley-Smith. Whereas several types are in use, no one device has gained universal acceptance as the sta ndard for use in all types of streams. Moreover, evidence suggests that sim ilar devices may collect substantially different amounts of bedload because of only slight modifications in design. In this study, sample weights coll ected by three types of pressure-difference samplers are compared to determ ine whether differences are statistically significant or whether sampler pe rformance is so irregular and overlapping that one might regard them as bei ng the same. The results confirm that the weights of samples collected by t he 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 a t 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 calibrati on. To place this finding in a larger context, sediment rating curves, dete rmined from weights of samples and measurements of flow, were integrated ov er available flow records and used to estimate annual yield. Three estimate s 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. Th e results indicate that despite differences between the devices, data obtai ned with pressure-difference samplers estimated annual accumulations of sed iment reasonably well. Predicted accumulations were within 40-50% of the me asured yield for two samplers whereas the third sampler predicted within 80 %. (C) 1999 Elsevier Science B.V. All rights reserved.