A Comparison of Coarse Bedload Transport Measured with Bedload Traps and Helley-Smith Samplers

Abstract

Gravel bedload transport rates were measured at eight study sites in coarse-bedded Rocky Mountain streams using 4-6 bedload traps deployed across the stream width and a 76 by 76 mm opening Helley Smith sampler. Transport rates obtained from bedload traps increased steeply with flow which resulted in steep and well-defined transport rating curves with exponents of 8 to 16. Gravel transport rates measured by the Helley- Smith sampler started with much higher transport rates during low flows and increased less steeply, thus fitted bedload rating curves were less steep with exponents of 2 to 4. Transport rates measured with both samplers approached similar results near or above bankfull flow, but at 50 % of bankfull, transport rates from the bedload traps were 2-4 orders of magnitude lower than those obtained from the Helley-Smith sampler. The maximum bedload particle sizes also differed between the two samplers. They were smaller in the bedload traps than the Helley-Smith sampler at low flows, while at higher flows bedload traps collected larger particles than the Helley-Smith sampler. Differences in sampler opening size and sampling time contribute to the measured differences, but the biggest effect is likely attributable to the bedload traps being mounted on ground plates thus avoiding direct contact between the sampler and the bed and preventing involuntary particle pick up.