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 HelleySmith 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.

Calculation of bedload transport in Swiss mountain rivers using the model sedFlow: proof of concept

“…At the Brenno, the minimum threshold dimensionless shear stress θ c50,Min for the initiation of bedload motion has been calibrated to a value of 0.1 (Table 2). This corresponds to the findings of Lamb et al (2008) and Bunte et al (2013), who showed that in mountain rivers θ c may well assume values in this order of magnitude.…”

“…This is partly due to the complex measurement conditions in gravel-bed rivers (Bunte et al, 2008;Gray et al, 2010). Because of these difficulties, there are relatively few data sets available for deriving conceptual models or for validating and testing numeric models.…”

“…They also suggested an approach to quantify the impact of macro-roughness based on the relative flow depth compared to a characteristic grain diameter. Lamb et al (2008) and Bunte et al (2013) have noted that in steep channels higher energies are needed for the initiation of bedload motion, compared to channels with gentle slopes. Turowski et al (2011) have shown that the conditions for the initiation of bedload motion vary in time and are strongly linked to the conditions at the end of the last bedload transport event.…”

Alluvial cover controlling the width, slope and sinuosity of bedrock channels

“…Measured m values are usually much larger than those derived from models. For example, Bunte et al (2008) reported m values ranging from about 7.5 to 16, using data obtained during the snowmelt period of American streams with portable bedload traps. Analysing bedload data sampled with Helley-Smith pressure difference samplers from a large number of streams, Barry et al (2004) found values of m in the range of about 1.5-4.0.…”

An overview of underwater sound generated by interparticle collisions and its application to the measurements of coarse sediment bedload transport

“…Many measurements have utilised box or tray samplers, and this approach is still common today. Hubbell (1964), Engel and Lam Lau (1981), Ergenzinger and de Jong (2003), Bunte et al (2008) and Holmes (2010), along with many others, have considered this technique. Some of the major shortcomings of this direct sampling method are the need for operators in the field, the limited range of conditions when measurements are possible, dangerous conditions during large floods, the impact on the flow introduced by the presence of the sampler, the lack of spatial/temporal resolution, the variable efficiency of samplers, the problems in obtaining continuous records and other difficulties specific to the particular samplers.…”