Effect of sampling time on measured gravel bed load transport rates in a coarse‐bedded stream

Abstract

[1] This study empirically quantified the effect of sampling time on measured transport rates and fitted rating curves based on results obtained from bed load traps deployed for 2, 10, and 60 min in a coarse-bedded stream. As expected for a skewed distribution of transport rates, 2 min deployment underpredicted transport rates obtained from 10 and 60 min deployment by factors of 2 and 3 at moderate flows (50-70% Q bkf ). At near-bankfull flow the underprediction by 2 min versus 10 min sampling increased to a factor of 5, while transport rates collected during 60 min deployments were reduced because of overfilled bags. At flows near incipient gravel motion, 2 min sampling overpredicted transport rates obtained by 10 and 60 min deployment by factors of 2.7 and 3.4. The overprediction is attributed to computational effects arising mainly from the lowest measurable transport rate for each sampling time. Rating curves fitted to transport rates from 2 min sampling were significantly less steep than those for longer deployment times. However, sampling time explains only a small degree of the large difference between rating curves from bed load traps and a Helley-Smith sampler (2 min sampling).Citation: Bunte, K., and S. R. Abt (2005), Effect of sampling time on measured gravel bed load transport rates in a coarse-bedded stream, Water Resour. Res., 41, W11405,

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

“…This may happen when sampling time is not reduced sufficiently at times when large amounts of coarse organic material are transported during the first rising limb of a highflow season in a forested watershed. Overfilling the bedload trap net reduces its sampling efficiency [61].…”

“…The effects of sampling time on transport rates measured with bedload traps were quantified by [61] in a coarse-bedded mountain stream. Transport rates computed from 2-minute deployment at the lowest transporting flows were about 5 times larger than for 1-hour sampling.…”

“…8). The collection of smaller particle sizes at high discharge is attributable to the small opening of the Helley-Smith sampler as well as the short (2-minute) sampling time that undersamples infrequently moving large particles [61]. Sampler placement on large rocks or blockage of the sampler opening [41] …”

“…One criterion was that the sampler can collect a wide range of gravel and cobble particle sizes over transport rates ranging from one small gravel particle per hour to hundreds of gravel particles per minute. Due to the typically skewed distribution of short-term transport rates with frequent small and infrequent large rates (e.g., [36,37,[54][55][56][57][58][59][60]), bedload traps had to be capable of long sampling times to minimize oversampling at low and undersampling at high transport events [61]. Due to lateral variability of transport over a stream channel cross-section and its change over time, bedload traps had to be able to collect samples at several locations within the cross-section simultaneously.…”

Formation, expansion and restoration of a sedimentation fan: the case of the Arroyo del Partido stream (Spain)

“…On the other hand, direct measurements of bedload transport requires a considerable field effort and are difficult to be conducted especially during floods, even if recent efforts have been made in developing and testing advanced field methods for the direct and indirect quantification of bedload transport [e.g. 1,10,19]. Additionally, representative measurements are difficult to obtain due to the significant spatial and temporal variability associated with bedload movement [6].…”