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

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-bank-full 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).