HR: 1340h
AN: H53B-0471    [Abstracts]
TI: Ground Plates May Improve Accuracy of Helley-Smith Bedload Samples
AU: * Bunte, K
EM: kbunte@engr,colostate.edu
AF: Engineering Research Center, Colorado State University, Fort Collins, CO 80523 United States
AU: Abt, S R
EM: sabt@engr.colostae.edu
AF: Engineering Research Center, Colorado State University, Fort Collins, CO 80523 United States
AB: A Helley-Smith bedload sampler (HS) with a 0.076 m opening is often used to collect bedload transport in mountain gravel-bed streams due to its ease of use. However, the relationship between transport and discharge computed from HS samplers in mountain gravel-bed bed streams is relatively flat. Rating curves exponents are typically 2-5 as opposed to values of 5-18 obtained by samplers capable of capturing all available sediment amounts and sizes. This study explores the reason for the relatively flat rating curves. Gravel bedload transport has been measured intensively at several field sites in Rocky Mountain gravel-bed streams during snowmelt runoff seasons using side by side a 0.076 m opening sheet metal HS sampler with a 3.22 expansion ratio and bedload traps. Bedload traps have an 0.3 by 0.2 m aluminum frame to which a 0.9-1.6 m long net with a 3.5 mm mesh width is attached, and they are fastened onto stationary ground plates (0.3 by 0.4 m) anchored to the stream bottom. This set-up permits sampling times of about 1 hour for bedload traps. The HS bedload samples were collected in the conventional manner of placing the HS sampler repeatedly directly onto the bed of the channel. Gravel transport rates resulting from the HS sampler deployed for 2 minutes at 12-18 locations on the bed were between 1.5 and 3.7 orders of magnitude higher at 50% of bankfull flow than those computed from bedload trap samples. Both samplers had similar results near bankfull flow. The difference in sampling time between the two samplers has been shown to contribute only a minor portion of observed differences. To test whether an occasional entrainment of unwanted bedmaterial particles caused by direct placement of the sampler onto the bed influences bedload measurements, results from conventional deployment of the HS sampler on the bed and from placement onto the ground plates were compared to those obtained from the bedload traps. The total collection time per sample was the same for both deployments (2 minutes on each of the 15 locations and 5 minutes on each of the 6 plates). The experiment was conducted at Hayden Creek, a steep (4%) mountain gravel-cobble bed stream in south central Colorado. When gravel transport was relatively high (90 g/m/min) near bankfull flow, transport rates were similar for all samplers and placements. When only a few small gravel particles were moving per hour at flows 50% of bankfull, gravel transport rates from the HS placed on the bed were approximately 400 times larger than those obtained from placing the HS sampler onto the ground plates, while gravel transport rates from the HS on the ground plates were generally similar to the ones computed from the bedload traps. Sand transport was higher by only a factor of 20 for the HS on the bed, suggesting that placement of the HS directly on the bed produced particularly high transport rates of gravel. As the HS sampler on the bed also collected larger gravel particle sizes than the HS on the ground plates, study results suggest that much of the difference in transport rates observed between a HS sampler placed onto the bed and bedload traps at low transport is due to the disturbance and entrainment of bedmaterial particles when setting the HS sampler onto the streambed. Placing the Helley-Smith sampler onto ground plates may thus greatly increase the accuracy of measured gravel transport rates collected at flows below bankfull and align them with transport rates derived from the bedload traps.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: Fall Meeting 2005