Field Comparisons of the Elwha Bedload Sampler and an Acoustic Gravel-transport Sensor: Middle Fork of the Piedra River, Colorado, USA

Ten simultaneous bedload measurements were made with an Elhwa sampler and an acoustic-gravel-transport sensor (GTS) on the Middle Fork of the Piedra River in southwestern Colorado near the end of the spring freshet in water year 2001. The purpose was to compare bedload samples with acoustic measurements acquired under field conditions. Upstream of the measurement site, the river drains 86 km² of andesite, ash flows, tuffs, and breccias in the San Juan Mountains, contributing a relatively high sediment load to the river system. The channel transitions from step-pools at high elevations to a plane bed with a slope of 0.018 in the study reach. Channel width, mean depth and bank-full velocity at the site are: 13.6 m, 0.52, and 1.5 m s^-1. The D₅₀ of the riverbed surface is 0.08 m which is 6 to 40 times larger than the D₅₀s of the bedload samples. D₁₆ and D₈₄ of the bed = 0.02 and 0.21 m respectively. Water discharges from 7.3 to 9.3 m³ s^-1 transported about 0.01 kg of gravel m^-1 s^-1 in the channel. Transport of coarse gravel (8-64 mm) ranged from 0.00063 to 0.024 kg m^-1 s^-1. The Elwha sampler is a portable, pressure-differential trap with a 0.2 m wide by 0.1 m high opening. The acoustic sensor is a 0.025-m wide by 0.1 m high strip of PVDF piezoelectric film connected to a signal processor and bonded to an aluminum pressure plate. When the plate is struck by stones, the GTS produces signal peaks with areas that are accurate measures of stone momentum. The GTS was calibrated with steels balls dropped on the pressure plate in still water to develop a curve of ball momentum as a function of peak areas. Based on these calibrations, the standard error of the GTS momentum estimates is 0.0017 kg m s^-1. Five transects with 30 verticals, each occupied for 60 s, were completed with the sampler and GTS separated by < 1 m. Five additional verticals were occupied for about 1800 s each with the instruments separated by < 0.5 m. The trapped material was sieved and weighed and the water slope and local depth were used to calculate the velocity and momentum for each size fraction. The total momentum of coarse gravel trapped during 273 minutes of sampling per meter of bed was 16.4 kg m s^-1. During the same time, the GTS measured total momentum of 14.9 kg m s^-1 per meter of bed. Paired Elwha and GTS measurements for transects and single verticals are poorly correlated (r² = 0.1 to 0.4), however, suggesting that additional samples and longer sampling times will be required to resolve cross-channel and temporal variations in transport.