2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 23-9
Presentation Time: 10:25 AM


GRAY, Harrrison, University of Colorado - Boulder, Geological Sciences, Boulder, CO, hjgray10@gmail.com

Luminescence equivalent doses of river sediment have been shown to follow a systematic decrease with downstream distance. We test the hypothesis that this reflects progressive bleaching of suspended sediment in turbulent, semi-opaque floodwater, by developing a stochastic model of combined grain transport and luminescence bleaching. We compare theoretical predictions with previously published luminescence single-aliquot data collected along the Mojave River, California, USA. The model assumes that bleaching occurs during fluvial transport, while suspended grains of sand undergo diffusive motion in a turbulent flow field. Grain motion is modeled using a random-walk particle tracking method. We find that the theory can account for the observed downstream decrease in luminescence signal along the Mojave River, but that the data require that (1) mean equivalent doses sediment along the study reach are augmented by lateral influx of unbleached grains from tributaries, and (2) the average sediment grain velocity is substantially slower than the fluid velocity. Water turbidity exerts the strongest control on the bleaching rate of grains during transport. We use the sediment transport parameters from the bleaching model constrained by field data to estimate the volumetric flux of sediment per unit width during a ‘characteristic’ flood. We estimate the mean virtual velocity of sediment during transport as approximately 10% of the mean fluid velocity, and a volumetric sediment flux per unit width of ~0.04 m2/s, consistent with values from similar rivers. Luminescence signals in fluvial sediment exhibit significant potential as a means towards obtaining sediment transport information.