Smartrock Transport From Seconds to Seasons: Shear Stress Controls on Gravel Diffusion Inferred From Hop and Rest Scaling

• Published in: Geophysical research letters Vol. 48; no. 9
• Main Authors: Pretzlav, Kealie L. G., Johnson, Joel P. L., Bradley, D. Nathan
• Format: Journal Article
• Language: English
• Published: 16-05-2021
• Subjects: anomalous diffusion; gravel tracer; Halfmoon Creek; hop and rest durations; sediment transport; smartrock
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL091991

Our ability to test probabilistic models linking clast movements to bedload diffusion is most limited by basic field data, because measuring transport statistics during natural floods is difficult. We embedded accelerometers and gyroscopes into artificial cobbles, and measured transport during 28 daily snowmelt floods in Halfmoon Creek, Colorado, USA. The tracers captured ≈6 orders of temporal magnitude of rest durations in one data set for the first time. Motions and rests suggest a scaling transition around ≈12.5 min from subdiffusion to superdiffusion with increasing shear stress and timescale. We interpret that diurnal hydrograph cyclicity may cause another diffusion scaling break at ≈12 h. Shear stress controls and scaling uncertainties may explain differences in diffusion exponents found in several field data sets, suggesting that gravel superdiffusion scaling may be relatively universal over minutes to seasons. Methodologically, “smartrocks” can quantify field transport probabilities previously only possible in laboratory experiments.
Plain Language Summary
During floods in mountain streams, gravel moves downstream but also spreads out. Predicting this spreading—called diffusion—is useful for river restoration and for assessing flood risks. We used “smartrocks” containing accelerometers and batteries to measure exactly when individual sediment grains moved during a month‐long flood in the Rocky Mountains of Colorado. The data were used to calibrate various equations to improve predictions of gravel diffusion during future floods.
Key Points
Sensor‐embedded cobble tracers quantify bedload rest and hop durations during a ≈month‐long 10‐year flood in a natural mountain stream
Shear stress is quantified as a first‐order control on bedload diffusion
A single field data set resolves scaling transitions from subdiffusion to superdiffusion with increasing shear stress and timescale