Particle size reduction in debris flows: laboratory experiments compared to field data from Inyo Creek, CA

Abstract

Rock particles in debris flows are reduced in size through abrasion and fragmentation. Wear of coarse sediments results in production of finer particles, which can alter the bulk material rheology influencing runout distance. Particle wear also affects the size distribution on hillslopes before delivering the sediment to the fluvial channel network. A better understanding of the controls on particle wear in debris flows is needed to infer flow conditions from debris flow deposits, estimate the initial size of sediments entrained in the flow, model debris flow dynamics, and map hazards. I used three rotating drums to create laboratory debris flows across a range of scales. Drum diameters range from 0.2 to 4.0 m, with the largest drum able to accommodate up to 2 Mg of debris, including boulders. I began the experiments with well-sorted, angular coarse particles, which evolved through particle wear in transport. The fluid was initially clear water, which rapidly acquired fine-grained wear products. After each 0.25 km of tangential travel distance, I quantified the particle size distribution. I calculated particle wear rates by fitting the Sternberg equation to the statistics of particle size and mass distributions. Mass wear rates are 2.9, 4.9. and 11%/km in the small, medium, and large drum, respectively. Rates of coarse particle wear and production of fragments and fine particles scale with the rate of energy expenditure per unit bed area, or unit drum power. I use this power scaling to estimate a mass particle breakdown rate of 13%/km at Inyo Creek, CA.