THE EVOLUTION OF PARTICLE SHAPE AND ANGULARITY IN GEOPHYSICAL GRANULAR FLOWS

The size and shape of particles entrained in debris and granular flows influence flow dynamics, affecting erosive power and run-out distance. Particle size distributions evolve through abrasion and fracturing, and by changes in sediment mass during transport. To tease apart these factors, the controls on rates and patterns of particle wear in debris and granular flows must be examined. This study focuses on changes in particle angularity with travel distance, combining laboratory experiments with field measurements. Angularity can indicate proximity to sediment source, assuming abrasion leads to progressive smoothing of particle surfaces; however, particle fracture can create fresh angular surfaces, confounding estimates of travel distance. Experiments conducted using a 1.63 m diameter rotating drum with vanes to prevent sliding to recreate geophysical granular flows. Initially well-sorted, angular coarse particles evolved through wear in transport. Clear water filled the pore spaces before the first run, with fine-grained wear products increasing with distance. Every 0.25 km of tangential travel distance, size is quantified with calipers for all particles above a minimum size. Other metrics include axis ratios and hand-placed equilibrium points. Angularity is determined with shape-measuring parameters including convexity, entropy, and isopometric ratios from 2-D binary images. The same techniques were used on samples collected in the field study of the downstream evolution of rock clasts along a 1 km length of Inyo Creek in the eastern High Sierra. In this catchment, underlain by granodiorite, sediment transport is dominated by debris flows, which leave deposits on the bed and channel margins at slopes >20%. Angularity of particles decreases without a change in axis dimensions. Rocks show evidence of smoothing that slows down after 1 km distance. In contrast, downstream evolution of angularity in the field is more variable, possibly due to mixing of particles from multiple source areas. Results confirm that downstream changes in angularity contain information about proximity to sources and intensity of particle interactions in transport, and may be useful for constraining predictive relations for particle size evolution in granular flows.