Leslie Hsu

Discrete element modeling and large scale experimental studies of bouldery debris flows

INTRODUCTION Bouldery debris flows –consisting of particles ranging from boulders to fine particles with a variety of potential interstitial fluids – are dramatic features in steep upland regions (e.g., iveRson, 1997) and references within). They play an important role in sculpting the landscape in steep upland regions and have the potential for causing tremendous loss of damage and property (e.g., stoCk & dietRiCH, 2006 and references within). Of additional interest is the wide variety of complex behaviors exhibited by debris flows. They exhibit a rich variety of dynamics including complex solid-like and fluid-like behaviour and dynamic spontaneous examples of pattern formation. Debris flows often start to flow under conditions such as a large rainfall event, but the initiation point is difficult to predict. Once they start to move, they exhibit a variety of behaviours from those similar to a shallow fluid flow, to that of an energetic granular material. Segregation of particles by size mediates the behaviour while the debris flow travels and also in the manner in which it comes to rest. Like a granular material, debris flows stop flowing over a bed of nonzero slope; in other words, they resist macroscopic shear. However, the angle of the slope at which they stop is significantly lower than the measured angle of repose of the debris flow giving rise to a so-called long-runout avalanches (PHilliPs et alii, 2006; linaRes-GueRReRo, 2007). This is likely due in part to a dynamic pore pressure effect giving rise to complex fluid-particle interactions ABSTRACT Bouldery debris flows exhibit a rich variety of dynamics including complex fluid-like behaviour and spontaneous pattern formation. A predictive model for these flows is elusive. Among the complicating factors for these systems, mixtures of particles tend to segregate into dramatic patterns whose details are sensitive to particle property and interstitial fluids, not fully captured by continuum models. Further, the constitutive behaviour of particulate flows are sensitive to the particle size distributions. In this paper, we investigate the use of Discrete Element Model (DEM) techniques for their effectiveness in reproducing these details in debris flow. Because DEM simulations individual particle trajectories throughout the granular flow, this technique is able to capture segregation effects, associated changes in local particle size distribution, and resultant non-uniformity of constitutive relations. We show that a simple computational model study using DEM simulations of a thin granular flow of spheres reproduces flow behaviour and segregation in an experimental model debris flows. Then, we show how this model can be expanded to include variable particle shape and different interstitial fluids. Ultimately, this technique presents a manner in which sophisticated theoretical models may be built which consider the evolving effects of local particle size distribution on debris flow behaviour.

Correction to “Experimental study of bedrock erosion by granular flows”

[1] Field studies suggest that bedrock incision by granular flows may be the primary process cutting valleys in steep, unglaciated landscapes. An expression has been proposed for debris flow incision into bedrock which posits that erosion rate depends on stresses due to granular interactions at the snout of debris flows. Here, we explore this idea by conducting laboratory experiments to test the hypothesis that bedrock erosion is related to grain collisional stresses which scale with shear rate and particle size. We placed granular material in a 56-cm-diameter rotating drum to explore the relationship between erosion of a synthetic bedrock sample and variables such as grain size, shear rate, water content, and bed strength. Grain collisional stresses are estimated as the inertial stress using the product of the squares of particle size and vertical shear rate. Our uniform granular material consisted of 1-mm sand and quartzite river gravel with means of 4, 6, or 10 mm. In 67 experimental runs, the eroded depth of the bed sample varied with inertial stresses in the granular flow to a power less than 1.0 and inversely with the bed strength. The flows tended to slip on smooth boundaries, resulting in higher erosion rates than no-slip cases. We found that lateral wall resistance generated shear across the channel, producing two cells whose widths depended on wall roughness. While the hypothesized inertial stress dependency is supported with these data, wear mechanics needs to account for grain dynamics specifically at the snout and possibly to include lateral shear effects.