Experimental Studies of the Influence of the Properties of the Matrix of a Debris Flow on Its Erosional Behavior

Abstract

Debris flows – massive flows of fluid and particles – create significant hazard for communities established in or near mountainous regions. There is evidence that the frequency of debris flows and hazards associated with them are increasing associated with changes in macro and micro climate and subsequent changes in rainfall patterns, soil moisture, and local sediment supply. Thus there is an increasing need for a mechanistic understanding of the manner in which physical parameters give rise to certain debris flow behaviors. Recent field and laboratory observations have indicated that the nature of the matrix of rocky debris flows – the muddy or watery interstitial fluid among the gravel and boulders – can have a significant influence on the flow behaviors of a debris flow, from local sorting behaviors to entrainment and depositional behaviors, to associated avulsion behaviors. In this presentation, we report on our experimental investigations of the influence of the rheology and relative density of the matrix of a debris flow on its behaviors using laboratory experiments of particle-fluid flows and their erosive behaviors. To do so, we perform laboratory experiments of erosive debris flows where we systematically vary the interstitial fluid in our debris flow and the erodible bed over which it flows. We track the particles throughout the experiment along with pore fluid pressure which enables us to determine the instantaneous flow dynamics and correlations in the flow and erosion behavior. We find that entrainment rate varies substantially with interstitial fluid viscosity, essentially increasing with decreasing interstitial fluid viscosity, yet this relationship is not strictly monotonic and varies in time. We use these variations as a basis to investigate three specific mechanisms previously suggested responsible for entrainment by debris flows: (1) bed shear stress, (2) granular temperature (essentially, correlated velocity fluctuations), and (3) excess pore fluid pressure (measured pore pressure that exceeds that associated with the weight of the interstitial fluid). We find that both average and instantaneous bed shear stresses and granular temperatures are poorly correlated with associated entrainment rates. Rather, the excess pore pressure is well correlated with entrainment rates, particularly under conditions when the entrainment rate is the greatest. We investigate the source of the excess pore pressure and find that through the majority of the experiment it is likely due to the suspension of the particles in the sparsest most energetic part of the flow, effectively fluidizing them and increasing the effective density of the interstitial fluid in that region. We conclude with a brief discussion of preliminary field work we performed to extend this understanding to field scale debris flows to provide a stepping stone to prevention and avoidance of loss to property and communities due to these dangerous phenomena.