Controls on Erosion and Transport of Mass by Debris Flows.

摘要

Debris flows and sediment-rich floods are common transport processes in steep valleys that dissect mountainous terrain. Rapid movement, high discharges, and the transport of large quantities of coarse-grained sediment characterize these hydrologically-driven processes. Despite the importance of debris flows for landscape evolution and natural hazards, there is not an agreed upon mechanical framework to describe how debris flows entrain sediment, erode bedrock, and transport mass. As a result, large uncertainties remain pertaining to the potential for a debris flow to grow through entrainment of loose sediment, the rate at which bedrock is eroded, and the manner in which changes in climate, tectonics, or land-use might affect steep landscapes.;I use a combination of in situ measurements of debris-flow dynamics from a natural laboratory located in the headwaters of a debris-flow dominated catchment, grain-scale numerical modeling of granular flows, and digital elevation model data to constrain the mechanics controlling erosion and transport of mass by debris flows. In particular, I quantify: (1) the characteristic flow properties of natural debris-flow surges and how they relate to total travel distance; (2) the mechanics controlling the rate of bed-sediment entrainment and growth of flow volume; (3) the degree to which debris flows erode the bedrock channel floor; and (4) how changes to channel or flow properties influence the erosive potential of a flow.;Monitored debris-flow events were composed of multiple surges, each with clear variation of flow properties along the length of the surge. Relatively fine-grained and water-rich tails that had a wide range of pore-fluid pressures pushed along steep, highly resistant, visually unsaturated surge fronts of coarse-grained material. Surges with large excess pore-fluid pressures, and thus lower frictional resistance, had longer travel distances. The dominant control of non-equilibrium pore pressure on flow resistance makes the prediction of travel distance based solely on channel properties problematic. During passage of dense granular-fronts as well as water-rich, inter-surge flow, bed sediment was entrained from the sediment-surface downward in a progressive fashion. Despite similar flow properties and thicknesses of bed sediment entrained across all events, time-averaged entrainment rates for bed sediment that was saturated prior to flow arrival could exceed entrainment rates for dry sediment by over an order of magnitude. As a result, a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment.;Once all shielding bed sediment was entrained, flow particles could directly impact the bedrock channel floor. Average bedrock erosion rates that resulted were ∼1 cm/yr-1. Variability in impact-stress magnitude increased linearly with the mean basal stress and measured probability density functions were generally best fit by Pareto or power law distributions with well-defined means and variances. Using the grain-scale numerical modeling, I observed a nonlinear increase in particle-bed impact forces and impact energy as a function of slope. In contrast, particle impact flux increased at small slopes, but then decreased linearly as slope increased beyond a threshold value. Predicted erosion rate, which scales as the product of impact energy and impact flux, increased as a nonlinear function of slope. Steep landscapes in which millennial scale erosion rates have been quantified display a similar nonlinear relationship between erosion rate and channel gradient. This suggests that the grain-scale mechanics quantified here place strong controls on steepland morphology that evolves over thousands to millions of years.