Debris flows are water-saturated masses of soil and fragmented rock that can rush down mountainsides, funnel into stream channels and form lobate deposits when they spill onto valley floors. These flows can be devastating to people and property. To understand the variables that contribute to debris flows, scientists use numerical models to test hypotheses about how flows begin and move, and compare the results to real-world examples and physical experiments.
A new approach to debris-flow modelling focuses on the impact of pore pressure within the flow and how it changes flow characteristics. Flow motion can be triggered in several ways, by gradually increasing pore pressure (simulating the effect of rainfall or snowmelt infiltration), gradually reducing the basal friction angle (simulating the effects of rock weathering or decay of roots that help bind soil), gradually changing the slope geometry (simulating erosion or human intervention), or rapidly changing the force of gravity (simulating earthquakes). What happens next depends on pore-pressure feedback that accompanies the expansion or contraction of the material as it moves. For loosely packed sediment, slope failure can lead to positive pore-pressure feedback, making partial liquefaction and runaway debris-flow motion almost inevitable. Alternatively, densely packed sediment with negative pore-pressure feedback may lead to slow or intermittent landslide motion, although it does not preclude debris-flow initiation.
This new depth-averaged numerical model allows feedbacks to develop as the simulation unfolds, to demonstrate that the evolving debris dilation rate, coupled to the evolution of pore-fluid pressure, plays a primary role in regulating debris-flow dynamics. The model helps to explain high mobility exhibited by many large debris flows. Read the two abstracts online: A depth-averaged debris-flow model that includes the effects of evolving dilatancy. I. Physical basis and A depth-averaged debris-flow model that includes the effects of evolving dilatancy. II. Numerical predictions and experimental tests. Also, watch videos of experiments at the USGS debris-flow flume.