iPlex Lunch spring-2013

Catastrophic debris flows are common following wildfire in steep, bedrock landscapes. However, we lack mechanistic, predictive models for sediment transport on bedrock hillslopes, debris-flow initiation and the role of wildfire in influencing these processes. I will present recent field, experimental and modeling work that shows that sediment transport on steep, rocky hillslopes is different than on soil-mantled slopes, and is controlled by transient sediment storage by vegetation. Upon incineration of vegetation dams, hillslope sediment is rapidly transported to channels by gravity even in the absence of rainfall. Flume experiments on fluvial sediment transport in steep channels show that grain stability is enhanced on steep slopes due to wake turbulence and particle emergence. At even steeper slopes, we find that debris flows can initiate in-channel prior to any fluvial transport, and this process is sensitive to perturbations in bed-sediment size. Consequently, I will argue that debris flows may be more common following wildfire in bedrock landscapes due to incineration of vegetation dams, rapid loading of channels with relatively fine particles, and the transition from fluvial sediment entrainment to mass failure.