WILDFIRE-INDUCED DRY RAVEL AND DEBRIS-FLOW INITIATION IN STEEP, BEDROCK LANDSCAPES

Catastrophic debris flows are common following wildfire in steep, bedrock landscapes, but the mechanistic connection between wildfire and debris flows is not well understood. I will present field 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 dry ravel in the absence of rainfall. Flume experiments and theory on fluvial sediment transport 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 by in-channel bed failure prior to any fluvial transport, and this process is sensitive to perturbations in bed-sediment size. This work suggests that debris flows may be more common following wildfire in bedrock landscapes because rapid loading of channels by dry ravel forces a transition from incipient sediment transport by fluvial processes to mass failure.