Flume tests in wettable and water repellent sands: insights into the initiation of wildfire-related debris flows
Japanese Geotechnical Society Special Publication
It is widely accepted that the majority of post-wildfire debris flows are triggered by surface runoff and sediment bulking in response to short duration, high intensity rainfall events. Infiltration-triggered debris flows that result from discrete bodies of soil dislodging from the slope are less frequent, and tend to appear after longer periods of time or with the vegetation re-growth. Understanding the processes that control the transition, from runoff to infiltration-initiated, will dictate
... ated, will dictate the approach for hazard assessment: rainfall intensity-duration relations for runoff-initiated and slope stability for the infiltration-initiated. Here, in two physical experiments we document the role of soil particle wettability in this transition. We found that the two extremes, fully wettable and water repellent gave opposite responses, slumps for infiltration-initiated in wettable sand and erosion by surface runoff in water repellent sand. From the tests, a continuous capping effect generated by strong water repellency was a necessary condition for erosion and sand bulking to occur i.e. the generation of runoff-initiated debris flows.