The scale of destruction resulting from the recent Montecito mudslides was historic, yet the landscape of the Santa Barbara region tells us that such 'debris flows' are common on geologic timescales. Numerical models are now capable of simulating some important aspects of debris flows; however, physics-based “predictions” of such flows remain elusive. In this talk I discuss what is known about the formation, growth and runout of the Montecito debris flows, and the frontier challenges in the physics of fluid-granular flows that limit predictability. The destructive capacity of these flows resulted from two combined effects: (1) soil on the burned hillsides "liquefied" under intense rain and sent high-concentration mudflows into the canyons; and (2) these dense mudflows moved downstream as a wave, that easily picked up boulders from the channels and concentrated them into a front. The mechanisms underlying these two effects are not well understood; however, emerging ideas from physics - including a current program at KITP - help to illuminate a path forward.
