Schedule May 24, 2018
Geophysical aspects of the 2018 Montecito debris flows
Tom Dunne (UCSB)

n December 2017, a series of wildfires devastated the slopes surrounding the Santa Ynez Mountains overlooking Ventura and Santa Barbara counties. They destroyed vegetation and loosened the soil, which enabled massive mudslides after a short but intense rainstorm in Montecito, CA in early January 2018. This event caused fatalities, injuries, and massive destruction of property. The creation of debris flows from the rainstorm - that is, muddy flows of silt, soil, and rock mixed with rainwater that rush down mountains and entrain large boulders - are not well understood. This study uses a range of measurements gathered soon after the event, which include laser-imaged before-and-after surveys of terrain, photographs, rock samples, and eye witness accounts, to explain the flow dynamics that caused the destruction. Initial analyses of the data indicate that formations of shale (fine-grained rock from compacted silt and clay) called rills (small, stream-like shapes) help explain the initial source of the mud. The observations imply complex flow characterized by mud flow depths that change as they move downstream, as yet unknown mud-boulder interactions, and scouring of terrain affected by the composition of the mud. Much of the data has yet to be analyzed, but will provide valuable information on the generation of the mud and what affects its behavior as it begins to move. This information will be essential for assessing hazard risks for debris flows in the future.

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