DEBRIS FLOW PATH CHANGE IN RESPONSE TO TOPOGRAPHY AND ACTIVE TECTONIC PROCESSES: IMPLICATIONS FOR HAZARD EXPOSURE

The ~M6 Montecito Debris flow of January 9, 2018 was a tightly linked wildfire / debris flow event that took 23 lives and damaged or destroyed several hundred homes. Relative flow chronology, based on boulder weathering with limited numerical dating, is used to correlate paths of prehistoric debris flows. Nearly the entire community on the piedmont of Montecito and Santa Barbara is impacted by debris flows that range in age from the late Pleistocene to present. The topography of the upper piedmont is significantly affected by the south-side-up reverse Mission Ridge fault system (MRFS). Examination of weathering rinds of the older Pleistocene debris flows confirm that the Rattlesnake Creek-Mission Ridge debris flow is folded over the ridge. Over time, faulting and lateral propagation, linked to uplift of marine terraces (uplift rate of ~ 0.5 to 1 m/ky), significantly altered debris flow paths, moving exposure to the hazard ~ 700m west. Paths of debris flows in Montecito are maintained across the relatively low folds and young fault scarps of the MRFS. Debris fans are prone to channel avulsions caused by radial fan channels being blocked by boulder deposition and changing channel morphology. A several thousand-year-old debris flow in the lower Montecito Creek fan flowed into the sea. The flow may be the penultimate event in Montecito. The flow (~ 2 m thick) consists of a lower boulder layer with a mud tail on top and is below an uplifted late Holocene marine wave-cut platform (earthquake terrace) with a prehistoric Native American site at the surface. The 2018 debris flow flowed into the sea about 430 m west of the present outlet of Montecito Creek (mapped in 1887 as an active channel on the lower fan). The main conclusion is that debris flows interact significantly with the Santa Barbara and Montecito landscape, which has been modified by uplift, folding, and faulting to change flow paths, and, thus, increase the exposure of people and property to the hazard.