WEATHERING-RIND CHRONOMETER: RECURRENCE INTERVAL OF HIGH-MAGNITUDE DEBRIS FLOWS IN SANTA BARBARA AND MONTECITO, CA

High-magnitude debris flows (HDFs) are a major sediment-related hazard in steep landscapes such as the mountainous regions of the southwest United States. On January 9th, 2018, a HDF event occurred in Montecito, California, USA, and caused 23 deaths and ~$1 billion in economic losses. Field evidence shows that Montecito and the adjacent Santa Barbara regions have experienced multiple HDF events in the past, but the timing and frequency of those events remain relatively unknown. Here, we use the thickness of weathering rinds on meter-sized boulders as a chronometer to estimate the frequency that HDFs are triggered (and subsequently preserved) in Santa Barbara, California. We measured the weathering-rind thicknesses of boulders that were emplaced as levees on debris flow fans and abandoned above the active channel. We measured 30 debris-flow deposits in Santa Barbara County and calibrated their thicknesses to a series of numeric ages based on radiocarbon analyses, soil chronology, ²¹Ne exposure dating, and incision rates. We find a strong positive correlation between weathering rind thickness and the numeric age dates (R² = 0.80). Statistical analyses of the weathering-rind data indicate that at least 17 distinct HDF events are represented in the 30 measured deposits. We estimate a recurrence interval for HDFs of ~1.7 ky using the last ~9 ky record, in which there have been at least 5 events. Our estimated recurrence interval translates into a ~6% chance of another HDF event in this area in the next 100 years.