UC Santa Barbara

Massive debris flows devastated Montecito, California, USA on January 9th, 2018. The damage from the flows resulted in 23 deaths and greater than $200 million in property damage. Given this destruction, the community fears the possibility of another event. Prior to this study, the recurrence interval of high-magnitude debris flows in the Santa Barbara, California area was unknown despite evidence of them in nearly every canyon. Certainly small to moderate flows following wildfire are common and very large events are rare, but approximately how rare? The evidence of past debris flows occurs as large boulder fields and boulder levees along the banks of the streams that flow out of the canyon mouths and into the city. A major limitation to understanding how often these events happen is the applicability of traditional dating methods. In order to assess the viability of alternative dating methods, data were collected on four different weathering indicators to be used as potential proxies for the time since deposition. These weathering indicators included weathering rind thickness, boulder compressive strength, clast roundness, and clast color. It is hypothesized that the boulders of the debris flow deposits have increased in weathering rind thicknesses, decreased in compressive strength, increased in roundness, and become redder in color with increased weathering time at rates that can be calibrated to numerical ages. To test these hypotheses, the data of each weathering indicator were correlated to a series of numeric ages based on radiocarbon analyses, soil chronology, 21Ne exposure dating, and incision rates. In the end, although the hypotheses for weathering rind thickness, boulder compressive strength, and clast color were accepted, weathering rind thickness was concluded to be the most successful relative age dating method as it has the highest R2 value when calibrated to numeric age - a value of 0.80.

The calibrated rate of weathering rind thickness development linked to numerical dates is used to estimate the amount of time since the deposition of boulders through debris flow processes. The equation that estimates the rate of weathering rind development is t=0.012(w^3.80), where t is the predicted age in thousands of years and w is the mean weathering rind thickness of the boulders within the deposit in millimeters. This analysis served to test the overarching hypothesis for this study that high-magnitude debris flows (on the relative order of magnitude as the 2018 Montecito event) occur once every 1 to 2 thousand years. Results of statistical paired tests and geospatial analyses of the weathering rind data and their respective predicted ages indicate that at least 17 distinct high-magnitude debris flow events are represented in the 30 measured deposits of our study area. These events are estimated to have occurred over the span of the last ~96 ky, indicating a maximum recurrence interval of 5.6 ky. However, since the age control is much better for the younger events, using the last ~9 ky instead, in which there have been at least 5 events, the resulting recurrence interval is ~1.7 ky, confirming the hypothesis that high-magnitude events occur on the order of every 1 to 2 ky. Based on these values, probability estimations suggest that there is approximately a 6% chance of another high-magnitude event happening in the next 100 years. Overall, this study has improved the understanding of the chronology of high-magnitude debris flows and the general understanding of the geomorphic history of the area.