Montecito debris flows of 9 January 2018: Physical processes and social implications

Abstract

Montecito, California, has a complicated Quaternary history of debris flows, the most recent being the Montecito debris flows of 9 January 2018, which were wildfire-debris flow–linked events that took 23 lives and damaged or destroyed several hundred homes. Relative flow chronology, based on boulder weathering, incision rates, and soil dates with limited numerical (radiocarbon and exposure) dating, is used to identify paths of prehistoric debris flows. Topography of debris flow fans on the piedmont is significantly affected by the south-side-up reverse Mission Ridge fault system. Examination of weathering rinds from Pleistocene debris flows confirms that the Rattlesnake Creek–Mission ridge debris flows are folded over the ridge, and that lateral propagation linked to uplift of marine terraces (uplift rate of ~0.5–1 m/k.y.) significantly altered debris flow paths. As communities continue to rebuild and live in these hazard-prone areas, disaster risk reduction measures must take into account both spatial and temporal components of vulnerability. This field guide includes four stops from Montecito to Santa Barbara. The first stop will be to observe debris flow stratigraphy over the past ~30 ka beneath an earthquake terrace and a prehistoric Chumash site on the beach near the Biltmore Hotel in Montecito. The second stop will be at San Ysidro Creek in San Ysidro Canyon, the site of the largest Montecito debris flow that occurred on 9 January 2018. We will discuss source area and processes of the debris flow, and take a short hike up the canyon to visit the debris flow basin and a ring net designed to reduce the future hazard. The final two stops will explore the debris flow chronology of Santa Barbara over the past ~100 ka. Figure 1 shows the location of the field-trip stops. There is no road log as field sites can be found with a search on a smartphone.

A Case Study and Numerical Modeling of Post-Wildfire Debris Flows in Montecito, California

“…In the north of the study area are the Santa Ynez Mountains, whose rock formations are predominantly made of weak sandstones and shale and are susceptible to weathering and incision [33]. There are considerably loose soil layers on the rock surface of these mountains due to excessive weathering.…”

Post-Wildfire Debris Flows in Montecito, California (USA): A Case Study and Empirically Based Debris Volume Estimation

“…It is in the Santa Barbara County of California; the Santa Ynez Mountains are in the north of the study area. These mountains are composed of weak sandstones and shale, which are especially susceptible to weathering and incisions [38]. Due to excessive weathering, these mountains have significant loose soil layers on the surface, which are often covered by vegetation.…”

Risk as a process: a history informed hazard planning approach applied to the 2018 post-fire debris flows, Montecito, California

“…These debris flows have been examined by researchers from a range of disciplines. Most papers and reports published on the events either look at the physical processes (Kean, 2019;Keaton, 2019;Alessio et al, 2021), the emergency management aspects (Goto et al, 2021) and short-term consequences (Keller et al, 2020a;Keller et al, 2020b;Goto et al, 2021), but not the processes that encouraged development in hazardous areas over recent decades.…”

“…Between the steep Santa Ynez Mountains and the coast sits a forested upland, a wide swath of urbanized areas, traversed by north-south trending creeks and east-west trending Highways 101 and 192. Debris flows after wildfires are common in Southern California (Keller et al, 2020b).…”

“…The debris flows were generated on lands recently burned by the Thomas Fire, the biggest fire recorded in California to that date, which burned 1,140 km 2 in Santa Barbara and Ventura Counties over the prior month (Diskin, 2020). The denuded slopes and impermeable soils resulting from the fire, together with intense precipitation, produced the debris flows (Keller et al, 2020b;Kean et al, 2019).…”