Overview of geotechnical effects of the January 9, 2018, debris-flow and flash-flood disaster in Montecito, California

Abstract

A strong winter storm moved across slopes above Montecito recently burned by the December 2017-January 2018 Thomas Fire, producing disastrous debris flows and flash flooding that killed 23 people, injured many others, and damaged or destroyed residential buildings and community infrastructure. Prior to the event, the National Weather Service issued a flash flood watch, forecasting periods of intense precipitation that had the potential to produce debris flows below recently burned slopes. The Geotechnical Extreme Event Reconnaissance (GEER) Association formed a team to document some of the effects of the Montecito disaster within San Ysidro creek and on bridges along State Route 192, which included geologists, engineers, and a graduate student with a background in public health. The Santa Ynez Mountains are comprised of steeply dipping Tertiary sedimentary rocks that include thick-bedded durable sandstone, with interbeds of shale, claystone, and silty sandstone, which weather to bouldery and cobbly clayey and silty sand sediments. The drainage basins have large upper subbasins separated from the coastal plain where Montecito is located by narrow steep-sided canyons. Alluvial fan deposits on the coastal plain contain boulders of local historic debris flow deposits. The debris flows overwhelmed debris-catch basins at the mouths of canyons and the shallow channels where homes and community infrastructure had been built. The debris flows damaged or destroyed the bridges observed for this study. Stream channels were blocked by accumulated boulder and woody debris in a number of locations during the debris flow event, which diverted flows away from the channels and onto adjacent land with residential and commercial developments. Above-grade stream crossings of some utility pipelines, including water supply, were on the upstream sides of the larger bridges, in part because of difficult and expensive excavation into bouldery alluvial-fan deposits. The flows also ruptured a natural gas pipeline and the leaking gas ignited, causing fire damage during the flood event. Damage in Montecito occurred to property adjacent to the five major stream channels, but life in all parts of the community, and to the region, was severely disrupted, highlighting the need for consideration of potential impacts from rare and extreme geologic processes in land-use decisions and infrastructure design.