Assessment of a post-fire debris flow impacting El Capitan Watershed, Santa Barbara County, California, U.S.A.

In the summer of 2016, the Sherpa Fire burned 30.2 km (super 2) in steep terrain in western Santa Barbara County, California. Rainfall events in the subsequent wet season produced damaging post-wildfire flooding and debris flows. This paper presents a case study along a watershed within the burned area, El Capitan Creek, that (1) describes the events and conditions that led to the post-wildfire flooding and debris flow events, and (2) documents the debris flow deposits and inundation zone impacted by the events. Observations compiled after three post-wildfire precipitation events indicate that three distinct flow processes impacted El Capitan Creek between 19 and 22 January 2017. These flow processes included watery flows, hyper-concentrated flows, and debris flows. The velocity and concentrated nature of these flows caused overbanking and channel avulsions that resulted in damaged roads, bridges, pipelines, and major infrastructure damage to the El Capitan Canyon Resort. These events occurred only 1 year prior to the devastating Montecito debris flows of 2018 and call attention to the conditions that produced these impactful flows and highlight the timing and conditions that generate post-wildfire debris flows. Information from case studies such as this can guide decision makers and emergency managers to understand the hazards and risks that floods and debris flows pose on communities below steep mountain drainages and support the development of sound protocols to help reduce the threat to life, property, and infrastructure in downstream communities.