UAS IMAGERY OF DEBRIS BASIN VOLUMES AND INFRASTRUCTURE SUPPORTS WATERSHED SCALE ASSESSMENT AND PREPARATION FOR CHANGING FLOOD AND DEBRIS–FLOW HAZARDS IN SANTA BARBARA COUNTY, CALIFORNIA

The “one-two punch” of fire followed by extreme rainfall can magnify the probability of large flooding and mass wasting events across mountainous semi-arid landscapes globally. Such a “one-two punch” occurred in Santa Barbara County, CA with the December 2017 Thomas Fire and Jan 8-9 2018 extreme rainfall event, causing powerful mud and debris flows with consequent loss of life, destruction of property, and severe impact to the local economy. A regional network of debris basins, built in the late 1960s-1970s, played an important role in protecting some downstream communities. Monitoring and assessment of basin infrastructure and capacity to retain debris volume over a large, rugged region – both as part of decadal-scale maintenance and before/after major flood events - is critical for disaster preparedness, but costly. To date, it has cost over $30 million to remove debris from the Jan 8-9 debris flows.

We use low-altitude UAS imagery as a low-cost, precise, and rapid assessment tool for geologic research, hazard planning/mitigation/response and outreach for those debris basin-managed catchments affected by the Thomas Fire and January debris flows. Across watersheds spanning an order of magnitude in area (340-2,060 ha) and debris basin design volumes (15,000-200,000 m³), we connect ground data, UAS imagery, airborne and satellite image data to assess relationships among geomorphologic variables (watershed area, slope), vegetation cover, fire history and debris flow volumes removed from basins across catchments. From low-altitude UAS aerial imagery and structure-from-motion (SfM) photogrammetry, we produced 3D terrain models to 1) compare image-based volume estimates to the last available basin survey data; 2) examine 40+ yr. changes in effective debris basin retention volumes - starting (design) volume to current volume – as functions of watershed geomorphology (area, average slope), vegetation cover, fire history, and basin design types, and 3) relate current basin volumes to USGS-estimated Preliminary Hazard Map predictions for debris flow volumes in extreme rainfall events. As climate change and continued land development expose larger populations to debris flow hazards in the region, we demonstrate the value of low-cost UAS imagery for geoscience and management of debris basin infrastructure.