Debris flows and sediment transport at Yucaipa Ridge and impacts to Oak Glen and Forest Falls area, southern California, following the 2020 El Dorado and Apple Fires

Abstract

The 2020 El Dorado and Apple Fires burned a combined area of ~225 km2 in the San Bernardino Mountains, California, USA, including much of the steep terrain of Yucaipa Ridge. Wildfire increases the potential for debris flows, flooding, and rockfall hazards. A California Watershed Emergency Response Team assessed these hazards to assist affected portions of San Bernardino and Riverside counties with their emergency preparedness efforts. These assessments identified high hazard sites in the Forest Falls and Oak Glen areas on the north and south flanks of Yucaipa Ridge, respectively. Subsequent monitoring by the California Geological Survey documented 16 flow events from nine study area basins and their impacts to downstream communities generated by six storms between fall 2020 and August 2023. Tropical Storms Kay (2022) and Hilary (2023) and monsoonal thunderstorms on August 13, 2022, produced the most widespread and severe impacts. Observations of flow characteristics were then used to determine flow type behavior and estimate flow velocities and discharge for 11 flow events, and the total volume of mobilized debris was estimated for three flow events. Debris flow velocities were found to range from 2.1 to 11.2 m/s (4.7–25 mph) and associated discharge values were estimated to range from 12 to 311 m3/s (~425–11,000 cfs). Observed discharges were then used to assess and calibrate previous flow bulking multipliers for post-fire emergency planning. Volumes of debris mobilized during three events were estimated based on hydraulic methods, field observations, and lidar differencing, respectively, with resulting volumes ranging from ~19,000 m3 to more than 100,000 m3. Rainfall data were also acquired and reviewed to assess rainfall intensity thresholds for the generation of post-fire debris flows. Concurrently, studies were conducted to assess sediment transport and yields based on the distribution of fractured rock outcrops, colluvial debris, and dry ravel generated by fire impacts, as documented by repeat surveys of selected source basin areas. The surveys showed a mixture of fractured rock outcrops, and steep soil mantled slopes provided both coarse (>3 m) boulders and fine sediment. The latter was preferentially delivered to debris flow prone channels as post-fire dry ravel. Debris flows generated by Tropical Storms Kay and Hilary mobilized all of these grain size fractions downstream to alluvial fans and developed areas.