An Evaluation of Post-Wildfire Erosional and Flooding Damage in Southern California

This paper presents a forensic investigation of a large flood event and resulting damage during a substantial rain storm event in 2019 that was preceded by wildfires in the previous two years. Wildfires throughout California in 2017 and 2018 resulted in damages estimated over $20 billion. The Rose Fire of 2017 and Holy Fire of 2018 burned over 23,000 acres of the Santa Ana Mountains in Southern California. About 1,700 acres of the Leach and Dickey Canyon watersheds were burned, denuding the slopes, and leaving the soils vulnerable to erosion. Less than six months after the Holy Fire was extinguished, a 20-year storm occurred at the Leach and Dickey Canyon watersheds, generating large amounts of erosion. The rainfall runoff and the eroded soil/debris caused damage to a dam spillway and discharged uncontrolled debris and stormwater into a 200-ft-wide floodway easement sized to convey runoff from a 100-year storm event. Large amounts of soil erosion occurred at localized areas within the floodway easement, resulting in extensive damage to private properties. A comprehensive study was performed to determine the cause of uncontrolled channel erosion, resulting in damage to the properties along the floodway easement. The study included evaluation of the soil erosion, debris flow, rainfall data, and historical floodway documentation, as well as capturing detailed UAV imagery, and performing a topographic survey. The findings of the study identified unpermitted modifications made by the various property owners along the floodway, including the placement of fill that extended into the floodway easement and other obstructions that had reduced the effective width of the channel by as much as 75%. A detailed review of the investigation methodology, technology utilized, and analysis performed is provided.