A SYSTEMATIC APPROACH TO EVALUATING POST-FIRE HAZARDS AND PRESCRIBING POST-FIRE TRIGGERING THRESHOLDS IN THE FIRST AND SECOND YEAR AFTER WILDFIRE, 2020 RIVER FIRE, MONTEREY COUNTY, CALIFORNIA (Invited Presentation)

The California Geological Survey (CGS), in partnership with other State and Federal agencies, conduct post-fire assessments to identify areas susceptible to post-fire floods and debris flows, and prescribe triggering rainfall thresholds tied to the anticipated runoff response. The triggering rainfall thresholds are incorporated into specific post-fire emergency response plans that define where and when to deploy resources and, in some cases, initiate evacuation orders. The effects of fire and their influence on the type and magnitude of runoff response are often greatest within the first year following fire. However, as conditions change with time, and the threat of post-fire runoff wanes, CGS is often requested by local emergency response agencies to re-assess and prescribe updated triggering thresholds. We introduce a multi-step process that CGS has developed to systematically identify potential hazard areas, document and evaluate post-fire responses in the first year following fire, and prescribe updated rainfall triggering thresholds for the second year following fire. To illustrate the process, we use the River Fire that started on August 16, 2020, and burned over 48,000 acres in Monterey County, California. The River Fire provides a good case study for the following reasons: 1) portions of the fire had not been previously burned in recorded history, which may influence the post-fire response to stressing storms; 2) the burn area was subjected to two storms, the most significant of which occurred on January 26-29, 2021, and was associated with an atmospheric river that triggered hyperconcentrated flows resulting in roads being overtopped, 25 damaged homes , and one person hospitalized; and 3) warm, dry conditions caused by a region-wide drought impeded post-fire recovery. Through applying the systematic process on the River Fire, we learned that additional research is needed to: 1) improve our understanding on how changed conditions within burned watersheds, such as sediment availability, soil cover, and infiltration capacity, may affect runoff and sediment and debris entrainment that may cause flash flooding and debris flows, and 2) develop and improve tools to assess post-fire recovery using remote sensing techniques.