Rainfall Triggering of Post-Fire Debris Flows over a 28-Year Period near El Portal, California, USA

Abstract

Wildfires frequently affect the steep hillslopes near El Portal, California (United States), a small community established during the California Gold Rush in the mid-1800s. In addition to the historical significance of El Portal, State Route 140 (SR 140) is a major transportation and economic corridor connecting the San Joaquin Valley to Yosemite National Park (YNP). In 2019, an estimated 4.5 million tourists visited and accessed YNP via SR 140. In the years after wildfires, the burned watersheds produced debris flows during intense rainfall, impacting the El Portal community and motorists traveling on SR 140 and local roads. The steepness of the hillslopes and confinement of the valley limit options for mitigating debris-flow risk. As such, emergency managers are left with evacuation orders or temporary road closures as the best options for risk reduction. The effectiveness of these options is highly dependent on establishing an accurate local rainfall intensity-duration threshold that officials can use to guide emergency response actions and timing. We present an overview of the rainfall conditions that initiated 12 post-fire debris-flow events near El Portal from 1991 to 2018 and objectively define rainfall intensity-duration thresholds from triggering rainfall rates. Our results highlight the modest rainfall rates that triggered debris flows in these steep watersheds, while radar data from more recent events (2012–2018) portray the spatial variability of intense rainfall in the area. Additional rainfall monitoring is needed to provide a robust rainfall threshold that will effectively mitigate risk for residents and motorists while minimizing the impact of road closures and evacuations.

How Long Do Runoff‐Generated Debris‐Flow Hazards Persist After Wildfire?

“…We compiled 536 runoff‐generated debris‐flow records from 25 fires that offered variable timing of debris‐flow response following the fire (28–807 days after fire ignition) and a wide regional coverage across five western U.S. states (Figure 1a). This data set combined published debris‐flow inventories (DeGraff et al., 2022; McGuire, 2021; McGuire et al., 2021; Michel et al., 2019; Neptune et al., 2021; Rengers et al., 2021, 2023; Staley et al., 2016; Swanson et al., 2022; Tang et al., 2019; Thomas, Lindsay, et al., 2023) with new records we synthesized from field observations, photos, and reports collected by U.S. Geological Survey (USGS) staff and/or partner agencies (Graber, 2023) (Table S1 in Supporting Information ). Refer to Figure 1b for an example debris‐flow inventory from the 2012 Waldo Canyon Fire, Colorado.…”

“…Runoff‐generated debris flows can threaten lives and property in the months to years following wildfires. Although most runoff‐generated debris flows occur within Year 1 (i.e., the first 365 days after fire ignition) (DeGraff et al., 2015), numerous cases have been documented in Year 2 (i.e., days 366–730 postfire) (DeGraff et al., 2015, 2022; Porter et al., 2021; Riley et al., 2013; Staley et al., 2016; Tillery & Rengers, 2019) or in Year 3 or later (Cannon et al., 2011; Jordan, 2016; Wagner et al., 2013; Wang et al., 2022). The likelihood of runoff‐generated debris flows following fire is thought to scale inversely with vegetation and soil‐hydraulic recovery of the burned area (Thomas et al., 2021).…”

The Rainfall Intensity‐Duration Control of Debris Flows After Wildfire

“…Rapid soil‐hydraulic recovery, however, may limit pathways for postfire debris‐flow initiation in this region compared to southern California (Perkins et al., 2022; Thomas et al., 2021). Overall, short‐duration rainfall thresholds have performed well for the central California coast and the central Sierra Nevada (De Graff et al., 2022; Neptune et al., 2021).…”

“…Rapid soil-hydraulic recovery, however, may limit pathways for postfire debris-flow initiation in this region compared to southern California (Perkins et al, 2022;Thomas et al, 2021). Overall, short-duration rainfall thresholds have performed well for the central California coast and the central Sierra Nevada (De Graff et al, 2022;Neptune et al, 2021). Documentation of postfire debris flows in northern California, approximately midway between southern California and western Oregon/Washington, is limited (e.g., Felling et al, 2019), or, in the case of our study region in the northern Sierra Nevada, nonexistent.…”