Characterization of a Runoff-initiated Post-fire Debris Flow in the Western Cascades, Oregon: Implications for high severity fire in the Pacific Northwest

Wildfires dramatically alter the hydrologic response of soil in forested hillslopes, which can result in the generation of debris flows. Two types of debris flows occur in response to wildfire, namely debris flows initiated via shallow landsliding and debris flows initiated by runoff and debris entrainment. In the Pacific Northwest, post-fire shallow landsliding has been well documented and studied, but the potential role of runoff-initiated debris flows is not well understood and no post-fire runoff-initiated debris flows have been documented in this region. On June 20th, 2018, approximately one-year after the Milli fire burned 24,000 acres, a post-fire runoff-initiated debris flow occurred on the flanks of Black Crater in the high Cascades in Oregon. High fire severities that occurred during the Milli fire resulted in dramatically reduced soil infiltration rates at the study site, as measured in the field with a mini-disk infiltrometer. We quantified rainfall rates for the storm event that triggered the debris flow, finding that peak 15-minute rainfall rates were 25.4 mmh-1, exceeding the measured infiltration rates at the study site, which had a geometric mean between 24.64 mmh-1 and 24.9 mmh-1 depending on soil texture. Field mapping shows that the combination of high fire severity and topography contributed to the occurrence of a debris flow initiated by runoff at this site. As wildfires increase in frequency and intensity across the western United States, the Pacific Northwest could become more susceptible to runoff-initiated debris flows. Therefore, understanding the conditions that resulted in this debris flow on Black Crater is necessary in understanding how runoff-initiated debris flows operate in the Pacific Northwest