Runoff-initiated post-fire debris flow Western Cascades, Oregon
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Abstract
Wildfires dramatically alter the hydraulics and root reinforcement of soil on forested hillslopes, which can promote the generation of debris flows. 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 only one previous to 2018 had been documented in the region. On 20 June 2018, approximately 1 year after the Milli fire burned 24,000 acres, a runoff-initiated debris flow occurred on the flanks of Black Crater in the Oregon Cascade Range. The debris flow was initiated via dispersed rilling on > 30-degree slopes near the crater rim and traveled > 1.5 km downslope. We measured exceptionally low soil infiltration rates at the study site, likely due to high burn severity during the Milli fire. Based on nearby 5-min rain gage data, we quantified rainfall rates for the storm event that triggered the debris flow. Our results show that peak 15-min rainfall rates were 25.4 mmh−1, equaling or exceeding the measured infiltration rates at the study site, which had a geometric mean of ~ 24 mmh−1. Field mapping shows that high burn severity resulted in the initiation of the debris flow and that convergent and steep topography promoted the development of a debris flow at this site. As wildfires increase in frequency and intensity across the western USA, the Pacific Northwest could become more susceptible to runoff-initiated debris flows. Therefore, characterization of the conditions that resulted in this debris flow is crucial for understanding how runoff-initiated debris flows may shape terrain and impact hazards in the Pacific Northwest.
Study Area
Publication type | Article |
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Publication Subtype | Journal Article |
Title | Runoff-initiated post-fire debris flow Western Cascades, Oregon |
Series title | Landslides |
DOI | 10.1007/s10346-020-01376-9 |
Volume | 17 |
Publication Date | April 01, 2020 |
Year Published | 2020 |
Language | English |
Publisher | Springerlink |
Contributing office(s) | Geologic Hazards Science Center |
Description | 13 p. |
First page | 1649 |
Last page | 1661 |
Country | United States |
State | Oregon |
Other Geospatial | Western Cascades |