Runoff-initiated post-fire debris flow Western Cascades, Oregon

By: , and 



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.

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Publication type Article
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
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
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