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.

中文翻译：

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径流引发的火灾后泥石流 俄勒冈州西部喀斯喀特

野火极大地改变了森林山坡上土壤的水力学和根系加固，这会促进泥石流的产生。在太平洋西北部，火灾后浅层滑坡已被充分记录和研究，但径流引发的泥石流的潜在作用尚不清楚，该地区仅记录了 2018 年之前的泥石流。2018 年 6 月 20 日，也就是米利大火烧毁 24,000 英亩土地大约一年后，俄勒冈州喀斯喀特山脉的黑火山口两侧发生了径流引发的泥石流。泥石流是通过在靠近火山口边缘的 > 30 度的斜坡上进行分散的钻孔而引发的，并向下移动了 > 1.5 公里。我们在研究地点测量了异常低的土壤渗透率，这可能是由于 Milli 火灾期间烧伤严重。根据附近的 5 分钟雨量计数据，我们量化了引发泥石流的风暴事件的降雨率。我们的结果表明，峰值 15 分钟降雨率为 25.4 mmh -1 ，等于或超过研究地点测得的渗透率，其几何平均值约为 24 mmh -1 。现场测绘显示，高烧严重程度导致了泥石流的开始，汇聚和陡峭的地形促进了该地点泥石流的发展。随着美国西部野火频率和强度的增加，太平洋西北部可能变得更容易受到径流引发的泥石流的影响。因此，描述导致这种泥石流的条件对于理解径流引发的泥石流如何影响太平洋西北部的地形和影响危险至关重要。