RAPID: Monitoring postfire geomorphic response on humid slopes of the North Cascade Range, Washington

最近炎热干燥的夏季加剧了美国西部的野火，将影响扩大到太平洋西北沿海地区凉爽潮湿的斜坡，这些斜坡很少被烧毁。越来越多的证据表明，更干燥的气候条件以及日益扩大的野火规模和严重程度可能会使PNW的凉爽潮湿的陡坡更容易受到径流驱动的泥石流、山体滑坡和岩石滑坡的影响。由于野火在沿海斜坡上很少见，人们对其火灾后的地貌反应了解有限。华盛顿州北卡斯喀特山脉最近发生的一场博尔特克里克大火在陡峭的森林斜坡上造成了严重的土壤烧伤，那里的山体滑坡和泥石流是地貌运输的主要形式。陡峭的坡度和土壤疏水性的证据为秋季和初冬期间预计的火灾后径流和地貌反应提供了一个易腐烂的数据收集机会，因为该地区大多数极端降水事件都发生在秋季和初冬。径流驱动的泥石流通常是在土壤渗透率随着土壤湿润和植被生长而改善之前预料到的。该小组将尽快使用地面和无人机激光雷达和航空图像、土壤拒水性和植被状态收集地理空间数据，以确定基线条件，并在触发地貌响应的重大风暴之后收集数据。将创建数字高程模型(DEM)时间序列，以使用DEM差分绘制侵蚀和沉积模式，并测试和改进现有的泥石流模型和数据驱动的滑坡危险制图方法。华盛顿自然资源部滑坡灾害项目将与他们在数据收集和分析方面进行合作。一名女本科生将参与调查和数据分析，接受快速设施工作人员的培训，发展数据后处理技能，并参与DEM差异和侵蚀制图分析。我们目前对火灾后景观反应的概念性理解表明，野火发生后，受影响地区立即面临山洪暴发和径流引发泥石流的高风险。这种高风险来自于在防水的山坡上渗透-过量径流引起的逐渐的泥沙膨胀。随着土壤渗透能力的恢复，渗透过量的径流驱动的泥石流风险在几个月内消退，而饱和驱动的滑坡风险在几年内达到峰值，原因是腐烂的树根失去了支撑土壤的抗拉强度。这一概念框架以及火灾后泥石流预警模型在很大程度上是根据从美国西部和南加州内陆地区获得的观察和数据制定和测试的，这些地区的火灾比太平洋西北沿海地区更频繁，气候更干燥。虽然径流驱动的泥石流事件过去很少被报道，但最近的证据表明，在太平洋西北部沿海发生此类事件的风险越来越大。该小组将开发的数据集将提供一个难得的机会来观察PNW海岸火灾后地貌的性质。如果发生径流驱动的泥石流，这些数据将提供独特的机会，根据地形变化和周围土壤和植被的特征，研究泥石流的启动和输送机制。这些数据还将为研究滑坡提供初始地形条件，预计滑坡将在根部腐烂的几年内发生，但实际上可能发生在极端降雨的观测期间。与植被和降水有关的落石观测非常罕见，他们的观测将有助于完善现有的落石模型。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Recent hot and dry summers have worsened wildfires in the western United States, broadening their impact well into the cool and wet slopes of coastal Pacific Northwest (PNW) that have rarely been burnt. Growing evidence suggests that drier climatic conditions and increasing wildfire size and severity could make cool and wet steep slopes of the PNW more susceptible to runoff-driven debris flows, landslides and rockslides. Because wildfires have been rare in coastal slopes, there is limited understanding with respect to their postfire geomorphic response. A recently burned Bolt Creek Fire within the North Cascade Range of Washington state produced high soil burn severity on steep forested slopes, where landslides and debris flows are dominant forms of geomorphic transport. Steep slopes and evidence of soil hydrophobicity present a perishable data collection opportunity for anticipated postfire runoff and geomorphic response during the fall and early winter, when most of the extreme precipitation events fall in the region. Runoff-driven debris flows are usually expected before soil infiltration rates improve with soil wetting and vegetation growth. This team will collect geospatial data using ground- and UAV-based lidar and aerial imagery, soil water repellency and vegetation states as soon as possible to establish baseline conditions, and following major storms that trigger geomorphic response. Digital Elevation Model (DEM) time series will be created to map erosion and deposition patterns using DEM differencing, and to test and refine existing debris flow models and data-driven landslide hazard mapping methods. The Washington Department of Natural Resources Landslide Hazard Program will collaborate with them in data collection and analysis. A female undergraduate student will participate in the surveys and data analysis, be trained by RAPID facility staff, develop skills for data post-processing and participate in DEM differencing and erosion mapping analysis.Our current conceptual understanding of postfire landscape response suggests that, immediately after a wildfire, impacted areas are at high risk for flash floods and runoff-initiated debris flows. This high risk comes from progressive sediment bulking caused by infiltration-excess runoff on water-repellent hillslopes. While infiltration excess runoff-driven debris flow risk subsides within months as soil infiltration capacity recovers, saturation-driven landslide risk peaks within several years due to decaying tree roots losing tensile strength to hold soils. This conceptual framework, as well as early warning models for postfire debris flows have been largely developed and tested on observations and data acquired from inland regions of the Western United States and Southern California, where fires are more frequent and climate is drier than the coastal PNW. While runoff-driven debris flow events were rarely reported in the past, more recent evidence suggests growing risk for such events in the coastal PNW. The data set this team will develop will provide a rare opportunity to observe the nature of postfire geomorphic in coastal PNW. If runoff-driven debris flows occur, the data will provide the unique opportunity to study their initiation and transport mechanics, as inferred from topographic change and the characterization of the surrounding soil and vegetation. The data will also provide initial topographic conditions for studying landslides, which are anticipated within several years of root decay, but may in fact happen during the observational period under extreme rainfall. Rockfall observations in relation to vegetation and precipitation have been rare and their observations will contribute to the refinement of existing rockfall models.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.