Some of the largest landslides can happen during the first rainy season after a fire. 6 While no major landslides have occurred as of early 2023, some portions of the Columbia River Gorge National Scenic Area remain closed for parts of the year. After the Eagle Creek Fire, the Oregon Department of Transportation, the Oregon Department of Geology and Mineral Industries, and the USGS participated in the creation of a post-fire landslide response plan and hazard map to address potential post-fire debris flow and flood emergencies. It receives over 100 inches of precipitation annually and was already at risk of landslides, with over 80 landslides recorded in the 20 th century. The Columbia River Gorge is made up of steep cliffs, forested areas, hydroelectric dams, and major transportation lines. An example of appropriate caution is the response to the Eagle Creek Fire, which consumed nearly 49,000 acres in Oregon’s Columbia River Gorge between Sept. Post-fire flash flooding and debris flow risks remain for several years after the fire. 1 Mitigate Short-term RisksĮmergency response teams may address risks that can be mitigated within 3 months of a fire, such as developing action plans, gathering resources like sandbags, notifying community members of emergency measures, and clearing drainage areas. Automated cameras may be installed to monitor post-wildfire flooding and debris flows in specific areas. While it is impossible to stop a debris flow, there are tools to give advanced warning of a flood or landslide. They consider local geology, soils, climate, and topography to develop a debris flow hazard assessment and mitigation plan. They utilize aerial imagery and satellite data, geological surveys, hydrologic and debris flow models, and in-person observations to assess site-specific conditions. Local geologists and hydrologists may also assist communities impacted by wildfires. This estimate considers the shape of the land, severity of the fire, soil properties, and rainfall characteristics. The USGS uses geospatial data related to historical debris flows in specific basins to assess the likelihood and severity of a post-burn debris flow in that area. After a fire, national, state, or local teams take the following steps to address debris flow hazards: Assess Risks in the Area They have an online map that notes post-fire debris flow risk locations. Geological Survey (USGS) assesses post-fire debris flow hazard conditions after major fires in the Western United States. 3 This creates a flash flood risk, and a risk of landslides and mudslides. These water-repellant soils cause intense runoff, more than doubling the rate at that water will flow into streams or water channels. The aftermath of burned vegetation and forest floor debris can also produce water-repellent soils, known as hydrophobic soil conditions. After a fire, communities, buildings, and roads that are in or near canyons, at the base of steep hillsides, or excavated hills are at serious risk for post-fire debris flows. In addition to clearing large areas, wildfires can destroy root systems that were holding dirt and rocks in place. When soil is exposed, it is vulnerable to erosion. Why are Debris Flows More Likely After a Fire? They can travel greater distances than landslides, move faster, and carry anything in their path, from boulders to cars. 1ĭebris flows are fast-moving and can cause human fatalities in addition to damaging property. The resulting shift of soil, rocks, mud, and water, which is usually triggered by intense rainfall, is called a post-fire debris flow. This leads to an increased risk of flooding, landslides, and mudflows in areas affected by a fire. Wildfires burn away low vegetation and trees, clearing large swathes of land. Consider risk in your area and learn the best way to respond to flash floods and mudslides that may occur after a wildfire. Communities throughout the western United States are developing strategies to mitigate destructive debris flows in areas that have been affected by a fire. There are also long-term effects on the area impacted by fire, including a higher risk of flash flooding and landslides. The immediate impact of a wildfire is clear: smoky air, burned forests, and possibly property damage. Debris flows are caused by heavy rain in vulnerable areas, such as those with steep slopes that have recently been affected by fire. Learn how debris flows caused by wildfires can have a devastating impact on communities, and what can be done to mitigate the risks with expert guidance from the WFCA.
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