While Oregonians welcomed rain in late September 2017, after the Columbia River Gorge Eagle Creek fire scorched large sections of forest, consuming 48,831 acres, destroying homes and forcing evacuations, landslides posed a new threat to the area.
The Eagle Creek fire carried smoke and ash across the Portland area to the western boundaries of Washington County, until an unusually hot, dry summer gave way to cooling fall temperatures and moist conditions. Although heavy rain in October was a welcome relief, it created a new danger of landslides and flash floods in the Gorge.
Gorge Geologic History Reveals Landslide Vulnerability
Bill Burns, engineering geologist with the Oregon Department of Geology and Mineral Industries (DOGAMI), predicted high risk for landslides throughout fall and winter and for several years to come. Compiling information from historic records and looking at landslides through aerial LIDAR images that reveal contours of past landslides in the area on the Oregon side of the River, Burns found evidence of 286 recent, historic, and prehistoric landslides that had traveled over a mile. The aerial images showed areas of the Gorge already susceptible to landslides overlapping with areas of the Eagle Creek fire, indicating where landslides will be more likely to occur in the future.
“We can’t predict when and where the next landslide events will occur,” Burns said. “But by improving information about existing landslide locations, we better understand what areas might be hazardous during storm events, or where taking action to reduce risk is a good idea.”
When Is a Landslide a Debris Flow?
When fire strips away trees, shrubs and grasses, water can infiltrate the ground, making it more prone to sliding. Ground movement can be expected after landslides, causing surface erosion first, followed by rock falls, and then debris flows.
A debris flow is an extremely destructive landslide that moves faster than a person can run. During a debris flow, masses of rock and earth saturated with water create a flowing river of mud that can travel suddenly with no warning, at avalanche speeds for a mile or more, growing in size as it picks up trees, boulders, cars and other materials.
Areas below steep slopes in canyons and near the mouths of canyons are especially hazardous for landslides and debris flows. The most dangerous places are:
- Canyon bottoms, stream channels, and areas of rock and soil accumulation at the outlets of canyons
- Bases of steep hillsides
- Embankments along roadsides
- Areas where slopes of hills have been excavated or over-steepened
- Places where slides or debris flows have occurred in the past
How to Protect Yourself from Landslides and Flash Floods
The National Weather Service offers the following advice. In the event of heavy rain, those traveling and hiking in the area should watch and listen for National Weather Service flash flood warnings, and be aware that an intense, short burst of rain may be particularly dangerous, especially after a longer period of heavy rainfall and damp weather. When you hear there is potential for debris flows and landslides in your area, stay alert and track the flood watch by radio, TV, weather radio or online.
If you are in an area susceptible to landslides and debris flows, consider leaving if it is safe to do so. Listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. Watch for a trickle of falling mud or debris that may precede larger landslides.
If the water in a stream or creek suddenly turns muddy or the amount of water flowing suddenly decreases or increases, it is a warning that the flow has been affected upstream by a debris flow that may soon be coming downstream.
Be alert while driving, watching the road for collapsed pavement, mud, fallen rocks, and other indications of possible debris flows. Remember that driving during an intense storm is hazardous. Travelers are advised to avoid driving at night, when there is low visibility.
For more information about protecting yourself from landslides and debris flows see: https://www.ready.gov/landslides-debris-flow
For weather alerts and sudden severe conditions in and around the Columbia River Gorge see: http://www.weather.gov/
Friends of the Columbia Gorge posts trail alerts at: https://gorgefriends.org/hike-the-gorge/trail-alerts.html
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