The parade of rainstorms flowing across Oregon and into Columbia County is increasing the risk of landslides, according to state officials.
The danger statewide is evident in the latest photos of huge boulders that nearly crushed a car along Highway 62 near Prospect in Southern Oregon to a landslide that closed the Historic Columbia River Highway this week between Multnomah Falls and the Angel's Rest trailhead in Northwest Oregon.
According to Robert A. Houston with the Oregon Department of Geology and Mineral Industries (DOGAMI), landslides are caused by a combination of factors, including the type of geology, the slope, and usually water.
“With repeating storms, the soils do not have enough time to drain and so the water builds up in the soil and leads to failure or a landslide,” Houston said.
According to a DOGAMI executive summary report about the landslide hazards in Columbia County, landslides and debris flows are common in the Oregon Coast Range due to the combination of high precipitation, steep slopes, and landslide-prone geologic units. Cutting through the northern Coast Range, the U.S. Highway 30 (Oregon State Highway 92) corridor is prone to slope instability.
The study indicates that the Highway 30 corridor in Columbia and Clatsop Counties is at significant risk from landslide hazards. Landslides cover 25% of the study area, and 33% of the City of Clatskanie is covered by large, deep landslides. The large number of people and structures residing on these deposits highlights the potential danger present and shows the need for public awareness on landslide hazards.
“The areas that have had landslides before (historic and prehistoric or ancient) are the areas of first concern,” Houston said. “This is because landslides tend to happen in the same places repeatedly through reactivation or the combination of factors talked about in the above question.”
People, structures and roads located below steep slopes in canyons and near the mouths of canyons may be at serious risk. Dangerous places can include:
- Canyon bottoms, stream channels and areas of rock and soil accumulation at the outlets of canyons.
- Bases of steep hillsides.
- Road cuts or other areas where slopes of hills have been excavated or over-steepened.
- Places where slides or debris flows have occurred in the past.
According to DOGAMI’s A Homeowners Guide to Landslides, a landslide is the downward slope movement of rock, soil or debris. Debris flow, earth flow, rock fall, mudflow, mudslide, and slump are also terms for landslide.
Landslides can take human life. However, even a few inches of slope movement can disrupt septic, sewer and water lines and crack foundations severely damaging or destroying your home, according to the guide.
If you live on or near a steep slope, the guide encourages you to look for warning signs of landslides by evaluating your property for signs of landslide movement. Many, but not all, signs of landslide activity are listed below. A high score may indicate the presence of a landslide.
Inside Your Home:
- Cracks in walls
- Nails popping out of walls
- Bulging walls
- Separation of chimney from walls
- Creaking/popping noises
- Light switches coming out of walls
- Doors/windows hard to shut
- Twisted beams
- Cracks in floors
- Water seeping into basement
Outside Your Home:
- Changes in surface drainage
- Bulges in retaining walls or tilting of walls
- Cracks developing in the soil
- Pistol-butted or bent trees
- Broken water, utility, or sewer lines
- Cracks in sidewalks or foundation
- Stretched or leaning utility lines
The guide also recommends actions property owners can take to reduce the chances of landslides, which include:
- Draining water from surface runoff, downspouts, and driveways well away from slopes.
- Planting native ground cover on slopes.
- Consulting with a professional before significantly altering existing slopes uphill or downslope of your home.
Oregon Department of Transportation spokesman Lou Torres said ODOT is also keeping a close watch on the landslide potential in Columbia County following the parade of rainstorms this winter.
Torres said there are a number of areas on all of the Coast Range highways that are being monitored daily for slides by ODOT crews, including on Highway 30 at milepost 33 and milepost 39 between Columbia City and Rainier.
"These are two potential problem areas but there are others," Torres said. "A major rockfall repair in 2018 solved a problem we had at milepost 63 just west of Clatskanie. However, we still get occasional rocks and debris that fall on to the road."
Torres said the major rock fall mitigation work at the milepost 63 site was a $1.3 million project to scale back the rock cliffs above that were causing rock slides, which closed lanes of Highway 30 over the years.
"But most of the time we are cleaning up the slides,” he said. “On occasion, when our geologists are analyzing a particular slide area, we will use some scientific instruments placed at locations."
According to Torres, the slides along the Coast Range routes and Highway 101 are caused by heavy rain that saturates the soils.
“That, combined with the steep slopes and weak underlying rock formations, are the main causes of the slides, rock falls and slumps,” he said.
Slides are part of the geologic history of Northwest Oregon, according to Torres, and drivers using the Coast range routes and Highway 101 should take precautions.
“Slides can occur at almost anytime on these highways," Torres said." However, during periods of heavy rainfall, travelers should recognize that there is a higher likelihood that slides can happen. Travelers should slow down, drive carefully, pay attention to their driving which means avoid distractions, and watch for hazards."
According to DOGMI, average annual repair costs for landslides in Oregon exceed $10 million, and severe winter storm losses can exceed $100 million. As population growth continues and development into landslide susceptible terrain occurs, damage and loss from this natural hazard will continue to grow.
For more information:
- View the DOGAMI web map to see landslide areas https://gis.dogami.oregon.gov/maps/slido/
- View the homeowner’s guide to landslides at https://www.oregongeology.org/Landslide/ger_homeowners_guide_landslides.pdf