The National Weather Service (NWS), Oregon Office of Emergency Management (OEM) and the Oregon Department of Geology and Mineral and Industries (DOGAMI) issued weather advisories late last week concerning heavy rain, flooding and the potential of landslides.
The Chief contacted DOGAMI geologist Bill Burns for insight into the landslide risks along the Oregon Coast and inland during and after such weather events.
The Chief: What specific areas are at risk for landslides and why?
- Canyon bottoms, stream channels, channel outlets
- Steep slopes and bases of steep hillsides
- Areas where slopes of hills have been altered by excavated or steepened or recently burned
- Places where slides or debris flows have occurred in the past
Our Statewide Landslide Information Database for Oregon (SLIDO) map can help people understand where the potential landslide risk is generally higher or lower. https://gis.dogami.oregon.gov/maps/slido/
As you will see much of western Oregon has a moderate to high susceptibility to landslides and thus our concern during large storms like the one happening now.
The Chief: Specifically, how does the heavy rainfall increase the risk of landslides?
Burns: During the next several days, the NWS is forecasting an atmospheric river type storm in northwestern Oregon. This means we could have intense rain over prolonged time. Storms like these are the ones that have caused landslides in the past and thus the concern right now. The soils can’t drain fast enough, and the combination of too much water too fast, steep slopes, and weak geology all contribute to the potential for landslides.
Even after the heavy rains, what should property owners and travelers watch for that could indicate further erosion and landslides?
We created a homeowners guide to landslide that can help with this questions. https://www.oregongeology.org/Landslide/ger_homeowners_guide_landslides.pdf
The Chief: We understand landslides can move quickly. How is that and what recommendations do have for personal safety?
Burns: Certain types of landslides, notably debris flows, can move rapidly (10s of miles per hour). These types of landslides tend to start in the upper portions of a drainage and can accelerate and grow as they flow down the channel. When they reach the mouth of the channel they tend to fan out and inundate the areas at the mouth of the channel.
Unfortunately, the channel outlets are sometimes where people and infrastructure are located. These types of areas are the most concerning. People that live or are traveling in these areas should be extra cautious and be prepared to evacuate. These types of landslides are common in post-fire areas. The combination of the potential long distance from where they start to where they end up and the speed they move is why they are of extra concern and can be life threating.
Since the 2020 Labor Day fires, the Oregon Department of Geology and Mineral Industries (DOGAMI) landslide team has been focused on the potential post-fire debris flow hazard. We were just awarded a large multiyear, multiagency FEMA grant to work on understanding the post-fire debris flow risk and identify areas of risk throughout many of these recent fires in western Oregon. The end goal of these projects is to use science to understand the post-fire debris flow landslide risk and collaborate with the communities in these wildfire areas to reduce the landslide risk.
The following information is provided by Oregon Department of Geology and Mineral Industries.
Landslides take place most often where they’ve occurred in the past. They’re also more common in areas that have recently been burned by a wildfire.
• Make an escape plan and build a kit.
• Be aware of warning signs of possible landslides:
• Increased pooling of water or newly wet ground.
• New cracks or unusual bulges in the ground, street pavements or sidewalks.
• Soil moving away from foundations.
• Tilted or bent trees.
• Sagging or taught utility lines.
• Sunken or broken road beds.
• Leaking or broken water pipes.
• Reduce the chances of landslides by:
• Draining water from surface runoff, downspouts and driveways well away from slopes.
• Planting native groundcover on slopes.
• Refraining from adding water to steep slopes.
• Avoiding placing fill soil and yard debris on steep slopes.
• If you suspect imminent danger, evacuate immediately. Inform affected neighbors if you can, and contact your public works, fire or police department.
• Listen for unusual sounds that might indicate moving debris, such as trees cracking or boulders hitting one another.
• If you are near a stream, be alert for any sudden increase or decrease in water flow and notice whether the water changes from clear to muddy. Such changes may mean there is debris flow activity upstream so be prepared to move quickly.
• Be especially alert when driving—watch for collapsed pavement, mud, fallen rocks and other indications of possible debris flow.
• If you are ordered to or decide to evacuate, take your animals with you.
• Stay away from the slide area. There may be danger of additional slides.
• Check for injured and trapped persons near the slide without entering the direct slide area. Direct rescuers to the person’s location.
• Listen to local radio or television stations for the latest emergency information.
• Watch for flooding, which may occur after a landslide or debris flow. Floods sometimes follow landslides and debris flows because they may be started by the same event.
• Look for and report broken utility lines to appropriate authorities. Reporting potential hazards will get damaged utilities turned off as quickly as possible, preventing further hazard and injury.
• Check the building foundation, chimney and surrounding land for damage. Signs of damage may help you assess the safety of the area.
• Replant damaged ground as soon as possible since erosion caused by loss of groundcover can lead to flash flooding.
Additional information about landslides is available on the Oregon Department of Geology and Mineral Industries (oregongeology.org) and the Washington Department of Natural Resources (dnr.wa.gov) websites.