Landslides: What homeowners should know
In 2009, a million-dollar home in the Marylhurst area was destroyed by a local landslide. The landslide was triggered by heavy rains, and a subsequent lawsuit claimed that the removal of trees on the slopes above the house decreased soil stability and caused the landslide.
Damage from landslides is not covered under normal homeowner's insurance, so this is an area where the average homeowner is often "on his or her own."
While landslides are unpredictable in some respects, that doesn't mean there is nothing homeowners can do to help protect themselves.
The starting point for protecting yourself is to understand what is meant by the term "landslide," and what can trigger one.
The Oregon Department of Geology and Mineral Industries has published an excellent educational document entitled "A Homeowner's Guide to Landslides."
A landslide refers to any downslope movement of soil, rock or slope debris. Mudslides, mudflows, debris flows, rock falls and slumps are all terms describing landslides. The two most common types of landslides are rotational slides and earthflows.
A rotational slide occurs when a large section of earth is transported downslope by sliding on a discrete detachment surface. The mass of soil and rock will partially disaggregate as it moves downslope. Rotational slides can occur when slopes are too steep or in areas where the base of the slope is undercut by either natural or man-made processes.
The developing landslide at Rattlesnake Ridge near Union Gap in Washington state is an example of a slow-moving, rotational slide that may have been initiated by quarrying activity, which undercut the base of the slope.
An earthflow occurs when water mixes with soil or debris, and the liquid-like mixture flows rapidly downslope.
The devastating mudslides in Southern California this winter are good examples of earthflow type landslides. Two of the common conditions that trigger this type of landslide are water-saturated ground and a loss of vegetation cover. In the case of the California mudslides, the late 2017 wildfires removed the vegetation cover, and heavy rain in January 2018 saturated the soil with water.
Earthquakes can also initiate both types of landslides. So, what can homeowners do to protect themselves?
When you buy a home
The ideal time to start thinking about landslide risk is when you are purchasing a home.
The presence of previous landslides in an area is an indicator of higher risk. The City of West Linn website contains natural hazard maps that show both areas of high landslide risk and areas where historic landslides have been mapped.
Consult these maps to understand if your prospective home is in a higher risk zone. If you get serious about buying in a higher risk area, then you may want to consider contacting a licensed engineering geologist or geotechnical engineer.
When you are viewing a home ask yourself several questions: Is the home on a steep slope? Is the slope forested, and are there any activities that are removing trees or vegetation from the slope above or below the house? Tree roots play a vital role in stabilizing a slope and preventing landslides.
Are the bases of the trees on the slopes around the house consistently curved and bending in a downslope direction? This type of curvature is an indicator that slow soil movement is occurring.
Is the base of the slope below the home being undercut by natural or man-made activities?
If the answer to any of these questions is "yes," then you should put a warning flag in your notes that a deeper investigation may be needed.
You should have a good look at the inside and outside of the home. On the inside check for cracked floors, water seeping into the basement or crawl space, bulging walls, or fixtures and windows that are out of alignment with the walls. On the outside look for open cracks in the soil, sidewalks, foundations, or driveways. Also look for tilted retaining walls or broken utility pipes. These signs all indicate potential problems related to soil movement or slope instability.
If you already own a home
Current homeowners may still want to check the West Linn City hazard maps. Even though the chances of a landslide are low in most areas, you should be vigilant for key warning signs and engage in proactive planting across your property.
Maintain healthy vegetation using trees and shrubs that take up water efficiently. Examine your drainage and direct water away from slopes when possible.
You need to be aware of landscape alterations both upslope and downslope from your property. Removal of trees and other vegetation on steep slopes creates a significant risk since it can destabilize the slopes, even in an established neighborhood.
If you notice any changes to drainage like new springs/seeps or newly forming drainage gullies, then you should be cautious since this is reflecting a change in the sub-surface hydrology. Soils that are oversaturated with water increase the chance of earthflows, and new springs indicate increased water in the local soil.
Remember that just because you live on steep slopes or in an old landslide area, it does not imply imminent danger. However, don't get so complacent that you fail to observe changes in the local neighborhood that could affect you.
Stay informed and observant, and if you suspect a problem contact the City at 503-657-0311 or by using the YourGOV app (http://westlinnoregon.gov/YourGOV). Contact the police or fire department in an emergency situation.
William House is an earth scientist and writer in West Linn.