During the crippling storm events of February 1996, over 800 landslides occurred within the Portland area, resulting in millions of dollars of damage to homes and property. Steep slopes combined with geologic conditions already susceptible to landslides leaves many Clackamas County properties facing two strikes in terms of landslide risk. The third strike results from the uncontrolled impact of water.
The factors that contribute to landslides can be either geophysical (a condition of the land at that site) or human-caused. Therefore, a landslide can occur on both developed and undeveloped land.
While many landslides are sudden and unexpected, resulting from extreme weather, others can take the form of an imperceptibly slow shifting of the landscape. Known as creeping slides, these slides are identified by curved tree trunks, cracked pavement or retaining walls, doors and windows that stick, leaning fence posts or utility poles, rippled ground or arc-shaped cracks in the soil.
To reduce the likelihood of a landslide, effective management of water and soil conditions on your property is critical. General steps may include:
- Minimize irrigation on slopes
- Check for leaks in underground plumbing and irrigation pipes
- Direct stormwater away from steep slopes, either to a storm drain or a safe natural drainage
- Avoid removing soil from the base of steep slopes
- Avoid dumping soil or yard waste on the top of slopes
- On exposed slopes, plant deep-rooted native vegetation such as oceanspray, ninebark or red-osier dogwood
Slopes where landslides have occurred in the past have a higher likelihood of future movement. To report a landslide, visit the Oregon Department of Geology and Mineral Industries landslide reporting form. In an emergency, contact your local police, fire or public works department.
Download the Oregon Geology Landslide Factsheet (PDF format).