Wet Winters, Slopes and Landslides in Portland: What Homeowners Should Know

How common are landslides in Portland?

The state geology agency, DOGAMI, estimates about 20 landslides a year in Portland, at least 1,700 between 1928 and 2016, and annual losses of $1.5 million to $3 million. Most are small, and most happen in winter. The 1996 storm season produced more than 700 landslides in the city, according to material linked to the agency.

The yearly average hides a wide range. Some years have a handful and a stormy year can have hundreds. Rain over several days matters more than a single downpour.

These figures come from DOGAMI's landslide hazard study for Portland, called IMS-57, which maps susceptibility across the city. The mapping covers Portland and Multnomah County. It does not cover Lake Oswego, West Linn or Happy Valley, so we have no local figures for those towns.

It helps to separate hazard from damage. A map shows where slides are more likely, and the loss figures show what slides have cost. Neither tells you whether your particular lot is safe, which only an engineer's site assessment can address.

When and where do slides cluster?

In wet winters, after days of heavy rain on already saturated ground, and mostly on steep slopes. The West Hills, also called the Tualatin Mountains, draw the most attention in Portland. Atmospheric rivers that deliver long, steady rain are the type of storm most associated with wet-ground slides.

Downtown averages 44.07 inches of rain a year, with 6.58 inches in November, 7.35 in December and 6.36 in January by the 1991 to 2020 normals. The ground is usually full by midwinter, so a late-season storm lands on soil that cannot absorb more.

We do not predict slides. The point is that the weeks of high rainfall between November and February are the weeks to watch your slope.

Because the pattern depends on prior rain, a dry fall followed by a wet January is different from a wet October that carries straight through. Watching the slope after a long wet spell is more useful than watching the forecast.

How does water on a slope end up in a house?

Water moves downhill through soil and presses against the uphill wall of a house, seeps through basement walls and floor joints, collects in crawl spaces and overloads drains. If soil moves too, it can crack a foundation, break a drain line and open new paths for water. The water damage can occur without any visible slide.

A basement cut into a slope has an earth wall on one side. That wall resists soil pressure and water at the same time, and a wet uphill wall may be carrying both.

Surface runoff adds to it. Water flowing over the slope can reach doors and windows, especially where a downspout or drain is blocked.

A leaking retaining-wall drain, a broken downspout line or an overflowing gutter can add water to a slope that would otherwise cope. Homeowners often cannot see these from the house, so a walk of the property after heavy rain is worth the time.

The same logic applies to crawl spaces on a hillside. Water entering at the uphill end can run the length of the space and collect at the low end, soaking insulation and joists. See crawl space water damage.

What are the warning signs on a slope?

New cracks in foundations, patios, driveways or retaining walls, doors and windows that start to stick, tilting fences, leaning trees or poles, soil or mud at the base of walls, springs or muddy seeps on the slope, and bulging ground. Any of these after prolonged rain is a reason to call an engineer.

Look after the first heavy rain of the season, and again after a long wet spell. Early signs are easier to deal with than late ones.

Photograph anything you see with a date, using a coin or ruler for scale on cracks. A record over time shows whether a crack is growing.

Not every crack is a slide. Houses settle and concrete shrinks. The pattern, the growth over time and the combination with other signs are what tell an engineer something is changing.

What should you do if soil is moving?

Leave if the house may be unsafe, keep clear of the slope and contact your local building or emergency authority. Do not go below or onto a moving slope. Once it is safe, call for water removal and drying, and arrange an engineer's inspection before major cleanup or rebuilding begins.

A moving slope is a safety and engineering problem before it is a restoration problem. A crew should not be sent into a building an engineer has not cleared.

After the engineer has seen it, a restoration crew can dry and document, and the engineer's report and the moisture log together tell your insurer what happened.

Tell neighbors if the slope above or below you is moving, since slides do not respect property lines. Local authorities may also want to know.

Does homeowners insurance cover landslide damage?

Usually not. Oregon's Division of Financial Regulation notes that landslide is a standard exclusion in homeowners policies, as is flood. Only your insurer can say what your policy does, and the decision is theirs. Some policies cover water damage that follows from a covered cause even where the movement itself is excluded.

That makes records valuable. Photograph the slope, the cracks, the water path and the damage, with dates, and keep any engineer's report.

Ask your agent now, in writing, how the policy treats landslide, earth movement and water that arrives from a slope. It is much better to know before a storm.

If you are buying or refinancing a hillside house, ask the agent how the policy treats earth movement, and whether any endorsement is offered. The answer may shape how much you are willing to spend on drainage.

What does a restoration crew do on a hillside house?

It reads the water pattern inside, removes water and mud, dries the structure and records cracks and movement for an engineer. It does not judge stability or stabilize the slope. The crew tells you what it sees and which specialists, such as geotechnical engineers, drainage contractors and excavators, should follow.

Drilling or cutting into a retaining basement wall is a matter for the right specialist, since the wall may be bearing soil load.

Doors, windows and floor levels tell their own story. Gaps at stairs, diagonal cracks and floors that slope are all worth recording. See slope and landslide water damage.

The records the crew keeps are useful to the engineer. Moisture readings by wall and floor, photographs of each crack with a date, and notes on where water entered show how the building has responded to the wet season. An engineer can compare them with what the slope is doing.

The sequence matters. Engineers and local authorities come first when movement is suspected. Drying follows once the building is safe to enter, and rebuilding comes last.

How can you reduce slope water around a house?

Keep gutters and downspouts clear and discharging well away from the foundation, maintain drains and retaining-wall weep holes, avoid dumping water, yard waste or fill on a slope, and ask a drainage contractor or engineer for a plan. These steps cut the water against the house but cannot guarantee stability.

Plantings can help some slopes and not others, and clearing vegetation without advice can make things worse. Ask a professional before major changes.

If you are buying a hillside house, ask about past water, slope repairs and drainage, and have it inspected with that in mind. Look at the slope after a heavy rain if you can.

Check your downspouts. A downspout that empties onto a slope, or into a pipe that has cracked, can concentrate water on a patch of ground that was never meant to carry it. Extending the discharge well away from the house, or into a proper drain, is one of the least expensive changes.

Walk the property in a heavy rain at least once. You will see where water runs, where it pools and where it leaves, which no map of the lot shows.