The 2009 Nile landslide blocked the Naches River and covered State Route 410.
Washington is one of the most landslide-prone states in the country, with hundreds to thousands of events each year. The direct cost of landslide damage includes the repair of roads and property and the loss of life. Indirect costs, such as loss of property value and tax revenue, and environmental effects, such as the degradation of water quality, can exceed direct costs. The Washington Department of Transportation routinely budgets $15 million a year for cleanup of landslides on highways. Nationally, landslides exceed $2 billion in loss each year and result in an estimated 25–50 deaths (1996 estimate).
WHAT WE DO
Our mission is to collect, develop, use, distribute, and preserve geologic information to promote the safety, health, and welfare of the citizens, protect the environment, and support the economy of Washington. To this end, we conduct and maintain an assessment of landslide hazards in Washington collected from numerous sources. This assessment is not comprehensive, nor is it consistent in terms of methodology or scale of collection. Whatever its limitations, this valuable data is regularly added as it becomes available. We also provide technical assistance to state and local government agencies on the interpretation and application of this assessment. In any given year, we respond to and (or) record hundreds to thousands of landslides.
Work to increase public understanding
The Geology and Earth Resources Division works to increase public and scientific understanding of landslide hazards in Washington State. Please visit our Geologic Information Portal and Geologic Hazard Maps page for the most up-to-date listing of all of our hazard maps.
Consider subscribing to our blog, Washington State Geology News, to receive notifications when new information is published. Also check out Ear to the Ground, published by the Department of Natural Resources.
LANDSLIDE HAZARDS IN WASHINGTON STATE
As our population expands into hilly and mountainous rural forests and agricultural lands, the need for mapping and understanding landslides and landslide hazards becomes increasingly urgent. Coupled with climatic change prediction for more frequent and intense storm events, mapping and understanding geologic hazards can greatly reduce impacts to infrastructure, loss of life, and property.
Landslides cause widespread damage but are often under-represented and understudied compared to other potential geologic hazards such as earthquakes, tsunamis, and volcanoes. Landslides are complex, often moving in numerous different ways, from small shallow slumps and rock topples to deep-seated landslides. Understanding how and why these landslides move help geologists develop mitigation techniques and determine future hazards for roads, houses, infrastructure, and human life.
LANDSLIDE WARNING SIGNS AND TRIGGERS
- Warning Signs of a Landslide
- Common Landslide Triggers
Warning Signs of an Impending Landslide
Landslides are dangerous and unpredictable. Some landslides may provide clues that they are about to happen; others may happen suddenly without any warning signs.
- Cracks growing in the ground; downslope movement of rock, soil, or vegetation.
- Sudden changes in creek water levels, sometimes with increased sediment, especially during or right after large or protracted storm events.
- Sounds of cracking wood, knocking boulders, groaning of the ground, or other unusual sounds, especially if the sound increases.
If you notice these signs or observe a landslide in progress...
Leave the area immediately if it is safe to do so! Landslides are dangerous, unpredictable, and can occur rapidly. A landslide can easily destroy or bury a car or house. Report the problem immediately to your county Emergency Manager.
Warning Signs of a Potential Landslide
- A hillside that has increased spring and (or) seep activity, or newly saturated ground, especially if it was previously dry.
- New or developing cracks, mounds, or bulges in the ground.
- Sagging or taut utility lines; leaning telephone poles, deformed fences, or or tilting of trees on a hillside.
- Sticking windows or doors; new and (or) growing cracks in walls, ceilings, or foundations.
- Broken or leaking utilities, such as water, septic, or sewer lines.
- Separation of structures from their foundation; movement of soil away from foundations.
- Changes in water well levels or water wells that suddenly run dry.
- Movement of the ground.
What to do if you think a landslide may occur
Signs of a potential landslide should be reported to your county Emergency Manager so that the area can be properly evaluated.
Landslides exist throughout Washington State and can be triggered in many ways.
- Rainfall—Prolonged or intense rainfall and rain-on-snow events can saturate soils and trigger landslides.
- Earthquakes—Intense shaking during earthquakes can cause the ground to fail. The earthquakes of 1949, 1965, and the Nisqually earthquake of 2001 produced numerous landslides throughout the Puget Sound region.
- Water-level changes—Rapid lowering of water levels can trigger landslides, especially along dams, coastlines, reservoirs, and rivers.
- Human activities—Vegetation removal, mining, loading on a slope, excavation at the base of a slope, and leakage from pipes can all trigger landslides.
- Geology—Easily weathered rock types and sandy or clay-rich soils are especially susceptible to landslides.
Union Pacific branch rail line collapse during the 1965 Puget Sound earthquake near Tumwater, Washington. Photo by G.W. Thorsen.
REDUCE YOUR RISK
The Aldercrest–Banyon landslide complex was active for nearly a year and eventually destroyed 138 homes. Photo by Tim Walsh, DGER.
Most shallow landslides and flows occur during or up to several days after a heavy rainfall or rapid snowmelt event. Deep-seated landslides can occur at any time. Earthquakes can also initiate landslides, so be alert if you feel the ground shake and you are near a slope.
- Educate yourself about your landslide risk. Locations that experienced landslides in the past are more likely to have future landslides. The Washington Geologic Information Portal contains the most comprehensive listing of landslides available. However, because not all landslides are mapped, the absence of a landslide in the database does not indicate the absence of risk.
- Make a landslide emergency plan. Know what areas near your home or work are at risk. Know which areas are safe and how to reach them in an emergency.
- Consider evacuating prior to storm events which can cause sudden flooding and (or) landslides.
- Avoid living in locations that are hazardous. Areas above or below steep slopes, or in areas known to have frequent landslides, are more likely to experience landslides in the future.
- Consult a licensed engineering geologist or licensed geotechnical engineer if you would like a site-specific evaluation.
- Control runoff from buildings and roads so it flows away from steep slopes and into natural drainages or storm drains.
During dangerous weather
- Seek out advisories and warnings during and after intense rainfall events. Check the NOAA Weather Radio, your local TV stations, and the Shallow Landslide Hazard Forecast website.
- Don't assume that highways are safe—watch for collapsed pavement, mud, fallen rock, or other debris on the roadway.
- Listen for loud or unusual sounds. These can be indicators of an imminent landslide. If you think there is danger, evacuate immediately.
- Keep away from landslide-prone areas.
The 2011 Pearrygin Creek debris flow north of Twisp, Washington spread out when it emerged from a confining canyon. Photo by Jack Powell, DGER.
WHY DO LANDSLIDES HAPPEN?
A landslide generally refers to the downhill movement of rock, soil, or debris. The term landslide can also refer to the deposit that is created by a landslide event. The data on this website are meant to provide general information only; real landslides have many variables.
The Role of Gravity
Landslides nearly always move down a slope. This is because the force of gravity—which acts to move material downhill—is usually counteracted by two things: (1) the internal strength of the material, and (2) the friction of the material on the slope. A landslide usually occurs because the internal strength of the rock, soil, or sediment becomes less than the force of gravity.
Steeper slopes have a greater proportion of downhill force.
The Role of Water
The addition of water to material on a slope can make landslides more common. This is because water adds significant weight to the slope as it seeps into the ground and becomes groundwater. This extra weight adds to the gravitational force. Water also lowers the strength of the material which can make it less able to withstand the force of gravity. Water also reduces friction and increases pore fluid pressure.
The addition of water can decrease friction, decrease strength, and increase the weight of the material. All of these properties are important for landslides.
Pore fluid pressure is the pressure that water exerts on all of the nearby grains. When there is little water and low pore fluid pressure, most grains are touching and the material is relatively strong. When there is a lot of water and the pore fluid pressure is high, the water buoys up the grains and forces them apart. This buoyant force weakens the material and is also why your feet sink into beach sand after a wave washes over and adds water. These processes help to explain why landslides are much more common during the rainy season, and especially common during or right after large storms.
Water weighs a lot (about 8 pounds per gallon). If 1 acre of land soaks up 1 inch of rain, the land gains about 234,000 pounds. The addition of water can increase pore fluid pressure and separate grains. This and other processes decrease strength and can promote landslides.
The Role of Friction
The amount of friction between a deposit of rock or soil and the slope that it rests on plays a large role in when landslides happen. Imagine trying to slide a large rock along a flat surface—it's very difficult because of the friction between the rock and the surface. Pushing the rock is easier if the surface slopes downhill or is slippery. The same is true for landslides—for the same kind of material, steeper slopes have less friction, making landslides more common. Any change to the Earth's surface that increases the slope (for example, river incision or the removal of material at the base of a slope by humans) or that reduces the friction of a slope (such as the addition of water) can increase the likelihood of a landslide.
The addition of water increases weight and reduces friction.
TYPES OF LANDSLIDES
Landslides can occur as flows, slides, or falls and topples. A major difference between the types of landslides is their depth and the amount of water they contain. The images in this section are modified from the U.S. Geological Survey.
It is important to note that the type or size of landslide is largely irrelevant if you happen to be the one in harm's way. Under the appropriate circumstances, any landslide type can pose a serious hazard to life or property.
- Shallow and Deep Landslides
- Falls and Topples
In general, landslides can be categorized as shallow or deep-seated and this difference can determine their speed and size. Shallow landslides typically occur during the winter months in western Washington and during the summer months in eastern Washington, but are possible at any time. Deep-seated landslides can also occur at any time. Many of the landslide areas in Washington are a mixture of different slide types.
Shallow landslides are rooted in the soil layer and often form slumps along roadways or fast-moving debris flows down valleys. These types of landslides are often called 'mudslides' by the news media. Shallow landslides also occur as flows, slides, or rockfalls and topples.
Deep-seated landslides are those that fail below the rooting depth of trees and vegetation. They are often slow moving, but can also move rapidly. Deep-seated landslides can cover large areas and devastate infrastructure and housing developments. These landslides usually occur as translational slides, rotational slides, or large block slides. Deep-seated landslides are typically much larger than shallow landslides, in terms of both surface area and volume.
A deep-seated landslide may appear stable for years, decades, or even centuries. These long-lived features can be partially or entirely reactivated for a variety of reasons.
Flows are generally a slurry mixture of water, soil, rock and (or) debris that moves rapidly downslope. Flows may or may not be confined to a channel.
Earthflows may have a characteristic 'hourglass' shape. The slope material loses strength and runs out, often forming a bowl or depression at the head. Flows usually occur in fine-grained material on moderate, water-saturated slopes.
Debris flows usually occur in steep gullies, move very rapidly, and can travel for many miles. They may contain more coarse material than a mudflow when channelized. Slopes where vegetation has been removed by fire or humans are at greater risk for debris flows and many other types of landslides.
Debris avalanches are unchannelized debris flows that move very rapidly. They typically do not mobilize far and sometimes move like a snow avalanche.
Lahars are debris flows that originate on volcanoes. A volcanic eruption can rapidly melt snow and ice, causing a deluge of rock, soil, ash, and water that accelerates down the slopes of a volcano, devastating anything in its path. They can travel great distances and damage structures in flat areas far from their source. Communities near rivers draining Mount Rainier and Glacier Peak are at greatest risk.
Lahar on Mount St Helens. Image courtesy of USGS.
Lateral spreads occur on very low-angle slopes toward a free face such as a cliff or embankment. Movement is accompanied by cracking of the ground. Failure is often caused by liquefaction (when soil is transformed from a solid to a liquid), usually because of an earthquake.
Soil creep is the very slow (inches/year), steady, downward movement of soil or rock. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges.
Slides are the downslope movements of soil or rock along a discrete or narrow failure surface and can be deep-seated or shallow. The initiation of slides, like flows or rockfalls, is sensitive to steep slopes, the additional weight of water or other loads, and friction along their base.
Translational slides usually fail along geologic discontinuities such as faults, joints, bedding surfaces, or the contact between two rock types. They move out or down along a planar surface with little tilting, and can travel great distances. Translational slides can contain loose sediments or large slabs of bedrock.
Block slides are a type of translational slide that occur when large and relatively intact slabs of rock or earth are rapidly transported downslope. These type of landslides can be large and damaging and occur where alternating layers of strong and weak rock slope downhill.
Rotational slides (slumps) are landslides that occur along a curved or spoon-shaped surface. Back-tilting may occur near the scarp of the landslide and there is often a toe of displaced material. Rotational slides often occur because the internal strength of the material is overcome by its own weight. They are usually composed of relatively loose or unconsolidated material.
Falls and topples
Falls and topples are usually rapid, downward movement of large pieces of rock or debris. Sometimes this is enough rock to cover a road or block a stream or river. Rockfalls and topples are common in Washington’s mountain highways.
Landslide figures in the "Types of Landslides" section are modified from U.S. Geological Survey Fact Sheet 2004-3072 and Cruden, D. M.; Varnes, D. J., 1996, Landslide types and processes, in Turner, A. K.; Schuster, R. L., editors, Landslides—Investigation and mitigation: National Academy Press; National Research Council Transportation Research Board Special Report 247, p. 36-75.
SOME HISTORIC LANDSLIDES IN WASHINGTON STATE
|Some historic landslides in Washington State|
|Slide or area name||Date|
|(A) SR 530 (aka Oso or Hazel)||Mar. 2014|
|(B) Nile||Oct. 2009|
|(C) Aldercrest–Banyon||Feb–Oct. 1998|
|(D) Mount St. Helens||May 1980|
|(E) Lake Roosevelt||1944–1953|
|(F) Tacoma Narrows||Apr. 1949|
|(G) Ribbon Cliffs||Dec. 1872|
A more comprehensive list of significant landslides in Washington is available here
City/County Emergency, Health, and Planning Departments
Washington Division of Geology and Earth Resources
- Fact Sheet: What are landslides?
- Fact Sheet: Landslide hazards
- Geologic Information Portal
- Shallow Landslide Hazard Forecast Map
- Washington State Forest Practices Board Manual Evaluating Unstable Slopes
Washington State Department of Transportation
Federal Emergency Management Agency (FEMA)
U.S. Geological Survey (USGS)
Washington State Department of Licensing