United States Search and Rescue Task Force

Landslides

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Damage To Homes By Landslides

 

WHAT ARE LANDSLIDES?

What Are Landslides?Debris Flow in San Mateo, California [Click on picture for full image]

Landslides are rock, earth, or debris flows on slopes due to gravity. They can occur on any terrain given the right conditions of soil, moisture, and the angle of slope.  Integral to the natural process of the earth's surface geology, landslides serve to redistribute soil and sediments in a process that can be in abrupt collapses or in slow gradual slides.  Such is the nature of the earth's surface dynamics.  Also known as mud flows, debris flows, earth failures, slope failures, etc., they can be triggered by rains, floods, earthquakes, and other natural causes as well as human-made causes, such as grading, terrain cutting and filling, excessive development, etc.  Because the factors affecting landslides can be geophysical or human-made, they can occur in developed areas, undeveloped areas, or any area where the terrain was altered for roads, houses, utilities, buildings, and even for lawns in one's backyard. They occur in all fifty states with varying frequency and more than half the states have rates sufficient to be classified as a significant natural hazard.

La Conchita, California, LandslideThe U.S. Geological Survey, working with other federal agencies, has efforts underway to study, plan, and mitigate landslide risks. So have some communities across the country. Many deal with landslides as part of flood control, erosion control, hillside management, earthquake hazard mitigation, road stabilization, and other programs. 

Perhaps the most common reminders of landslide risks are those "Watch For Falling Rocks" highway signs. Although "sliding rocks" is more apt, very few get to see a land slide. Occasionally we see small rocks or debris on the pavement, but a large size slide usually starts with such small incidents. Visually, a landslide resembles a snow avalanche, only with a louder rumbling noise, and is capable of generating enough force and momentum to wipe anything in its path. One such devastating landslide wiped entire towns and villages in Columbia in 1985 when 20,000 died.

Rockslide on Interstate 70, Colorado [USGS Picture, Click to see larger version]The pictures you see on this web site (including the background of this page), are recent examples from around the country. They show what's left after a slide. In some cases, only the rail or pavement is mangled, in others a house or building crushed, but in almost every aftermath, the losses are real, the damages total, and the terrain changes permanent.

Landslides cause one to two billion dollars in damage each year in the US and claim as many as fifty lives.  That's more devastating than all the other natural hazards combined.  They affect utilities, transportation, and all other forms of infrastructure, whether public or private. 

What are Landslides? by David Miller, USGS [Click to see the document]As development pressures around the country increase, so does the likelihood of building in areas susceptible to landslides. Such areas are neither isolated nor far in-between. They span the entire eastern part of US, from New England to the Appalachian region encompassing some of the most scenic areas in the east as well as large urban areas. Landslide risks loom through them all.  Pittsburgh and Cincinnati are two examples of urbanized areas with frequent landslides where developments on hills and hillsides are common. In the Great Plains, heavy rains combined with loss of vegetation due to wildfires trigger landslides in clay-rich rocky areas. On the west coast, earthquakes add to the causes of landslides. For example, the 1994 Northridge earthquake triggered many thousand landslides in the Santa Susanna Mountains.  In short, no region of the country is safe from landslides, whether caused by geophysical or human-made factors.

Although the term landslide is often used somewhat loosely to mean any fairly rapid movement of rocks and sediment downslope, it is actually more accurate to use the term mass wasting to refer to the wide variety of mass movement processes that wear away at the Earth's surface.

What Factors Affect The Mass Wasting Process?

There are three main factors that control the type and rate of mass wasting that might occur at the Earth's surface:

Slope gradient: The steeper the slope of the land, the more likely that mass wasting will occur.

Slope consolidation: Sediments and fractured or poorly cemented rocks and sediments are weak, and more prone to mass wasting.

Water: If slope materials are saturated with water, they may lose cohesion and flow easily.

What Kinds Of Mass Wasting Processes Do We See?

There are three basic types of mass wasting:

Falls - rocks fall or bounce through the air

Slides - rock and/or sediment slides along Earth's surface

Flows - sediment flows across Earth's surface

FALLS

While driving your vehicle through mountain country, such as parts of eastern California, you may have noticed signs such as: WARNING! ROCK FALL HAZARD.

Because weathering is an ongoing process, steep mountain slopes are constantly wasting away, often in the form of rocks falling and bouncing down slopes. Such falls can be triggered by freezing of water, the growth of plants (and their roots), earthquakes, or by people hiking on the slope. Rock falls occur in just a matter of seconds, so they are difficult (though fun) to observe. But, you can tell where rock falls occur on a mountain slope by looking for talus, a buildup of loose, angular rocks at the base of a steep slope.

Typically cone-shaped, these piles of rock debris are found at the base of the San Gabriel and San Bernardino Mountain systems, the former almost within view of the 210 freeway (on a clear day, of course!).

For those of you who may have some difficulty in finding the talus slopes, take a look below:

Again, note the cone shape of the talus slope.

SLIDES

Whenever a mass of slope material moves as a coherent block , we say that a slide has taken place. There are several types of slides, but one of the most common is a slump. A slump occurs when a portion of hillside moves downslope under the influence of gravity. A slump has a characteristic shape, with a scarp or cliff at the top of the slump, and a bulge of material (often called the toe of the slump) at the base of the slump. In the image shown below, see if you can spot the location of the scarp and toe of the slump.


What is particularly interesting about this slump picture is that it was taken in the year 1907 in the Berkeley Hills just east of San Francisco. Needless to say, this area has now been built up. Houses probably now stand where the slump took place. Question: Do you think that the danger of slumping is over for the Berkeley Hills region, now that it has been built up? Has the danger perhaps increased? What factors might increase the risk of slumping in areas such as the Berkeley Hills, or the Palos Verdes Peninsula and Pacific Palisades regions?

For those of you wondering where the scarp and toe of the slide might be, check out the image shown below.

FLOWS

We say that a flow has occurred if the material moving downslope is being transported as a very thick fluid (like a river of debris, rock, and/or soil), rather than as a coherent unit. Often, water is the primary transport agent for the flow.

Lahars

For sheer drama and destruction, you can't beat the mud flows that can occur when a volcano erupts. These flows are called lahars. After the Mount St. Helens eruption, for example, the heat of the volcanic products melted the snowcap. The resulting liquid water rushed down the volcano flanks, incorporating debris as it progressed. The damage to forests and humans was extensive.

Look at that mudline -- up to the window!

Imagine what would have happened, though, if Mt. St. Helens had been closer to major urban centers, such as Seattle, Washington, or Portland, Oregon. The eruption at Mount Pinatubo, in the Philippines, was an instance where a volcano erupted close to a densely populated area. Luckily, geologists had predicted that the eruption would occur, and were able to advise a timely evacuation of the region.

In this aerial view, you can see the destruction of a bridge by a lahar from Mount Pinatubo. The white dots, by the way, are humans crossing makeshift bridges over the river.

Areas That Are Generally Prone To Landslides

Areas That Are Generally Safe From Landslides

Features That Might Be Noticed Before Major Landslides

What To Do If You Suspect Immediate Landslide Danger

If You Live Near Steep Hills

Before Intense Storms

During Intense Storms

After Intense Storms

DID YOU KNOW...

FACT SHEET: LANDSLIDES AND MUDFLOWS

Landslide and mudflows usually strike without warning.  The force of rocks, soil, or other debris moving down a slope can devastate anything in its path.  Take the following steps to be ready.

BEFORE

Get a ground assessment of your property.

Your county geologist or county planning department may have specific information on areas vulnerable to landsliding. Consult a professional geotechnical expert for opinions and advice on landslide problems and on corrective measures you can take.

Minimize home hazards.

Learn to recognize the landslide warning signs.

Make evacuation plans.  Plan at least two evacuation routes since roads may become blocked or closed.

Develop an emergency communication plan.   In case family members are separated from one another during a landslide or mudflow this is (a real possibility during the day when adults are at work and children are at school), have a plan for getting back together.

Ask an out-of-state relative or friend to serve as the "family contact". After a disaster, it's often easier to call long distance. Make sure everyone knows the name, address, and phone number of the contact person.

DURING


If inside a building:

If outdoors:

Sinkholes

A sinkhole occurs when groundwater dissolves a vulnerable land surface such as limestone, causing the land surface to collapse from a lack of support.  In June 1993, a 100-foot wide, 25-foot deep sinkhole formed under a hotel parking lot in Atlanta, killing two people and engulfing numerous cars.

AFTER

 

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Landslide Damage

 

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