Landslide Hazards

A landslide is defined as, the movement of a mass of rock, debris, or earth down a slope. Landslides are a type of "mass wasting" which denotes any down slope movement of soil and rock under the direct influence of gravity. The term "landslide" encompasses events such as rock falls, topples, slides, spreads, and flows. Landslides can be initiated by rainfall, earthquakes, volcanic activity, changes in groundwater, disturbance and change of a slope by man-made construction activities, or any combination of these factors. Landslides can also occur underwater, causing tidal waves and damage to coastal areas. These landslides are called submarine landslides.

Failure of a slope occurs when the force that is pulling the slope downward (gravity) exceeds the strength of the earth materials that compose the slope. They can move slowly, (millimeters per year) or can move quickly and disastrously, as is the case with debris-flows. Debris-flows can travel down a hillside of speeds up to 200 miles per hour (more commonly, 30 - 50 miles per hour), depending on the slope angle, water content, and type of earth and debris in the flow. These flows are initiated by heavy, usually sustained, periods of rainfall, but sometimes can happen as a result of short bursts of concentrated rainfall in susceptible areas. Burned areas charred by wildfires are particularly susceptible to debris flows, given certain soil characteristics and slope conditions. (From http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=304)

This type of maps ranks slope stability of an area into categories that range from stable to unstable. Susceptibility maps show where landslides may form. Many susceptibility maps use a color scheme that relates warm colors (red, orange, and yellow) to unstable and marginally unstable areas and cool colors (blue and green) to stable areas. (From http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=314)

This type of map shows the locations and outlines of landslides. A landslide inventory is a data set that may present a single event, a regional event, or multiple events. Small-scale maps may show only landslide locations whereas large-scale maps may distinguish landslide sources from deposits and classify different kinds of landslides and show other pertinent data. (From http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=313)

This type of map shows the expected annual cost of landslide damage throughout an area. Risk maps combine the probability information from a landslide hazard map with an analysis of all possible consequences (property damage, casualties, and loss of service). (From http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=316)

A landslide hazard map indicates the possibility of landslides occurring throughout a given area. A hazard map may be as simple as a map that uses the locations of old landslides to indicate potential instability, or as complex as a quantitative map incorporating probabilities based on variables such as rainfall thresholds, slope angle, soil type, and levels of earthquake shaking. An ideal landslide hazard map shows not only the chances that a landslide may form at a particular place, but also the chance that it may travel downslope a given distance. (From http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=315)

Debris and mud flows are rivers of rock, earth, and other debris saturated with water. They develop when water rapidly accumulates in the ground, during heavy rainfall or rapid snowmelt, changing the earth into a flowing river of mud or "slurry." They can flow rapidly, striking with little or no warning at avalanche speeds. They also can travel several miles from their source, growing in size as they pick up trees, boulders, cars, and other materials. (From http://www.fema.gov/areyouready/landslide.shtm)

(From http://www.fema.gov/hazard/landslide/ls_before.shtm)

Protect yourself from the effects of a landslide or debris flow:

• Do not build near steep slopes, close to mountain edges, near drainage ways, or natural erosion valleys.
• Get a ground assessment of your property.
• Contact local officials, state geological surveys or departments of natural resources, and university departments of geology. Landslides occur where they have before, and in identifiable hazard locations. Ask for information on landslides in your area, specific information on areas vulnerable to landslides, and request a professional referral for a very detailed site analysis of your property, and corrective measures you can take, if necessary.
• If you are at risk from a landslide talk to your insurance agent. Debris flow may be covered by flood insurance policies from the National Flood Insurance Program (NFIP).
• Minimize home hazards:
  • Have flexible pipe fittings installed to avoid gas or water leaks, as flexible fittings are more resistant to breakage (only the gas company or professionals should install gas fittings).
  • Plant ground cover on slopes and build retaining walls.
  • In mudflow areas, build channels or deflection walls to direct the flow around buildings.
  • Remember: If you build walls to divert debris flow and the flow lands on a neighbor's property, you may be liable for damages.

Recognize Landslide Warning Signs

• Changes occur in your landscape such as patterns of storm-water drainage on slopes (especially the places where runoff water converges) land movement, small slides, flows, or progressively leaning trees.
• Doors or windows stick or jam for the first time.
• New cracks appear in plaster, tile, brick, or foundations.
• Outside walls, walks, or stairs begin pulling away from the building.
• Slowly developing, widening cracks appear on the ground or on paved areas such as streets or driveways.
• Underground utility lines break.
• Bulging ground appears at the base of a slope.
• Water breaks through the ground surface in new locations.
• Fences, retaining walls, utility poles, or trees tilt or move.
• A faint rumbling sound that increases in volume is noticeable as the landslide nears.
• The ground slopes downward in one direction and may begin shifting in that direction under your feet.
• Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.
• Collapsed pavement, mud, fallen rocks, and other indications of possible debris flow can be seen when driving (embankments along roadsides are particularly susceptible to landslides).

Residents near mountain slopes, canyons, and landslide prone areas should stay alert even after heavy rain subsides. While the likelihood of debris flows begins to decline after a day or more of dry weather, some deep-seated landslides may occur days-even weeks to months-after long periods of intense rainfall. (From http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=899)

Landslides can and do occur in every state and territory of the U.S.; however, the type, severity and frequency of landslide activity varies from place to place, depending on the terrain, geology, and climate. Major storms have caused landslides in Washington state, Oregon, California, Colorado, Idaho, Hawaii, Virginia, Ohio, Pennsylvania, Puerto Rico, and elsewhere. Rapid snowmelt has caused landslides in Utah and Washington State. (From http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=901)