Floods, coastal erosion and heavy precipitation aren’t the only predicted consequences of the El Nino phenomena. Landslides and debris flows could happen in areas where intense rainfall occurs.
To aid in awareness of these hazards, the U.S. Geological Survey has published a digitized version of a national landslide map for the conterminous United States. The map, at a scale of 1:3,750,000, shows landslide susceptibility and incidence. A high-resolution, image file of the map can be downloaded from its website, http://geohazards.cr.usgs.gov,or a paper copy can be ordered through USGS publication outlets beginning in mid-October. Information about the map can be obtained by calling the National Landslide Information Center at 303/273-8588.
Landslides, including rockfalls, debris flows, and a variety of other slope movements cause billions of dollars in property losses in the United States, and between 25 and 50 deaths annually, according to USGS scientists. All 50 states have landslide problems to some extent.
"Many people think of landslides as isolated, local events brought about by special sets of unique conditions, occurring sporadically across the landscape," said Lynn Highland, coordinator for the USGS Landslide Information Center in Golden, Colo. "While this perception is partly true, many of the most significant landslides are produced by powerful geologic systems operating on much larger scales."
In addition to landslides and debris flows that occur following heavy rainfall, landslides can be triggered by earthquakes, volcanic eruptions, coastal and river bank erosion, or the buildup of underground water pressures by an accumulation of seasonal rainfall.
The following guidelines reflect what areas are generally prone to landsliding and what residents of these areas should do before and after intense storms. Additional information is provided for residents who live near steep, sloping hills.
WHAT TO DO AND LOOK FOR IMMEDIATELY FOLLOWING A SEVERE STORM AND FEATURES THAT MAY INDICATE CATASTROPHIC LANDSLIDE MOVEMENT
Areas that are generally prone to landslide hazards:
- On existing old landslides
- On or at the base of slopes
- In or at the base of minor drainage hollows
- At the base or top of an old fill slope
- At the base or top of a steep cut slope
- Developed hillsides where leach field septic systems are used
Areas that are typically considered safe from landslides:
- On hard, non-jointed bedrock that has not moved in the past
- On relatively flat-lying areas away from sudden changes in slope angle
- At the top or along the nose of ridges, set back from the tops of slopes
Features that might be noticed prior to major landsliding:
- Springs, seeps, or saturated ground in areas that have not typically been wet before
- New cracks or unusual bulges in the ground, street pavements or sidewalks
- Soil moving away from foundations
- Ancillary structures such as decks and patios tilting and/or moving relative to the main house
- Tilting or cracking of concrete floors and foundations
- Broken water lines and other underground utilities
- Leaning telephone poles, trees, retaining walls or fences
- Offset fence lines
- Sunken or down-dropped road beds
- Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content)
- Sudden decrease in creek water levels though rain is still falling or just recently stopped.
- Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb
What to do if you suspect imminent landslide danger:
- Contact your local fire, police or public works department
- Inform affected neighbors
For further information on landslides in your area:
- Contact your state or county geologist-some cities also have staff geologists
- If a very detailed site analysis is desired, contract with a private consulting company that specializes in earth movement. Such companies would likely be those specializing in geotechnical engineering, structural engineering, or civil engineering. Your local, state or county geologist can usually advise you as to the best kind of professional to contact.
IF YOU LIVE NEAR STEEP SLOPING HILLS
Prior to Intense Storms:
Become familiar with the land around you. Learn whether debris flows have occurred in your area by contacting local officials, state geological surveys or departments of natural resources, and university departments of geology. Slopes where debris flows have occurred in the past are likely to experience them in the future.
Support your local government in efforts to develop and enforce land-use and building ordinances that regulate construction in areas susceptible to landslides and debris flows. Buildings should be located away from steep slopes, streams and rivers, intermittent-stream channels, and the mouths of mountain channels.
Watch the patterns of storm-water drainage on slopes near your home, and note especially the places were runoff water converges, increasing flow over soil-covered slopes. Watch the hillsides around your home for any signs of land movement, such as small landslides or debris flows or progressively tilting trees.
Contact your local authorities to learn about the emergency response and evacuation plans for your area, and develop your own emergency plans for your family and business.
During Intense Storms:
Stay alert and stay awake! Many debris-flow fatalities occur when people are sleeping. Listen to a radio for warnings of intense rainfall. Be aware that intense short bursts of rain may be particularly dangerous,especially after longer periods of heavy rainfall and damp weather.
Listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of flowing or falling mud or debris may precede larger landslides. If you are near a stream or channel, be alert for any sudden increase or decrease in water flow and for a change from clear to muddy water. Such changes may indicate landslide activity upstream, so be prepared to move quickly. Don’t delay! Save yourself, not your belongings.
If you are in areas susceptible to landslides and debris flows, consider leaving if it is safe to do so. If you remain at home, move to a part of the house farthest away from the source of the debris flows, such as an upper floor, but keep an escape route open should it become necessary to leave the house.
Be especially alert when driving. Embankments along roadsides are particularly susceptible to landslides. Watch the road for collapsed pavement, mud, fallen rocks, and other indications of possible debris flows.