Landslides occur in all 50 states and territories and they affect lives, property, infrastructure, and the environment. Understanding when, where, and how landslides occur can help to reduce the risk of living with these natural hazards.
What is a landslide?
Landslides occur in all 50 states and territories, and they affect lives, property, infrastructure, and the environment. Landslides are the downslope movement of earth materials (rock, debris, and soil) at rates that range from inches per year to tens of miles per hour. Some landslides can move faster than a person can run. Landslides can happen with no notice or can take place over a period of days, weeks, or longer.
Where do landslides occur?
Landslides occur in every state and U.S. territory including the Appalachian Mountains, the Rocky Mountains, the Pacific Coastal Ranges, and some parts of Alaska and Hawaii. Any area composed of very weak or fractured materials resting on a steep slope can and will likely experience landslides.
Why Study Landslides?
Landslides are a serious geologic hazard common to almost every State in the United States. It is estimated that in the United States, they cause in excess of $1 billion in damages and from about 25 to 50 deaths each year. Globally, landslides cause many billions in damages and thousands of deaths each year.
Additional Resources
- A Homeowner's Guide to Landslides for Washington and Oregon - Washington Department of Natural Resources and the Oregon Department of Geology and Mineral Industries
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Puerto Rico Landslide Hazard Mitigation Project - Educational and preparedness resources in Spanish and English
What is a Debris Flow?
Debris flows, sometimes referred to as mudslides, mudflows, lahars, or debris avalanches, are common types of fast-moving landslides. These flows generally occur during periods of intense rainfall or rapid snowmelt. They usually start on steep hillsides as shallow landslides that liquefy and accelerate to speeds that are typically about 10 mph, but can exceed 35 mph. The consistency of debris flows ranges from watery mud to thick, rocky mud that can carry large items such as boulders, trees, and cars. They are among the most numerous and dangerous types of landslides in the world. They are particularly dangerous to life and property because of their high speeds and the sheer destructive force of their flow.
Vegetation and soil changes after a fire increase the runoff and erosion in a watershed, and significantly increase the likelihood of debris flows and flash flooding. Flash flooding and debris flows can initiate during even moderate rainstorms over burn areas and often occur with very little warning. Post-fire flow can alternate between flood and debris flow. Debris flows are more dangerous and more destructive and dangerous than floods because:
- 10-50 times greater peak discharge
- flow height up to 5 times greater
- flow velocity same or greater
- greater than 50% sediment content
- coarse-grained surge fronts
Distinguishing between debris flow and flood deposits can be difficult, but certain sedimentary structures and textures can give clues to the mechanism of deposition. The following provides more details on the differences.
| Characteristic | Debris Flows | Floods |
|---|---|---|
| Grain size | Clay to boulder, greater 50% of sediment is sand-size or greater | Clay to boulder |
| Sorting | Extremely poor | Moderate to good |
| Grain shape | Angular to subangular | Round to subround |
| Texture | No bedding, normal or inverse grading is common, matrix-supported | Distinct planar to cross-laminated beds and laminae voides are common between larger clasts |
| Sedimentary structures | Levees, terminal lobes, fans, coarse clasts on surface, sandy mud coatings on boulders, logs, banks | Dunes, ripples, and longitudinal bars are common, imbricated gravel clasts |
| Sediment load | 40-50% sediment by volume, greater than 50% of sediment load is sand-size or larger, sediment load controls flow behavior | 5-10% sediment by volume, water controls flow behavior |
Grain Size and Sorting
Debris flows are capable of transporting material or grains that range from very fine mud- or clay-size particles to boulders that are as large as cars. This wide variation in grain size results in accumulations or deposits of material at the outlet of a drainage basin that are poorly sorted, or in other words, all mixed up together.
Sedimentary Structures
Levees, or ridges of coarse cobbles, boulders and even tree trunks, often line channels that debris flows have passed through. Levees in the images to the right contain pebble to boulder-sized rocks and a tree trunk that has been stripped of its branches and is aligned parallel to the axis of the channel.
As debris flows move from steep, mountainous terrain to gently sloping or flat terrain, they slow down considerably, causing sediment to settle out and form fan-shaped deposits. Alluvial fans formed by debris flow are also poorly sorted and lack any kind of layering or stratification.
