What are landslides & how can they affect me?
Dixie Fire Debris-Flow Deposit
New Zealand Landslides
Home damaged by rainfall-triggered debris flow - Carrol County, Kentucky
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 and can happen with no notice or can take place over days, weeks, or longer
Landslides can bury homes, damage critical infrastructure, block or damage roads and rail lines, and disrupt vital utilities and communication lines. Landslides can happen with no notice or can take place over a period of days, weeks, or longer. Landslides are unpredictable. A slow-moving landslide can rapidly change to a fast-moving landslide. Some fast-moving landslides can travel thousands of feet, even across flat ground. A landslide can remain inactive for centuries and then suddenly start moving again. Although not as dangerous to human life as fast-moving landslides, a slow-moving landslide that may be moving a few inches per year can, over periods of months to years, severely damage and destroy building, roads, pipelines, and other utilities built on and adjacent to the landslide.
Types of landslides
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Landslides come in many varieties, from small rockfalls or debris flows (mudflows) that occur quickly to mountain-sized slides that move for centuries. Many landslides are complex and involve a variety of landslides themselves, such as large, slow-moving landslides that produce smaller but rapid debris flows.
Causes of landslides
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Landslides can be triggered by rainfall from hurricanes and rainstorms, ground shaking from earthquakes, and volcanic activity. Human activity commonly triggers landslides, such as modifying (grading) a slope, removing vegetation, or channeling water onto a slope. Local terrain conditions, such as slope steepness and curvature, and geologic materials make hillslopes more or less likely to experience landslides.
A landslide that moves into water can produce a displacement wave, which is a type of tsunami. The largest displacement wave in the world occurred in 1958 in Lituya Bay, Alaska when an earthquake triggered a landslide that rapidly moved into the bay. The resulting displacement wave was over 1,700 feet high and stripped trees and soils from the slopes along the Lituya Bay.
Areas recently burned by wildfires can be more susceptible to landslides from rainfall. Fire can destroy vegetation and alter soil properties, such that water is repelled by a soil. During a rain event over a burned area, the soils may not be able to absorb water and instead, the water beads on the surface. If the soils are on a slope, the water will flow downslope and can rapidly increase in volume, picking up debris.
In the right conditions, the water can collect sufficient soil, rocks, and other debris and become a debris flow, which are very dangerous to those living downslope. Debris flows can damage or destroy roads, building, and other infrastructure far outside of the burned area.
Related Information - Postfire Debris Flows
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Postfire Landslide Monitoring Station: “Maria Ygnacio” (2019 Cave Fire) near Santa Barbara, California
Wildfire can increase landslide susceptibility in mountainous terrain. The USGS maintains postfire landslide monitoring stations to track hillslope hydrologic conditions in the years following fire.
Impacts of landslides
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Landslides can have cascading consequences; for example, a landslide can form a debris dam that blocks a stream channel, forming a pond. The rising pond water can eventually breach the debris dam which can lead to downstream flooding. Or a landslide-damaged road can cause closures, forcing vehicles to take alternate routes for weeks to months. This can impact local economies and hinder emergency response. The frequency and size of landslides are expected to grow in areas due to climate change, which is increasing rainfall intensity that can trigger landslides. Climate change is also linked to more frequent and severe wildfires and recently burned areas can experience increased occurrence of landsliding due to the fire altering the soil and vegetation.
In the United States, financial loss from landslide damages is not well-documented; however, landslides are estimated to cause billions of dollars in damage and multiple deaths annually. In additional, indirect loss from landslides, such as cost to commercial traffic due to extended road closures by landslide damage, are also not well understood.
More importantly, if a home is damaged by a landslide, the typical homeowner’s insurance policy does not cover earth movement. So, the damage to a home from a landslide may be entirely covered by the homeowner.
People who live in areas with landslide hazards should learn the signs of a landslide and learn what to do after a landslide occurs.
