Landslides and Avalanches: When Slopes Fail

Mass movement events - where soil, rock, snow, or ice suddenly mobilize and flow downslope - represent some of the most geographically concentrated yet locally catastrophic natural hazards. Landslides and avalanches share the common feature of rapid, often unpredictable slope failure, with consequences ranging from road closures to complete community destruction.

Landslides: Types and Triggers

Landslides encompass a range of mass movement processes, classified by the type of material involved and the nature of the movement:

• Slides: A coherent mass of soil or rock moves along a failure surface, either as a rotational (slump) or translational movement.
• Flows: Material behaves in a fluid-like manner. Debris flows - fast-moving mixtures of water, rock, and soil - are among the most dangerous, capable of reaching speeds of 35 mph or more and burying entire structures.
• Falls: Rock or soil falls freely from a cliff or steep slope.
• Lateral spreads: Saturated fine-grained soils or weak rock spread laterally, often triggered by seismic shaking.

Primary triggers include heavy or prolonged rainfall (the most common trigger globally), rapid snowmelt, earthquake shaking, volcanic activity, and human activities such as slope excavation, overloading, vegetation removal, and changes to drainage patterns.

Post-Fire Debris Flows: A Compounding Hazard

One of the most dangerous and increasingly common hazard cascades involves wildfires followed by debris flows. Wildfire burns away protective vegetation and creates a water-repellent layer in the soil (hydrophobicity), dramatically increasing runoff and the potential for debris flows during subsequent rainfall. The January 2018 Montecito, California debris flow, which killed 23 people and destroyed more than 100 homes, occurred just weeks after the Thomas Fire burned through the hills above the community - illustrating how hazards compound in succession.

Avalanches: The Physics of Snow Failure

An avalanche is a rapid flow of snow down a slope, often starting when the cohesion within a snow layer is exceeded by the stresses acting upon it. Slab avalanches - where a cohesive layer of snow breaks free and slides on a weaker layer below - are responsible for the vast majority of avalanche fatalities. Triggering factors include recent heavy snowfall, wind-deposited (wind slab) snow, rapid temperature changes, and mechanical loading from skiers, snowboarders, or snowmobiles.

In the United States, the vast majority of avalanche fatalities occur in recreationists - skiers, snowboarders, snowmobilers, and backcountry travelers - who trigger slides on slopes typically between 30 and 45 degrees. The Colorado Avalanche Information Center and regional avalanche centers throughout the western U.S. provide daily avalanche forecasts during the winter season.

Risk Reduction

• Landslide awareness: Know whether you live in a landslide-prone area by consulting USGS landslide hazard maps. Watch for warning signs: new cracks in walls or ground, tilting trees, changes in spring or stream behavior, or unusual sounds (cracking, rumbling).
• Post-fire vigilance: If you live downstream of a recently burned area, be especially cautious during rainfall and monitor your county's emergency alerts.
• Avalanche safety: Never travel in avalanche terrain alone. Carry and know how to use an avalanche transceiver, probe, and shovel. Check the local avalanche forecast before every outing and avoid avalanche terrain under high or extreme danger ratings.