Landslides

Landslides are the movement of rock, soil, and debris down a slope under the influence of gravity. They occur in every climate and can range from small soil slips to massive rock avalanches. They can be triggered by rainfall infiltration, earthquakes, volcanic activity, frost wedging, thawing permafrost, or human activities that destabilize slopes such as deforestation, road construction, mining, or improper irrigation.

Landslides are categorized by type of movement: falls (rockfalls), slides (translational or rotational), flows (debris flows, earthflows, mudflows), and avalanches. In a fall, blocks detach and descend through the air; in a slide, material maintains approximate contact with a curved or planar failure surface; in a flow, material behaves as a viscous mixture and moves downslope as a liquid.

Initiation occurs when shear strength is reduced below shear stress. Pore-water pressure increases, seismic shaking, or rapid loading can trigger initiation. Once movement starts, the flow or slide may extend rapidly, entrain more material, and accelerate to high speeds before decelerating and depositing as a debris apron or landslide dam.

Hazards include loss of life, destruction of buildings and roads, disruption of utilities, and changes to drainage and river courses. Landslides can generate downstream floods or tsunamis if they enter bodies of water, and they can create dammed lakes that pose downstream breach risks.

Mitigation and management involve land-use planning, slope stabilization (drainage improvements, surface grading, retaining structures, rock bolts), vegetation, installation of nets and barriers, and early warning systems using rainfall thresholds, inclinometers, and shaking sensors. Hazard assessment relies on landslide inventories and susceptibility mapping to inform development restrictions and emergency preparedness.