Causes of landslides

Natural causes:

• groundwater pressure acting to destabilize the slope
• Loss or absence of vertical vegetative structure, soil nutrients, and soil structure.
• erosion of the toe of a slope by rivers or ocean waves
• weakening of a slope through saturation by snowmelt, glaciers melting, or heavy rains
• earthquakes adding loads to barely-stable slopes
• earthquake-caused liquefaction destabilizing slopes (see Hope Slide)
• volcanic eruptions

Human causes:

• vibrations from machinery or traffic
• blasting
• earthwork which alters the shape of a slope, or which imposes new loads on an existing slope
• in shallow soils, the removal of deep-rooted vegetation that binds colluvium to bedrock
• Construction, agricultural, or forestry activities which change the amount of water which infiltrates into the soil.

Types of landslide

sync: Landslide classification main: Landslide classification

Debris flow

Debris flow is often mistaken for flash flood, but they are entirely different processes.

Muddy-debris flows in alpine areas cause severe damage to structures and infrastructure and often claim human lives. Muddy-debris flows can start as a result of slope-related factors, and shallow landslides can dam stream beds, provoking temporary water blockage. As the impoundments fail, a "domino effect" may be created, with a remarkable growth in the volume of the flowing mass, which takes up the debris in the stream channel. The solid-liquid mixture can reach densities of up to 2 tons/m³ and velocities of up to 14 m/s (Chiarle and Luino, 1998; Arattano, 2003). These processes normally cause the first severe road interruptions, due not only to deposits accumulated on the road (from several cubic metres to hundreds of cubic metres), but in some cases to the complete removal of bridges or roadways or railways crossing the stream channel. Damage usually derive from a common underestimation of mud-debris flows: in the alpine valleys, for example, bridges are frequently destroyed by the impact force of the flow because their span is usually calculated only for a water discharge. For a small basin in the Italian Alps (area = 1.76 km²) affected by a debris flow, Chiarle and Luino (1998)Fact: date=May 2007 estimated a peak discharge of 750 m3/s for a section located in the middle stretch of the main channel. At the same cross section, the maximum foreseeable water discharge (by HEC-1), was 19 m³/s, a value about 40 times lower than that calculated for the debris flow that occurred.