Landslides are downslope movement of earth, debris, and rock mass under the impact of gravity. These have differing proportion of water as also movement mechanism; fall, slide, topple, flow, slump and, spread, and are common in mountainous areas. Landslide initiation requires an external trigger that is often provided by presence of water, and seismic shaking.
Landslides sometimes occur as debris, and mud flows that are rivers of rock, earth, and other debris saturated with water. These develop during intense rainfall, runoff, or rapid snowmelt, changing the earth into a flowing river of mud or “slurry.” These can flow rapidly, striking with little or no warning at avalanche speeds (faster than a person can run). These also can travel many miles from their source, growing in size as they pick up trees, boulders, cars and other materials. Debris flows don’t always stay in stream channels and they can flow sideways as well as downhill.
Once instability is introduced in the hill slope landslides often take place repeatedly at one place, and can thus be avoided by going through past records.
Though often attributed to anthropogenic intervention 1998 Okhimath and Malpa landslides in Uttarakhand occurred in areas free of anthropogenic intervention. Landslides are common during the monsoon period but these can occur in other seasons as well; 2003 Varunavat landslide and 2005 Ramolsari landslides in Uttarakhand occurred during non-monsoonal period.