Floods are the most common natural hazards affecting people, infrastructure and the natural environment. They can occur in many ways and in many environments. Riverine floods, the most prevalent, are due to heavy, prolonged rainfall, rapid snowmelt in upstream watersheds or the regular spring thaw of snow in river catchments. Other floods are caused by hefty rainfall occurring over a short period, the backup of estuaries due to high tides coinciding with riverine flooding or storm surges, dam failures, dam overtopping due to landslides into a reservoir, and seiche and wind tide effects in large lakes. Occasionally an eruption under a glacier or snow-covered volcanic peak can cause a flood or a mudflow in which the terrain is radically changed and any rural development is destroyed, frequently with much loss of life.
