Landslides and Water

Slope saturation by water is a primary
cause of landslides. This effect can occur
in the form of intense rainfall, snowmelt,
changes in ground-water levels, and waterlevel
changes along coastlines, earth dams,
and the banks of lakes, reservoirs, canals, and
rivers.
Landsliding and flooding are closely allied
because both are related to precipitation,
runoff, and the saturation of ground by water.
In addition, debris flows and mudflows usually
occur in small, steep stream channels and
often are mistaken for floods; in fact, these
two events often occur simultaneously in the
same area.
Landslides can cause flooding by forming
landslide dams that block valleys and stream
channels, allowing large amounts of water
to back up. This causes backwater flooding
and, if the dam fails, subsequent downstream
flooding. Also, solid landslide debris can
“bulk” or add volume and density to otherwise
normal streamflow or cause channel blockages
and diversions creating flood conditions
or localized erosion. Landslides can also cause
overtopping of reservoirs and/or reduced
capacity of reservoirs to store water