Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally have bulk densities comparable to those of rock avalanches and other types of landslides, but owing to widespread sediment liquefaction caused by high pore-fluid pressures, they can flow almost as fluidly as water. Debris flows descending steep channels commonly attain speeds that surpass 10 m/s (36 km/h), although some large flows can reach speeds that are much greater. Debris flows with volumes ranging up to about 100,000 cubic meters occur frequently in mountainous regions worldwide. The largest prehistoric flows have had volumes exceeding 1 billion cubic meters. As a result of their high sediment concentrations and mobility, debris flows can be very destructive.
Debris flow channel with deposits left after 2010 storms in Ladakh, NW Indian Himalaya. Coarse bouldery levees form the channel sides. Poorly sorted rocks lie on the channel floor.
Scars formed by debris flow in Ventura, greater Los Angeles during the winter of 1983. The photograph was taken within several months of the debris flows occurring.
Ancient debris flow deposit at Resting Springs Pass, California
A debris flow in Ladakh, triggered by storms in 2010. It has poor sorting and levees. Steep source catchment is visible in background.
The Armero tragedy occurred following the eruption of the Nevado del Ruiz stratovolcano in Tolima, Colombia, on November 13, 1985. The volcano's eruption after 69 years of dormancy caught nearby towns unprepared, even though volcanological organizations had warned the government to evacuate the area after they detected volcanic activity two months earlier.
Nevado del Ruiz seen from space. The summit ice cap and glaciers surround the dark Arenas crater.
The summit of Nevado del Ruiz in late November 1985
Armero was located in the center of this photograph, taken in late November 1985
Only a few buildings and structures remained standing after the mud and debris flows ravaged the town of Armero