Debris flow
They generally have bulk densities comparable to those of rock avalanches and other types of landslides (roughly 2000 kilograms per cubic meter), but owing to widespread sediment liquefaction caused by high pore-fluid pressures, they can flow almost as fluidly as water.
Debris flows with volumes ranging up to about 100,000 cubic meters occur frequently in mountainous regions worldwide.
Notable debris-flow disasters of the twentieth century involved more than 20,000 fatalities in Armero, Colombia, in 1985 and tens of thousands in Vargas State, Venezuela, in 1999.
By definition, “debris” includes sediment grains with diverse shapes and sizes, commonly ranging from microscopic clay particles to great boulders.
In all cases the chief conditions required for debris flow initiation include the presence of slopes steeper than about 25 degrees, the availability of abundant loose sediment, soil, or weathered rock, and sufficient water to bring this loose material to a state of almost complete saturation (with all the pore space filled).
Debris flows can be more frequent following forest and brush fires, as experience in southern California demonstrates.
They pose a significant hazard in many steep, mountainous areas, and have received particular attention in Japan, China, Taiwan, USA, Canada, New Zealand, the Philippines, the European Alps, Russia, and Kazakhstan.
In Japan a large debris flow or landslide is called yamatsunami (山津波), literally mountain tsunami.
Debris flows are accelerated downhill by gravity and tend to follow steep mountain channels that debouche onto alluvial fans or floodplains.
The front, or 'head' of a debris-flow surge often contains an abundance of coarse material such as boulders and logs that impart a great deal of friction.
Ancient debris-flow deposits that are exposed only in outcrops are more difficult to recognize, but are commonly typified by juxtaposition of grains with greatly differing shapes and sizes.
These include: A lahar is a debris flow related in some way to volcanic activity, either directly as a result of an eruption, or indirectly by the collapse of loose material on the flanks of a volcano.
The word lahar is of Indonesian origin, but is now routinely used by geologists worldwide to describe volcanogenic debris flows.
Downvalley of the breach point, a jökulhlaup may increase greatly in size through entrainment of loose sediment from the valley through which it travels.
Calibrating and validating such sophisticated models require well-documented data from field surveys or minute laboratory experiments.
Under these conditions of hydrodynamic support of the particles by the fluid, the debris mass is fully fluidized (or lubricated) and moves very economically, promoting long travel distances.
In this case the force due to the pressure gradient is altered, the drag is high and the effect of the virtual mass disappears in the solid momentum.
[19] In 1989, as part of his large-scale piece David Gordon's United States, and later, in 1999, as part of Autobiography of a Liar, choreographer David Gordon brought together the music of Harry Partch and the words of John McPhee from The Control of Nature, read by Norma Fire, in a dance titled "Debris Flow", a "harrowing taped narrative of a family's ordeal in a massive L.A. mudslide..."[20]
