Debris flows are geological phenomena in which water-laden masses of
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
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
Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, ...
(roughly 2000 kilograms per cubic meter), but owing to widespread sediment
liquefaction
In materials science, liquefaction is a process that generates a liquid from a solid or a gas or that generates a non-liquid phase which behaves in accordance with fluid dynamics.
It occurs both naturally and artificially. As an example of t ...
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 (i.e., 1 cubic kilometer). As a result of their high sediment concentrations and mobility, debris flows can be very destructive.
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 )
, anthem = ''Carmañola Americana''
, image_map = Vargas in Venezuela.svg
, map_alt =
, map_caption = Location within Venezuela
, pushpin_map =
, pushpin_map_alt ...
,
Venezuela
Venezuela (; ), officially the Bolivarian Republic of Venezuela ( es, link=no, República Bolivariana de Venezuela), is a country on the northern coast of South America, consisting of a continental landmass and many islands and islets in ...
, in 1999.
Features and behavior
Debris flows have volumetric
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
concentrations exceeding about 40 to 50%, and the remainder of a flow's volume consists of water. By definition, “debris” includes sediment grains with diverse shapes and sizes, commonly ranging from microscopic
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay part ...
particles to great
boulder
In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive.
In ...
s. Media reports often use the term
mudflow
A mudflow or mud flow is a form of mass wasting involving fast-moving flow of debris that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/second. Mudflows contain a significa ...
to describe debris flows, but true mudflows are composed mostly of grains smaller than
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...
. On Earth's land surface, mudflows are far less common than debris flows. However, underwater mudflows are prevalent on submarine
continental margin
A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental marg ...
s, where they may spawn
turbidity current
A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
s. Debris flows in forested regions can contain large quantities of
woody debris such as logs and tree stumps. Sediment-rich water floods with solid concentrations ranging from about 10 to 40% behave somewhat differently from debris flows and are known as
hyperconcentrated floods.
[Pierson, Thomas C]
Distinguishing between debris flows and floods from field evidence in small watersheds
US Department of the Interior, US Geological Survey, 2005. Normal stream flows contain even lower concentrations of sediment.
Debris flows can be triggered by intense rainfall or snowmelt, by dam-break or glacial outburst floods, or by landsliding that may or may not be associated with intense rain or earthquakes. In all cases the chief conditions required for debris flow initiation include the presence of slopes steeper than about 25
degree
Degree may refer to:
As a unit of measurement
* Degree (angle), a unit of angle measurement
** Degree of geographical latitude
** Degree of geographical longitude
* Degree symbol (°), a notation used in science, engineering, and mathemati ...
s, 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
Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, ...
is called
''yamatsunami'' (
山津波), literally ''mountain
tsunami
A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater exp ...
''.
Debris flows are accelerated downhill by gravity and tend to follow steep mountain channels that debouche onto
alluvial fan
An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...
s or
floodplain
A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...
s. 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
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding (motion), sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative la ...
. Trailing behind the high-friction flow head is a lower-friction, mostly liquefied flow body that contains a higher percentage of
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...
,
silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel wh ...
and clay. These fine sediments help retain high pore-fluid pressures that enhance debris-flow mobility. In some cases the flow body is followed by a more watery tail that transitions into a hyperconcentrated stream flow. Debris flows tend to move in a series of pulses, or discrete surges, wherein each pulse or surge has a distinctive head, body and tail.
Debris-flow deposits are readily recognizable in the field. They make up significant percentages of many alluvial fans and
debris cone
A debris cone consists of debris deposited in a conical shape with a surface slope greater than 10 degrees (perpendicular to contours), usually transported by small streams or snow avalanches. A debris cone is also called a dirt cone or cone of d ...
s along steep mountain fronts. Fully exposed deposits commonly have lobate forms with boulder-rich snouts, and the lateral margins of debris-flow deposits and paths are commonly marked by the presence of boulder-rich lateral
levees
A levee (), dike (American English), dyke (Commonwealth English), embankment, floodbank, or stop bank is a structure that is usually earthen and that often runs parallel to the course of a river in its floodplain or along low-lying coastl ...
. These natural levees form when relatively mobile, liquefied, fine-grained debris in the body of debris flows shoulders aside coarse, high-friction debris that collects in debris-flow heads as a consequence of grain-size segregation (a familiar phenomenon in
granular mechanics
A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when grains collide). The constituents that compose g ...
). Lateral levees can confine the paths of ensuing debris flows, and the presence of older levees provides some idea of the magnitudes of previous debris flows in a particular area. Through dating of trees growing on such deposits, the approximate frequency of destructive debris flows can be estimated. This is important information for land development in areas where debris flows are common. Ancient debris-flow deposits that are exposed only in
outcrop
An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth.
Features
Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...
s are more difficult to recognize, but are commonly typified by juxtaposition of grains with greatly differing shapes and sizes. This poor
sorting of sediment grains distinguishes debris-flow deposits from most water-laid sediments.
Types
Other
geological
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Eart ...
flows that can be described as debris flows are typically given more specific names. These include:
Lahar
A
lahar
A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.
Lahars are extreme ...
is a debris flow related in some way to
volcanic activity
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
, either directly as a result of an eruption, or indirectly by the collapse of loose material on the flanks of a volcano. A variety of phenomena may trigger a lahar, including melting of glacial ice, intense rainfall on loose
pyroclastic
Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...
material, or the outburst of a lake that was previously dammed by pyroclastic or glacial sediments. The word lahar is of Indonesian origin, but is now routinely used by geologists worldwide to describe volcanogenic debris flows. Nearly all of Earth's largest, most destructive debris flows are lahars that originate on volcanoes. An example is the lahar that inundated the city of
Armero, Colombia.
Jökulhlaup
A
jökulhlaup
A jökulhlaup ( ) (literally "glacial run") is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages.
It originally referred to the well-known subglacial outburst floods ...
is a glacial outburst flood. Jökulhlaup is an Icelandic word, and in Iceland many glacial outburst floods are triggered by sub-glacial volcanic eruptions. (Iceland sits atop the Mid-Atlantic Ridge, which is formed by a chain of mostly submarine volcanoes). Elsewhere, a more common cause of jökulhlaups is the breaching of ice-dammed or
moraine
A moraine is any accumulation of unconsolidated debris ( regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice sh ...
-dammed lakes. Such breaching events are often caused by the sudden calving of glacier ice into a lake, which then causes a displacement wave to breach a moraine or ice dam. Downvalley of the breach point, a jökulhlaup may increase greatly in size through
entrainment of loose sediment from the valley through which it travels. Ample entrainment can enable the flood to transform to a debris flow. Travel distances may exceed 100 km.
Theories and models of debris flows
Numerous different approaches have been used to model debris-flow properties,
kinematics, and
dynamics. Some are listed here.
*Rheologically based models that apply to mud flows treat debris flows as single-phase homogeneous materials (Examples include:
Bingham,
viscoplastic
Viscoplasticity is a theory in continuum mechanics that describes the rate-dependent inelastic behavior of solids. Rate-dependence in this context means that the deformation of the material depends on the rate at which loads are applied. The ine ...
,
Bagnold-type
dilatant fluid,
thixotropic
Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow (become thinner, less viscous) over time when shaken, agitated, shear-stressed, or otherwise stressed ( ...
, etc.)
* Dam break wave, e.g. Hunt, Chanson et al.
* Roll wave, e.g., Takahashi, Davies
* Progressive wave
* A type of translating rock dam
Calibrating and validating such sophisticated models require well-documented data from field surveys or minute laboratory experiments.
Two-phase
The
mixture theory Mixture theory is used to model multiphase systems using the principles of continuum mechanics generalised to several interpenetrable continua. The basic assumption is that, at any instant of time, all phases are present at every material point, an ...
, originally proposed by Iverson
and later adopted and modified by others, treats debris flows as two-phase solid-fluid mixtures.
In real two-phase (debris) mass flows there exists a strong coupling between the
solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structura ...
and the
fluid
In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shea ...
momentum transfer
In particle physics, wave mechanics and optics, momentum transfer is the amount of momentum that one particle gives to another particle. It is also called the scattering vector as it describes the transfer of wavevector in wave mechanics.
In the ...
, where the solid's normal
stress is reduced by
buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
, which in turn diminishes the
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding (motion), sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative la ...
al resistance, enhances the
pressure gradient
In atmospheric science, the pressure gradient (typically of air but more generally of any fluid) is a physical quantity that describes in which direction and at what rate the pressure increases the most rapidly around a particular location. The ...
, and reduces the
drag
Drag or The Drag may refer to:
Places
* Drag, Norway, a village in Tysfjord municipality, Nordland, Norway
* ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania
* Drag (Austin, Texas), the portion of Guadalupe Street adj ...
on the solid component. Buoyancy is an important aspect of two-phase debris flow, because it enhances flow mobility (longer travel distances) by reducing the frictional resistance in the
mixture. Buoyancy is present as long as there is fluid in the mixture. It reduces the solid normal stress, solid lateral normal stresses, and the basal
shear stress
Shear stress, often denoted by (Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. '' Normal stress'', on ...
(thus, frictional resistance) by a factor (
), where
is the density
ratio
In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
between the fluid and the solid phases. The effect is substantial when the density ratio (
) is large (e.g., in the natural debris flow).
If the flow is neutrally buoyant, i.e.,
, (see, e.g., Bagnold, 1954) the debris mass is fluidized and moves longer travel distances. This can happen in highly
viscous
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
natural debris flows. For neutrally buoyant flows,
Coulomb friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of tw ...
disappears, the lateral solid pressure gradient vanishes, the
drag coefficient
In fluid dynamics, the drag coefficient (commonly denoted as: c_\mathrm, c_x or c_) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. It is used in the drag e ...
is zero, and the basal slope effect on the solid phase also vanishes. In this
limiting case, the only remaining solid force is due to
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...
, and thus the force associated with buoyancy.
Under these conditions of
hydrodynamic
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) and ...
support of the
particle
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, fro ...
s by the fluid, the debris mass is fully fluidized (or
lubricated) and moves very economically, promoting long travel distances. Compared to buoyant flow, the neutrally buoyant flow shows completely different behaviour. For the latter case, the solid and fluid phases move together, the debris bulk mass is fluidized, the front moves substantially farther, the tail lags behind, and the overall flow height is also reduced. When
, the flow does not experience any buoyancy effect. Then the effective frictional shear stress for the solid phase is that of pure
granular flow. 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. All this leads to slowing down the
motion
In physics, motion is the phenomenon in which an object changes its position with respect to time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed and frame of reference to an observer and mea ...
.
Damage prevention
To prevent debris flows reaching property and people, a debris basin may be constructed. Debris basins are designed to protect soil and water resources or to prevent downstream damage. Such constructions are considered to be a last resort because they are expensive to construct and require commitment to annual maintenance.
Also, debris basins may only retain debris flows from a fraction of streams that drain mountainous terrain.
Before a storm that can potentially nucleate debris flows, forecasting frameworks can often quantify the likelihood that a debris flow might occur in a watershed; however, it remains challenging to predict the amount of sediment mobilized and therefore, the total size of debris flows that may nucleate for a given storm, and whether or not debris basins will have the capacity to protect downstream communities. These challenges make debris flows particularly dangerous to mountain front communities.
In popular culture
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
Harry Partch (June 24, 1901 – September 3, 1974) was an American composer, music theorist, and creator of unique musical instruments. He composed using scales of unequal intervals in just intonation, and was one of the first 20th-century co ...
and the words of
John McPhee
John Angus McPhee (born March 8, 1931) is an American writer. He is considered one of the pioneers of creative nonfiction
Creative nonfiction (also known as literary nonfiction or narrative nonfiction or literary journalism or verfabula) is ...
from ''
The Control of Nature
''The Control of Nature'' is a 1989 book by John McPhee that chronicles three attempts (with varying success) to control natural processes. It is divided into three long essays, "Atchafalaya", "Cooling the Lava", and "Los Angeles Against the Mount ...
'', 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..."
[Tobias, Tobi]
"Dance: Burning the Flag"
''New York
New York most commonly refers to:
* New York City, the most populous city in the United States, located in the state of New York
* New York (state), a state in the northeastern United States
New York may also refer to:
Film and television
* '' ...
'' (November 20, 1989), p.116; Jowitt, Deborah
"Rush Forward. Look Back."
Village Voice
''The Village Voice'' is an American news and culture paper, known for being the country's first alternative newspaper, alternative newsweekly. Founded in 1955 by Dan Wolf (publisher), Dan Wolf, Ed Fancher, John Wilcock, and Norman Mailer, th ...
(December 21, 1999)
See also
*
Colluvium
Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combinatio ...
*
Illhorn, below which lies Illgraben a popular debris flow tourist spot
*
Rheology
Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an appli ...
References
Notes
Further reading
*
McPhee, John. ''
The Control of Nature
''The Control of Nature'' is a 1989 book by John McPhee that chronicles three attempts (with varying success) to control natural processes. It is divided into three long essays, "Atchafalaya", "Cooling the Lava", and "Los Angeles Against the Mount ...
''. New York: Noonday Press (Farrar, Straus & Giroux, 1989 )
External links
Dongchuan Debris Flow Observation and Research Station, ChinaVideo documentation of experiments at the USGS debris-flow flume, Oregon, USAMass Movements. Section of the information platform "Natural hazards in Switzerland"*
ttp://www.debrisflow.ru/en/ Debris Flow Association
{{Authority control
Geological hazards
Geomorphology
Landslide types
Articles containing video clips
Natural disasters