An earthflow (earth flow) is a downslope viscous flow of fine-grained materials that have been saturated with water and moves under the pull of

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 stro ...

. It is an intermediate type of

mass wasting Mass wasting, also known as mass movement, is a general term for the movement of rock or soil down slopes under the force of gravity. It differs from other processes of erosion in that the debris transported by mass wasting is not entrained in ...

that is between

downhill creep Downhill creep, also known as soil creep or commonly just creep, is a type of creep characterized by the slow, downward progression of rock and soil down a low grade slope; it can also refer to slow deformation of such materials as a result of p ...

and

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 ...

. The types of materials that are susceptible to earthflows are

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 par ...

, fine

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 textural class of s ...

and

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 when ...

, and fine-grained

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.Easterbrook, D: “Surface Processes and Landforms”, page 78-79. Prentice-Hall, Inc, 1999 When the ground materials become saturated with enough water, they will start flowing (

soil liquefaction Soil liquefaction occurs when a cohesionless saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake or other sudden change in stress condition, in ...

). Its speed can range from being barely noticeable to rapid movement. The velocity of the flow is dictated by water content: the higher the water content is, the higher the velocity will be. Because of the dependency on water content for the velocity of the flow, it can take minutes or years for the materials to move down the slope.

Features and behavior

Earthflows are just one type of

mass movement Mass movement may refer to: * Mass movement (geology), the movement of rock and soil down slopes due to gravity * Mass movement (politics) A mass movement denotes a political party or movement which is supported by large segments of a population. P ...

that can occur on a hill slope. It has been recognized as its own type of movement since the early 20th century. Earthflows are one of the most

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 shear ...

types of mass movements. Earthflows occur on heavily saturated slopes like

mudflows 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 ...

or a

debris flow 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 ...

. Though earthflows are a lot like mudflows, overall they are slower and are covered with solid material carried along by flow from within. Earthflows are often made up of fine-grained materials so slopes consisting of

and

materials are more likely to create an earthflow. As earthflows are usually water-dependent, the risk of one occurring is much higher in humid areas especially after a period of heavy rainfall or

snowmelt In hydrology, snowmelt is surface runoff produced from melting snow. It can also be used to describe the period or season during which such runoff is produced. Water produced by snowmelt is an important part of the annual water cycle in many part ...

. The high level of

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

, which saturates the ground and adds water to the slope content, increases the pore-water pressure and reduces the shearing strength of the material. As the slope becomes wet, the earthflow may start as a creep downslope due to the clay or silt having less friction. As the material is increasingly more saturated, the slope will fail, which depends on

slope stability Slope stability analysis is a static or dynamic, analytical or empirical method to evaluate the stability of earth and rock-fill dams, embankments, excavated slopes, and natural slopes in soil and rock. Slope stability refers to the condition of i ...

. In earthflows, the slope does not fail along a clear shear plane and is instead more fluid as the material begins to move under the force of gravity as friction and slope stability is reduced.

Velocity

Earthflows vary in velocity of flow depending partly on the consistency of the flow for the speed of the entire movement, usually meaning how much water is in the material of the hill slope before the slope fails. Though water is often the key factor in slope failure, triggering an earthflow, there can also be dry granular flows made up of

granular material 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 gra ...

. The speed also depends on the angle of slope as earthflows can happen on moderate or steep slopes. Because earthflows are usually water-dependent, they can take many years or just minutes to move a significant amount. An earthflow may affect as few as several square meters or up to several hectares in either time frame.

Effects

Earthflows can have sudden impacts on the amount of sediment that is deposited into a river system, which can have effects on the life in and around the river itself. They can also cause damage to roads and constructions built near the slope. One of the best mitigation techniques to avoid serious earthflow and

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 grade (slope), slope failures, mudflows, and debris flows. Landslides occur in a variety of ...

damage is properly draining the slope of water, especially in places of high levels of precipitation.

Areas of risk

The areas most at risk for earthflows are: * Slopes that have been undercut or loaded with more sediment for human construction * Slopes that have been undercut by

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...

s or stream beds * Areas that receive heavy rainfall or snowmelt * Hill slopes made up of clay, silt, or other fine-grained materials * Areas with limited vegetation on hill slopes * Areas with evidence of past earthflows

References

External links

Slumgullion earthflow, USGS Bulletin 2130
a

National Natural Landmark The National Natural Landmarks (NNL) Program recognizes and encourages the conservation of outstanding examples of the natural history of the United States. It is the only national natural areas program that identifies and recognizes the best ...

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...