Lahar, Mud & Quicksand Hazards
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Recommended:   Mass Wasting - Landslides & Debris Flows,  Volcanoes & Volcanism Hazards,  Earthquake Preparedness


Lahar, Mud & Quicksand Hazards


Which do you consider to be more dangerous — mud or quicksand? You may be surprised to discover that quicksand is not the terror portrayed in the movies but mudslides can prove especially deadly.


Quicksand

Quicksand is sand that, through infusion with water, has become saturated until the mixture forms a suspension. Upward hydrostatic pressure applied to the mix, such as from an underground spring, causes the suspension to go into motion. The motion forces water molecules between the sand grains, reducing intergranular pressure and frictional forces. This quickening greatly reduces the ability of the sand to support the weight of anything resting on its surface.

Because quicksand is denser than water, it is possible for you to float in quicksand. However, the tendency for both people and animals that venture into quicksand is to panic and struggle, which causes them to dig deeper into the quagmire. Further exacerbating this situation is the potential for quicksand to create a partial vacuum as an object is removed (because air is unable to fill the void left by the object).

You can escape from quicksand, even without assistance, by moving very slowly and attempting to bring your body to a horizontal position, facing upward, so you can float. Once floating is achieved, you can gradually “swim” toward firmer ground. Having an object to lever yourself against, such as a large walking stick, can make the task easier.

Quicksand can range in depth from a few inches to many feet. It is most often found near rivers, streams and lakes, but may also be encountered in hilly regions where loose soils are present and the water table is near the surface. Being cautious and observant in such areas and probing the soil ahead with a walking stick are the best ways to avoid quicksand.


Mudslide, Debris Flow, Lahar

Mudslides and mudflows can range in consistency from muddy-looking water to a course, pasty substance resembling wet concrete. Deep mud at rest presents a hazard similar to that of quicksand, but may be more difficult to escape because it possesses a more solid structure and, like quicksand, can form a partial vacuum as an object is removed.

Mud is most dangerous when it flows. Because both water and earth are dense, they possess high kinetic energy when in motion. A mudslide or mudflow, being a combination of these two materials, can acquire a great deal of momentum as it travels and exerts considerable force on objects it comes in contact with. What begins as a mudslide may become a debris flow as the moving mud picks up additional soil, rocks, trees, and even cars and buildings.

Mudflows involving volcanic material are known as lahars. Volcanic ash is very light, fine and loose material which flows easily in the presence of water. Even the courser material derived from ejecta on the slopes of most volcanoes is loosely packed and can easily be swept into a lahar. Because volcanic ash is so susceptible to flow and because volcanic slopes are devoid of vegetation for many years following an eruptive event, lahars can be especially sudden, violent and unpredictable. The threat of lahars can exist for many dozens of years following a volcanic eruption.

Mudslides and debris flows pose a threefold hazard; they have the ability to crush, bury and dislocate. The force exerted by flowing mud and the debris it may carry can crush structures, people and animals. Mud can flow around and over objects to entrap them or bury them entirely. For humans and animals, this may result in death due to asphyxiation or hypothermia. Finally, mudslides dislocate objects they come in contact with. Dislocation can include the uprooting of trees, severing of utility lines such as electric, telephone, gas, water and sewer (resulting in additional hazards), tossing of vehicles off of roadways, and destruction of roads, railways and bridges.

Numerous factors are instrumental in enabling mud to flow. Some of these factors also relate to the behavior of quicksand, while others influence the initiation and behavior of landslides.
  • Gravity and slope:   Mud cannot readily begin to flow on a flat surface, and too steep a slope will not permit large volumes of loose material to accumulate. A moderate slope, especially one having a slight inverted bowl shape, is optimal for initiating a mudslide.

  • Water:   Either a rapid influx of water capable of stripping loose soil or a sufficient volume of water to saturate the existing soil structure is required to initiate a mudslide. Mudslides usually occur when sustained rains or frequent storms occur which eventually saturate soils. Heavy convective downpours, such as from thunderstorms, can result in mudslides or debris flows, usually in conjunction with flash flooding. Lahars may occur in a similar fashion or as a result of rapid snowmelt or glacier melting due to volcanic heating.

  • Soil structure:   Loose but relatively non-porous soils are most susceptible to sliding. The ideal soil structure for creation of a mudslide consists of a shallow layer of loose soil or shale above a zone of impermeable rock or clay.

  • Viscosity:   Viscosity is resistance to flow. More viscous materials such as clays are less likely to slide.

  • Permeability:   Highly permeable soils allow water to percolate through and be removed; these will not generally slide unless they sit atop a layer of lower permeability. Slopes composed of moderately permeable soils, or those containing highly permeable soils astride a less permeable layer, enable water to be absorbed but may not be able to release water at a rate equal to the absorption rate. Eventually, these slopes may slide.

  • Hydrostatic pressure:   Underground springs or water percolating downward and collecting from points higher up a hillside may exert upward hydrostatic pressure on a slope. If the pressure is sufficient to overcome frictional forces, a mudslide will result.

  • Vibration:   The large earth movements caused by earthquakes are often responsible for both mudslides and landslides. Earthquake shaking can also result in liquefaction, a condition in which groundwater is forced to the surface, causing ordinarily solid soil to temporarily behave in a manner similar to quicksand. Structures built on soil undergoing liquefaction can actually sink into the ground, which returns to its solid state once the shaking subsides. The vibration generated by a large mudslide or landslide may contribute to additional sliding.

  • Cycles of heating and cooling:   Diurnal (daily) and seasonal cycles of heating and cooling, especially when freezing and thawing take place, can break down rock into soils and shales prone to sliding.

  • Removal of vegetation:   Reduction or removal of vegetation through natural disasters such as fire, flood, prior landslides or avalanches, or volcanic activity, or by human activity such as lumbering, mining or home building makes hillsides far more susceptible to mudslides and debris flows.

  • Volcanic and geothermal heating:   Heat from beneath the Earth’s surface can result in snowmelt and melting of glacial ice. During volcanic eruptions or movement of magma close to the surface, the resulting lahars can be catastrophic.

It becomes clear that a complex array of conditions must be present in order for a mudslide to take place. The severity of a mudslide will be directly proportional to the intensity of the prevailing conditions; however, not all of these conditions may be apparent or predictable. It can often be difficult and expensive to determine whether a particular location may slide.

Regions in which slides or slumps have occurred (See Mass Wasting - Landslides & Debris Flows for definitions.) should be avoided during periods of moderate to high mudslide danger, and should never be built upon or disturbed unless the intent is to improve drainage. Residents or travelers in mountainous or hilly regions must be extremely vigilant during periods in which flash flood or slide warnings are present or when local downpours occur.


Authored by Kenneth L. Anderson.  Original article published 21 May 2004, updated 20 October 2004.


Follow links to the right to learn more about mud, quicksand, mudslides, debris flows and lahars. At the left margin, Related Links address topics of interest pertaining to geologic hazards and other security issues. View the Security & Consumer Protection SiteMap for a complete list of security and consumer protection topics.


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