Mass wasting

Mass wasting, also called mass movement, is the process through which the layers of soil and rock that sit atop the bedrock are moved in bulk downslope because of gravity and other factors, such as moisture level and soil composition. Such movement of significant amounts of matter can have a profound effect on humans and other species.

Introduction

Earth's landscape is in a constant state of flux because of the effects of gravity and a geological process known as mass wasting. Mass wasting occurs when rock and regolith at the earth's surface move downslope. The movement can occur gradually, as slowly as 1 centimeter per year, or it can occur rapidly, as quickly as 1 kilometer per hour or more.

In mass wasting the material that was at a higher elevation moves to a lower elevation. In wilderness areas this may or may not have a significant impact on the environment or on local animal populations. Mass wasting also affects humans, often resulting in the loss of property and human life.

Slope Failure and Sediment Flow

Mass wasting occurs as the result of two major processes: slope failure and sediment flow. In a slope failure, the failure is sudden and can be caused by a variety of means, but the result is that debris is transported downslope by sliding, rolling, falling, or slumping.

Slumping occurs with the downward motion of materials in a concave, upward-curved surface area. As a result of this underlying formation, the upper surface of each block, along with the material in the block, remains intact. (It looks as if the face of the slope moves downward and leaves the impression of steps in the hillside.) The material accumulates at the base of the slope. This type of mass wasting is often seen along coastlines or riverbanks, where water has eroded the base of the slope, transporting the materials that supported the slope to a place downstream. With nothing to serve as a footing, the material above that point is prone to sliding, causing the characteristic depressions. Any houses or other structures atop the moving material will slide with the material and will often be destroyed by the rotation that is part of this type of movement.

Rock falls occur when a piece of rock on a steep slope is dislodged. As it falls down the slope it may bounce against other material that breaks free or it may simply fall by itself. It will join the talus at the base of the slope. Debris also falls in this way. The debris falls often include vegetation and other materials. These types of falls are dangerous because they can occur without any warning and because they often occur along roads that have been built between steep areas of rock. Because the rocks fall, and even “bounce,” they can travel far and can rest on the road. As a precaution, wire mesh is often anchored at the top of these steep slopes. When wire meshing is not practical, tall fencing is used.

Slides can be composed of rock and other debris. Slides are not the same as slumping because they do not move along a concave, upward-curved surface. Instead, slides move down the bedding plane or frontal surfaces of the underlying structures. They literally slide down the slope and end up at the bottom. Anything that sits atop the bedding or joint planes slides along with the material but is not necessarily destroyed in the process because a slide lacks rotation.

Sediment Flows

Slurry Sediment Flows. Slurry sediment flows occur when the regolith and rocks are mixed with water and end up containing about 20 to 40 percent water. A sediment flow often occurs after heavy rains or flooding. These water-saturated flows include mudflows. Mudflows are highly fluid, fast-moving mixtures of sediment and water that tend to travel across valley floors. Often they are caused by stream flooding in valley floors. As mudflows travel, they accumulate significant amounts of loose material. Because they move so swiftly and are made of such dense material, they can be dangerous. They also can cause significant structural damage. The slowest form of slurry flow is solifluction, in which rates of flow are measured in centimeters per year. These flows occur in areas where the soil has been saturated for years. They leave behind distinctive impressions on a hillside.

Granular Sediment Flows. Granular flows involve materials that include little or no water. Usually these flows take on the characteristics of fluids because of the introduction of air. Creep is the slowest form of granular flow, taking place at rates of less than 1 centimeter per year and occurring as a continuous downslope movement. Because it occurs so slowly, creep is rarely the cause of significant damage, but the steeper the hill, the faster the rate of creep. Generally, creep occurs at a rate that is not noticeable to the human observer. One way to confirm creep in an area is to look for trees that are not growing at an angle to the hillside. In such cases, the tree root structure is instead adapting to the downward movement of the slope.

Earthflows occur during heavy rains and cause the saturated material to move rapidly downslope, leaving tongue-like marks to show their paths. Earthflows usually remain active for long periods of time. Grain flows are similar in nature but they involve dryer materials moving at a faster rate. Not much disturbance is needed to set off a grain flow; an animal or a hiker on a trail can easily serve as the catalyst.

Avalanches, another type of granular flow that can be made of snow or rock, move vast amounts of material at high rates of speed, causing destruction to all habitat forms. Snow avalanches, which can be precipitated by animals or humans moving across susceptible areas, are quite common in high mountain areas that have significant snowfall; they lead to scores of deaths around the world each year. Debris avalanches are often triggered by earthquakes and volcanic eruptions. The slope of the mountain does not have to be especially steep for a debris avalanche to take place.

Human Activity

Human activity can play a significant role in mass wasting events. When humans create steep slopes or build homes on bluffs along the coast, for example, they are creating opportunities for mass wasting. Situations that are prone to mass wasting are easy to anticipate. When a work crew blasts a landscape to create a pathway through a mountain or hill, creating steep sides in the process, they are setting the stage for mass wasting. Also, water and ice will form cracks and fissures in rock, weakening the face of the rock and making materials susceptible to the effects of gravity. Soon, the weight of the material and the force of gravity will move that material downward. Once it hits the ground, that material may join the talus at the slope bottom, but it is just as likely to continue bouncing and rolling for some distance from where it lands.

Just as predictable is the erosion that occurs at the foot of a bluff. Over time, coastal processes erode the sediment that makes up the beach between the bluff and the ocean. Large pieces of the beach can be eroded by significant weather events. Even if the erosion occurs more slowly, the base of the bluff can be weakened by wind, currents, and waves. As the bluff base is weakened significantly, so too is the soil at the top of the bluff weakened by heavy rains, seismic activity associated with road traffic, building projects, and tremors and earthquakes. When the force of gravity becomes too great, the soil beneath structures and other objects atop the bluff will slide downslope. This type of sliding is inevitable unless steps are taken to reinforce the integrity of the bluff.

Preserving the Environment

The mechanisms involved in mass wasting are well understood. As a result, it is possible for engineers to build structures and roadways in areas that are less likely to be lost to mass wasting. Also, relocating structures and roadways is possible because engineers can determine an area's susceptibility to a mass wasting event. Red flags for such events include the angle of the slope, the makeup and weight of the regolith and rock, and the amount of precipitating activity. Another red flag is heavy rain and snowfall.

Even with thorough study and planning, engineers cannot ensure that mass wasting will not occur in a given area. Gravity is an ever-present force, and the composition of regolith changes with the weather. The best that engineers can do is locate roads and structures in areas calculated to be the least influenced by mass wasting.

Principal Terms

avalanche: a type of granular flow with a high velocity of large volumes of materials

creep: a type of granular flow that exhibits a gradual and continual downslope movement of regolith

debris flow: a type of slurry flow that often results from heavy rains and has velocities between 1 meter per year and 100 meters per hour

free fall: a type of slope failure where individual rocks on a steep slope break loose and fall to the ground; also known as rock fall

granular flow: a type of sediment flow that contains between 0 and 20 percent water

mudflow: a slurry flow with soup-like consistency that moves at speeds greater than 1 kilometer per hour along valley floors

regolith: loose rock, sediment, and soil that sits atop bedrock

sediment flow: debris that flows downslope; mixed with air or water

slide: a type of slope failure that occurs when rocks or debris slide down a preexisting surface; also known as translational slide

slope failure: a sudden failure of a slope resulting in transport of debris downslope

slump: a type of slope failure involving the downward rotation of rock or regolith along a concave upward curved surface; also known as rotational slide

slurry flow: a type of sediment flow that contains between 20 and 40 percent water

solifluction: a slurry flow measured at a rate of centimeters per year

talus: an accumulation of fallen material

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