10 Mass Wasting
At the end of this chapter, students should be able to:
- Explain the basic processes and triggers of mass wasting.
- Identify types of mass wasting.
- Identify risk factors for mass wasting events.
- Evaluate landslide risks for particular landscapes.
Mass wasting is the downhill movement of rock and soil material due to gravity. The term “landslide” is often used as a synonym for mass wasting, but mass wasting is a much more broad term referring to all movement downslope. Technically, landslide is a general term for faster mass wasting where the regolith moves. Unconsolidated fragments of rock located on top of bedrock is called regolith. This loose material along with overlying soils are often what typically moves during a mass wasting event, but bedrock can move (rock topples) or a more liquid-driven movement can occur in mudflows. Movement by mass wasting can range from slow to rapid where it can be dangerous, such as during debris flows. Areas with steep topography and rapid rainfall, such as the California coast, Rocky Mountain Region, and Pacific Northwest, are particularly susceptible to hazardous mass wasting events. This chapter will discuss the fundamental mechanisms driving mass wasting processes, types of mass wasting, examples and lessons learned from famous mass wasting events, how mass wasting can be predicted, and how people can be protected from this potential hazard.
10.1 Slope Strength
Mass wasting occurs when a slope is too steep to remain stable. Slope stability is ultimately determined by two principal factors: the angle of the slope and the strength of the underlying material. In the figure, a block of rock situated on a slope is being pulled down toward the Earth’s center by the force of gravity (fg). The force of gravity is, for the most part, constant on the Earth’s surface (variations depend on the local elevation and the density of the underlying rock). The gravitational force acting on a slope can be divided into two components: one pushing the block down the slope (the shear force or driving force, fs), and the other pushing into the slope (the normal force or resisting force, fn). The relationship between shear force and normal force is called shear strength. For the block to move, the shear force has to be greater than the normal force; that is, the driving force has to be greater than the resisting force. When the normal force is greater than the shear force, then the block does NOT move downslope. However, if the slope becomes steeper or if the earth material is weakened, causing the shear force to exceed the normal force, then downslope movement can occur.
In the above figure, the force vectors change as the slope angle increases. The gravitational force doesn’t change, but the shear force increases while the normal force decreases. The steepest angle at which rock and soil material is stable (and will NOT move downslope) is called the angle of repose and is measured from horizontal. When a slope is at the angle of repose, the shear force is in equilibrium with the normal force. If the slope becomes just slightly steeper, the shear force exceed the normal force, and the material would start to move downhill. The angle of repose varies for all materials and slopes depending on many factors such as grain size, grain composition, and water content. The figure below shows the angle of repose for sand that is poured onto a flat surface. The sand grains cascade down the side of the pile until coming to rest at the angle of repose.
The shear strength of a slope also depends on the water content of the material. Water can significantly change the shear strength of a particular slope. Water is located in pore spaces, which are empty air spaces in sediments or rocks. For example, assume a dry sand pile has an angle of repose of 30 degrees. If water is added to the sand, the angle of repose will increase, possibly to 60 degrees or even 90 degrees, such as a sand castle being built at a beach. But if too much water is added to the pore spaces of the sand castle, then the water decreases the shear strength, lowers the angle of repose, and the sand castle collapses.
Another factor influencing shear strength, are planes of weakness in sedimentary rocks. Bedding planes can act as significant planes of weakness when they are parallel to the slope and less so if they are perpendicular to the slope. At locations A and B, the bedding is nearly perpendicular to the slope and the bedding is relatively stable. At location D, the bedding is nearly parallel to the slope and the bedding is quite unstable. At location C the bedding is nearly horizontal and the stability is intermediate between the other two extremes . Additionally, if clay minerals form along bedding planes they can absorb water and become slick. When a bedding plane of shale (clay and silt) becomes saturated, it can lower the shear strength of the rock mass and cause a landslide such as at the Gros Ventre Slide discussed below.
10.2 Mass-Wasting Triggers
Mass wasting events often have a trigger, something that changes that causes a landslide to a occur at a specific time, such as rapid snow melt, intense rainfall, earthquake shaking, volcanic eruption, storm waves, or rapid stream erosion. In addition, modifications to a slope during human activities can provide a trigger. An increase in water content within the slope is the most common mass-wasting trigger. Water content can increase due to rapid melting of snow or ice in winter, spring, or early summer, or as the result of a intense rain event. These intense rain events can more often occur during El Niño year storms, when the west coast of North American receives more precipitation than normal and landslides become more common. Changes in surface water conditions resulting from earthquakes, previous slope failures that dam up streams, or human structures that interfere with runoff (e.g., buildings, roads, or parking lots) can provide additional water to a slope. In other cases, shear strength can be weakened by earthquake shaking such as in the case of the Hebgen Lake slide discussed below.
An oversteepened slope can trigger landslides. Slopes can be made excessively steep when humans modify the landscape for building construction, or by natural processes of erosion. An example of how a slope may be oversteepened during development would be where the bottom of the slope is cut into, perhaps to build a road or level a building lot, and the top of the slope is modified by depositing the material excavated from the lower slope. If done carefully this practice can be very useful in land development, but in some cases this can result in an oversteepened slope that is prone to landslides. For example, this might have been a contributing factor in the 2014 North Salt Lake City, Utah slide. A former gravel pit was regraded to provide a road and several building lots. These activities may have resulted in the oversteepening of the slope. This slow moving landslide destroyed one home at the bottom of the slope. Natural processes such as excessive stream erosion from a flood or coastal erosion during a storm can also oversteepen slopes. For example, riverbank undercutting was proposed as part of the trigger for the famous Gros Ventre slide in 1925. See case studies below for details.
10.3 Types of Mass Wasting
Mass wasting events are classified based on the type of movement and type of material. Since there are several ways to classify mass wasting events, the USGS Figure and the table below don’t match up perfectly. In addition, mass wasting types often share common morphological features observed on the surface such as the head scarp, hummocky surface, and toe of slope.
Type of Movement
|Primary Material Type and Common Name of Slide|
|Mostly Coarse-Grained||Mostly Fine-Grained|
|Rock Avalanche||Rock Avalanche||—||—|
|Rotational Slide (Slump)||—||Rotational Debris Slide (Slump)||Rotational Earth Slide (Slump)|
|Translational Slide||Translational Rock Slide||Translational Debris Slide||Translational Earth Slide|
|Flows||—||Debris Flow||Earth flow|
The most common mass wasting types are falls, rotational and translational slides, flows, and creep . Falls are abrupt movements of rock that detach from steep slopes or cliffs. Separation occurs along existing natural breaks such as fractures or bedding planes, and movement occurs as free-fall, bouncing, and rolling. Falls are strongly influenced by gravity, mechanical weathering, and the presence of water. Rotational slides show movement along a curved rupture surface with a commonly slow movement rate. Translational slides are movements along a plane of distinct weakness between the overlying slide material and more stable underlying material, and are often rapid. Slides can be further subdivided into rock slide, debris slide, or earth slides depending on the type of the material involved (See above table). The largest and fastest slides are called sturzstroms, or long-run-out landslides. They are still poorly understood, but are known to travel for long distances, even in places without significant atmospheres like the Moon.
Flows are mass wasting events in which the material is mixing internally, sometimes with abundant water, and moving at rapid speeds with long runouts at the slope base. Flows are commonly separated into debris flow (coarse) and earthflow (fine) depending on the type of material involved and the amount of water (See above table). Soil creep is the imperceptibly slow downward movement of material caused by shear stress sufficient to produce permanent deformation in unconsolidated material . A type of soil creep is solifluction, that is the slow movement on low-angle slopes due to repeated freezing and thawing of soil in high-latitude cold locations. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges.
10.4 Examples of Landslides
Landslides in United States
1925 Gros Ventre, Wyoming: On June 23, 1925 a 38 million cubic yard translational rock slide occurred next to the Gros Ventre River (pronounced “grow vont”) near Jackson Hole, Wyoming. The rock slide consisted of large boulders that dammed the Gros Ventre River and ran up the opposite side of the valley several hundred vertical feet. The dammed river created Slide Lake and two years later in 1927 lake levels rose high enough to destabilize the dam. The dam failed and caused a catastrophic flood that killed six people in the small downstream community of Kelly, WY .
The cause of the rock slide was a combination of three factors: 1) heavy rains and rapidly melting snow that saturated the Tensleep Sandstone, causing the underlying Amsden Shale to lose its shear strength, 2) the Gros Ventre River had cut through the sandstone creating an oversteepened slope, and 3) soil on top of the mountain became saturated with water due to poor drainage . The bedding planes between the Tensleep Sandstone and Amsden Shale were parallel with the surface and an earthquake may have been the trigger.
1959 Madison Canyon, Montana: In 1959 the largest earthquake in Rocky Mountain recorded history, magnitude 7.5, struck the Hebgen Lake, Montana area. The earthquake caused a rock avalanche that dammed the Madison River and ran up the other side of the valley hundreds of vertical feet. Today, there are still house-sized boulders visible on the slope opposite their starting point. The slide moved at a velocity of up to 100 miles per hour creating an incredible air blast that swept through the Rock Creek Campground. The slide killed 28 people, most of which were in the campground . Similar to the Gros Ventre slide, planes of weakness in metamorphic rock outcrops were parallel with the surface.
1995 and 2005 La Conchita, California: In 1995 a fast moving earth flow damaged nine houses in the southern California coastal community of La Conchita. A week later a debris flow in the same location damaged five more houses. Surface “tension” cracks at the top of the slide gave warning signs the year before in the summer of 1994. As the rainy season began for the 1994/1995 winter, the cracks grew larger. The likely trigger of the 1995 event was unusually heavy rainfall during the winter of 1994/1995 and rising water table (groundwater levels). Ten years later, in 2005, a debris flow occurred at the end of a 15–day period of near–record rainfall in southern California. Vegetation remained relatively intact as is was rafted on the surface of the rapidly flowing mass, which indicates that much of the landslide mass simply was being carried on the fluidized layer beneath, which presumably was much more saturated. The 2005 slide damaged 36 houses and killed 10 people . Use several figures from Jibson (2005).
2014 Oso Landslide, Washington: On March 22, 2014, a landslide of 10 million cubic yards traveled nearly a mile at 40 miles per hour, dammed the North Fork of the Stillaguamish River, covered 40 homes, and killed 43 people in the Steelhead Haven community near Oso, Washington. This volume of material is equivalent to 600 football fields covered in 10 feet of material. The winter of 2014-2015 was unusually wet with almost double the average amount of precipitation. The location of the slide was an active area with many slides throughout the Stillaguamish River Valley but previous slides had been small in size .
Markagunt Gravity Slide: 21-22 million years ago, more than 1700 cubic kilometers of material was displaced in what is thought to be one relatively fast event . This is one of the biggest land-based landslide yet discovered in the geologic record. Evidence for this slide includes a basal breccia, pseudotachylytes (glass created from friction heat), slip surfaces (similar to faults), and clastic dikes). The landslide is thought to encompass an area the size of Rhode Island and extends from near Cedar City, Utah to Panguitch, Utah. This landslide was likely the result of material on the side of a growing laccolith being triggered by a eruption-related earthquake.
1983 Thistle Slide: Starting in April of 1983 and continuing into May of that year, a slow moving slide traveled 1,000 feet downhill and blocked Spanish Fork Canyon with an earth flow 200 feet high. This caused disastrous flooding upstream in the Soldier Creek and Thistle Creek Valleys, submerging the town of Thistle. As part of the emergency response a spillway was constructed to prevent the newly formed lake from breaching the dam. Later a tunnel was constructed to drain the lake and currently the river flows continues to flow through this tunnel. The rail line and Utah State Highway 6 had to be relocated at a cost of more than $200 million .
2013 Rockville Rock Fall:
Rockville, Utah is a small community near the entrance to Zion National Park. In December of 2013, 2,700 tons of of blocky Shinarump Conglomerate fell from the Rockville Bench cliff, landed on the steep 35-degree slope below, and shattered into several large pieces that continued downslope at a high speed. These boulders completely destroyed a house located 375 feet below the cliff and killed two people inside the home. As can be seen in the figure (Figure), there have been other rock falls in the area, including the site of this catastrophic event .
Reports from residents suggested that ground cracks had been seen near the top of the slope at least a year prior to the catastrophic movement.The presence of easily-drained sands and gravels overlying more impermeable clays weathered from volcanic ash, along with recent regrading of the slope, may have been contributing causes of this slide. Local heavy rains may have provided the trigger. In the two years after the landslide the slope has been partially regraded to increase its stability. Unfortunately, in January 2017, parts of the slope have shown reactivation movement. Similarly, in 1996 residents in a nearby subdivision started reporting distress to their homes. This distress continued until 2012 when 18 homes became uninhabitable due to extensive damage and were removed. A geologic park was constructed in the now vacant area.