Landslides

The term “landslide” refers to a wide range of mass wasting events that result in rock, soil, or fill moving downhill under the influence of gravity (Figure 10.9). These events occur when friction between the earth material (i.e., rock and soil) and the slope is overcome, allowing the earth material to fail and move downslope. Landslides may be triggered by high rainfall, earthquakes, erosion, deforestation, groundwater pumping, or volcanic eruptions. They may occur rapidly, such as in some mud and debris flows, or they can be as slow as soil creep: slow land movement that usually does not cause loss of life, but can still destroy roads and buildings. In mountainous areas, avalanches, landslides, and rockfalls can be dangerous, moving downslope and then crossing roads and moving into areas that contain homes and other buildings. In the Rocky Mountains, every year at least one road will be temporarily closed as the result of an avalanche, earth movement, or rockfall event. Mass wasting events can also dam streams and rivers, creating lakes. If such dams fail, a flood will result somewhere downstream.

Figure 10.9: Common types of landslides.

Landslides are common in mountainous regions of the Northwest Central thanks to a combination of steep terrain, poorly consolidated sediments, and tectonic activity (Figure 10.10). They often occur in high glacial valleys with little vegetative cover. In the winter, many of the same mountainous areas that are prone to landslides during the year are subject to avalanches—rapid flows of snow, ice, and rock. Avalanches occur when the strength of the snow is overcome, or when a weak layer in the snow fails. These snow failures can result from storms, warming weather, sunny slopes, earthquakes, and people moving over the snow. Thousands of avalanches occur every winter in the mountains of Idaho, Montana, and Wyoming.

Figure 10.10: Landslide incidence and risk in the Northwest Central.

In Montana, landslides are among the state’s most common geologic hazards. The largest landslide in Montana history, triggered by the Hebgen Lake Earthquake of August 1959, carried 80 million tons of mud, rock, and debris down Sheep Mountain at an estimated 160 kilometers per hour (100 miles per hour) (Figure 10.11). The slide killed 28 people and buried sections of Montana Highway 287 beneath almost 122 meters (400 feet) of rock, as well as formed a major dam across the Madison River (Figure 10.12). Landslides are also common occurrences in the mountains of Wyoming. In 1925, more than 38 million cubic meters (50 million cubic yards) of waterlogged soil was dislodged from a mountainside, crossed the Gros Ventre River, and moved 90 meters (300 feet) up the other side of the valley. The landslide blocked the river, creating Lower Slide Lake. Two years later, the dam failed, and the subsequent flash flood killed six people and destroyed a nearby town.

Figure 10.11: Damage from the Hebgen Lake Landslide is still visible today in Madison Valley, Montana.

Figure 10.12: The landslide dam that led to the formation of Quake Lake (also known as Earthquake Lake). Today, the lake is 58 meters (190 feet) deep and 10 kilometers (6 miles) long.

In Idaho, a variety of geological features combine to increase the likelihood of slope failure. Throughout the Snake River Plain and Columbia Plateau, basalt is interbedded with unconsolidated sediments, fractured metamorphic rocks, and loose volcanic material along deep canyons. Rocks fractured by folding and faulting are common, and ice-age floods deposited loose gravel and sand as well as undercut slopes. All these factors contribute to slope instability, and tremors from earthquakes associated with Idaho’s several fault lines often produce landslides throughout the state. Intense storms and heavy winter rains, generated by moisture carried eastward from the Pacific Ocean, can also waterlog soils and lead to mudflows or debris flows.

Mudflows or earthflows are fluid, surging flows of debris that have been fully or partially liquefied by the addition of water. They can be triggered by heavy rainfall, snowmelt or high levels of ground water flowing through cracked bedrock. High groundwater pressures and soil liquefaction due to nearby roadwork are thought to have generated the 1998 mudflow in Bonners Ferry, Idaho, in which 306,000 cubic meters (400,000 cubic yards) of earth materials flowed across Highway 95 and a Union Pacific railway track, burying more than a million dollars’ worth of equipment (Figure 10.13).

Figure 10.13: Mudflow in Bonners Ferry, Idaho.

Debris flows are a dangerous mixture of water, mud, rocks, trees, and other debris that moves quickly down valleys. The flows can result from sudden rainstorms or snowmelt that creates flash floods. In Glacier National Park, Montana, debris flows regularly occur where rock fragments like talus have built up on steep slopes and cliff faces. These debris flows can travel hundreds of meters (feet), and regularly impact trails and roads within the park.

Slumps and creep are common problems in parts of the Northwest Central with a wetter climate and/or the presence of unstable slopes, such as North Dakota’s Red River Valley, the Fort Randall Reservoir in South Dakota, and the Niobrara River in Nebraska. These areas contain expansive soils generated from clay-rich shales. Certain clay minerals can absorb water and swell up to twice their original volume—an amount of expansion that can exert enough force to cause damage, such as cracked foundations, floors, and basement walls. An estimated $9 billion of damage to infrastructure built on expansive clays occurs each year in the United States. In addition, when the clay dries and contracts, the particles settle slightly in the downhill direction. This process can cause soil creep, a slow movement of land that causes fences and telephone poles to lean downhill, while trees adjust by bending uphill (Figures 10.14 and 10.15). Human development can exacerbate this process when homes are built along river bluffs, disturbing vegetation that would otherwise stabilize the slope and adding water to the land in the form of yard irrigation or septic systems.

Figure 10.14: Some influences of soil creep on surface topography.

Figure 10.15: These fenceposts along the Sheyenne River Valley in North Dakota lean downhill under the influence of soil creep, while the trees near them bend uphill to compensate.

Slumping occurs when expansive minerals are present on steeper slopes, and involves the downward movement of a larger block of material along a surface that fails when the weight of the saturated soils can no longer be supported. Thanks to rain and heavy spring snowmelt runoff, slumps are a significant problem in some areas of North Dakota. In 2011 alone, this type of mass wasting caused more than $3 million of damage to roads and trails in Theodore Roosevelt National Park. Slumping is common near roads and highways throughout the state, thanks to the presence of steeper hills, roadcuts, and construction (Figures 10.16 and 10.17).

Figure 10.16: This slump near Interstate 29 in Fargo, North Dakota occurred in clay-rich materials used to construct the nearby overpass.

Figure 10.17: This slump occurred along a North Dakota roadcut after a spring thaw melted piles of snow on the upper bank, saturating the clay-rich soil and increasing its weight.

While expansive soils can be found all over the US, nearly every state in the Northwest Central has bedrock units or soil layers that are possible sources, with central Montana, North Dakota’s Red River Valley, and South Dakota’s Cretaceous shales being the most susceptible (Figure 10.18). Significant or repeated changes in moisture, which can occur in concert with other geologic hazards such as earthquakes, floods, or landslides, greatly increase the hazard potential of expansive soils. The key to reducing this hazard is to keep the water content of the soil constant. There are also chemical stabilizers, including lime, potassium, and ionic agents, that can reduce the potential for soil volume changes by increasing the clay’s structural stability.

Figure 10.18: Approximate distribution of expansive soils in the Northwest Central US. This map is based on the distribution of types of bedrock, which are the origin of soils produced in place. (Where substantial fractions of the soil have been transported by wind, water, or ice, the map will not be as accurate.)

Damage to life and property from mass wasting events can be reduced by avoiding landslide hazard areas or by restricting access to known landslide zones. Hazard reduction is possible by avoiding construction on steep slopes or by stabilizing the slopes. There are two main ways to accomplish stabilization: 1) preventing water from entering the landslide zone through runoff, flooding, or irrigation and 2) stabilizing the slope by placing natural or manmade materials at the toe (bottom) of the landslide zone or by removing mass from the top of the slope.