Is Montana at Risk?
Identified Hazards for the State of Montana
Basic Disaster Information:
LANDSLIDES / SOIL FAILURE
Landslides are among the most common natural hazards. As with most natural hazards, they are often characterized by the catastrophic examples, such as the Hebgen Lake Quake Lake slide of 1959. Unlike most natural hazards, however, most damage is not caused by extreme events, but by uncounted (and often unreported) minor events.
Slopes with the greatest potential for sliding are between 34 degrees and 37 degrees. Although steep slopes are commonly present where landslides occur, it is not necessary for the slopes to be long. Therefore, landslides are one major natural hazard that is as likely, or perhaps more likely, to occur in eastern Montana as in the mountainous western part of the state.
Two criteria are typically used to classify landslides - types of movement and types of material. The types of movement are falls, topples, slides, spreads, flows, and combinations or two or more of these. Bedrock and soils are considered the principle material types. The soils type is further divided into debris and earth.
Landslides occur when the shearing of forces exceed the resisting forces of earth materials. Several factors contribute to either increased shear stress or reduced shear strength, thus initiating the landslide process. Factors which can increase shear stress are:
- Removal of lateral support, such as previous slope failure, erosion by rivers, streams or tidal currents and construction.
- Loading by natural or human means. Loading may occur with the added weight of rain, hail or snow, the accumulation of loose rock fragments or volcanic material, stockpiles of ore or rock, or the weight of buildings.
- Vibrations caused by earthquakes, blasting, machinery, traffic or possibly thunder.
Factors which may contribute to reduce shear strength are:
- Characteristics of the earth materials, composition, texture, structure and slope geometry.
- Material alteration through weathering and other physio-chemical reactions.
- Changes in direct water content and pore pressure and in structure.
The hazards associated with landslides are as diverse as the types of failure. Falls may damage roads ro buildings at the base of a steep slope, injure climbers, or remain on a road as a hazard to drivers. Slumps usually damage utilities within or below the slide mass, but seldom cause a threat to life. Translational slides can be the most catastrophic of all. In addition to presenting a local hazard to structures and utilities, they can cause damage and death both far from and only slightly below the source. Flows, in addition to the above hazards, can flow around well-built structures, preserving them but causing damage from water and mud.
In addition to the direct hazards of a landslide moving out from under or onto structures or utilities, there is a major indirect hazard. Large slides generally do not stop moving until they reach the bottom of a valley. At which point they block streams, usually resulting in flooding.
Much of the topography of the State of Montana was shaped by landsliding. Some slides occurred under a different climate than that which presently exists and are now stable. Others have been de-stabilized by human activity or natural causes and are currently active. The evidence of past landslides is among the most important factors in the prediction of future failures as it is inevitable that landslides will recur in most parts of the state. The areas of greatest susceptibility are those underlain by Cretaceous shales, with steep slopes, where the geologic structure parallels the hillslope, and where water availability is highest. The hazard decreases with deviation from those characteristics. The hazard is greatest in the spring, when more water is usually present, and during earthquakes, which may trigger the failure of unstable slopes. The variability of the hazard with structure, rock type, and terrain is such that only a site-by-site analysis will serve to delineate areas of hazard.
Landslides pose some degree of hazard to virtually the entire State of Montana. They become disastrous when intensive land development occurs in their proximity and people and property are endangered. It is therefore reasonable to expect an increase in potential levels of damage as the population grows, unless mitigative strategies are implemented. Identification of problem areas and avoidance of these problem areas will greatly reduce the potential for a catastrophic event. In addition, improved building designs and enforced building codes, control and stabilization necessary for the area, and the purchase of insurance will help to properly mitigate against this hazard.
