Interesting Facts
Types of Landslides
There are many different types of landslides. Different scientists classify landslides in different ways. I classified them by the material they contain and by the mechanism of the slide.
Translational slide
This type of slide can contain rocks, gravel, sand or soil. When the slide contains rocks and when the rocks slide very quickly, it is called a rock avalanche. The Hope Slide, which happened 20 km. east of Hope, B.C. on Route #3, the Hope-Princeton Highway, in 1965, was this type of slide.
Hope Slide, BC.
Rotational Slide
In this type of landslide the surface where the slide material breaks free is concave.
Toppling
Big blocks of rock fall like dominoes. This has occurred on Mount Cheam in the Fraser Valley. Usually this type of slide doesn't occur in British Columbia except in the Rocky Mountains.
Mount Cheam, BC. Image courtesy of Drummond Cavers, AMEC Earth and Environmental, 1991.
Debris Flow
When rocks and mud are combined with a lot of water and they slide, it is called a debris flow. In this type of slide only the top layer of a slope becomes fluid. Debris flows usually happen on steep slopes. Many debris flows have occurred on Route # 99, the Sea to Sky Highway, north of Vancouver in British Columbia.
Rock Falls
Rock falls occur when rocks on a steep slope or on an eroded slope do not have enough support. The rocks fall, bouncing and rolling, at a high speed. In this type of slide there is a much smaller amount of rock than in a rock avalanche.
Groundwater Blow-Off Failure
Sometimes a lot of groundwater can collect in the area of a slope where the drainage is restricted on the surface of that slope. If the groundwater pressure keeps rising it will finally erupt through a weaker area of the surface of the slope. A lot of soil above this eruption will be displaced creating a valley or a ravine. This type of landslide occurred in 1935 on the slopes in front of the Museum of Anthropology at the University of B.C.
The Causes of Landslides
The most important cause of landslides is gravity. Rocks, mud, gravel or sand have a tendency to move down a slope, especially when a slope is steep.
Four important factors in determining whether a landslide will occur are:
1. friction between the sliding surfaces
2. the steepness of the slope
3. water pressure
4. the volume of sliding material present
The characteristics of a slope are also important. The cracks, layers and articulations are areas which are naturally weak and where rock can break into pieces. When there is a lot of groundwater present there is a lot of water pressure between layers of rock or in cracks or fissures in rock. That can cause the rock to break apart. Natural hazards like an earthquake or the eruption of a volcano can cause landslides and unfortunately humans can sometimes cause landslides also. For example, if logging roads are not built correctly or if certain areas are logged by "clear-cutting", erosion of the slope or mudslides can occur.
Destruction Caused By Landslides
In British Columbia, many highways, railways and natural gas pipelines have to be built on steep slopes and in other areas where slides occur. Also, sometimes villages or small towns are built on steep slopes. This means that landslides can cause a lot of damage to the environment and to buildings, highways and railways. For example, in 2003, near Prince Rupert B.C., a debris flow destroyed the natural gas pipeline for the city. The weather was very cold and lots of people in the city were left without heat in their houses until a temporary gas pipeline was completed.
When a landslide happens in a river or a stream, the debris can cause a dam. Then the water can't flow properly. Sometimes a landslide damages a salmon river. For example, in 1914, in the Fraser Canyon in B.C., in the area known as "Hell's Gate", there was a debris dam in the Fraser River. This prevented the salmon from migrating to the ocean and caused a big drop in the number of salmon.
Unfortunately, landslides sometimes kill people. For example, in the village of Frank, Alberta, in 1903, a huge rock avalanche of limestone on Turtle Mountain killed 75 people. A landslide sometimes kills people indirectly. If a bridge is destroyed by a debris flow, people driving in a car who do not know the bridge has been destroyed can fall into the river.
Reducing the Damage From Landslides
There are ways to reduce the damage caused by landslides. One way is to use enormous bolts to attach rocks together on a rock face. It is also possible to cover slopes with a strong wire mesh. In order to help promote the drainage of groundwater, engineers often drill holes into the face of a slope. That helps to reduce the water pressure inside the slope. Building cement "spillways" in areas where debris flows occur, helps the debris to fall without causing more destruction. A structure like this has been built at Alberta Creek between Horseshoe Bay and Squamish in British Columbia. It is possible to install sensors to detect and monitor the movements of rocks and fissures. In October 2004, geotechnical engineers finished installing sensors at the location of the Frank Slide in Alberta which occurred in 1903. These engineers think that another slide could occur here in the future.