Page 479 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 14 Weathering, Karst Landscapes, and Mass Movement 443
Drier
Water content
Wetter
Soil creep
Solifluction
(a)
Drier
(b) Translational slide
Rotational slide, or slump
Rockfall
Earthflow
(c) Debris avalanche
Mudflow
▲Figure 14.24 Mass-movement classification. Principal types of mass movement produced by variations in water content and rates of movement. (a) A 1995 slide in La Conchita, California, also the site of a 2005 mudslide event. (b) The St. Boniface, Québec, mass movement, a retrogressive landslide, occurred in late April 1996. Here, the slide debris forms a series of concentric ridges, effectively damming the river when it occurred. (c) A 1999 muddy debris flow triggered by localized rainfall buried the Trans Canada Highway
5 km west of Banff. [(a) Robert L. Schuster/USGS. (b) Courtesy of the Geological Survey of Canada. Photo 2002-703 by Greg Brooks. Reproduced with the permission of Natural Resources Canada. © 2012. (c) Courtesy of the Geological Survey of Canada. Photo 2002-584 by Réjean Couture. Reproduced with the permission of Natural Resources Canada. © 2012.]
Wetter
Animation
Mass Movements
49 homes or structures. Soon after the event, debate ensued regarding whether the slide could have been foreseen given past events in the area, and whether log- ging operations upslope affecting groundwater flows may have contributed. What is known is that there was prolonged rainfall in the days preceding the slide which would have weakened slope materials. A report on this slide by the USGS, with a variety of images and explanatory materials, is found at www.usgs.gov/blogs/ features/usgs_top_story/landslide-in-washington-state/.
Rotational slides, also called slumps, occur when surface material moves along a concave surface. Fre- quently, underlying clay presents an impervious barrier to percolating water. As a result, water flows along the clay’s surface, undermining the overlying block. The overlying material may rotate as a single unit, or it may
acquire a stepped appearance. Continuing rotational mudslides in response to heavy rainfall plague La Conchita, California (Figure 14.24). In 1995, a slump landslide buried homes there, and in January 2005, a mudslide episode following a period of heavy rains bur- ied 30 homes and took 10 lives.
Flows When the moisture content of moving material is high, the suffix -flow is used, as in earthflows and more fluid mudflows. Heavy rains can saturate barren mountain slopes and set them moving, as was the case in the Gros Ventre River valley east of Jackson Hole, Wyoming, in the spring of 1925. About 37 million m3 of wet soil and rock moved down one side of the canyon and surged 30 m up the other side, damming the river and forming a lake. The water content of this
Slower Faster
Faster Rate of mass movement Slower