Geologic, Hydrologic, and Urban Hazards for Design in Desert Environments - Design with the Desert: Conservation and Sustainable Development - page 96

5.3 Mass Wasting: Debris Flows and Rockfalls
Mass wasting is a general term used by geologists and hydrologists to describe a
continuum of geologic processes ranging from avalanches to soil creep (see Chapter 2).
These processes involve the three-phase media of solids (e.g., rocks and soil), air, and water
moving at velocities ranging over several orders of magnitude. 26 Although the full range
of mass wasting represents general hazards, the principal hazards of this group in arid
and semiarid regions are rockfalls, avalanches, and debris flow. Landslides, perhaps the
most widely recognized mass-wasting hazard, generally do not occur in this region under
present-day climate except in certain localities after extreme rainfalls.
Rockfalls are relatively small failures of bedrock or slopes that generate relatively fast-
moving two-phase mixtures of solids and air. The resulting coarse-grained, poorly sorted
deposits are called colluvium and are stored on steep hillsides known as colluvial or talus
slopes. Rockfalls are an isolated hazard for certain buildings or infrastructure below cliffs
or steep slopes throughout arid regions (Figure 5.4), but this hazard becomes nonexistent
away from mountain fronts. Avalanches are larger-scale slope failures. The Blackhawk
Landslide, in Lucerne Valley of the Mojave Desert of California, is an air-cushion avalanche
that traveled about 5 miles from its source area to its depositional area more than 17,000
years ago.* Air-cushion avalanches are large enough that they trap air beneath the mass of
moving debris, much like a hovercraft, and move over the landscape with less friction and
higher speeds than if the avalanche moved in contact with the ground surface. ∗ Although
this type of extremely large avalanche occurred in a different climatic regime during the
Pleistocene, smaller avalanches remain a common occurrence in arid regions, particularly
where road cuts or other construction activities undermine slopes. Snow avalanches are
FIGURE 5.4
Debris-flow deposition in Soldier Canyon, Tucson basin, Arizona. This debris flow, which occurred in July 2006,
filled an existing channel with coarse sediment. (From Webb, R.H. et al., Debris flows and floods in southeastern
Arizona from extreme precipitation in late July 2006: Magnitude, frequency, and sediment delivery, U.S.
Geological Survey Open-File Report 2008-1274, Denver, CO, 2008.)
* http://3dparks.wr.usgs.gov/landslide/big/43.htm (accessed June 26, 2009). 27