Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 257

5.11). Mudflows may occasionally move several kilometers onto adjacent lowland and
may transport boulders weighing several tons.
Mudflows have caused considerable destruction of life and property. Mudflows initi-
ated at the peaks of inactive volcanoes were believed to contribute to the tragic mud-
slide at Guinsaugon, Philippines, in 2006. In the Andes of Peru, earthquakes and the
melting of glacial ice during the past century have resulted in numerous major mud-
flows. Glacial meltwater collects in lakes behind moraine dams; if there is an earth-
quake or the dam is breached, a huge mudflow may result. A long history of such cata-
strophic mudflows can be interpreted in the geomorphology of the base of the Andes
(Lliboutry et al. 1977).
Debris flows contain material ranging in size from clays to boulders, and may contain
woody debris, such as logs and tree stumps. Available material is an important control
on debris flow initiation; if material has been removed from a recent flow, a flood is
more likely to occur (Glade 2005). The character of the surface cover and response to
intense rainfall, rather than contributing area or slope, are more important for debris
flow formation (Godt and Coe 2007). Evidence from a debris flow in the Italian Alps in-
dicates a maximum flow depth of 7 to 8 m and a peak discharge between 350 and 400
m 3 per second, with a fluid-mud and grain flow behavior (Sosio et al. 2007).
Slumping
Slumping is the slippage of material moving as a unit or as several subsidiary units
along a concave surface of rupture (Fig. 5.12). Slumping begins with a rapid movement
at depth. The movement may evolve into a flow or slide. Slumping typically takes place
along a zone of weakness where the area downslope has been disturbed and support
has been removed. Consequently, this process is important along road cuts and where
lakes or streams have undercut their banks. The rate of movement by slumping is rapid
enough to be observable, but is not as rapid or destructive as mudflows or landslides.
Rockfalls
Rockfalls simply involve the rapid falling of rock downslope (Sharpe 1938). Rocks may
fall directly from a cliff or headwall, or they may tumble downward and initiate oth-
er movement (Fig. 5.13). In many mountain areas, rockfall is frequent and is thought
to be of considerable geomorphic significance (Gardner 1973; Luckman 1976). It is a
localized and sporadic process, occurring most frequently in spring and autumn. Dur-
ing spring, rockfalls occur with greater frequency as the ice-cemented bond that holds
cliffs together is lost. In autumn, rockfall occurs in a process similar to that of fractur-
ing a pipe. Pressures increase because of the volumetric expansion of ice, but also be-
cause water cannot escape readily (Williams and Smith 1989). The increased pressures
lower the freezing point, and as the temperature cools, the pressure rises. Eventually,
the pressure becomes so great that rupture occurs, initiating a rockfall event.