Environmental Engineering Reference
In-Depth Information
Figure 25.17
Evolution of an alpine mountain slope after
glacier retreat: (a) block section and landforms, (b) the
related slope profile development.
oversteepened and therefore mechanically unstable rock (see Chapter 13). Major
rockslides usually occur as a result of catastrophic destabilization along deep-seated
failure surfaces involving very large rock volumes between 10
5
and 10
10
m
3
. They depend
on postglacial unloading, as in the 1963 Vaiont slide in northern Italy (2·5 × 10
8
m
3
), or
seismo-tectonic activity such as the 1964 Sherman Glacier slide, Alaska (2·3 × 10
7
m
3
).
Frost weathering works directly, sending small fragments into free fall, and indirectly by
widening discontinuities and weakening joint fill. This increases susceptibility to
unloading and intense run-off. Unloading generates tension cracks in upper rock walls,
whereas intense run-off can operate across their entire face. Stimulated by rainfall or melt
episodes, with power enhanced by high elevation and steep slopes, it flushes out loose
debris and may promote rockslides. A combination of all three processes tends to work
the rock wall into a series of chutes and intervening rock pinnacles which can locally
intensify any one of them.
Debris delivered to the rock-wall foot accumulates initially as an extensive apron or
series of
talus cones
. Progressive dissection of the rock wall leads to the formation of an
'hourglass' shape with chutes feeding cones, which coalesce as the pinnacles are finally
eroded (Plate 25.9). Debris supply to developing talus slopes can be relatively steady or
markedly episodic. Seasonal melt is a major influence which affects their form and
onward transfer processes. Most rockfall and shallow slide debris enters the slope near
the rock-wall foot and is then reworked by a combination of slow, talus-wide processes
involving creep, solifluction and slope wash, and fast, more concentrated debris slide,
mudflows and snow/slush avalanches. Rapid movement is common during spring melt,
especially in the active layer when permafrost is present, and intense summer rainstorms
after dry spells. Debris flows are the most rapid means of reworking talus and usually
originate at the sharp contrast in slope angle and permeability at the rockwall-talus
boundary below chutes (see box, p. 276). Flowing at 5-15 m sec
−1
, they form the clearest
contrast with debris creep at 10-100 cm yr
−1
. Progressive incorporation of glacial debris
towards valley floors creates a geotechnically complex colluvium. The profile of the
