Geoscience Reference
In-Depth Information
Another kind of flow associated with heavy rainfall is a
debris flow.
. Debris flows differ from mudflows because, in
addition to mud, they also contain a mix of boulders, trees,
and even buildings as they flow rapidly downhill. These rap-
idly moving and powerful flows can be triggered by extremely
heavy rains in mountainous regions or by a volcanic eruption
that instantly melts summit glaciers.
Debris flows are very serious natural hazards that can im-
pact people catastrophically. An unfortunate example was the
massive debris flow that swept down a steep hillslope into the
coastal community of La Conchita, California, in January 2005
(Figure 14.23). This debris flow was triggered by a three-day
storm that produced nearly 25 cm (10 in.) of rain in the region.
The slopes above La Conchita consist of ancient debris
flow and landslide deposits that had failed to a smaller extent
in 1995 when a large slump destroyed nine homes and blocked
a county road. When these deposits became saturated again in
early 2005, they failed on a much larger scale, releasing ap-
proximately 14,000 m
3
(18,311 yd
3
) of material that flowed
more than 100 m (330 ft) into the community. This debris flow
destroyed 13 homes and killed ten people. Given that slope
failures frequently occur in California during winter, this disas-
ter demonstrates why it is important to understand how mass-
wasting processes operate and how they can be triggered by
passing weather systems.
Figure 14.23 The La Conchita debris flow.
This massive de-
bris flow along the coast of Southern California was triggered by
heavy rains over the course of three days. Thick deposits of mud,
trees, and other debris swept into the community of La Conchita
and unfortunately killed several people.
Avalanches
Another mass-wasting process that you have probably heard of
is an
avalanche
, which is a large mass of snow or rock that
suddenly slides down a mountainside (Figure 14.24a). Ava-
lanches are most commonly associated with snow and typically
occur when thick deposits of snow accumulate at steep angles
on mountain slopes. These steep-angled snow deposits can be-
come unstable when a subsequent large storm produces another
layer of snow of lighter density than the snow it buries. Under
these conditions, an avalanche will occur because the two layers
of snow are not firmly bound together to form a uniform mass.
When the uppermost layer is somehow destabilized, a section
will break free as a
slab avalanche
. Such an avalanche may
release over 225 million m
3
(
∙
300 million yd
3
) of snow, which
is equivalent to about 20 football fields covered with about 3 m
(10 ft) of snow.
VISUAL CONCEPT CHECK
14.2
What type of mass-wasting process is pictured
here?
a)
Debris flow.
b)
Mudslide
c)
Rockfall
d)
Slumping
e)
Avalanche
Debris flow.
A rapidly flowing and extremely powerful mass
of water, rocks, sediment, boulders, and trees.
Avalanche
A large mass of snow or rock that suddenly slides
down a mountainside.
