Geoscience Reference
In-Depth Information
as low as possible through pumping and excava-
tion of a drainage trench. Eventually a 3.7 km
long bypass tunnel for the Vispa River was
completed (Fig. 2.12c). Following the event, road
and rail connections were rerouted away from
the site. Costs of works and surveys exceeded
110 million Swiss Francs (
c
. 88 million US$).
The rock slides were produced by structural
weaknesses in the valley-side rock slopes. Relief
joints parallel to the surface developed up to
200 m below the surface. At the base of the slope
a steep continuous fracture traversed the slope and
three joint sets divided the face into a large block
(Fig. 2.12d). The face was loaded from above by
pre-existing instabilities which also increased
water infiltration into the rock mass, promoting
weathering and build-up of porewater pressures.
The rock's mass was estimated to have 7 to 15%
void content (Schindler et al. 1993). Prior to the
event small rock-falls had been observed with the
final 1991 failure occurring during a snowmelt
period. Jets of water were observed near the basal
slip-plane just before and during the vent. Rock
slides of this magnitude are relatively rare, and
over the past 100 years in the Alps only a dozen
events of similar size have occurred.
Events such as this in Switzerland, although
infrequent, are not unusual. One of the most
famous historic landslide events is the Elm
landslide in 1881 (Heim 1882). This developed
in unstable, highly deformed sedimentary rocks.
Local slate mining was thought to be partly
responsible for the failure. The mass movement
occurred in three stages. Following failure the
rock-fall debris hit a rocky ledge on its steep
descent, the debris then cascaded like a waterfall
on to lower slopes where the main mass con-
tinued downslope towards the village of Elm.
A small part of the landslide continued under
its own momentum and surged 100 m up the
opposite hillslope. The landslide lasted an estim-
ated 40 seconds and travelled about 2 km. The
total volume is estimated to have been about
10 million m
3
. Although termed a landslide the
debris did not slide but flowed as a granular
mass (sediment-gravity flow) with a dispersive
stress developing as a result of grain collisions
in the mobile mass. There were 115 fatalities
and local communities were devastated by the
event.
2.3.4 Volcanically triggered mass movements and
long-term sediment delivery
Sediment delivery following major volcanic
events is conditioned by the nature and extent of
sediment deposited from the initial eruption and
how these are reworked over time. An import-
ant element is how geomorphological processes
act to determine the local dominance of sediment
transfer processes. Figure 2.13 shows a schematic
diagram of the changing dominance of different
sediment transfer processes with distance from
the Mount St Helens crater following the 1980
eruption (Scott 1988). The section shows a 60 km
transect from the crater down the Toutle River.
Over this distance there is a transition from pyro-
clastic surge and avalanche behaviour to debris
flows and finally hyperconcentrated streamflow.
In order to assess the impact of a mass movement
event of the like seen at Mount St Helens a useful
way of classifying catastrophic mass movements
is in terms of the runout distances (Siebert 1984).
A suitable relationship for describing the beha-
viour of different large-scale mass movements
types is the ratio of vertical drop to travel dis-
tance (
H
/
L
). Figure 2.14 compares various types
of mass movement using this simple relationship.
Although there is considerable scatter in the data
it is clear that volcanic landslides have greater
travel distances, with some pyroclastic flows and
lahars travelling great distances (
H
/
L
0.02). Hsu
(1975) developed an index to describe the exces-
sive travel distance (
L
e
) of large mass movements
that travel distances greater than the maximum
point expected by sliding of a body with a 0.62
friction coefficient (the value of 0.62 is the friction
coefficient applicable to sliding of a rigid block).
The relationship can be expressed as
L
e
=
<
H
/0.62.
In terms of Mount St Helens the distances
travelled by the pyroclastic flows and lahars were
excessive, resulting in an extensive tract of deva-
station. This special quality of extended runout
means that large-scale mass failures in volcanic
mountainous terrain should be considered dif-
ferently in terms of their sedimentology.
