Geoscience Reference
In-Depth Information
on 24 -25 August. Over 600 debris flows occurred
during this period and because most of the pre-
cipitation fell as rain even at high altitudes, over
60% of the debris flows, including 82 large events
(volumes
Table 2.10
Comparison of the probability of permafrost
occurrence in the 1987 debris flow starting zones for
contemporary and Little Ice Age conditions in the Swiss Alps.
(Source: Zimmermann & Haeberli 1992.)
1000 m
3
), originated above 2300 -
2400 m a.s.l. in the periglacial belt; on the lower
slopes conditions remained stable. Rainfall inten-
sities were relatively low, however the period of
precipitation was long and most debris flows were
reported during or shortly after the maximum
downpour. Starting zones were typically 28° to
33° for slope-type debris flows, but a substantial
proportion (25%) were initiated in torrents,
gorges and rocky ravines (Haeberli et al. 1990;
Rickenmann 1990; Zimmermann & Haeberli
1992; Rickenmann & Zimmermann 1993).
The event has been investigated in detail by
an interdisciplinary team of Swiss scientists
examining the causes and impacts of the storms
(Haeberli et al. 1990).
1
Debris flows were initiated in thick and extre-
mely loose debris and bedrock. No sedimentary
structure could be considered a factor that limited
the depth of erosion. Erosion depth was more
likely caused by the dynamics of the flow. The
sediments affected appeared to be highly per-
meable and hydraulically non-homogeneous.
The widespread occurrence of such material in
starting zones makes these areas prone to fur-
ther instabilities (Haeberli et al. 1990).
2
The regions affected by the debris flows are
mainly located in crystalline bedrock covered
in massive and stratified, loose glacial and talus
deposits with poor sorting and low clay and
limited silt contents (Roesli & Schindler 1990).
3
Most of the debris flow activity in 1987
occurred on unstable slopes or in gullies pre-
disposed to instability. The spatial distribution
of debris flows shows a distinct concentration
in the periglacial zone as well as small clusters
in small tributary valleys. The occurrence of
the events could not be predicted from a
knowledge of meteorological conditions alone
(Zimmermann 1990).
Zimmermann & Haeberli (1992) did a
comparison of the probability of permafrost
occurrence in the 1987 debris-flow starting
zones and conditions at the same locality in
>
Occurrence of
Percentage of all cases (
n
=
82)
permafrost in areas
of 1987 debris flows
1850
1987
Certain
6
6
Probable
27
9
Possible
23
21
Unlikely
44
64
the Little Ice Age. Table 2.10 indicates that
the 1987 debris flows predominantly occurred
in localities where permafrost is marginal or
absent. Significantly in about 20% of cases con-
ditions over the historical period changed from
probable to possible permafrost occurrence and
in such areas permafrost degradation and thaw
instability are likely to have taken place prior to
the 1987 debris flows.
Another form of mass movement which can
have a large impact in developed mountain coun-
tries are massive rock-slides. The 1991 Randa
rock slide in southern Valais, Switzerland caused
extensive environmental damage and a heavy
economic burden. The rock slide occurred in
the tourist valley leading to Zermatt and the
Matterhorn (Fig. 2.12). Approximately 30
×
10
6
m
3
of rock fell from a south-east facing rock
face near the village of Randa approximately
10 km north of Zermatt. The rock fall occurred
in two main parts: on 18 April 1991 and again
on 9 May 1991. There were no fatalities except
for loss of some livestock and 31 chalets were
buried in the events (Quanterra 2003). The first
event occurred without warning and caused major
disruption and devastation to the valley infra-
structure. The first event deposited approxim-
ately 20
10
6
m
3
of rock and initial assessments
showed there was still a considerable mass of
unstable rock in place. This prompted authorities
to install a seismographic and geodetic warning
system. As a result the second event was forecast
from field evidence and geodetic and seismic
surveys, and the area was successfully evacuated.
Prior to the second slide geodetic displacement
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