Geoscience Reference
In-Depth Information
Table 2.9
Characteristics of glaciers
susceptible to outburst floods -
Mount Rainier, Washington, USA.
(Source: Driedger & Fountain 1989.)
Glacier
Surface area
Mean surface slope
Number of floods in
(km
2
)
(degrees)
record up to 1988
Nisqually
4.6
25
9
South Tahoma
2.8
23
12*
Kautz-Success
1.8
29
5
Carbon
11.2
18
1
Winthrop
9.1
21
2
*Revised estimate (Walder & Driedger 1995) 22 floods since 1967.
favouring subglacial cavity formation: increased
exposure to storms and more intense solar
radiation (melt) (Driedger & Fountain 1989). At
Mount Rainier water originates from snow and
ice melt and liquid precipitation, and is stored
within glacier cavities and at the bed of the glacier.
Ice movements deform these cavities, resulting
in the catastrophic release of the water. Hydro-
logical studies of the South Tahoma glacier
(Driedger & Fountain 1989) suggest that water
is stored at the bed of the glacier and the flood
magnitudes originating from such storage are in
the order of 1
in places. On the Nisqually River (Fig. 2.11a),
outburst flooding from the Nisqually glacier
destroyed and damaged bridges in 1926, 1932,
1934 and 1955, resulting in a high-level bridge
eventually being built. Since then large floods in
1968, 1970, 1972 and 1985 resulted in massive
rearrangement of the stream bed but have left
the bridge unscathed. Other activity has also been
noted at the Winthrop glacier, and the Carbon,
South Mowich and Emmons glaciers are also
suspected of being susceptible to this kind of
activity (Fig. 2.11a). The most recent activity
reported from Mount Rainier was on 14 August
2001 and constituted a moderate debris flow
in the Van Trump drainage within the Kautz
Glacier area.
The outburst floods tend to occur in late sum-
mer or autumn; usually in the late afternoon
or early evening, often during rainstorms. The
flood waves have some common characteristics.
They have been described as a noisy, churning
mass of mud and rock. Local winds can develop
along the flows and thick dust clouds can accom-
pany the events. There is often a smell of freshly
cut vegetation and chipped rock as boulders
smash trees and collide with bedrock and other
sediment (Fig. 2.11b). The flows are very rapid,
often in excess of 4.5 m s
−1
. Observers have said
there is generally less than 2 minutes between
hearing the flow and it passing the observer
(Driedger 1988) (Table 2.7).
10
5
m
3
. These floods threaten life
and property because they occur without warning
and quickly develop into rapidly moving debris
flows as they entrain loose volcanic debris.
Kautz Creek (Fig. 2.11a) is the catchment
affected by the largest recent event, which occurred
on 2-3 October 1947. This was triggered by
heavy rain and it was estimated that 38
×
10
6
m
3
of material was moved in the event, with some
boulders up to 4 m in diameter being trans-
ported (Driedger 1988). The highway 9 km
downstream from the glacier was engulfed by
the flow, which deposited over 3 m of mud and
debris. Since 1947 smaller significant floods have
occurred in 1961, 1985 and 1986. Tahoma Creek
shows clear evidence of frequent outburst floods
from South Tahoma glacier. At least 22 outburst
floods have been recorded since 1967, including
14 in the years 1986 -1992 (Walder & Dreidger
1995) (Table 2.9). In 1967, 1971, 1973, 1986
and 1988 small floods triggered debris flows that
destroyed trail bridges and campground-picnic
areas. Recent activity has resulted in valley scour
of 2 m in places, with boulders up to 0.5 m
transported and local deposition exceeding 1 m
×
2.3.2 Seismically triggered slope instabilities
Landslides or rock falls occur when a surficial
mass fails along a steeply dipping fracture plane.
The rock mass breaks up and moves downslope,
