Avalanche Formation, Movement and Effects (Proceedings of the Davos Symposium,
September 1986). IAHS Publ. no. 162,1987.
Slushflow release mechanism: A first approximation
LAWRENCE J. ONEST1
Department of Geology, Indiana University, Bloomington, Indiana 47405, USA
ABSTRACT
Starting zone slopes, snowpack depths and
snow densities prior to and after the slushflow were
measured for-13 separate events. Undrained and
drained slush was also-measured. Based upon these
data and field observation a model was developed which
attempts to determine the release mechanisms of
slushflows.
Mécanisme du dégagement des flots de neige fondante:
Approximation préliminaire
RESUME
On a mesure a treize reprises la hauteur de
la neige a la cime de la pente avant le glissement des
neiges, la profondeur et le poids de la neige avant et
après le flot de neige fondante. La neige fondante,
drainée et non-drainee, a ete également mesurée. Base
sur ces faits et sur des observations sur place, on a
développe un modèle qui cherche a établir les
mécanismes du dégagement des flots de neige fondante.
Introduction
Slushflows are described as the predominantly linear flow of
water-saturated snow. A review of the literature (Washburn 1980
193_195) indicates no information exists concerning slushflow
release mechanisms. Research has been done on the formation of
wet snow avalanches (Moskalev, 1966), and there is a large body
of literature on wet snow metamorphism, movement of water through
snow and wet snow stability, which appears to be related to the
release conditions of slushflow (Colbeck 1978, 1979, and 1982,
Kattelmann, 1984 and Nyberg 1985).
Observations of slushflows in the Central Brooks Range, Alaska
suggest that adequate snowpack and accelerated snowmelt are
conditions necessary to initiate flowage (Onesti, 1985). Major
factors causing accelerated snowmelt are:
(a) Twenty-four hours of radiation input.
(b) Intrusion of warm continental air into the Arctic during
early spring.
(c) Above freezing temperatures for extended periods during
early breakup.
(d) Sensible heat transfer and. turbulent heat exchange on side
slopes.
(e) Multiple free water sources (see Fig. 1 ) .
(f) Aspect of slopes and/or chutes, since it affects radiation
331
332 Lawrence J. Onesti
intensity.
Hestnes (1985) indicates that rainfall is a major contributor
to slushflows in Norway, which further supports the concept that
rapid accumulation of free water in the snowpack is an important
criteria for slushflow initiation.
The purpose of this paper is to present investigations on
starting zone characteristics and snowpacks and slush densities
in the Central Brooks Range. It will also attempt to develop a
first approximation for slushflow release mechanisms.
FIG. 1 Schematic representation of the manner in
which free water accumulates in the snowpack
prior to slushflow initiation.
Starting Zone Characteristics
Rapp's (1960) and Raup's (1965) descriptions of slushflow
starting zones emphasize the importance of a collection basin for
free water, usually a barrier that traps melt water and
facilitates the saturation of the snowpack such as a snow
avalanche deposit or a frozen waterfall.
The above conditions were not observed in the Brooks Range.
The gradients of chutes in which most slushflows occur are
generally concave downward. A subdued stair-step configuration
is superimposed on these concave gradients due to varying
resistance in the bedrock and differential erosion (Fig. 1).
Slushflow release mechanism: A first approximation 333
Collection basins are located in the flatter segments of the
chutes. The slopes of the starting zones range from 15° - 20° in
steep mountain chutes and are as low as 2° on broad open
floodplains. Starting zones are located on impermeable surfaces
of either bedrock, permafrost or a layer of ice located at the
base of the snowpack.
Snowpack and Slush Characteristics
The greatest accumulation of snow is found in the troughs of
chutes and valley bottoms. Snow depths in the chutes range from
1.5m to 3.7m and average 2.7m; in the floodplains they average
0.6m.
Snowpack densities were measured prior to breakup periods in
chutes which had a history of slushflow production. Preflow snow
densities range from 490 kg/nr to 685 kg/nP averaging 517 kg/nr,
which is considerably higher than those found in mid latitude
alpine snowpacks (Perla and Martinelli, 1976). Slush samples
were collected, weighed, gravity drained and reweighed. Slush
densities ranged from 900 kg/n, to 970 kg/rrr and averaged 927
kg/m^. The weight of the drained slush ranged from 540 kg/mP to
570 kg/nr and averaged 550 kg/rrP indicating at least a H0%
saturation of water by weight. The author is aware that
additional waters remain in the slush sample given the method of
drainage. Postflow snow densities measured in the crown surface
immediately after flowage ranged from 445 kg/nr to 690 kg/nr and
averaged 606 kg/nA The increase in snow density from the
preflow to postflow period is related to the metamorphism which
takes place when free water accumulates in the snowpack prior to
release, and to the free water which was retained by surface
tension.
Crown surfaces are always perpendicular to the bed of the chute
or valley floor (Fig. 2). In all cases full slab release was
observed suggesting the snowpack was saturated throughout its
entire thickness. In steep chutes crown surfaces ranged in width
from 14.6 m to 6.4 m and averaged 11.6 m. However, these widths
are primarily controlled by chute width. In the broad open
floodplain crown surface width was observed to be as great as
30m.
Tension cracks are normally observed above the crown surface,
an indication of creeping and gliding prior to failure. Sawtooth
fracture patterns are typically displayed on the flank surfaces.
The main conditions prior to slushflow release may be summarized
as follows:
1) Isothermal snowpack.
2) Accelerated rate of freewater input into snowpack.
3) Multiple freewater sources
a) overlying snowpack
b) upslope snowpack
c) sideslope snowpack
4) Input of free water exceeds output
5) Lower slope angles in starting zone
6) Total saturation of snowpack
334 Lawrence J. Onesti
FIG. 2
Crown surface showing full slab release which
is typical for slushflows. Sawtooth pattern
can be seen on flank surface.
Release Mechanism
With the onset of the snowmelt season, small amounts of water
appear in the snowpack and the pack becomes isothermal.
Capillary forces and intergrain bonding maintain the mechanical
strength of the snowpack and free water is held in the snowpack
by capillary attraction. This condition is classified as the
pendular regime. As melting continues, water completely
surrounds each grain and the snowpack is then in the funicular
regime (Colbeck, 1979). Later smaller grains disappear and the
large grains grow (Wakahama, 1968a). The ultimate of this
condition is fewer intergrain contacts, melting at the
interfaces, and as a result the mechanical strength of the snow
is reduced dramatically (Wakahama, 1975). As the grain size
enlarges so does pore size and free water retaining capabilities
of the snowpack are reduced.
It appears as though the metamorphosis which takes place within
the snowpack in conjunction with the accelerated rate of snow
melt is closely linked to slushflow activity. A model presented
by Nobles (1965) suggests that saturation of the local snowpack
is dependant upon snow permeability and the rate of melting. As
melting accelerates each downslope segment has to pass larger
volumes of free water than the segment above it. A water table
Slushflow release mechanism: A first approximation 335
develops and rises to the surface thus totally saturating the
snowpack. As the snowpack becomes saturated inter-grain cohesion
is reduced and pore water pressure increases thus a decrease in
the shear strength of the snowpack is experienced and may be
expressed as follows:
r= c + (CT-U) tancp
where r is shear strength, c cohesion, o compressive stress,
u hydrostatic pressure and <f> the angle of internal friction.
Starting zones on more gentle slope segments would fit the
Nobel s model. In addition, meltwater added to the snowpack from
the chute side slopes further accelerates the rise of the water
table. Larger pore spaces which develop after smaller grains
have melted would provide large conduits which would assist the
rapid free water buildup and saturation of the snowpack.
The snowpack would not necessarily metamorphose uniformly.
When snow immediately downslope from the starting zone is in the
pendular regime or in the early stage of the funicular regime,
it may act as a barrier to the free water collecting in the
basin. Another factor which could be part of the scenario
leading to mechanical failure is added weight to the snowpack due
to the addition of increased accumulation of free water.
Certainly all factors must be considered. Release mechanisms to
this date are not fully understood.
References
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snow. Advances in Hydrosciences. 11, 165-206.
Colbeck, S.C. (1979) Water flow through heterogeneous snow.
Cold Region Sciences and Technology. 1, 37_65.
Colbeck, S.C. (1982) An overview of seasonal snow metamorphism.
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Hestnes, E. (1985) A contribution to the Prediction of Slush
Avalanches. Annals of Glaciology 6, 1-4.
Kattelmann, R. (1984) Wet slab instability. Proceedings of the
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Moskalev, Y.D. (1966) On the mechanism of the formation of wet
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Nobles, L.H. (1965) Slush avalanches in Northern Greenland and
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Lawrence J. Onesti
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