Saturation of a Valley Snowpack

Meltwater production by atmospheric effects on a snowpack is restricted to the uppermost part. Once meltwater is produced in an amount exceeding the irreducible content, it starts to percolate through the snow. Meltwater that is held by pore pressure yields only a few per cent by volume of snow (Reference ColbeckColbeck, 1972). Subsequent metamorphism of the snow in the presence of the liquid phase leads to grain growth and the disappearance of smaller grains. This wet-snow metamorphism takes place within several hours to some days, especially when the whole pore volume is occupied by water, i.e. in saturated conditions (cf. reviews by Reference Male and ColbeckMale (1980) or Reference ColbeckColbeck (1987)). Snow metamorphism triggered by melting and percolating of water leads to increased hydraulic conductivity due to: (1) the coarsening of the grains and increasing distances between grains; (2) the higher pore volume and diameters; and (3) the weight lost of the ice matrix in saturated snowpacks (Reference Male and ColbeckMale, 1980; Reference ColbeckColbeck, 1987; Reference KattelmannKattelmann, 1987). Fully saturated snow with low bond attraction between grains therefore has low resistance against erosion by the concentrated flow of water. An effect caused by this process is the erosion of drainage pipes within the snowpack.

Nevertheless, slope inclination remains an important invariant factor, since it determines the gradient at which gravity is effective. This implies that, on steep slopes, water drainage is faster than on shallower slopes.

Water movement within the snowpack

Movement of water within a saturated snow layer can be computed by adapting Darcy’s formula. The melt water-flow velocity is given by

where k, ρ and μ are the intrinsic permeability, density and viscosity of water; g represents gravity. Δh and Δx are the vertical and horizontal differences of the water-table surface, i.e. the hydraulic gradient. The field measurements in Kärkerieppe offer adequate data for the hydraulic gradient but not for Darcy’s k. Direct measurements of the permeability in fully saturated snow are rare, so suitable values must be approximated. Reference ColbeckColbeck (1972) calculated 10–40 × 10⁻¹⁰ m² and Reference KattelmannKattelmann (1987) quoted approximately 3 × 10⁻¹¹ m² associated with high wave speeds, based on measurements by different authors. Both values are similar to the permeability of sand for similar grain-sizes. Therefore, a value of 10⁻¹⁰ m² is assumed to be appropriate, although snow metamorphism under saturated conditions should lead to even higher permeability. Tracer measurements of water drainage through a snow-filled channel indicated velocities of 10⁻¹ ms⁻¹ (Reference Gude and Scherer.Gude and Scherer, 1995), which implies an intrinsic permeability of approximately 10⁻⁸ m². We assume this value is valid only for concentrated flow at the snow base, presumably accompanied by piping.

The Darcy formula allows calculations of flow velocities and discharge rates on the basis of measurements of depth and slope of the saturated layer in the cirque bottom of the Kärkerieppe (Table 1). The discharge rates are valid for the gently inclined upper part, where the water pressure was measured, as well as for the lower section with a steeper slope (Fig. 1). This holds because no remarkable water gains or losses occur between. The calculated depths of the discharge layer in the steeper part are significantly lower than in the low-gradient section. The mean decrease of angle in the water table from full saturation of 3.5 m (on 3 June) to a saturation depth of 0.7 m almost equals the angle of the water table in the steep part. Nevertheless, maximum decreases of angle are even higher, when the propagation of single water waves is considered. Whereas in the steep part the snow is saturated only to approximately 20% of the depth, the saturation in the upper part is 100%.

Fig. 1.

Longitudinal profile of the Kärkerieppe valley section with slope change (water stages shown as dots; interpolation assumed and calculated).

Table 1.

Caculated velocities and discharge rates in Kärkerieppe, 1995

A significant saturation layer has been present in the low-gradient section of Kärkerieppe since 31 May, and wet-snow metamorphism has subsequently weakened the snow cover. Preservation of former weak layers such as depth hoar is unlikely, because saturated metamorphism has developed well since this time (Reference Male and ColbeckMale, 1980; Reference ColbeckColbeck, 1987). Reference Elder and Kattelmann.Elder and Kattelmann (1993) have reported a “homogeneous snowpack of large rounded non-cohesive grains with a high free water content” prior to the release of a slushflow. The release of the slush torrent at 19.00 h on 3 June therefore is temporarily and spatially connected to the accelerated increase in the water table and the resulting hydraulic gradient. Further snow metamorphism is assumed to have had small influence, because rupture of the snow cover occurred at the base of the snowpack, where full saturation had developed for several days but not in the upper part with the increasing water level.

This type of rupture zone could be observed on Spitsbergen and also in the Kärkevagge and its surroundings. It has to be stressed that the prevention of open flow in a channel due to snow-filling is prerequisite for this sort of hydraulic release (cf. Reference NoblesNobles, 1966; Reference NybergNyberg, 1985; Reference Elder and Kattelmann.Elder and Kattelmann, 1993).

Relation to Fluvial Processes and Avalanches

Slush-flows are closely connected to the fluvial environment, which refers to the alfcctcd areas as well as to the hydrological process characteristics. The first publication with a definition of this topic, drafted by Reference Washburn and Goldthwait.Washburn and Goldthwait (1958), had already indicated this relation. Since then, about 30 publications in international journals have quoted slush movements and most of them have been confined to channels, valley bottoms or chutes (cf. e.g. Reference HestnesHestnes, 1985; Reference NybergNyberg, 1985; Reference OnestiOnesti, 1985; Reference AndréAndré, 1993).

Since saturation of a deep snowpack is a prerequisite for the release of a slushflow, the preferred area of initiation is a valley-section with a high inflow from the adjacentslopesbutlow outflow through a channel. On straight slopes, a remarkably long distance with a gentle gradient is needed to generate a saturated layer of a certain depth, as described by Reference Male and ColbeckMale (1980). In Arctic, and especially in Alpine environments, this type of long and gently inclined slope is rare except for glacier surfaces. Therefore, it is not surprising that slushflows have been reported from ice shields, and the slush zone of valley glaciers is also a well-known phenomenon (Reference NoblesNobles, 1966).

Besides the release zone and the flow path, the run-out zone fans of slushflows are akin to fluvial fans. Analyses of fans in northwestern Spitsbergen have stressed that slushflow fans are built up at the outlets of valleys similar to those of fluvial fans but they can be identified because of their characteristic shape and sediment features (Reference NybergNyberg, 1985; Reference Barsch, Gude, Mäusbacher, Srhukraft, Schulte and Strauch.Barsch and others, 1993). Consequently, sediment fans can offer important data on the magnitude and frequency of slushflows (cf. e.g. Reference AndréAndré, 1993).

The observed slushflow tracks in northern Sweden and northwestern Spitsbergen (a total number of approximately 20 tracks) were restricted to channel sections with almost intact snow cover. These channels are opened by slushflows and herebyopen-waterflow is enabled. This conrrsponds to the initial definition given by Reference Washburn and Goldthwait.Washburn and Goldthwait (1958). In this context, slushflows represent an important element of the seasonal course of fluvial activity. Since high amounts of meltwater can be stored temporarily in channel snowpacks, peak floods in the form of slush torrents can be several orders of magnitude higher than fluvial floods (Reference Barsch, Gude., Mäusbaeher, Schukraft and Schulte.Barsch and others, 1994).

The occurrence of slushflows has often been discussed in connection with avalanching, as a result of which several authors have used the term “slush avalanche” (e.g. Reference RappRapp, 1960; Reference HestnesHestnes, 1985). However, the term implies a mixing with wet-snow avalanches, although several process characteristics indicate that a separate definition is to be recommended. As demonstrated above, snow metamorphism is assumed to be a necessary but not sufficient factor in the development of a critical mass of slush, but the factor determining the release is the hydraulics of the water table. These release conditions are not covered by the avalanche definition. Furthermore, avalanches and slushflows affect different areas, depending on the initiation factors. The typical slope thresholds for the release areas of avalanches are greater than 25° but slushflows most frequently occur on slopes of less than 15° and have been reported to have been released on slopes of 2° (Reference RappRapp, 1960; Reference NoblesNobles, 1966; Reference SchererScherer. 1994).

Despite the different release conditions, transitional processes are possible. Prominent is the triggering of slushflows by avalanches that run into saturated snow covers in channels and mobilize slush masses. Without accounting lor the significant amount of meltwater incorporated into the flow, the overcoming of long and gently inclined valley sections cannot be explained.

Consequently, slushflows represent a transitional process related to fluvial floods and avalanches, but have to be separated from these processes because of their characteristic mechanisms of initiation, movement and run-out (summarized in Table 3). On the other hand, it is recommended that slush torrents (slush avalanches) be included in the overall definition of slushflows, because these mass movements have similar release and flow features.

Table 3.

Summary of typical characteristics of fluvial foods, slushflows and avalanches, based on field studies and publications

Hazard Assessment

In summary, the discussion of the release processes of slushflows and their relation to fluvial activity and to avalanches provides information suitable for the development of a hazard-assessment system. An assessment of hazardous impacts requires an evaluation of affected areas as well as the dangerous periods.

Consideration of affected and endangered areas comprises both the hydrological and terrain features, as well as the flow characteristics. They can be evaluated by accounting for the following subjects:

The periods that possess high risks of slushflow release have been given by the meteorological and snow hydrological conditions.

The most important factors are:

A hazard evaluation covering hazardous areas and periods requires both terrain analyses and meteorological and snow-hydrological monitoring. Information from these data sources must then be included in a numerical model.

Comprehensive experience in avalanche-risk assessment is an enormous benefit for the development of a model for slushllow risk evaluation but information on the release and flow characteristics is still limited.