The role of forest in slab avalanche formation

References

Atkins, D., 2012. Skiers, trees and avalanches: A murderous triad, in Proceedings of 2012 International Snow Science Workshop, Anchorage, Alaska, 736–739.

Frehner, M., Wasser, B., and Schwitter, R., 2007. Appendix 1 Natural hazards, in Sustainability and success monitoring in protection forests: Guidelines for

silvicultural interventions in forests with protective functions, Federal Office for the Environment, Bern, Switzerland, 1–26.

Frey, W., and Salm, B., 1990. Snow properties and movements in forests of different climatic regions, in Proceedings of the XIX IUFRO World Congress,

Montreal, Canada, 328–339. Not seen.

Gubler, H., and Rychetnik, J., 1991. Effects of forests near the timberline on avalanche formation, in Proceedings of the Vienna Symposium, Snow, hydrology

and forests in alpine areas, IAHS, Vienna, Austria, 19–38.

Imbeck, H., and Ott, E., 1987. Verjüngungs.kologische Untersuchungen in einem hochstaudenreichen subalpinen Fichtenwald, mit spezieller

Berücksichtigung der Schneeablagerung und der Lawinenbildung: Mitteilungen des Eidgenössischen Institutes für Schnee- und Lawinenforschung, 202 pp.

Not seen.

In der Gand, H., 1978. Verteilung und Struktur der Schneedecke unter Waldbaumen und im Hochwald, in IUFRO Working Party on Snow and Avalanches,

Swiss Federal Institute for Snow and Avalanche Research, Davos, 98–119. Not seen.

McClung, D.M., 2001. Characteristics of terrain, snow supply and forest cover for avalanche initiation caused by logging: Ann. Glaciol., 32:223–229.

McClung, D.M., and Schaerer, P.A., 2006. The Avalanche Handbook, The Mountaineers, Seattle, 271 pp.

Schaerer, P., 1977. Analysis of snow avalanche terrain: Canadian Geotechnical Journal, 14(3):281–7. Not seen.

Shea, C., Jamieson, B., 2010. Spatial distribution of surface hoar crystals in sparse forests. Natural Hazards and Earth System Sciences 10(6):1317–1330.

Schneebeli, M., and Bebi, P., 2004. Snow and avalanche control: Hydrology, v. Snow and avalanche control, 397–402.

Schneebeli, M., and Meyer-Grass, M., 1992. Avalanche starting zones below the timber line - structure of forest, in Proceedings of the International Snow

Science Workshop, Breckenridge, Colorado, 176–181.

Schweizer, J., Jamieson, B., and Schneebeli, M., 2003. Snow avalanche formation: Reviews of Geophysics, 41(4):1–25.

Teich, M., Marty, C., Gollut, C., Grêt-Regamey, A., and Bebi, P., 2012. Snow and weather conditions associated with avalanche releases in forests: Rare

situations with decreasing trends during the last 41 years: Cold Reg. Sci. Technol., 83–84:77–88.

Viglietti, D., Letey, S., Motta, R., Maggioni, M., and Freppaz, M., 2010. Snow avalanche release in forest ecosystems: A case study in the Aosta Valley Region

(NW-Italy): Cold Reg. Sci. Technol., 64(2):167–173

Weir, P., 2002. Snow Avalanche Management in Forested Terrain: Res. Br., B.C. Min.For., Victoria, B.C.

Part III

Critical stem density

Part I

Tree v. snowpack

Part II

Critical opening size

Density

How dense is dense enough?

canopy cover (%)

stem density (stems/ha)

Extreme critical slab length

Critical density in stems/ha (and spacing) where stems stabilize the snowpack

How open is too open?

Gubler and Rychetnik 1991

• formation limited to weak layer extent between supports

Critical canopy coverage (%) for inhibiting forest avalanches. After Schneebeli and Meyer-Grass 1992

• Same avalanche characteristics (slab, weak layer, slope) as with open terrain
• Forest openings influential in formation, despite limited research

• Objective: What is critical opening length and width?

Tree height

• important in determining density
• minimum height = 1.5 x snow depth
• minimum height = 3 m

(Saeki and Matsuoka 1969)

"Dense trees on steep slopes are likely protective if they cannot be enjoyably skied" - Dale Atkins (2012)

Key research:

• Saeki and Matsuoka (1969)
• Gubler and Rychetnik (1991)
• Schneebeli and Meyer-Grass (1992)
• McClung (2001)
• Schneebeli and Bebi (2004)
• Viglietti et al (2010)

Critical opening size

(Frey et al. 1987; Meyer-Grass 1987; Schneebeli and Meyer-Grass 1992)

Schneebeli and Meyer-Grass 1992

• Compared 118 start zones against 130 non-avalanche slopes

Proportion of vegetation height on slopes where avalanches occured. After McClung (2001)

Critical parameters for avalanches in five classes of forest. After Schneebeli and Meyer-Grass 1992

Species

Does species affect slope stability?

Minimum gapwidth size

Pfister (unpublished)

• 112 avalanches in coniferous forest

not necessarily

yes

• Gubler and Rychetnik (1991)
• Frehner et al (2007)

• Schneebeli and Meyer-Grass (1992)
• Schneebeli and Bebi (2004)
• Viglietti et al (2010)

After Pfister (unpublished) sourced from Schneebeli and Bebi (2004)

Precipitation

Wind

Radiation & temperature

Terrain (roughness)

Stratigraphy

Stratigraphy

Notes and numbers

Radiation / Temperatures

Notes and numbers

Wind

Precipitation

Terrain (roughness)

• Forest undergrowth reduces risk of sliding snow
• Exception: DH and FC around rocks and shrubs

• Fewer weak layers, and fewer fine layers and more fracture tough areas
• Main contributory factors: cold temperatures, heavy and intense snowfall and high wind speeds

(Gubler and Rychetnik 1991)

• incoming shortwave reduced by 90 % for dense stands and 30 % for sparse stands
• air temperature, humidity and diurnal variations unchanged between open and forested terrain

(Schneebeli and Bebi 2004)

(Frehner et al. 2007)

• wind patterns change and slow down in trees
• fewer slabs created than in open areas
• less fine layering of snowpack
• exception: localized slabs at stand edges

(Teich et al 2012)

Any forest cover reduces wind speed

• open terrain (mean wind speed 10 m/s), dense (0.1 m/s), sparse (0.9 m/s)

Wind Index

• W1 = completely sheltered, dense forest
• W2 = sheltered, open forest

• forests create their own climate - reduce incoming shortwave and outgoing longwave radiation
• air, tree and snow surface temperature close to equilibrium - inhibit weak layer formation
• small temperature gradients - encourage rounding
• exceptions:
• SH in areas with good skyview and moisture supply, albeit spatially varied
• DH and FC from high temperature gradients in some areas

(Viglietti et al 2010)

(Schneebeli and Bebi 2004)

(Weir 2002)

(Schaerer 1977)

(Gubler and Rychetnik 1991)

Distribution and accumulation

Formation and evolution of weak layers

Highest interception rates:

• few cm already on branches (holds 40 - 50 cm)
• during light snowfall (70%)
• deciduous trees in light snowfall only and conifers in heavy and light snowfall

Highest dumping rates:

• increased wind gusts, incoming shortwave and temperature spikes
• dumping effect up to 1.5 x crown projection
• water equivalency increases at 2 - 5 x tree height

(Frehner 2007)

(Shea and Jamieson 2010)

(Imbeck and Ott 1987)

(Gubler and Rychetnik 1991)

(Teich et al. 2012)

D

(Schneebeli and Bebi 2004)

High textural and layer variability in freiland (open), kronenrand (band) and bestand (stand) treed zones. After Imbeck (1987).

The effect of trees on the forest snowpack. After Meyer-Grass (1987)

(Gubler and Rychetnik 1991)

Forests serve protective function:

The role of forest in slab avalanche formation

• moderate precipitation, wind, radiation
• act as anchors

Consequences

• stripped forest cover and soil
• destroyed infrastructure
• 10,000 clear cuts in BC
• 32% of fatalities in US from trauma since 2007

(McClung 2001)

(Atkins 2012)

Alexandra Sinickas

ENCI 753

Fall 2012

photo: thestoke.ca