Weather and snowpack conditions essential to slushflow release and downslope propagation

Abstract

Rapid mass movement of water-saturated snow, usually known as slushflow or slush avalanche, is a major natural hazard in Norway. Slushflows occur in winter during heavy rainfall, as well as due to intense thaw in spring. The basic interrelations between ground conditions, snowpack properties and water supply, critical to slushflow release and downslope propagation, has been studied. The analytical approach to these problems and the major conclusions are summarized. Altogether 31 slushflow periods and more than 80 slushflow sites, critical to human activity and located close to meteorological stations, were examined. The geomorphic and vegetational site characteristics, snowpack development and flow path morphology were established by field work as well as interpretation of maps and meteorological records. Estimation of precipitation and meltwater were based on meteorological data transformed to the starting zones. Drainage channels and water-saturated snowfields are the typical starting zones of slushflows. Slushflows due to cyclonic activity are normally released within 24 hours of rain and snowmelt. Five main combinations of winter snowpack and current weather were identified to characterize the slushflow periods. Cohesionless new snow and coarse grained snow are most liable to start flowing, and tend to spread out downslope. The most favourable conditions for large slushflows are coarse grained snowpacks with depth hoar at the base, and when water is in abundance during spring break-up. Acute hazard may be predicted based on meteorological records and weather prognosis.