Powder Snow Avalanches Research Articles

A Model Study of Powder-Snow Avalanches

AbstractA powder-snow avalanche is a particular type of turbulent gravity current which it is proposed to treat using methods developed in contexts where the Boussinesq approximation is valid. Hence, it is assumed that the air entrainment rate E primarily depends on the internal Froude number (Tochon-Danguy and Hopfinger, [1975]) and that the large density variation in an avalanche reduces only the drag resulting from the air entrainment. In order for this drag to dominate over the wall stress it is necessary that the slope angle be greater than a certain minimum value which depends on the density ratio of the flow. For a low-density flow, for example, a minimum slope angle of about 5° is necessary. The more unusual aspect is that an avalanche may also entrain snow along its path which increases its mass and can produce in certain cases an accelerating flow. This aspect is included in the theoretical treatment. Experiments have been carried out in an inclined water channel using brine as dense fluid. Visual observations as well as measurements of the velocity and density profiles have been made. Various states of flow are proposed depending on whether the avalanche is “long" (having the structure of a usual gravity current) or “short" (having the structure of a thermal) or whether snow is entrained. The practical consequences which can be drawn from these results are discussed, in particular in the context of the design of defence structures, and flow visualizations of a gravity current impinging on an obstacle give an idea of the protected region.

A Model Study of Powder-Snow Avalanches

Abstract A powder-snow avalanche is a particular type of turbulent gravity current which it is proposed to treat using methods developed in contexts where the Boussinesq approximation is valid. Hence, it is assumed that the air entrainment rate E primarily depends on the internal Froude number (Tochon-Danguy and Hopfinger, [1975]) and that the large density variation in an avalanche reduces only the drag resulting from the air entrainment. In order for this drag to dominate over the wall stress it is necessary that the slope angle be greater than a certain minimum value which depends on the density ratio of the flow. For a low-density flow, for example, a minimum slope angle of about 5° is necessary. The more unusual aspect is that an avalanche may also entrain snow along its path which increases its mass and can produce in certain cases an accelerating flow. This aspect is included in the theoretical treatment. Experiments have been carried out in an inclined water channel using brine as dense fluid. Visual observations as well as measurements of the velocity and density profiles have been made. Various states of flow are proposed depending on whether the avalanche is “long" (having the structure of a usual gravity current) or “short" (having the structure of a thermal) or whether snow is entrained. The practical consequences which can be drawn from these results are discussed, in particular in the context of the design of defence structures, and flow visualizations of a gravity current impinging on an obstacle give an idea of the protected region.