Deflecting dams and the formation of oblique shocks in snow avalanches at Flateyri, Iceland

Snow avalanches are a threat in many populated mountainous regions, and deflectingdams are often built to divert them away from people, and infrastructure, into lessharmful areas. When an avalanche is deflected by a dam or wedge, it often generates rapidchanges in the flow thickness and velocity, which can be modeled as an obliqueshock wave. This paper reviews classical oblique shock theory, which was originallydeveloped for shallow water flows, and uses it to make predictions of the maximum runupheight on a deflecting dam, the downstream flow velocity, and the width of thechannelized stream. The theory is used to investigate field observations of snowavalanches at Flateyri in Iceland, where a dam has deflected two avalanches away from thetown and produced a channelized stream that flowed parallel to the dam. The resultsindicate that there is no one single set of upstream flow conditions that parameterizes theflow behavior, but the solution evolves as the avalanche propagates along the dam inresponse to the deceleration imposed by the slope. Fully time-dependent shock capturingnumerical simulations of the Skollahvilft avalanche, which hit the dam on 21 February1999, are used to show how the channelized stream widens as the avalanche slows downand thickens toward the end of the runout zone. The oblique shock relations neverthelessprovide useful local order of magnitude estimates for the flow conditions immediatelyupstream of the shock