Comparison of avalanche front velocity measurements and implications for avalanche models

- A series of front velocity measurements of snow avalanches are presented.
- The presented data can provide a guidance for model validation.
- Based on the measurements constraints for the parameters of common avalanche models are derived.
- These constraints imply that the effective velocity dependency of those models is overstated.

Besides the runout distance of an avalanche information on avalanche intensity along the path is often required for hazard zoning or planning of mitigation measures. The avalanche (front) velocity is a common intensity measure as it can be linked to expected impact pressures. Furthermore, the velocity of an avalanche determines if the avalanche stays in its usual track or if the avalanche unexpectedly deviates and thus endangers areas believed to be safe. Therefore, a reliable prediction of the expected velocities is most important. However, many of the prevailing avalanche models either tend to underestimate velocities or they overestimate the runout distances.In this paper, several avalanche front velocity measurements from the Ryggfonn test site are presented. The measurements are derived from photo and/or video analyses of mainly dry-mixed avalanches. Some of these observations are partly supplemented by Doppler radar measurements.Additionally, the measurements from the Ryggfonn test site are compared with published velocity measurements from other locations to obtain a wider spectrum of avalanche conditions. By analyzing these velocities, constraints for possible rheological models of avalanche flows are obtained.The measurements of the presented avalanches, of which the most were relatively large to their path and can be classified as dry-mixed avalanches (i.e. they were partially fluidized and accompanied by a powder cloud), suggest a lower (apparent) velocity dependency of the bottom friction term than generally believed. The measurements show that the maximum front velocities of these avalanches scale with the square root of the total fall height of the avalanche front, that is, the maximum velocity Um∝Hsc.