Gravitational wet avalanche pressure on pylon-like structures

Low-speed wet avalanches exert hydrostatic forces on structures that are flow-depth dependent. However, the pressure amplification experienced by smaller structures has not been quantified previously. In particular, recent wet avalanche pressure measurements, performed with small cells at the "Vallée de la Sionne" test site, indicate significantly higher pressures than those considered by engineering guidelines and common practice rules based only on the contribution of inertial forces. In order to gain a deeper understanding and investigate the relevance of these measurements for structural design, we analyzed data measured on obstacles of different shapes and dimensions. The pressure measured on a 1 m² pressure plate was, on average, 1.8 times smaller than the pressure measured on a 0.008 m² piezoelectric cell installed on a 0.60 m wide pylon and 2.9 times smaller than the pressure measured on a 0.0125 m² cantilever sensor extending freely into the avalanche flow. Further, avalanches characterized by a gravitational flow regime exerted pressures that increased linearly with avalanche depth. For Froude numbers larger than 1, an additional square-velocity dependent contribution could not be neglected. The pressure variations encountered by the different obstacles could be explained quantitatively with a granular force model, that assumes the formation of a mobilized volume of snow granules extending from the obstacle upstream whose dimensions depend on the incoming flow depth and the obstacle width. This mobilized volume is associated with the formation of a network of gravity-loaded grain-grain contacts, also called granular force chains, which densifies in front of the obstacle, producing force amplification. Our results underscore the fundamental influence of the dimensions of both the sensor and the obstacle on pressures in the gravitational flow regime and may help to improve rules for structural design.