Effects of flow regime and sensor geometry on snow avalanche impact-pressure measurements D. BAROUDI, 1,2 B. SOVILLA, 3 E. THIBERT 1 1 Cemagref, UR ETGR, 2 rue de la Papeterie, BP 76, 38402 Saint-Martin-d’He `res Cedex, France E-mail: emmanuel.thibert@cemagref.fr 2 School of Science and Technology, Aalto University, Department of Civil and Structural Enginering, PO Box 12100, FIN-00076 Aalto, Finland 3 WSL Institute for Snow and Avalanche Research SLF, Flu ¨elastrasse 11, CH-7260 Davos-Dorf, Switzerland ABSTRACT. Impact pressures of snowavalanches have been measured at the Swiss Valle ´e de la Sionne experimental test site using two kinds of sensor placed at different locations in the avalanche flow. Pressures measured in a fast dry-snow avalanche and a slow wet-snow avalanche are compared and discussed. The pressures recorded using the two types of sensor in the dense flow of a dry-snow avalanche agree well, showing negligible dependence on the measurement device. On the other hand, significantly different pressures are measured in the slow dense flow of a wet-snow avalanche. This is attributed to the slow drag and bulk flow of this type of avalanche, leading to the formation and collapse of force-chain structures against the different surfaces of the sensors. At a macroscopic scale, limit state analysis can be used to explain such a mechanism by a shear failure occurring between freely flowing snow and a confined snow volume against the sensor, according to a Mohr– Coulomb failure criterion. The proposed model explains (1) how impact pressure can be up to eight times higher than hydrostatic snow pressure in wet cohesive slow avalanches and (2) its dependence on sensor geometry. 1. INTRODUCTION Knowledge of how the impact pressure that avalanches exert on obstacles varies with time and location is of fundamental importance in the design of defense structures. As no well- established equation is available to describe avalanche impact pressure on structures of different dimensions and shapes, experimental investigations still represent an import- ant approach in avalanche science. Experiments can be performed in the laboratory using granular materials (Wieghardt, 1975; Albert and others, 1999; Chehata and others, 2003; Faug and others, 2009, 2010) that simulate the granular flow of dry snow relatively well (Naaim and others, 2003; Rognon and others, 2008). However, to validate the results of these small-scale avalanche experiments and extrapolate them to both full- scale avalanche scenarios and wet and viscous flow requires full-scale experiments (Gauer and others, 2007; Sovilla and others, 2008b; Thibert and others, 2008). Most experimental studies dedicated to snow avalanche impact-pressure measurement have used small load cells to obtain information on the structure of the avalanche and the pressure distribution over its depth (Lang and Brown, 1980; Schaerer and Salway, 1980; McClung and Schaerer, 1985; Norem and others, 1985; Kawada and others, 1989; Nishimura and others, 1989; Abe and others, 1992; Schaer and Issler, 2001; Sovilla and others, 2008a). Some full-scale investigations have used large obstacles providing spatially integrated impact pressure over scales of the same order of magnitude as the avalanche depth. In the late 1970s, Kotlyakov and others (1977) provided some initial measurements. Since then, impact-pressure measure- ments have been made on macroscopic structures such as towers (Norem, 1991) and large plates (Gauer and others, 2007; Sovilla and others, 2008b). Recently, inverse analysis has been used to reconstruct impact pressure on a 1 m 2 plate sensor using deformation signals (Thibert and others, 2008; Baroudi and Thibert, 2009). In spite of numerous experimental investigations, measur- ing avalanche impact pressure remains a difficult task because loading is the result of complex interactions between the avalanche and the measurement device (sensor and supporting structure). Furthermore, force measurement is intrusive and therefore potentially dependent on the adopted method. In addition, avalanche impact pressure is also influenced by the flow regime. Pressures from a dry dense or dilute avalanche generally display high temporal variations (Sovilla and others, 2008b) with high peak values (McClung and Schaerer, 1985). These fluctuations have been interpreted mostly as impulses from particles or snow blocks on the small surfaces of the sensors. On the other hand, the impact pressure from a wet dense slow avalanche is characterized by fluctuations that seem to be generated by the formation and destruction of chains of stress around the infrastructure (Sovilla and others, 2008b, 2010). In this paper, we investigate how both sensor technology and flow regime may influence impact-pressure measure- ments. For this, we compare impact-pressure data from classical piezoelectric load cells (Schaer and Issler, 2001; Sovilla and others, 2008a,b) and small steel cantilevers being developed and tested at the Lautaret full-scale avalanche test site in France (Berthet-Rambaud and others, 2008; Thibert and Baroudi, 2010). We analyze both sensor responses for a dry dense and a wet dense avalanche triggered at the Valle ´e de la Sionne test site in Switzerland. 2. STUDY SITE AND EXPERIMENTAL SET-UP The experiments were carried out at the real-scale avalanche test site of Valle ´e de la Sionne in the Swiss Alps, where natural and artificially released snow avalanches are studied Journal of Glaciology, Vol. 57, No. 202, 2011 277