Slow drag in wet-snow avalanche flow B. SOVILLA, 1 M. KERN, 2 M. SCHAER 3 1 Avalanches, Debris Flows and Rockfall Research Unit, WSL Institute for Snowand Avalanche Research SLF, Flu ¨ elastrasse 11, CH-7260 Davos Dorf, Switzerland E-mail: sovilla@slf.ch 2 Department of Snow and Avalanches, BFW Institute for Natural Hazards and Alpine Timberline, Hofburg-Rennweg 1, A-6020 Innsbruck, Austria 3 Warning and Prevention Research Unit, WSL Institute for Snow and Avalanche Research SLF, Flu ¨elastrasse 11, CH-7260 Davos Dorf, Switzerland ABSTRACT. We report impact pressures exerted by three wet-snow avalanches on a pylon equipped with piezoelectric load cells. These pressures were considerably higher than those predicted by conventional avalanche engineering guidelines. The time-averaged pressure linearly increased with the immersion depth of the load cells and it was about eight times larger than the hydrostatic snow pressure. At the same immersion depth, the pressures were very similar for all three avalanches and no dependency between avalanche velocity and pressure was apparent. The pressure time series were characterized by large fluctuations. For all immersion depths and for all avalanches, the standard deviations of the fluctuations were, on average, about 20% of the mean value. We compare our observations with results of slow-drag granular experiments, where similar behavior has been explained by formation and destruction of chain structures due to jamming of granular material around the pylon, and we propose the same mechanism as a possible microscale interpretation of our observations. 1. INTRODUCTION In avalanche engineering, the pressure, p, exerted on an obstacle by an extreme avalanche is assumed to be p ¼ c f & U 2 , i.e. p is proportional to the square of the impact velocity, U, and to the avalanche bulk density, &. Shape and rheology effects are taken into account by a drag coefficient, c f . This formulation is also used by the Swiss Guidelines (Salm and others, 1990) and applies to extreme dry avalanches where impact forces are largely correlated with the avalanche kinetic energy (Sovilla and others, 2008a). In practice, as there is no established alternative, the same formula is also used by practitioners for slow, wet-snow avalanches. Recent measurements indicate that, for avalanches with low Froude numbers, this formula may heavily underesti- mate the impact pressures (Norem, 1991; Gauer and others, 2007; Baroudi and Thibert, 2009). However, no satisfactory explanation of these discrepancies in terms of possible underlying processes has been given so far. Sovilla and others (2008a) suggested that the impact pressure of avalanches in the subcritical flow regime may be governed by other processes than those involved in the supercritical flow regime. They reported a preliminary analysis of pressure and velocity data from the Valle ´e de la Sionne (canton Valais, Switzerland) test site, which shows that the measured impact pressures did not have a signifi- cant velocity dependency for Froude numbers less than 1, i.e. for subcritical flows. Furthermore, they observed that the total drag on the pylon strongly depends on the avalanche flow depth. Finally, Sovilla and others (2008a) observed that the amplitude of pressure fluctuations in wet–dense ava- lanches increases with flow depth, in contrast to pressure fluctuations in dry–dense avalanches, which are larger close to the avalanche surface and smaller lower down in the avalanche body. Our measurements of drag forces in slow wet avalanche flow may be consistent with theories for granular systems which predict that drag in slow flow varies linearly with flow depth and is independent of fluid velocity (Wieghardt, 1975; Albert and others, 1999, 2000; Chehata and others, 2003). Thus, the calculation of avalanche slow drag on structures may require a different analytical approach than the avalanche dynamics models currently used. In this paper, we present additional data of slow drag in avalanche flow and use new analysis methods to improve the preliminary analysis of Sovilla and others (2008a). A comparison of our results with slow granular flow experi- ments provides further evidence of the granular flow behavior of wet-snow avalanches. 2. INSTRUMENTATION AND METHODS We present data from the full-scale avalanche experimental test site Valle ´e de la Sionne, which is described in detail by Sovilla and others (2008a). Here we analyze large, wet- snow avalanches that started spontaneously in the release zone at 2700 m a.s.l. and followed a track of 1500 m length down to a zone with artificial obstacles at 1600 m a.s.l. (Fig. 1a). Several obstacles equipped with instruments were installed on a large open slope with an average inclination of about 218 (Sovilla and others, 2008b). At this location, large wet avalanches have a maximum flow depth of 3.5–5.5 m and flow velocities of 1–10 m s –1 (Table 1). As there is no lateral confinement, the avalanches may expand to a width of up to 100 m. They finally run out in the wide open slope below the obstacle zone, or reach the bottom of the valley and stop at the slope on the other side of the valley. One of the obstacles is an oval-shaped, 20m high pylon, equipped with pressure and velocity sensors (Fig. 1b). Piezoelectric load cells are mounted on the pylon at 0.5–5.5 m above the ground with 1m vertical spacing (Schaer and Issler, 2001). During winter 2003/04, sensor diameters were 0.1 and 0.25 m. Since winter 2004/05, only Journal of Glaciology, Vol. 56, No. 198, 2010 587