PROPERTIES OF WEAK LAYERS COMPOSED OF PRECIPITATION PARTICLES IN AND NEAR AVALANCHE STARTING ZONES IN JAPAN

Shinji Ikeda and Tomoyuki Noro

Public Works Research Institute, Snow Avalanche and Landslide Research Center

ABSTRACT: To gain further insight into the properties of weak layers composed of precipitation particles(PP weak layer), we analyzed 18 measurement datasets. All analyzed data were collected from avalanche fracture lines or flat sites near avalanche starting zones; these data correspond to notable PP weak layer-related avalanche activities observed in Japan. PP weak layers were found at depths of 6–117 cm, and these layers were 2–22-cm thick. At all study sites, the PP weak layers were composed of large (diameter: 1.0–2.5 mm) rime-less stellar or dendrite planar crystals. The density of the observed layers ranged between 63 and 190 kgm-3. The shear frame index–density relationship of the studied layers resembled that observed by Jamieson and Johnston (2001) rather than observed by Yamanoi and Endo (2002) and Perla et al. (1982). Further analysis of larger datasets obtained from various regions with snow climate is necessary of the properties of weak layers. However, our results suggest that the identification of the types of precipitation particles present in the snowpack under consideration is important for snow stability evaluation and avalanche forecast when weak layers composed of precipitation particles are involved. 1. INTRODUCTION The properties of weak layer are an important factor for stability analysis of snowpack and avalanche prediction. However, most of existing studies about property of weak layers focused on persistent weak layers (Jamieson,1995) composed of facetted crystals, surface hoar and depth hoar, which maintain weakness for a long period (ex. Akitaya, 1974; Fukuzawa and Akitaya, 1993; Birkeland, 1998; Hachikubo and Akitaya, 1998). It is known that relatively large and little rimed precipitation particles form typical non-persistent weak layers. For such weak layers composed of precipitation particles (hereafter: PP weak layer), an approach to estimate their stabilities using a physically based model (with empirical parameterizations) is developed (Endo, 1991, Conway and Wilbour, 1999) as a support tool for avalanche forecasting (hereafter: SNOSS = SNOw Slope Stability model). SNOSS estimate a stability index which is a ratio of the shear strength of weak layers and the shear stress produced by overburden of snow on the weak layers for given depth and time as an output. The shear strength of *Corresponding and presenting author: Ikeda H. 2-6-8, Nishiki-cho, Myoko-City, Niigata-Pref, 944-0051, Japan Phone: +81-255-72-4131 Fax: +81-255-72-9629 e-mail: s-ikeda55@pwri.go.jp

weak layers is estimated by using empirical equation of the relation of snow density and shear strength, and snow density is estimated from initial density, overburden stress and temperature and compactive viscosity. For SNOSS, even the same situation may result in very different outputs of stability when using different empirical equation of the relation of snow density and shear strength and compactive viscosity. It have been pointed out that the necessity of examination of applicability of the properties of PP weak layer in conjunction with the climatic conditions by some studies (ex. Broun, et al, 2008, Hirashima, et al., 2008, Marshall et al., 2008). Hirashima, et al. (2008) has shown that the empirical equations of relationship between snow density and shear strength by Jamieson and Johnston (2001), which were developed using the observation data in Canada, tend to over-estimate the instability compared to the empirical equations by Yamanoi and Endo (2002), which were developed using the observation data in Japan, in their study on the snowpack stability analysis of Japanese snow using the SNOWPACK model. The differences in crystal types are considered to give a major influence on the properties of weak layers, such as the relationships between density and shear strength and compressive viscosity. Casson et al. (2008) attempt to be clear the properties for each type of precipitation particles by field measurement and pointed out importance of identification of each crystal types of fallen snow for SNOSS. Because the crystal type of precipitation particles depend not on the climate but on the weather

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conditions when it fall, and same types of snow crystals may fall in regions whose climate differ largely each other, even though the frequencies are different. Therefore, if we only factor the climate for the properties of PP weak layers, we may miss-estimate the instability of snowpack. On the other hand, most of existing studies on PP weak layers depend on the data observed at the study plots which are located on lowland and are at a distance from the avalanche start zone and those data are collected regardless of avalanche occurrence (Perla et al., 1982; Yamanoi and Endo, 2002; Brown and Jamieson, 2008; and Casson et al., 2008). Therefore, it is hard to say these studies had grasped the properties of PP weak layers that actually causing the avalanche. This study aims to clarify the properties of PP weak layers that actually caused avalanches in Japan where has quite different climatic condition from North America and Europe. To this end, data was collected and analyzed on the weak layers of precipitation particles, which were observed on or near the starting zones of avalanches under the obvious avalanche cycles rerated with PP weak layers in Japan. 2. METHODS We collected 18 cases of observations of PP weak layers that actually caused avalanches in Japan. 12 cases of observations were performed by author. One case of them was observed at fracture line of avalanche and 11 cases were observed near the avalanche starting zone when obvious avalanche cycles rerated with PP weak layer had been reported on the Snow bulletin board (Degawa et al., 2008). The Snow bulletin board is a web-based information exchange system operated by Japan Avalanche Network (JAN) to contribute to back country avalanche safety. The registered members who provide data are mountain guides, patrols and experienced recreational back country users and they have received training for observing and reporting data on avalanche activities, snowpack conditions and weather conditions in conformity with the Observation Guidelines and Recording Standards for Weather, Snowpack and Avalanches (JAN, 2009) which is drawn up based on Observation Guidelines and Recording Standards for Weather, Snowpack and Avalanches drawn up by Canadian Avalanche Association: OGRS (CAA, 2007). Snow pit observations were performed in accordance with both of OGRS (JAN, 2009) and OGRS (CAA, 2007). The observation items are as follow.

Layer structure: visually observed Type of snow: visually observed Particle diameter: measured with a grading

gauge with 1, 2 and 3 mm grids Hardness: determined by the hand test that

classifies hardness into four classifications of F, 4f, 1f, P, K and I

Density: average of three samplings collected by a cubic density sampler (height: 3 cm and volume:100 cm3) When thickness of the target layer is less than 3 cm, the snow of the upper layer is sampled together. Then, the snow of upper layer is sampled exclusively. The density of the target layer is calculated according to the following equation. ρw={3ρwu - (3-tw) ρu}/tw (1) ρw: density of weak snow layer ρwu: density of weak snow layer ρu: density of upper layer tw: thickness of weak snow layer ρu: density of upper layer

Shear strength of the weak layer: shear strength index is measured using a shear frame of 250 cm2; More than 10 measurements were taken for one sample. The average of measurements were used.

The weak layer was identified by visual observation of the boundary between remaining slab and sliding surface, when observation is performed at fracture line. In the other observations, the layers that may correspond to the weak layers that were reported on the Snow bulletin board were identified by confirming to the layer structures (the weak layer locations were specified relative to the index layers, such as crust layers). Father, One case and five cases were obtained from existing research reports and from the Snow Profile Information Network (SPIN) which is a web-based snow profile data base operated by JAN (Degawa et al., 2008), respectively. Some of the observation items may lack on those observations stated above. 3. RESULTS Table 1 shows observation results of 18 cases of PP weak layers that actually rerated avalanche activities in Japan and fig. 1 shows picture of examples of observed crystals. 7 cases are observed at avalanche fracture line and 10 cases are observed near avalanche starting zone under the obvious avalanche cycles rerated with PP weak layer.

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