| The ratio of the shear strength to the shear stress of snow accumulated on a slope (known as the stability index) is often used to determine avalanche risk. While the shear stress can be found from the snow load on an interested snow layer、 the shear strength varies by snow grain form. For example、 snow with a low level of shear strength (such as depth hoar) is known as a weak layer. The shear strength of snow begins to increase with the passage of time immediately after it falls、 and snow on slopes gradually stabilizes. However、 the quantitative relationship of temporal changes in the shear strength of snow with snow grain form、 temperature and other factors has not yet been clarified. In this study、 therefore、 measurements were conducted to clarify temporal changes in shear strength and the stability index of snow containing graupel ? a type of weak layer. [*] Measurements were taken in the Sapporo area of Hokkaido in Japan. The shear strength、 density、 upper snow load and temperature of snow containing graupel and that without it were measured over a period of two weeks. The results revealed that、 for several days after snowfall、 the rate of increase in the shear strength of snow containing graupel was about half that of snow without it. As the grain shape of graupel became vague thereafter、 the shear strength of snow containing graupel then increased at a rate similar to that of snow without it.[*] Next、 a theoretical study was conducted to examine the influence of the difference in shear strength for these two types of snow on the stability index. The study used the viscous compression theory、 which is a simple calculation method that takes snow densification into account. The compressive viscosity of these snow layers、 found from temporal changes in the upper snow load and density、 were used to calculate temporal changes in the stability index when there was a certain amount of snowfall. The calculation results indicated that the stability index of snow containing graupel was smaller than that of snow without it during snowfall. It was also found that、 after snowfall stopped、 the stability index of snow containing graupel increased more slowly than that of snow without it.[*] The above results indicate that it is necessary to pay attention to the stabilization of snow accumulating on slopes over a longer period of time in the case of snowfall containing graupel than in the case of that without it. |
