Snow survey by aerial photographs has become common from the time of heavy snowfall in January 1963. Photographs on a scale of 1 : 20, 000 were used for measuring snow depth and for interpreting snow avalanche, cornice and so on.
This paper deals with how to expect the degree of danger for snow avalanche on each slope and how to classify the mountain slope by the degree of danger and intends to make a standard of mountain slope classification from the point of the degree of danger. Degree of danger for snow avalanche is divided into that of occurrence ratio and that of impulsive force.
To calculate the occurrence ratio of snow avalanche, mountain slopes are divided into minimum units by the type of slope, abrupt change of slope angle and difference of vegetation.
Picking up model areas including about 3, 000 units of slope, the occurrence ratio of snow avalanche is calculated by the degree of slope, slope type, vegetation and snow depth (Table 2-6), and also by the combination of elements mentioned above.
The standard for mountain slope classification is considered and proposed as shown table 8 considering table 2-7. It is considered that the slope which has the angle between 40°and 60°, which is valley type and is bare land or grass, shrub or sparse forest, and is covered with snow of over 2m is the most dangerous slope, which will be called as common avalanche slope from the point of occurrence ratio. The slope, under 25°or over 60°, artificial dense forest, or under 1m snow depth is not a dangerous slope looking from occurrence ratio of snow avalanche.
Figure 5 is the example of mountain slope classification using the standard and figure 4 shows the existing snow avalanche at the time of aerial photographing.
