東ネパールヒマラヤにおける落石に関する研究

カンチェンジュンガ・ヒマールの事例

抄録

IntroductionRockfall is one of the major geomorphic processes acting on steep mountain slopes. The role of rockfall in the total denudation of mountain slopes is significant in cold areas where vegetation is sparse and frost action is active. Several weathering processes may contribute to rockfall generation in cold mountains. Many studies suggest that rockfall originates from diurnal and seasonal freezing and thawing; some explain deglaciation as a major cause, while some highlights on the thermal stress. Despite such numerous studies, studies focusing on the seasonal difference of rockfall in different slope aspects and altitudes are sparse. This study aims at: describing the rockfall activity in alpine cliff in different slope aspects in relation to the rock temperature and frost wedging in the upper Kanchenjunga valley, northeast corner of Nepal.Research MethodologyRockfalls were observed directly in the field in the east-, west-, north- and south-facing slopes; the exposed bedrock area was calculated from the photographs taken at different seasons. Rock temperatures at 2 cm depth, on the free face at different aspects and altitudes (ranging from 4947 m to 6005 m), were monitored using automatic data loggers. The air temperatures were measured at the altitudes of 4755 m and 6012 m, and precipitation were measured at the altitudes of 4648 m and 5235 m. Freeze-thaw cycles at different altitudes were computed to evaluate the effect of freeze-thaw in rockfall activity. Effect of frost wedging was derived using crack extensometer. Rock samplings were made at the base of each rockfall-counting sites to evaluate the affected geology and, at last, a rockwall retreat rate was calculated using the rockfall number and the average volume of the rocks sampled below the rockwalls at four aspects.Results Results based on rockfall/km2 show distinct difference in rockfall amount in terms of- season, aspect and the time of the day. In the presence of moisture in rock, which varies with season, the magnitude of rockfall activity significantly varies with the change in the ratio of snow-free area to snow-covered area. Different slope aspects show the seasonal difference in temperature and freeze-thaw activities, which are supposed to be the major factors controlling rockfall activity. Monitoring of frost wedging shows the seasonal expansion and contraction of the rocks, i.e., expansion in winter and contraction in dry and rainy seasons. Rockwall retreat rates show significant variations in the different aspects. ConclusionsThe rockfall activity in the Kanchenjunga valley started from the middle of March and reaches maximum during rainy season. This seasonal change in the amount indicates that the activity depends mainly on the seasonal difference of thermal and hydrological regime in the bedrock. Maximum rockfall occurs in the east-facing slope and then in south-, west- and north-facing slopes, respectively. From March to May the maximum rockwall retreat rate was found in the east-facing slope followed by north-, west-, and south-facing slope.