How can needle ice transport large stones? Twenty‐one years of field observations

Abstract

AbstractDownslope movements of 12 stones (10–27 cm in diameter) were determined from time‐series images for 21 years on a Japanese alpine debris slope (inclination 12°). The process of stone movements was analysed on the basis of frost heave and soil temperature records, with particular attention to the stone size transported by needle ice and the effect of climate change on stone movements. Soil heaving mainly due to needle‐ice growth occurred 24–85 times yr−1 with an annual maximum and cumulative amounts of 1.8–5.5 cm and 17–58 cm yr−1, respectively. Stones moved downslope at rates of 5–20 cm yr−1 (mean 11.7 cm), the velocity correlating with the stone size, although small stones with a height of less than about 3 cm do not reduce needle‐ice activity very much. On an assumption of a linear relationship between the two variables, needle ice can transport stones with a diameter of as large as 30 cm. Both annual mean air/soil temperatures and needle‐ice activity slightly increased through the monitoring period. Climatic warming may have raised the frequency of needle ice by shortening the snow‐covered period.