Abstract
Owing to severe rainfall on 11 and 12 July, 1995, a debris flow occurred and flowed down on the Shirouma-daisekkei snow patch, the Northern Japanese Alps. As a result, the snow patch was widely covered with debris flow deposits, the debris flow incised the snow patch, and a large-scale trench (1.3 km in length, 6-8 m in width, 10-20 m in depth) was formed. The trench was formed at the center of debris covered area, from the uppermost part to near the terminus of the snow patch. The width of the trench became more than 15 m on 18 October as a result of continuous snowmelt in summer and the area of the trench reached 20% of the entire snow patch area. Internal structures of the snow patch, such as firn aquifer and dirt layers which were composed of sand and gravel, were recognized on the side walls of this trench. However, no ice body was found on the side walls.
Formation processes of this trench are discussed on the bases of distribution and shape of the trench and debris covered area which were obtained from aerial photographs and field observations. It is concluded that the trench was formed since the debris flow eroded firn layers. On this snow patch, the similar kind trench was also formed in 1952. In order to understand formation and maintenance processes of perennial snow patches, it must be important to evaluate such influences of debris flow on snow patches.
