Altitudinal Changes in Slope Angle and Profile Curvature in the Japan Alps: A Hypothesis Regarding a Characteristic Slope Angle

Abstract

Previous research indicated that the average slope angle in Japanese mountains tends to increase with increasing altitude. This paper examines the details of the change for the three ranges of the Japan Alps using 2.25''×1.5'' (ca. 50×50m) DEMs provided by the Geographical Survey Institute of Japan. Altitudinal changes in profile curvature were also analyzed. The results indicate that the ranges can be divided into three altitude zones. Zone 1 (< ca. 1, 000m) is characterized by an increase in mean and modal slope angle with altitude as well as concave slope profiles. Zone 2 (ca. 1, 000 to 2, 800m), which occupies the broadest area of the ranges, is characterized by increases in the mean slope angle with increasing altitude, but the modal slope angle is around 35 degrees regardless of altitude. The modal profile curvature is also stable around null throughout the zone, reflecting straight slope profiles. The ratio of hillslopes with an angle of ca. 35 degrees to all hillslopes increases with increasing altitude, suggesting that hillslopes converge into a characteristic form after long-term erosion. Zone 3 occurs in narrow zones near mountain summits (> ca. 2, 800m) where hillslopes are subjected to periglacial actions, wind erosion and Pleistocene glacial erosion. This zone is characterized by the decreasing slope inclination with altitude and convex slope profiles.