Soil creep process and its role in debris slide generation — field measurements on the north side of Tsukuba Mountain in Japan

We measured the ground inclination movement, moisture content, and amount of rainfall on a valley-head slope and a side slope along the valley on the northern side of Tsukuba Mountain, and elucidated the process and role of soil creep in debris slide generation. Soil creep usually occurs during and after rain. The amount of creep caused by one rainfall has a positive correlation with the amount of rain and increase in soil moisture. For the valley-head slope, soil creeps down and accumulates at the bottom bottleneck part of the head hollow, in which soil is compressed and forms a wavy landform. The lower end of the head hollow gradually becomes steeper and unstable. Side slopes along a valley-head slope show differences in topsoil thickness and creep speed between sections near and below a knick line. The amount of soil creep per year is larger in a section near a knick point than that below it. Soil gradually concentrates directly below the knick point, and the slope becomes steeper and unstable. The total amount of soil movement in 1 year was several millimeters for both the valley-head and side slopes. This means several liters/year of soil squeeze out for every slope section of 1 m in width. This value is similar to the general speed of erosion caused by debris slide (mainly topsoil failure, or shallow landslide) in Japan. Furthermore, neither surface erosion nor internal erosion has been observed in this site. Soil creep seems to carry most of the debris to places where debris slides will occur in this site. Soil creep prepares future debris slides and dominates the cycle of debris slide as well.