The Relationship Between Underground Erosion and Landslide Movement in a Crystalline Schist Landslide, Zentoku, Tokushima, Japan

Abstract

The authors have thought that one of the most effective factors on the movement of a large crystalline schist landslide “Zentoku” in TokushimaPrefecture, Japan is the underground erosion, and have monitored the sediment discharge due to the erosion and subsequent transportation by the ground water at spring point in there. This paper aims to reveal the relationship between underground erosion and landslide movement on the basis of the monitoring results by the apparatus to catch the sediment, the extensometers and the rain gauge.Due to the results of the preceding monitorings, the sediment discharge is roughly proportional to the precipitation. However, sediment data scatter, which seems due to the factor other than the precipitation. In order to examine the condition of the sediment yield, the authors have induced two parameters, one is the displacement ratio (D/DAVn) to evaluate the degree of landslide movement and the other is the sediment discharge ratio (W/WP) to evaluate the degree of the sediment discharge affected by the factor other than the precipitation. The relationship between the discharge sediment and the landslide movement, which based on the result of the time dependent analysis of these two parameters, leads to the following conclusions;(1) Undoubtedly the precipitation is the most effective factor on the sediment discharge. The landslide movement affects the sediment discharge as the subsequent factor, which causes the scattering of data on the sediment discharge versus precipitation diagram.(2) According to the relationship between the landslide displacement under active period (D/DAVn>2) and the sediment discharge ratio (W/WP), the sediment discharge at the spring point has two factor groups, first group is under the effect of precipitation and the second is affected by the precipitation and the landslide movement itself. The sediment discharge, which produced one month after the active period of the slide, shows good proportionality to the landslide displacement.