Journal of Natural Disaster Science

THE EFFECTS OF TOPOGRAPHIC CONVERGENCE AND LOCATION OF PAST LANDSLIDES ON SUBSURFACE WATER MOVEMENT ON GRANITIC HILLSLOPE

Yuichi ONDA
Laboratory of Soil and Water Conservation, Department of Forestry, Nagoya University
Ayako MORI
SUNCOH Consultants
and
Shizuo SHINDO
Department of Earth Science, Chiba University

(Received 16 June 1992 and in revised form 21 October 1992)

Abstract

Detailed soil moisture tension data of convergence of subsurface flow into a hollow were obtained in Obara Village, central Japan, where many landslides had occurred in 1972. For a heavy rainfall, the shape of the hillslope plan favours convergence of subsurface water flowing into the center of the hollow and especially small contour curvature radius is essential for the upslope area. The flow concentration in cross section was found: intense storm throughflow was detected just upslope of the past landslide scar. This was due to the combination of the trenching effect, due to the creation of a face caused by past landslides, and by the abrupt change in regolith properties, which induces saturated conditions. Therefore, such a concentration of subsurface water results not only from contour curvature of a hollow but also from the location of past landslides.

Key words

convergent flow, trenching effect, subsurface storm flow, hollow, landslide scar, landform evolution, granitic hillslope