Valley Density as a Geomorphic Control on Rainfall-Induced Shallow Landslides in Japan

Abstract

Shallow landslides triggered by intense rainfall are a common geomorphic phenomenon, and their occurrence is largely controlled by local topography including valley incision. We refine a plan curvature-based method to extract valleys from 10–20 m digital elevation models (DEMs) and evaluate the relationship between valley density and landslide source areas associated with recent heavy rainfall events. Valleys were extracted by a contour crenulation method with thresholds corresponding to contour inflection angles of <90°, and drainage lines were generated using the D8 flow direction algorithm. Seven rainfall-induced landslide events across Japan were investigated, with landslide scars delineated using post-disaster aerial photographs. Grid-based analyses were conducted at multiple resolutions for regional-scale evaluation; we then focused on 1 km grids. The results show a positive correlation between valley density and landslide area at a grid scale of 1 km, with R² values of 0.53–0.90 across different regions. In this study,　hilly terrain with valley densities of ~4 km/km² is expected to produce 1000–2000 m² of shallow landslide area per km² during a single disaster event. These results highlight valley density as a robust indicator for assessing rainfall-induced landslide susceptibility.