Tsukuba, Japan—Debris supplied by rockfall and related slope processes is a key factor controlling the frequency and magnitude of debris flows. However, estimating the amount of debris supplied over several decades has been technically challenging. In this study, the research team focused on abandoned mountain roads and successfully estimated decadal-scale debris supply by measuring deposits accumulated on these road surfaces using UAV-LiDAR technology.
The team conducted a high-resolution topographic survey along a closed section of Shizuoka Prefectural Road 288 near the Shizuoka-Nagano prefectural border. The road became impassable after a disaster in 1991, and rockfall-derived debris has accumulated steadily since then. By dividing the surveyed road into segments, the researchers analyzed the relationship between slope topography and debris supply. The results showed that debris supply increases with steeper mean slope angles and larger contributing areas. Based on these analyses, headwater slopes in the region were estimated to supply approximately 70-93 m3 of debris per year, indicating that a sufficient volume of material to trigger a debris flow can accumulate within several decades.
Across Japan's mountainous regions, the number of abandoned roads is increasing due to route realignment. These unused roads have rarely been utilized for scientific observation. This study demonstrates that UAV-LiDAR surveys of abandoned roads can provide valuable baseline data to support forecasting and risk assessment of debris-flow and landslide hazards.
