A research group led by Associate Professor Tsuyoshi Hattanji from the Institute of Life and Environmental Sciences at the University of Tsukuba, together with researchers from Hyogo University of Teacher Education and Hokkaido University, announced that they have successfully measured the volume of debris deposited on an abandoned road using a drone equipped with LiDAR (laser survey) capability, and estimated the rate of debris supply from slopes. The technology is expected to contribute to predicting debris flow disasters. The findings were published in the journal Geomorphology on February 4.
The rate of debris supply from slopes due to rockfalls is one of the key factors controlling the frequency and scale of debris flows. However, direct measurement is difficult, and available means for measurement have been limited to the use of past records and periodic monitoring using rockfall traps.
The research group conducted topographic surveys using a UAV LiDAR (drone equipped with laser survey capability) along a roughly 4.75 km section of a closed road (Shizuoka Prefectural Road No. 288) near the border of Shizuoka and Nagano Prefectures. The target road section was divided into 96 segments, and the group examined the relationship between topographic conditions on the slope behind each segment and the amount of debris supplied.
The road is located along the Tenryu River in the southern part of the Akaishi Mountains (also known as the Southern Alps), which has one of the highest uplift rates in the Japanese archipelago. Steep, rugged slopes extend behind the road. The road became impassable due to a disaster in 1991, and since then, rockfall supplied from the slopes has continued to accumulate on the road surface.
The survey found that debris supply increases both as the average slope gradient increases and as the contributing (catchment) area grows larger.
In segments with a contributing area of 10,000 m2 or more, sediment transported by water flow also began to accumulate. All segments with a contributing area of 26,000 m2 or more showed accumulation of debris transported by debris flows or water flow.
Analysis of the debris supply data estimated that in topographic settings with a contributing area of 10,000 to 20,000 m2—corresponding to the upper valley areas prone to becoming debris flow source zones—approximately 70 to 90 m3 of debris is supplied per year. It was found that in the valleys of this region, enough debris to trigger a single debris flow event can accumulate over a period of several decades.
The method is said to be applicable to abandoned roads and disused railway tracks in other regions as well.
Hattanji commented: "We used drone-mounted LiDAR surveys for this research, but the underlying approach can also be applied to detailed topographic data with a resolution of around 0.5 m. In recent years, detailed topographic data has been made publicly available by the Geospatial Information Authority of Japan and by prefectural governments across the country. Japan also has a large number of abandoned roads and disused railway tracks throughout its mountainous regions. We therefore plan to apply the same method to estimate debris supply rates on slopes across Japan."
Journal Information
Publication: Geomorphology
Title: An abandoned road as a debris trap: Estimating debris-supply rate from steep slopes based on UAV-LiDAR DEMs
DOI: 10.1016/j.geomorph.2026.110193
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.