令和6年度砂防学会研究発表会概要集

P-218

Assessing grain size distribution characteristics of debris flow torrents using UAV photogrammetry
Debris flow, Grain size, UAV photogrammetry, BASEGRAIN*Samikshya Dahal 1, Fumitoshi Imaizumi 2, Shoki Takayama 2
1. Graduate School of Integrated Science and Technology, Shizuoka University
2. Faculty of Agriculture, Shizuoka University

1. Introduction
Debris flow is significant geohazard in mountainous regions thatpose a serious threat to people’s life and property. Debris flow
ranges from mudflows and lahars with a high fraction of fineparticles such as clay, silt, and sand, to granular flows containing
significant coarse particles, some exceeding a meter in diameter(Phillips and Davies 1991). Investigating the spatio-temporalchanges in grain size within debris flow channels is necessary forunderstanding the dynamics of debris flows and their potential
downstream impacts. This study aims to measure distributions ofboulders in the debris flow torrent of the Ohya landslide in central
Japan using UAV-based Structure from Motion (UAV-SfM)
photogrammetry for capturing aerial images and granulometricanalysis using BASEGRAIN software.

Fig. 1 Map and photographs of Ichinosawa
catchment in Ohya landslide, central Japan

2. Methodology
The study was conducted in Ichinosawa catchment in the northern part of Ohya landslide, central Japan (Fig. 1). The
Ichinosawa catchment comprises upper Ichinosawa, debris-flow initiation zone, and lower Ichinosawa, transportation anddeposition zones (Imaizumi et al. 2005). UAV photographs captured before and after debris flow events on April 21, 2021,and August 24, 2021, and before and after sediment supply seasons on August 24, 2021, and April 22, 2022, were used toanalyze the effect of debris flow and sediment supply on the grain size of deposits. Photographs taken using DJI Phantom
4 RTK drone were processed using Agisoft Metashape Software to create orthomosaics and DEM. In upper Ichinosawa,orthomosaics were extracted at 10-m intervals along the channel based on slope gradient, while in lower Ichinosawa,
multiple 10x10 m areas were extracted in the channel based on DEM of difference (DoD). These clipped images were thenanalyzed using BASEGRAIN software to calculate the coverage ratio of boulders. The ratio of surface coverage (𝑅𝐵𝐺,𝑥 )for sediments larger than size threshold x mm in BASEGRAIN was calculated by dividing the total area of sediment greaterthan x mm ( 𝐴𝑥 ) by the total area of the analysis site (𝐴𝑡𝑜𝑡𝑎𝑙 ). Validation was conducted using grid sampling with 6
sections of 10 x 10 m (100 m2 in area). The coverage ratio was calculated by dividing the total number of sediments larger
than x ( 𝑁𝑥 ), by the total number of sediments sampled (𝑁𝑡𝑜𝑡𝑎𝑙 ). Impact of image resolution was also assessed usingphotographs captured from 80 m and 15 m altitude.
3. Result3.1. Detection of boulders using
BASEGRAIN
Surface coverage ratio of larger sedimentparticles obtained by BASEGRAIN analysis
was different between the high and low
altitude photographs (80 and 15 m above theground, respectively; Fig. 2). BASEGRAIN
shows better performance in detecting finersized sediments in photographs captured
from lower altitude (Fig. 2). However, thisadvantage is accompanied by a potential
drawback; the algorithm overlooks larger
sediments (Fig. 3). On the other hand, whenemploying photographs taken from thehigher altitude, BASEGRAIN tends to missFig. 2 Surface coverage of sediments
for different altitude greater than thethreshold size

- 771 -Fig. 3 Detection of sedimentfrom higher altitude (up) andlower altitude photographs(down).

finer sediments but exhibits greater accuracy in detecting coarser
sizes (>250 mm) (Fig. 2). When comparing the results for surface
coverage by different sediment size threshold, BASEGRAIN andgrid sampling data for sediments greater than 250 mm showed greatalignment (Fig. 4).
3.2. Spatio-temporal changes in the boulder size
The spatial distribution of boulders along the debris flow channel
section shows considerable variation across different analysis
periods (Figs. 5, 6). Boulder ratio is comparatively higher justbelow the waterfall zone at the centre of study site than downstream
area. This is because, below the waterfall, in lower Ichinosawa,channel gradient starts to decrease, slowing down the flow velocity.
Fig 4 Surface coverage ratio between gridThis likely causes selective sorting, with large boulders settlingsampling and BASEGRAIN (high altitude)
while finer particles are transported downstream withmuddy flows. During the debris flow season, there was anincrease in boulder ratio in upper Ichinosawa, while itdecreased in almost all parts of lower Ichinosawa,remaining relatively constant in the end zone of the debris
flow channel (Fig. 5). This is due to selective transport offine sediments from upper to lower region (Imaizumi et al.
2005). In the upper section of upper Ichinosawa, due tosignificant sediment erosion, boulder ratio increased, andchannel got narrowed due to deepening of channel (Fig.
5). The sediment supply season led to an overall increasein boulder coverage on the surface of the debris flow
channel with significant increase in middle section ofupper Ichinosawa due to active boulder supply from
Fig. 5 Changes in the boulder ratio in debris flow
surrounding active hillslopes.
season: a on April 21, 2021, prior to the debris flow
season. b on August 24, 2021, after the debris flow
4. Conclusion
season. c temporal changes in boulder ratios
Combined use of BASEGRAIN software and UAV
photogrammetry can be valuable solution for analyzing
boulder-sized sediments in physically inaccessible areas. In
Ohya Landslide, there is significant differences in boulder
distribution between the upper and lower catchments acrossdifferent seasons due to difference in channel morphology andheterogeneous sediment supplies from adjacent hillslopes.
The spatial distribution of boulders in debris flow channels
reveals insights into the behavior of past debris flows.
Therefore, investigating their spatial distribution and change
patterns is crucial for planning effective mitigation andprevention measures.
References
Imaizumi F, Tsuchiya S, Osaka O (2005) Behaviour of debris
flows located in a mountainous torrent on the Ohyalandslide, Japan. Canadian Geotechnical Journal 42:919–931.
Phillips CJ, Davies TRH (1991) Determining rheologicalparameters of debris flow material. Geomorphology 4:101–110.

- 772 -Fig. 6 Changes in the boulder ratio in sediment supply
season in upper Ichinosawa: a on August 24, 2021,
prior to the sediment supply season. b on April 22,2022, after the sediment supply season. c temporalchanges in boulder ratios