5:15 PM - 7:15 PM
[HDS09-P10] Application to topographic analysis to identify collapse points to prevent landslide disasters
1. Introduction
Recently, the intensification of disasters due to increased rainfall has led to large-scale damage beyond people's expectations. In the face of such disasters, underestimation due to confirmation bias among affected residents can delay evacuation and exacerbate damages. This trend is also observed in Nagano Prefecture, a region with low rainfall. In 2021, a debris flow of approximately 20,000 m³ occurred in the Gebazawa River basin in Takabe, Chino City, Nagano Prefecture. In this case, early evacuation was achieved before evacuation instructions were issued by the municipality, thanks to land traditions, resulting in no human casualties. If early evacuation can be consistently achieved, the loss of life due to disasters can be minimized. Therefore, this study focuses on this debris flow disaster. It investigates land traditions, archaeological sites, shrines, and monuments and identifies terrains prone to collapse. Furthermore, by comparing these terrains with surrounding landscapes, the study attempts to share historical disaster information with the community.
2. Method
The study focuses on the Takabe area, where a landslide occurred in 2021, and the Ankokuji area, which suffered major debris flow disasters in 1983. Investigations included interviews and surveys of archaeological sites, shrines, and monuments. Using the 0.5m-DEM owned by the Nagano Prefecture Forestry Department, a terrain analysis was conducted for both areas. This study focused on slope angle, extracting areas with a slope angle exceeding 30 degrees, which correspond to steep slopes in debris flow warning zones. Additionally, aerial photos from the Geospatial Information Authority of Japan were displayed chronologically to observe regional development.
3. Results
Among the analysis results, Figure 1 shows the slope gradient map for the Takabe area. Areas with a slope less than 30 degrees (steep slope threshold) are shown in white, while areas of increasing steepness are shown progressively darker in blue. By overlaying this with the distribution of archaeological sites, shrines, monuments, and the 2021 disaster damage zone, the elements and terrain characteristics are visually comprehensible. Figure 2 shows a slope gradient map of the Ankokuji area using a similar display method, revealing differences in the shape of the sediment transfer zones. Particularly, in Takabe, the river is narrow, and the transition from steep slopes to the alluvial fan is close to the main river (Miyagawa). The debris flow is concentrated on the right bank of the Gebazawa River. In contrast, in the Ankokuji area, the deposition area spreads from the midstream, and river width increases. Furthermore, the uneven distribution of archaeological sites, shrines, and monuments indicates slight damage on the left bank of Gebazawa River in Takabe. This suggests a relationship between debris flow disasters and the distribution of archaeological sites, shrines, and monuments. Given that the distribution of sites is closely related to regional culture, these features could serve as accessible indicators for residents.
4. Future Prospects
In terms of terrain analysis, incorporating indicators such as the Terrain Wetness Index (TWI) to analyze surface water flow can contribute to creating hazard maps. These maps can be communicated to communities, supporting disaster prevention and mitigation efforts.
Recently, the intensification of disasters due to increased rainfall has led to large-scale damage beyond people's expectations. In the face of such disasters, underestimation due to confirmation bias among affected residents can delay evacuation and exacerbate damages. This trend is also observed in Nagano Prefecture, a region with low rainfall. In 2021, a debris flow of approximately 20,000 m³ occurred in the Gebazawa River basin in Takabe, Chino City, Nagano Prefecture. In this case, early evacuation was achieved before evacuation instructions were issued by the municipality, thanks to land traditions, resulting in no human casualties. If early evacuation can be consistently achieved, the loss of life due to disasters can be minimized. Therefore, this study focuses on this debris flow disaster. It investigates land traditions, archaeological sites, shrines, and monuments and identifies terrains prone to collapse. Furthermore, by comparing these terrains with surrounding landscapes, the study attempts to share historical disaster information with the community.
2. Method
The study focuses on the Takabe area, where a landslide occurred in 2021, and the Ankokuji area, which suffered major debris flow disasters in 1983. Investigations included interviews and surveys of archaeological sites, shrines, and monuments. Using the 0.5m-DEM owned by the Nagano Prefecture Forestry Department, a terrain analysis was conducted for both areas. This study focused on slope angle, extracting areas with a slope angle exceeding 30 degrees, which correspond to steep slopes in debris flow warning zones. Additionally, aerial photos from the Geospatial Information Authority of Japan were displayed chronologically to observe regional development.
3. Results
Among the analysis results, Figure 1 shows the slope gradient map for the Takabe area. Areas with a slope less than 30 degrees (steep slope threshold) are shown in white, while areas of increasing steepness are shown progressively darker in blue. By overlaying this with the distribution of archaeological sites, shrines, monuments, and the 2021 disaster damage zone, the elements and terrain characteristics are visually comprehensible. Figure 2 shows a slope gradient map of the Ankokuji area using a similar display method, revealing differences in the shape of the sediment transfer zones. Particularly, in Takabe, the river is narrow, and the transition from steep slopes to the alluvial fan is close to the main river (Miyagawa). The debris flow is concentrated on the right bank of the Gebazawa River. In contrast, in the Ankokuji area, the deposition area spreads from the midstream, and river width increases. Furthermore, the uneven distribution of archaeological sites, shrines, and monuments indicates slight damage on the left bank of Gebazawa River in Takabe. This suggests a relationship between debris flow disasters and the distribution of archaeological sites, shrines, and monuments. Given that the distribution of sites is closely related to regional culture, these features could serve as accessible indicators for residents.
4. Future Prospects
In terms of terrain analysis, incorporating indicators such as the Terrain Wetness Index (TWI) to analyze surface water flow can contribute to creating hazard maps. These maps can be communicated to communities, supporting disaster prevention and mitigation efforts.