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[HDS09-04] Analysis of landform development, natural hazards, and disaster situations using high-density aerial laser point clouds and a proposal for the use of Relative Elevation Model
Keywords:GIS, high-density aerial laser point clouds, REM (Relative Elevation Model), landform development and hazard, river, sediment, and volcanic disasters, VIRTUAL SHIZUOKA
Shizuoka Prefecture is the first prefecture in Japan to acquire and publish a high-density 3D point cloud (VIRTUAL SHIZUOKA) based on aerial laser measurements of almost the entire prefectural area in 2019-2021 (https://virtualshizuokaproject.my.canva.site). Using this data, the author has been conducting research in various fields of topography, geology, and disaster mitigation, and is discovering new findings and developing methods for analysis and public awareness.
1. Analysis of the internal structure and formation process of volcanic edifices
The author and co-researchers analyzed the detailed topography and geology of Omuroyama Scoria Cone, which erupted 4,000 years ago, and used muography together to clarify the internal structure and developmental processes of the volcano (Nagahara et al., 2022, Bulletin of Volcanology) (Fig.A). The author also analyzed the detailed topography and geology of the proximal deposits of the 1707 Hoei eruption of Fuji Volcano and revealed a new stratigraphy and eruptive history that differs from the conventional view (Koyama, 2023, Fujiology, No. 3) (Fig.B).
2. Understanding the river flooding and sediment disasters
Based on the field survey on the flood of Tomoe River, Shizuoka City, caused by Typhoon No. 15 in 2022, the author constructed a detailed inundation depth map, and clarified the relationship with microtopography and artificial obstacles (Koyama, 2023, Geoscience Reports. Shizuoka Univ.) (Fig.C). The point cloud has also been of great help in analyzing the collapse and sediment transport of the Atami debris flow in 2021 (Suzuki et al., 2021, Annual Journal of Urban Disaster Reduction Research).
3. Hazard Analysis of River Flooding Using Relative Elevation Models
Current flood hazard maps are drawn by repeating numerical simulations of inundation flows at each breach point and superimposing the results and thus is time-consuming and expensive. This situation resulted in the delay of preparation of hazard maps especially for small and medium-sized rivers. In contrast, a simplified hazard map that shows the areal risk distribution of flooding in the form of an elevation distribution from the nearest river floor can be created in a short time and at low cost by using a relative elevation model (REM) based on the river floor. Moreover, by expressing the map including buildings data, the inundation risk for each building can be expressed (Fig.D).
4. Clarification of landform development history using REM
REM is an effective tool not only for hazard analysis and expression, but also for clarifying the developmental history of river terraces and fans. Longitudinal profiles of landforms along river channels have often been used to correlate river terraces and analyze their developmental history. By using REM based on the river floors, it is easier to make areal correlation and tracing of fluvial terraces. In addition, by drawing REM based on the river floors surrounding a fan, it is also useful for estimating the development process of the fan.
Fig. A: Precise topography and density structure of the Omuroyama Scoria Cone of the Izu Tobu Volcano Group (after Nagahara et al., 2022). The high-density part (dikes) that branched off from the vent just below the summit crater contributed to the formation of Lava flow IV and the South Crater.
Fig. B: Precise slope distribution on the southeastern flank of Fuji Volcano. The stripes around Hoeizan indicate depositional structures of tephra deposited by the 1707 Hoei eruption.
Fig. C: Part of the inundation depth map of the Tomoegawa River flood caused by Typhoon No. 15 in 2022 (Koyama, 2023).
Fig. D: An example of 3D relative elevation maps based on the floor of the Kanogawa River (Koyama, unpublished data).
(Both maps were drawn using Shizuoka Prefecture point cloud data; Geographical Survey Institute maps were used as background for Figs. C and D)
1. Analysis of the internal structure and formation process of volcanic edifices
The author and co-researchers analyzed the detailed topography and geology of Omuroyama Scoria Cone, which erupted 4,000 years ago, and used muography together to clarify the internal structure and developmental processes of the volcano (Nagahara et al., 2022, Bulletin of Volcanology) (Fig.A). The author also analyzed the detailed topography and geology of the proximal deposits of the 1707 Hoei eruption of Fuji Volcano and revealed a new stratigraphy and eruptive history that differs from the conventional view (Koyama, 2023, Fujiology, No. 3) (Fig.B).
2. Understanding the river flooding and sediment disasters
Based on the field survey on the flood of Tomoe River, Shizuoka City, caused by Typhoon No. 15 in 2022, the author constructed a detailed inundation depth map, and clarified the relationship with microtopography and artificial obstacles (Koyama, 2023, Geoscience Reports. Shizuoka Univ.) (Fig.C). The point cloud has also been of great help in analyzing the collapse and sediment transport of the Atami debris flow in 2021 (Suzuki et al., 2021, Annual Journal of Urban Disaster Reduction Research).
3. Hazard Analysis of River Flooding Using Relative Elevation Models
Current flood hazard maps are drawn by repeating numerical simulations of inundation flows at each breach point and superimposing the results and thus is time-consuming and expensive. This situation resulted in the delay of preparation of hazard maps especially for small and medium-sized rivers. In contrast, a simplified hazard map that shows the areal risk distribution of flooding in the form of an elevation distribution from the nearest river floor can be created in a short time and at low cost by using a relative elevation model (REM) based on the river floor. Moreover, by expressing the map including buildings data, the inundation risk for each building can be expressed (Fig.D).
4. Clarification of landform development history using REM
REM is an effective tool not only for hazard analysis and expression, but also for clarifying the developmental history of river terraces and fans. Longitudinal profiles of landforms along river channels have often been used to correlate river terraces and analyze their developmental history. By using REM based on the river floors, it is easier to make areal correlation and tracing of fluvial terraces. In addition, by drawing REM based on the river floors surrounding a fan, it is also useful for estimating the development process of the fan.
Fig. A: Precise topography and density structure of the Omuroyama Scoria Cone of the Izu Tobu Volcano Group (after Nagahara et al., 2022). The high-density part (dikes) that branched off from the vent just below the summit crater contributed to the formation of Lava flow IV and the South Crater.
Fig. B: Precise slope distribution on the southeastern flank of Fuji Volcano. The stripes around Hoeizan indicate depositional structures of tephra deposited by the 1707 Hoei eruption.
Fig. C: Part of the inundation depth map of the Tomoegawa River flood caused by Typhoon No. 15 in 2022 (Koyama, 2023).
Fig. D: An example of 3D relative elevation maps based on the floor of the Kanogawa River (Koyama, unpublished data).
(Both maps were drawn using Shizuoka Prefecture point cloud data; Geographical Survey Institute maps were used as background for Figs. C and D)