10:45 〜 11:00
[AHW19-06] Postfire heavy rain in Japan as precondition type of compound disaster
★Invited Papers
キーワード：林野火災、複合災害、Hagibis、UAV
Wildfire enhances the damage caused by heavy rain due to ground surface changes. For example, runoff rates change after a wildfire since the surface becomes water-repellent (DeBano 2000). The resulting changes in hydrological processes change the risks of floods downstream and landslides, known as postfire heavy rains (Cannon et al., 2008; Cui et al., 2019). In Japan in 2019, heavy rain accompanied by Typhoon Hagibis caused severe damage at the site of the Ozaki Peninsula wildfire that occurred in Kamaishi City in 2017. The postfire heavy rain is a compound disaster and can be classified as a precondition type, in which the previous disaster strengthens the damage of the next one. There is concern about the impact of changes in the intensity and frequency of heavy rain and dryness hazards.
Therefore, in this study, we examine the evaluation of postfire heavy rain as a precondition type of compound disaster, targeting the severe postfire heavy rain that occurred in the largest wildfire in Japan. Specifically, (i) the ground surface before and after heavy rain was recorded in detail by multitemporal Unmanned Aerial Vehicle (UAV) observation, and (ii) slope stability was calculated by numerical simulation (Tozato et al., 2022) and further required evaluation was considered.
The target area is the burned area of the Ozaki Peninsula wildfire in Kamaishi City, Iwate Prefecture in May 2017. The burned area of 413 ha is the largest in Japan since 1995. In addition, heavy rain by Typhoon Hagibis in October 2019 was 317.5 mm for two days, causing many landslides in the area.
In the survey, multitemporal UAV observations were performed in October 2018 (after the wildfire and before heavy rain) and in March 2020 (after heavy rain). High-resolution DEMs were made before and after heavy rain using Structure from Motion (SfM) processing. Figure 1 shows changes in DEM before and after heavy rain. The significant changes in the ground surface were stream shifting and landslides. This observation data is beneficial as a record of the damage caused by a postfire heavy rain in Japan and as a high-resolution DEM before the disaster, which can be utilized as an input for further numerical simulation (Touge et al., 2023).
Tozato et al. (2022) performed a reproduction analysis of slope stability using the pre-disaster high-resolution DEM as input. Slope stability was calculated at 1 m resolution in the entire catchment area, and verification was performed based on changes in the ground surface shown in Figure 1. As a result, it is reproduced that the slope stability became low at the points where the landslides occurred. Since this slope stability calculation uses a physical model with rainfall as an input, it can be utilized for various scenarios under different compound conditions. For example, the runoff rate increases due to the water repellency of the ground surface after a wildfire, but the effect recovers with time. Therefore, the damage of postfire heavy rain is determined by the intensity of disasters and the time difference between wildfire and heavy rain.
Reference:
Cannon, S.H., Gartner, J.E., Wilson, R.C., Bowers, J.C., Laber, J.L., 2008. Storm rainfall conditions for floods and debris flows from recently burned areas in southwestern Colorado and southern California. Geomorphology 96, 250–269.
Cui, Y., Cheng, D., Chan, D., 2019. Investigation of post-fire debris flows in Montecito. ISPRS International Journal of Geo-Information 8.
DeBano, L.F., 2000. The role of fire and soil heating on water repellency in wildland environments: a review. Journal of Hydrology, 231–232, 195–206.
Tozato K., Dolojan N.L.J., Touge Y., Kure S., Moriguchi S., Kawagoe S., Kazama S., Terada K., 2022. Limit equilibrium method-based 3D slope stability analysis for wide area considering influence of rainfall. Engineering Geology, 308, 106808.
Touge Y., Hasegawa M., Minegishi M., Kawagoe S., Kazama S., 2023. Multitemporal UAV surveys of geomorphological changes caused by postfire heavy rain in Kamaishi city, northeast Japan. Catena, 220, 106702.
Therefore, in this study, we examine the evaluation of postfire heavy rain as a precondition type of compound disaster, targeting the severe postfire heavy rain that occurred in the largest wildfire in Japan. Specifically, (i) the ground surface before and after heavy rain was recorded in detail by multitemporal Unmanned Aerial Vehicle (UAV) observation, and (ii) slope stability was calculated by numerical simulation (Tozato et al., 2022) and further required evaluation was considered.
The target area is the burned area of the Ozaki Peninsula wildfire in Kamaishi City, Iwate Prefecture in May 2017. The burned area of 413 ha is the largest in Japan since 1995. In addition, heavy rain by Typhoon Hagibis in October 2019 was 317.5 mm for two days, causing many landslides in the area.
In the survey, multitemporal UAV observations were performed in October 2018 (after the wildfire and before heavy rain) and in March 2020 (after heavy rain). High-resolution DEMs were made before and after heavy rain using Structure from Motion (SfM) processing. Figure 1 shows changes in DEM before and after heavy rain. The significant changes in the ground surface were stream shifting and landslides. This observation data is beneficial as a record of the damage caused by a postfire heavy rain in Japan and as a high-resolution DEM before the disaster, which can be utilized as an input for further numerical simulation (Touge et al., 2023).
Tozato et al. (2022) performed a reproduction analysis of slope stability using the pre-disaster high-resolution DEM as input. Slope stability was calculated at 1 m resolution in the entire catchment area, and verification was performed based on changes in the ground surface shown in Figure 1. As a result, it is reproduced that the slope stability became low at the points where the landslides occurred. Since this slope stability calculation uses a physical model with rainfall as an input, it can be utilized for various scenarios under different compound conditions. For example, the runoff rate increases due to the water repellency of the ground surface after a wildfire, but the effect recovers with time. Therefore, the damage of postfire heavy rain is determined by the intensity of disasters and the time difference between wildfire and heavy rain.
Reference:
Cannon, S.H., Gartner, J.E., Wilson, R.C., Bowers, J.C., Laber, J.L., 2008. Storm rainfall conditions for floods and debris flows from recently burned areas in southwestern Colorado and southern California. Geomorphology 96, 250–269.
Cui, Y., Cheng, D., Chan, D., 2019. Investigation of post-fire debris flows in Montecito. ISPRS International Journal of Geo-Information 8.
DeBano, L.F., 2000. The role of fire and soil heating on water repellency in wildland environments: a review. Journal of Hydrology, 231–232, 195–206.
Tozato K., Dolojan N.L.J., Touge Y., Kure S., Moriguchi S., Kawagoe S., Kazama S., Terada K., 2022. Limit equilibrium method-based 3D slope stability analysis for wide area considering influence of rainfall. Engineering Geology, 308, 106808.
Touge Y., Hasegawa M., Minegishi M., Kawagoe S., Kazama S., 2023. Multitemporal UAV surveys of geomorphological changes caused by postfire heavy rain in Kamaishi city, northeast Japan. Catena, 220, 106702.