10:45 〜 11:05
[MIS10-07] Comparison of Shallow Landslide Caused by Heavy Rainfall in 1967 and 2022 in Murakami City, Niigata Prefecture, Japan
★Invited Papers
キーワード：表層崩壊、崩壊の免疫性、豪雨
Heavy rainfall on August 3-4, 2022, caused a lot of shallow landslides and many houses were damaged by debris flows in Murakami City and Sekikawa Village, Niigata Prefecture, Japan. In the same region, heavy rainfall on August 28, 1967, also caused many shallow landslides and debris flows, resulting in extensive damage.
It has been pointed out that on the slope, where landslides have occurred due to heavy rainfall, landslides are less likely to occur even after rainfall of the same intensity that caused landslides. This phenomenon is called landslide “immunity” [Koide, 1973]. Yamagishi et al. (2008) reported that the landslides caused by heavy rainfall in 2004 were little overlapped with the landslides caused by past heavy rainfalls since 1961 in the Izumozaki area of Niigata Prefecture. This result might support the existence of landslide immunity. In contrast, Aniya (1968) reported in the Sumiyoshi Valley, Mts. Rokko, there is no immunity to landslide as the soil layers regenerate and vegetation recovers quickly after landslides, and slopes are steep. As mentioned above, there are conflicting opinions about the immunity of landslide. In this study, the spatial distribution of landslide caused by 2022 heavy rainfall is compared with the landslide caused by 1967 heavy rainfall in Koiwauchi Osawa River basin, Murakami City, Niigata Prefecture, and the landslide immunity is discussed.
The Koiwauchi Osa River basin is located on the right bank of the Arakawa River . Its area is 0.75 km2, the elevation ranges from 20 m to 370 m, the average slope is 27.8°, and the lithology is conglomerate and sandstone [Kurita and Ishikawa, 2010]. Both heavy rainfall in 2022 and 1967 caused a lot of landslides and debris flows that damaged houses located in the downstream area of the basin. Although the total rainfall amount is almost same(approximately 550mm/day), the rainfall in 2022 is more concentrated in a shorter period than in 1967, and the maximum hourly rainfall in 2022 is significantly larger, about 150 mm compared to about 80 mm in 1967.
Spatial distribution of landslides occurred in 1967 and 2022 heavy rainfall in Koiwauchi Osawa River basin were detected by arial photo interpretation. For the landslide detection in 2022, orthophotos generated from the aerial laser measurement by Aero Asahi Corporation on August 5, 2022, were used. For the landslide detection in 1967, the 1:20,000 scale monochrome aerial photos taken on August 4, 1971, by the Geospatial Information Authority of Japan (GSI), were used.
The number and area of landslide in 2022 were 119, 8.75 ha (landslide ratio is 11.7 %), in 1967 were 85, 4.59 ha (landslide area ratio is 6.1 %), respectively. The spatial association analysis revealed that 25 of the 119 landslides (20 %) in 2022, overlapped with landslides in 1967. These were classified into only three classes of overlapping landslides spatial association: “flank”, “upper” and “enlarging”. And there was no landslide that reoccurred at residual soil (inside or lower).
Field Observation revealed that the soil layer developed inside of the landslide occurred area in 1967 was thinner than the other area, about 10 cm thick. It is considered that thickness of the soil layer in landslide occurred area in 1967 was not enough thick to unstable the slope and occur landslide even if it is fully saturated by the heavy rainfall, and this is the reason why landslides in 2022 occurred at different slope from those in 1967.
These results, obtained in this study, support the existence of landslide "immunity", which means that once a landslide has occurred, it even for a certain period.
It has been pointed out that on the slope, where landslides have occurred due to heavy rainfall, landslides are less likely to occur even after rainfall of the same intensity that caused landslides. This phenomenon is called landslide “immunity” [Koide, 1973]. Yamagishi et al. (2008) reported that the landslides caused by heavy rainfall in 2004 were little overlapped with the landslides caused by past heavy rainfalls since 1961 in the Izumozaki area of Niigata Prefecture. This result might support the existence of landslide immunity. In contrast, Aniya (1968) reported in the Sumiyoshi Valley, Mts. Rokko, there is no immunity to landslide as the soil layers regenerate and vegetation recovers quickly after landslides, and slopes are steep. As mentioned above, there are conflicting opinions about the immunity of landslide. In this study, the spatial distribution of landslide caused by 2022 heavy rainfall is compared with the landslide caused by 1967 heavy rainfall in Koiwauchi Osawa River basin, Murakami City, Niigata Prefecture, and the landslide immunity is discussed.
The Koiwauchi Osa River basin is located on the right bank of the Arakawa River . Its area is 0.75 km2, the elevation ranges from 20 m to 370 m, the average slope is 27.8°, and the lithology is conglomerate and sandstone [Kurita and Ishikawa, 2010]. Both heavy rainfall in 2022 and 1967 caused a lot of landslides and debris flows that damaged houses located in the downstream area of the basin. Although the total rainfall amount is almost same(approximately 550mm/day), the rainfall in 2022 is more concentrated in a shorter period than in 1967, and the maximum hourly rainfall in 2022 is significantly larger, about 150 mm compared to about 80 mm in 1967.
Spatial distribution of landslides occurred in 1967 and 2022 heavy rainfall in Koiwauchi Osawa River basin were detected by arial photo interpretation. For the landslide detection in 2022, orthophotos generated from the aerial laser measurement by Aero Asahi Corporation on August 5, 2022, were used. For the landslide detection in 1967, the 1:20,000 scale monochrome aerial photos taken on August 4, 1971, by the Geospatial Information Authority of Japan (GSI), were used.
The number and area of landslide in 2022 were 119, 8.75 ha (landslide ratio is 11.7 %), in 1967 were 85, 4.59 ha (landslide area ratio is 6.1 %), respectively. The spatial association analysis revealed that 25 of the 119 landslides (20 %) in 2022, overlapped with landslides in 1967. These were classified into only three classes of overlapping landslides spatial association: “flank”, “upper” and “enlarging”. And there was no landslide that reoccurred at residual soil (inside or lower).
Field Observation revealed that the soil layer developed inside of the landslide occurred area in 1967 was thinner than the other area, about 10 cm thick. It is considered that thickness of the soil layer in landslide occurred area in 1967 was not enough thick to unstable the slope and occur landslide even if it is fully saturated by the heavy rainfall, and this is the reason why landslides in 2022 occurred at different slope from those in 1967.
These results, obtained in this study, support the existence of landslide "immunity", which means that once a landslide has occurred, it even for a certain period.