5 Report on the Landslide Disaster in the Upper Part of the Abukuma River, Fukushima Prefecture, Due to Heavy Rainfall in August 1998 By Takashi INOKUCHI National Research Institute for Earth Science and Disaster Prevention Abstract A large number of landslides occurred in Nishigo and Taishin villages in Fukushima Prefecture on August 27, 1998 as a result of heavy rainfall. Nine persons were killed by the debris from the landslides, and many houses were destroyed. This report describes landslide disasters and the distribution of landslides based on field surveys and aerial photograph interpretation. The geology of this area mainly consists of Shirakawa welded tuff and volcanic sediments. More than one thousand landslides have occurred in this region. These landslides have been classified into 4 types; landslides in the volcanic ash layer on gentle slopes, surface landslides on steep hillside slopes, collapse in weathered welded tuff, and collapse of artificial slopes. Each type of landslide has been described, and the occurrence conditions examined. The problems connected with landslide disaster prevention are also discussed. Key Words : Rainfall disaster, Surfacial landslide, Pyroclastic-flow deposit, Fukushima Prefecture 107
1. 1998 27 31 2. 1998 8 26 27 90 mm 24 600 mm 29 31 11 80 3,753 11 27 1 30 10 5 2 3,000 Fig. 1 Locality map of the investigation area. 3. 3.1 NE SW Fig. 2 Topographical relief map of the investigation area. 108 109
Fig.3 Cross section of the investigation area 3.3 1998 800 mm 26 27 29 30 27 27 27 27 3.2 26 90 mm 27 40 mm/h 27 500 mm 1,200 mm 500 mm 4. 100 300 m 4.1 1999 10 Fig. 4 1959 Schematic geological column. 150 m K 10 1992 1998 100 m 110 111
Fig. 5 Precipitation at Mabune and Kumado from August 26th to 31st. m Fig. 7 Landslide distribution around Taiyo-no-Kuni. Fig. 6 Total Precipitation in Fukushima Prefecture from August 26th to September 1st. 10 1998100 m 15 25 3 112 113
Photo 1 () An overview of the Karamatsu-sou landslide. Photo 4 Damaged section of Karamatsu-sou. Photo 2 Karamatsu-sou directly hit by landslide debris. Photo 5 An overview of the Keyaki-sou landslide. Photo 3 Destroyed ceiling in Room 5 of Karamatsu-sou. 4.2 27 20 m 114 115
Photo 6 Main source of the Karamatsu-sou landslide. 10 15 m 20 25 m 1975 11 Fig. 8 Plan view of landslides at Karamatsu-sou. 11 A B 11 D 1975 1975 4.3 27 20 15 m 25 m m 600 m 3 12 Photo 7 Right side source of the Karamatsu-sou landslide. Fig. 9 Outline sketch of Yoshinome. 116 117
10 Photo 10 Source of the Yoshinome landslide. Photo 8 Flow route of the Karamatsu-sou landslide. 11 Photo 11 Aerial photos of Yoshinome before the disaster. Photo 9 Damaged area of the Yoshinome landslide. 12 Photo 12 Stereo pair photos on landslide at Akanita. 4.4 28 13 m 10 118 119
10 Fig.10 External view of the landslide at Taishin Junior high school. 1 1998 14 13 Photo 13 Damaged building of Taishin junior high school. 17 1975 700 1000 11 12 13 11 17.5 km 2 300 14 Photo 14 Damaged houses at Kashiwano. 30 12 100 13 100 120 121
11 Fig.11 - ( 1/25,000 ) K Y Landslide distribution in the Odakura-Mabune district. 122 123
12 13 1/25,000 Fig.12 Landslide distribution in the Habuto district. Fig.13 Landslide distribution in the Kumado district. 124 125
6. 50 1,000 6.1 11 15 6.2 40 11 10 40 20 30 30 m 6.3 6.4 13 10 30 m 15 40 m 50 m 7. 7.1 14 15 30 m 10 16 10 cm m m 1.5 m 30 35 45 1955 126 127
Table 1 Classification of the landslides and their features. 15 Photo 15 Flow type landslide. 14 Fig.14 Schematic sections of the landslide. 1999 20 25 17 18 19 7.2 1971 1953 35 40 1983 35 401984 1995 1993 8 45 50 15 25 30 15 128 129
17 Photo 17 Surface failure in weathered tuff. 16 Photo 16 Surface landslide on Shirakawa welded tuff. 20 25 18 Photo 18 Weathered Shira-kawa welded tuff. 8. 8.1 19 Photo 19 Landslide on artificial slope of roadside. 130 131
8.3 30 4. 2 15 Fig.15 Distribution of gradients on landslide slopes. 16 Fig.16 1978 Extent of damage to human life due to landslide disasters (Revision to Mizutani,1978). 10 13 9,000 10 27 28 8.2 1978 16 10 9. 1998 1,000 50 132 133
1 1999 1998 11 61 64 2 19991998 40 35 42 3 1999 1998 148 162 4 19951993 5 1998 1998 No. 91 98 6 195568 102 7 1978 No. 29 8 1984 58 114 9 1959 5 1 82p 10 1992 37 251 263 11 1971 85p 12) 19991998 182 196 1 98 10 26 139,000 9,000 8,000 77 38 71 98 10 WEB 134 135
2 10 10 11 300 600 98 11 11 WEB 136