HR: 0830h
AN: S51A-03    [Abstracts]
TI: Earthquake Induced Landslides In The Watershed Of The Imo River
AU: * Watanabe, N
AF: Research Institute for Hazards in Snowy Areas, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AU: Marui, H
AF: Research Institute for Hazards in Snowy Areas, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AU: Yoshimatsu, H
AF: Sabo Technical Center, 4-8-21 Kudan-Minami, Chiyoda-ku, Tokyo, 102-0074 Japan
AB: The 2004 Niigata Chuetsu earthquake induced >1600 landslides in various types and dimensions. It has been general understanding until now that because of strong ground shaking during earthquakes a lot of slope failures occur on the steep slopes but only few reactivated landslides occur on gentle slopes. However, also many reactivated landslides occurred in the neighborhood of the epicenter area especially in the Yamakoshi Village. Landslides induced by the earthquake can be classified manly into the following categories: (1) Shallow slope failures on steep slopes near ridges, (2) Shallow slope failures on steep slopes along river channels, (3) Reactivated landslides on gentle hillslopes, (4) Landslide dams were formed by the displaced soils mass by the previous three categories. For the mitigation of subsequent disaster, it was a matter of great urgency to implement emergency countermeasures against overtopping and failure of the landslide dams. The Yamakoshi Village locates on the Higashiyama Hills trending a NNE-SSW direction with the altitude between 400m and 700m. The Imo River is flowing through eastern part of the village. The geology around the village belongs to Pliocene and Pleistocene formations that consist mainly of mudstone, sandstone and their alternation. The geological structure is controlled by the anticlines and synclines almost parallel to the direction of the hills. Landslides occurred especially densely in the watershed of the Imo River along the Kajigane Syncline and slipped along dip direction of geologic formations. Originally, heavily landslide prone areas are widely distributed in the Tertiary mudstone areas in Niigata Prefecture. Reactivated landslides occur frequently in the western part of the village where the mudstone is distributed. This time most of landslides occurred in the western part of the village where the sandstone is distributed. More than 30 landslide dams were formed along the main channel of the river and its tributaries. There are two critical dams because of their dimensions among them, namely Higashitakezawa landslide dam and Terano landslide dam. Both of them have a length of about 350m and a volume of >10⁶ m³. The length of the buried river channel is about ten times of the maximum water depth of the reservoir in both cases. Thus the possibility of the destructive collapse of the dams by water pressure and piping were estimated to be low. However, there remained apparent danger of overtopping and successive collapse of the dams. Therefore, the inhabitants of the downstream area had to evacuate. It was urgently necessary to lower the water table of the reservoir. The reservoir at Higashitakezawa located most downstream has a critical significance. In order to mitigate the danger of overtopping, the following emergency maneuvers were arranged. At first, the water table was tried to lower by pumps and siphons. At the beginning 6 pumps and after then additional 6 pumps were installed. Because of maintenance problems of the pumps, later an alternative diversion pipelines were installed as reserve measure for unexpected alarm cases. Finally, an open channel with a sufficient cross section area also for water discharge including snow melting during early spring was constructed. It was absolutely necessary to keep the stability of the displaced soil mass against the secondary landslide during construction works of the channel. Therefore, cutting operation of the upper part of the soil mass was immediately carried out as an appropriate emergency countermeasure.
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2005 Joint Assembly