HR: 0800h
AN: H41B-0300    [Abstracts]
TI: The Effect of Deep Ground Water on the Initiation of Kumanodaira Landslide in Central Japan
AU: * Tanaka, T
EM: t\_tanaka@ies.life.tsukuba.ac.jp
AF: Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8572 Japan
AU: Onda, Y
EM: onda@atm.geo.tsukuba.ac.jp
AF: Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8572 Japan
AU: Shimamura, M
EM: m-shimamura@jreast.co.jp
AF: Safety Research Laboratory, Research and Development Center of JR East Group EAST JAPAN RAILWAY COMPANY, 2-0 Nisshin-cho, Kita-ku, Saitama, 331-8513 Japan
AU: Togari-Ohta, A
EM: togari@jreast.co.jp
AF: Safety Research Laboratory, Research and Development Center of JR East Group EAST JAPAN RAILWAY COMPANY, 2-0 Nisshin-cho, Kita-ku, Saitama, 331-8513 Japan
AU: Tsujimura, M
EM: mktsuji@atm.geo.tsukuba.ac.jp
AF: Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8572 Japan
AU: Hattori, S
EM: blue20mosc@hotmail.com
AF: College of Natural Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8572 Japan
AU: Uchida, T
EM: uchida-t92rv@nilim.go.jp
AF: Erosion and Sediment Control division, Research Center for Disaster Risk Management, National Institute for Land and Infrastructure Management, 1 Asahi, Tsukuba, 305-0804 Japan
AB: The hydrometric and isotopic analyses were carried out to study the groundwater flow processes at the Kumanodaira landslide scars in Gunma Prefecture, central Japan. Kumanodaira landslide, which occurred on 8th and 9th in June 1950, consists of two slope failure events and killed 50 people; total 2 mm of rainfall was monitored for two days before the disaster, however, 320 mm/day of rainfall had been monitored nine months before. Precipitation and stream flow have been monitored in four small catchments (K1, K2, K3, and K4: Kumanodaira landslide occurred in K4) in Kumanodaira area to elucidate the mechanism of Kumanodaira landslide. To realize the data use of antecedent precipitation, effective rain was defined as R$_{w}$=$\Sigma$a$_{1i}$R$_{1i}$; in which R$_{w}$ is the effective rain, a$_{1i}$ is the reduction coefficient of i hour before (a$_{1i}$=0.5$^{i/T}$: T is a half value period), and R$_{1i}$ is the one hour precipitation of i hour before. Two wells were also installed in K4; the Well A located relatively downslope of the catchment reaches the soil/bedrock interface, and Well B located upslope penetrates under the soil/bedrock interface. These data show that the rainfall-runoff response in K4 is much slower than that in the other catchments. The groundwater in shallow soil (Well A) was found to be corresponded to the effective rain with half value period of 2400 hours, whereas the groundwater in the bedrock (Well B) never corresponds to the effective rain with any half value periods. The stable isotopic ratio ($\delta^{18}$O) of rainfall, surface water, and groundwater samples from each catchment were analyzed. This analyses show that only $\delta^{18}$O of the Well B differs considerably from that of the others, implying that the water by heavy rainfalls during the summer season would supply to the Well B. Therefore these results suggest that groundwater flows as "piston flow" in the bedrock, responding several months after the rainfall. This caused significant difference in residence time from soil water, and outflow of the bedrock groundwater should triggered Kumanodaira landslide.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting