Katsumi OHNO

Study on Slope Failures of 7.13 Niigata Downpour Disaster in 2004

Satoru Ohtsuka

Niigata prefecture downpour disaster occurred on July 13, 2004 caused a serious damage by the flood of the Igarasi and the Kariyata. Moreover, it caused two persons dead, one person injured and much damage on houses and traffic lifelines by slope failures.
Recently downpour disasters have caused much damage every year. Slope disaster prevention law was therefore newly established and software countermeasure has been applied to prevent the people from disaster since the occurrence of Hiroshima downpour disaster in 1999. However, it is important to investigate the characteristics of slope failures since the disaster is greatly affected by not only rainfall, but also geographical and geological features. Warning and refugee system has been expected to reduce the disaster, but the background data has been a problem in reliability.
This study recorded the damages in 7.13 Niigata downpour disaster and analyzed the features of disaster. It investigated 2.5km mesh rainfall data of the Meteorological Agency and the collapse times of slopes to establish a warning and refugee system. The followings were obtained in this study.
(1)There was observed a strong relationship between the slope failure and the amount of total rainfall.
(2)There were about 10% cases of total slope failures caused in the area outside the slope failure warning area prescribed by the slope failure prevention law. It was found the house was unsafe when the slope height was greater than the distance between the house and the slope.
(3)Failure times of 59 slopes were obtained by field investigation. The way to evaluate a rainfall index for slope failure prediction was discussed and made clear. Necessary conditions for half-value period of rainfall index were clarified. It was shown that the prediction accuracy increased by handling the threshold magnitude for individual slope conditions.