Sediment disasters caused by the Mid Niigata Prefecture Earthquake and changes of sediment movement after snow melt

Abstract

Many slope failures and landslides occurred in the focal region by the mid Niigata prefecture earthquake, 2004. Along the River Imo several landslide dams were formed and the channel were occluded. The mid Niigata prefecture area is a very eminent area of heavy snowfall and especially the maximum depth of snow of the basin of the River Imo which suffered an enormous quake damage exceeds 3 m. Therefore, the enough attention against sediment movement which will occur during snow thawing season is necessary. From the field investigation, though remarkable new slope failures and expanded old slope failures were not found, unstable sediment which was carried by snowmelt water from the upper part of slope is accumulating at the foot of slope failures by the earthquake. There is a great possibility that the unstable sediment will be moved rapidly again at the time of rainy season or typhoon and develop into debris flows. Therefore, it is necessary to intensify a monitoring system against sudden sediment movement and aggradation at the lower reaches in the flood season.