Abstract
The Mid Niigata prefecture Earthquake (M6.8 at 17: 56 JST on October 23, 2004) triggered many landslides especially in Yamakoshi Village (currently merged into Nagaoka City), and blocked Imo River stream at several places, which caused landslide dam lakes.Urgent construction of channel works showed immediate effects by avoiding collapse of the landslide dams.However, high amounts of recorded snowfall in the winter brought another fear that collapse of the dams might occur when snow melts and water level of the dam lakes rise in the spring.Therefore, it was important to know how much snow was in the whole Imo River basin in advance.
The Geographical Survey Institute measured the snow depth in the study area from the upper basin to the confluence of Uono River by comparing two airborne LIDAR (light detecting and ranging) data sets, which were collected before and after the snowfall.Average snow depth of the study area, which is 37.8km2in area, was 2.84m, and total volume of snow was 108.6 million cubic meters. Then total snow water equivalent volume was calculated as 49.6±5.1 million cubic meters using sample snow weight data collected at five points over the basin. These results were sent to the earthquake disaster countermeasures headquarters and other organizations concerned such as local government offices.
