Characteristics of a Landslide Occurred in May 2015 in Mt. Hakusan and its Influence on Downstream System

Seiji YANAI

In May 2015, a large landslide occurred in the headwaters of the Tedori River at the foot of Mt. Hakusan. Airborne LiDAR data and a series of aerial photographs taken over 60 years were used to analyze the topographical features and geomorphological factors that triggered the landslide. The landslide was 800 m long and 300 m wide, with a total sediment volume of 1.3 million m3. Most of this accumulated at the bottom of the slope and buried the stream channel. However, half of the sediments washed into the river and generated very turbid water for 6 months after the landslide. This influenced the downstream area when the very turbid water entered rice paddies, fresh water, and coastal fisheries, and led to groundwater depletion. Countermeasures were extremely difficult to institute because there was no road access to the landslide, which occurred in a strictly protected national park. Although the Forestry Agency currently uses a helicopter to scatter anti-erosion material, fundamental measures should be taken to ensure the early recovery of native vegetation species, eliminate the source of turbid water, and enhance sedimentation by using sediment ponds.

2018/1 124-131