Seasonal variations in groundwater level and pore water pressure at the Gampei landslide, Joetsu City, Japan, a heavy-snowfall area

 In Joetsu City, Niigata Prefecture in Japan, the Gampei landslide is located in a heavy-snowfall area, where snow depth can exceed 5 m in heavy snow season. In this regions, the influence of snow cover on landslide activity has not yet been fully clarified. This study investigates the factors controlling landslide motion during the snow-covered season at the Gampei landslide in a heavy-snowfall area. Field observations included continuous monitoring of displacement using GNSS surveys and vertical extensometers, as well as measurements from pore water pressure gauges, groundwater level loggers, soil moisture sensors, ground temperature sensors, and time-lapse cameras for snow depth. In addition, surface deformation was analyzed using LiDAR surveys conducted with an unmanned aerial vehicle (UAV).

 The results indicate that pore water pressure and groundwater level remain high and stable with small fluctuations from the snow-covered period through the snowmelt season. In contrast, during snow-free periods, both parameters respond to rainfall and show rapid fluctuations, particularly in summer. These observations suggest that soil saturation induced by continuous autumn rainfall is maintained by snow cover, creating hydraulic conditions favorable for landslide movement during winter. Such conditions are more easily sustained during winter, resulting in larger displacement.

 Furthermore, in winters with low snow accumulation, a rapid rise in groundwater level was observed during the snowmelt period compared to heavy snowfall years. During winter, however, no clear increase in pore water pressure associated with groundwater level rise was detected, indicating that groundwater level and pore water pressure may be governed by different processes during the snow-covered period.