Seasonal change in permeability of surface soils on a slow-moving landslide in a heavy snow region

This study demonstrated seasonal changes in the topsoil permeability due to vegetation growth in summer and snow loading in winter of a landslide in a temperate snowy area of northern Japan, which is underlain by soft Neogene sedimentary rocks. The decrease in saturated hydraulic conductivity due to loading was also examined for undisturbed and remolded specimens of surface soils from the landslide, using a newly developed apparatus for the consolidation permeability test. Infiltration capacity measured in the field increased gradually after snowmelt to a peak in midsummer and decreased exponentially with increasing snow load in winter. In the laboratory experiment, saturated hydraulic conductivity declined as an exponential function of loading. The topsoil was permeable enough to infiltrate rainwater into the ground, even for intense rain storms in summer, but became less permeable under heavy snow load during winter and was not capable of full infiltration of meltwater during the snowmelt period. This suggested that Hortonian overland flow should occur in late winter, at least patchily, under the melting snowpack.