3:45 PM - 4:00 PM
[HCG23-07] Investigating sediment transport on disaster-affected slopes caused by wet snow avalanches and debris flow
Keywords:Hokkaido, geohazard, wet snow avalanche, debris flow
Ensuring safe road traffic requires significant attention to addressing slope hazards. Warning about slope hazards caused by debris flow in Hokkaido, Japan, is required to be paid for both associated with rainfall in summer and wet snow avalanches in winter. In recent years in Hokkaido, despite the substantial snow that remains in winter, wet snow avalanches leading to debris flow triggered by temperature rises and snowmelt by rainfall have been observed. The possibility of this climate situation persisting due to global warming, taking measures against debris flow involving wet snow avalanches, corresponding snowmelt, and rainfall urgent. One of the basic strategies for mitigating slope hazards is understanding the type of disaster of debris flow through the detailed observation of geomorphological and geological characteristics in geohazards area.
In this study, we analyze the difference between the digital elevation models (DEM) obtained from airborne laser surveys taken in July 2006 and October 2019 to identify surface elevation change in Hiroo Town, Hokkaido, which around damaged by debris flow and wet snow avalanches. Detailed field surveys and soil sampling were conducted at identified sites of surface elevation change by DEM analysis.
Comparison of the DEMs from the two periods revealed significant changes within the valley of one of the five valleys in the study area, including a maximum decrease of 4.5 meters in surface elevation upstream, and a maximum increase of 4.8 meters in surface elevation in the midstream area. Field surveys revealed evidence of slope collapses and talus in the upstream, exhibiting a V-shaped cross-section. Conversely, the valley in the midstream area exhibited a widened concave shape, with easily erodible sediment covering the surface. These sediments formed tongue-shaped lobes extending downslope, showing head-sized boulders at their tips and surfaces covered with fist-sized boulders, including wood fragments. These characteristics align with those of debris flow deposits. Resulting of the soil test, internal friction angles of soil samples collected at three points with different elevations showed a slightly higher tendency at the lowermost part of the slope, no significant differences in grain size composition and moisture content were observed among the samples. Therefore, the deposits on the slope have relatively homogeneous physical properties.
Considering field survey results, surface elevation changes detected by DEMs analysis were primarily due to slope collapses in the upstream and sediment deposition in the midstream by debris flow. These results indicate significant debris flow occurring between July 2006 and October 2019. The sediment distribution and various parameters obtained from soil samples (such as average grain size and internal friction angle) provide crucial insights for understanding sediment transport processes through numerical simulation of debris flow and stability calculations, contributing to future disaster mitigation.
In this study, we analyze the difference between the digital elevation models (DEM) obtained from airborne laser surveys taken in July 2006 and October 2019 to identify surface elevation change in Hiroo Town, Hokkaido, which around damaged by debris flow and wet snow avalanches. Detailed field surveys and soil sampling were conducted at identified sites of surface elevation change by DEM analysis.
Comparison of the DEMs from the two periods revealed significant changes within the valley of one of the five valleys in the study area, including a maximum decrease of 4.5 meters in surface elevation upstream, and a maximum increase of 4.8 meters in surface elevation in the midstream area. Field surveys revealed evidence of slope collapses and talus in the upstream, exhibiting a V-shaped cross-section. Conversely, the valley in the midstream area exhibited a widened concave shape, with easily erodible sediment covering the surface. These sediments formed tongue-shaped lobes extending downslope, showing head-sized boulders at their tips and surfaces covered with fist-sized boulders, including wood fragments. These characteristics align with those of debris flow deposits. Resulting of the soil test, internal friction angles of soil samples collected at three points with different elevations showed a slightly higher tendency at the lowermost part of the slope, no significant differences in grain size composition and moisture content were observed among the samples. Therefore, the deposits on the slope have relatively homogeneous physical properties.
Considering field survey results, surface elevation changes detected by DEMs analysis were primarily due to slope collapses in the upstream and sediment deposition in the midstream by debris flow. These results indicate significant debris flow occurring between July 2006 and October 2019. The sediment distribution and various parameters obtained from soil samples (such as average grain size and internal friction angle) provide crucial insights for understanding sediment transport processes through numerical simulation of debris flow and stability calculations, contributing to future disaster mitigation.