4:00 PM - 6:00 PM
[HDS07-P06] Preparation of a landslide hazard map with residents’ participation in Aruga district, Suwa City, Nagano Prefecture
Keywords:landslide hazard map, residents’ participation, landslide risk slope, Soil layer thickness measurement, Debris flow simulation
In this presentation, we report on a landslide hazard map prepared based on the requests of residents in the Aruga district of Suwa City, Nagano Prefecture. The study area is located southwest of Lake Suwa, and the landslide hazard map was prepared for nine mountain streams where debris could flow into the village. In the Odaizawa River basin, located 2 km northwest of the study site, heavy rains in 2006 caused a debris flow that resulted in seven human casualties. In addition, many landslides occurred due to heavy rains in 2006 in Tochikubozawa (target mountain stream) and in 1983 in Hodosawa (target mountain stream) . Therefore, it can be said that the target area has a high risk of debris flow. The mountainous area of the study site is based on the Enrei Formation from the Pliocene to the early Pleistocene, and volcanic ash soil is often distributed on the ground surface.
The survey was conducted in the following order. (1) Identification of landslide risk slopes, (2) detailed investigation of representative slopes, (3) preparation of the landslide hazard map, and (4) implementation of residents’ briefing. Steps (1) and (2) were conducted using a method based on Matsuzawa et al. (2021), and residents were invited to participate in Step (3).
As a result of the survey, 206 risky landslide slopes were identified in the nine target streams. Of these, a detailed survey was conducted in Kankozawa, which was highly requested by the residents and could cause severe damage to the village due to debris flows caused by landslides. In the detailed investigation, the thickness of the soil layer in the vertical and lateral directions was measured using a soil strength probe and an SH-type penetration tester on slopes with high landslide risk, and the scale of the landside was estimated. As a result, it was found that the following two types of landslides could occur: a relatively large landslide of approximately 3300 m3 and a small landslide of approximately 600 m3. Next, a debris flow simulation was conducted assuming a small landslide, which is expected to occur frequently. Finally, the information on landslide disaster risk was used to generate a landslide hazard map.
The following opinions were expressed at the residents’ briefing session held after preparing the landslide hazard map. “It became clear that no place would be safe in the long run, and we felt that debris flow simulations would be necessary to understand the risk level of each mountain stream” and “figuring out how to evacuate is a major issue.” Therefore, it can be said that the landslide hazard map prepared in this survey contributed to further understanding of the characteristics of local landslide risk.
The survey was conducted in the following order. (1) Identification of landslide risk slopes, (2) detailed investigation of representative slopes, (3) preparation of the landslide hazard map, and (4) implementation of residents’ briefing. Steps (1) and (2) were conducted using a method based on Matsuzawa et al. (2021), and residents were invited to participate in Step (3).
As a result of the survey, 206 risky landslide slopes were identified in the nine target streams. Of these, a detailed survey was conducted in Kankozawa, which was highly requested by the residents and could cause severe damage to the village due to debris flows caused by landslides. In the detailed investigation, the thickness of the soil layer in the vertical and lateral directions was measured using a soil strength probe and an SH-type penetration tester on slopes with high landslide risk, and the scale of the landside was estimated. As a result, it was found that the following two types of landslides could occur: a relatively large landslide of approximately 3300 m3 and a small landslide of approximately 600 m3. Next, a debris flow simulation was conducted assuming a small landslide, which is expected to occur frequently. Finally, the information on landslide disaster risk was used to generate a landslide hazard map.
The following opinions were expressed at the residents’ briefing session held after preparing the landslide hazard map. “It became clear that no place would be safe in the long run, and we felt that debris flow simulations would be necessary to understand the risk level of each mountain stream” and “figuring out how to evacuate is a major issue.” Therefore, it can be said that the landslide hazard map prepared in this survey contributed to further understanding of the characteristics of local landslide risk.