5:15 PM - 6:30 PM
[G01-P01] Considering limits of hazard map: A case of a debris flow disaster that struck a village at the southern foot of Yatsugatake in September 1898.
Keywords:hazard map, sediment disaster, historical documents of disaster
Hazard maps that describe sediment disaster warning areas and inundation areas are extremely important sources of safety information for residents. For sediment-related disasters, the warning area (and the special warning area) due to collapse of steep slopes and those for debris flow are designated. The steep slopes mentioned here refer to slopes with an inclination of 30 degrees or more, but flat areas within 10 m from the upper end of each slope and flat areas within 50 m from the lower end of each slope are also included in the warning area (Sediment Disaster Prevention Law Enforcement Regulations). However, the designation is made for "the area of land where it is recognized that there is a risk of harm to the lives or bodies of residents, etc. in the event of a collapse of a steep slope" (Sediment Disaster Prevention Law). Therefore, steep slopes in the mountains are not generally designated as a warning area. However, if a slope collapse occurs when heavy rain continues for a long time in a mountainous area, the debris flow may extend with a plenty of water to a region that is not expected as a sediment disaster warning area. It may consequently occur a significant disaster. Since such a case of the slope collapse under continuing heavy rain is not assumed in the current hazard map, inhabitants have to know about past catastrophes occurred in their area in detail.
In this presentation, we will discuss one of the debris flow disasters that once occurred at the southern foot of Yatsugatake as an example. Around 1:30 am on September 7, 1898, heavy rain brought about by a typhoon caused a debris flow in the Yato Area of Oizumi Village (Hokuto-city Yamanashi) at the southern foot of Yatsugatake. Consequently, 57 were killed, and 51 were injured. The author identified the locations of the damaged houses based on the "Storm Damage Investigation Report" compiled in the Meiji era and archived in the Imperial Household Agency, and the records of the land register stored in the Legal Affairs Bureau. It was found that most of the damaged houses had been located along an agricultural canal that flowed through the village. The author also compared the distribution of damaged houses in the disaster area with the results of debris flow simulations, which was made using the Hyper KANAKO system, and found that the local topographic features were the primary cause of the concentration of victims in a specific group of houses.
Based on the investigation of this historic disaster, it is pointed out that the debris flow may extend beyond the warning area designated on the hazard map, and that a minimal topography of the residential area could cause enormous damage. Farther, these findings suggest that general commentaries on the hazard map is not only insufficient as the disaster prevention education, but it may instill a dangerous sense of security in the inhabitants.
In this presentation, we will discuss one of the debris flow disasters that once occurred at the southern foot of Yatsugatake as an example. Around 1:30 am on September 7, 1898, heavy rain brought about by a typhoon caused a debris flow in the Yato Area of Oizumi Village (Hokuto-city Yamanashi) at the southern foot of Yatsugatake. Consequently, 57 were killed, and 51 were injured. The author identified the locations of the damaged houses based on the "Storm Damage Investigation Report" compiled in the Meiji era and archived in the Imperial Household Agency, and the records of the land register stored in the Legal Affairs Bureau. It was found that most of the damaged houses had been located along an agricultural canal that flowed through the village. The author also compared the distribution of damaged houses in the disaster area with the results of debris flow simulations, which was made using the Hyper KANAKO system, and found that the local topographic features were the primary cause of the concentration of victims in a specific group of houses.
Based on the investigation of this historic disaster, it is pointed out that the debris flow may extend beyond the warning area designated on the hazard map, and that a minimal topography of the residential area could cause enormous damage. Farther, these findings suggest that general commentaries on the hazard map is not only insufficient as the disaster prevention education, but it may instill a dangerous sense of security in the inhabitants.