夜間瀬川の土砂災害史と砂防事業

抄録

Originating in the Shiga Highlands, which is the remnants of an ancient volcano, the Yomase River has a catchment basin of 117 km² and runs for a total length of 26 km. The downriver area is characterized by a vast alluvial fan measuring 6 km in length and 25 km² in area. Hot spa communities, such as Yudanaka and Shibu, in the midstream area of the Yomase River and the urban area of Nakano city in the downstream area have frequently experienced serious damage due to sediment and flood disasters. The prefectural government of Nagano started work on sabo facilities in 1906, but the sabo facilities constructed then were destroyed by heavy rainfall in 1909 to 1910 and the sabo project was suspended. As such, the topography, geology, and history of sediment disasters along the Yomase River were investigated, and their relationship with sabo projects was studied. The location maps of the sabo facilities constructed from 1906 to 1910 were found in storage at the Nagano Prefectural Museum of History, so they were reviewed in detail. This paper identified the location mentioned in an anecdote by Professor Masao Akagi, who contributed to the sabo works along the Yomase River, where he slipped and fell during a field survey. Based on the relationship between the catchment area and the bed slope, useful knowledge was obtained that will help in reviewing the marginal conditions related to the dry masonry dam work conducted back then and conserving these facilities in the future. A symposium was held in Yamanouchi town in 2018 to commemorate the 100th anniversary of the start of the sabo project for the Yomase River, as a direct project of the central government.