The History of Sediment Disasters and Sabo Projects in the Yomase River Basin, Nagano Prefecture, Japan

Makoto KITAHARA, Masashi TASHITA, Masaru NISHIZAWA, Masaaki KINOSHITA, Takao YAMAZAKI, Kunihiro HASEGAWA, Junichi KANBARA, Kimio INOUE, Kouji OYA and Kazuhiko NAKANE

The Yomase River originates from the Shiga Highlands, which is an ancient volcano, and runs for a total length of 26 km with a catchment area of 117 km2. A vast alluvial fan of 6 km in length and 25 km2 in area is formed at the downstream area of the river. There are hot springs located along the Yomase River, including the Yudanaka/Shibu Hot Spring in the midstream reach of the river, and they have often suffered devastating sediment and flood disasters. The prefectural government of Nagano started sediment control projects in 1906, but the sabo facilities constructed by the prefecture were destroyed by heavy rainfall between 1909 and 1910, and the hot spring resorts also suffered serious damage. The authors studied the topography and geology of the Yomase River, the history of sediment disasters, and the relationship between the disasters and the sabo projects, and learned that the location maps of the sabo facilities constructed from 1906 to 1910 are stored at the Nagano Prefectural Museum of History. Then, we performed a detailed review of those documents. Analysis of the relationship between the catchment area and the bed slope provided knowledge useful in our review of the limit conditions related to the past construction of dry masonry dams and to the future preservation of those old dams.

2018/1 311-319