Journal of the Japan Society of Erosion Control Engineering, Vol.60,No.3,2007

The Tobata landslide dam and outburst floods at the Azusa River in 1757

Toshio MORI, Kimio INOUE, Takahisa MIZUYAMA and Toshiyasu UENO


Abstract

As many as 19 landslide dams have been formed in the northern region of Nagano prefecture, central Japan, in last 500 years except one case. Of this number, seven were formed when the Zenkoji Earthquake occurred in 1847. This abundance is likely because of the geotectonic background of this area which is located at the western end of the gFossa Magna," or Japan's central graben belt.
The Tobata landslide occurred in the early morning of June 24, 1757 due to heavy rain. Blocking the Azusa River,which is upstream of the Shinano River, a landslide dam with a height of 130m and a storage capacity of 85 million m3 was formed. Around 10 a.m. on the third day (54 hours later), this landslide dam burst and its water flooded the area up to the confluence with the Narai River. According to calculation using the Manning's formula, it is estimated that the flood water ran down the river in a concentrated path with a velocity of 12 m/s and a peak flow of 27,000 m3/s. When the dam burst, local people were quickly ordered to evacuate and no casualties were caused during this flood.


Key wordsFAzusa river, Tobata landslide, landslide dams, outburst flood
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