The River Joganji is one of the most wasted rivers in Japan because of its rapid flow and the great mass of debris eroded from volcanic mountain of Tateyama. In the tributaries of Joganji such as Yukawa, Makawa, Shomyo-gawa and others, Yukawa is most wasted.
The main reason why Yukawa was so wasted is the big landslide“Tombi-Kuzure” by the great earthquake in 1858. This landslide occured in the upper stream of Yukawa and its debris-flow filled up Doro-dani and Dashibara-dani. The vollume of its debris deposits is estimated about 3.3×108m3. For more than 100 years, these deposits“Tombi-doro” has been eroded violently. Thus debris deposits from Tombi-Kuzure have been main supply source of earth and sand to Joganji River ever since.
In 1964, approx. 2.9×106m3 of debris were moved by the big landslide and further in 1969 Yukawa basin got remarkably wasted because of the heavy rainfall.
Authors have been investigating this wasted area from the geological standpoint through photo-interpretation, photogrammetric method and field survey since 1965.
In this paper we first related on the state of landslide in Yukawa basin especially about Tombi-doro, and next on the effects of the disaster in 1969 including volume variation.
