When a volcano erupts in the snow season, mudflow by snow melting occasionally occurs, causing destructive disaster. On May 24, 1926, after an explosive eruption of Mt.Tokachi, hot debris avalanche melted the remaining snow, and caused big mudflows that attacked a hot-spring hotel and a village of Kamifurano. The village was buried under mudflow sediment and completely destroyed. 144 people were killed in total. Recently, on November 13, 1985, similar disaster was experienced after an eruption of Nevado del Ruiz Volcano, Columbia. As a pyroclastic flow from the summit crater covered the surface of glacier, rapid melting of ice triggered disastrous mudflows. The mudflow travelled about 50km and devastated the city of Armero, where 21000 of 29000 inhabitants were killed. About one month before this eruption, a detailed hazard map of Nevado del Ruiz Volcano had been circulated by INGEOMINAS ,the Geological Survey of Columbia, to local autonomies. This map was well prepared and the mudflows flowed down along the predicted courses. But each autonomy could not put the map to practical use for real disaster prevention. As soon as hearing the news about the disaster at Nevado del Ruiz, Kamifurano town prepared a new hazard map for emergency evacuation on the assumption that mudflow attacks the town after an eruption of Mt. Tokachi. Actually, when the volcano erupted on December 25, 1988, evacuation of inhabitants was directed on the basis of this hazard map. In Japan, mudflow disaster by snow melting with volcanic eruption has not been caused more than 80 years after the disaster of Mt.Tokachi in 1926. But, if in future, volcanoes including Mt.Fuji erupt on a large scale at the snow season, surrounding areas may face the danger of destructive mudflow. Accordingly, autonomies around active volcanoes have to prepare the disaster prevention planning against mudflows by snow melting with eruption.
