Risk of artificial dam breaching by volcaniclastic flows in Japan / Risque de rupture de barrages artificiels par des écoulements volcaniques au Japon

Abstract

The risk of artificial dam failure associated with volcaniclastic flows has been investigated. Hazardous volcanoes with a high possibility of imminent eruptions can be distinguished from other volcanoes by detailed studies of the last 10,000-year-eraption histories of the volcanoes in Japan. Most andesitic volcanoes in Japan have erupted periodically, once in 100 to 3000 years on average, and have sometimes generated volcaniclastic flows (pyroclastic flows, debris avalanches and debris flows) which have affected river channels. Many artificial dams have been constructed close to the hazardous volcanoes of Japan. The author has identified 17 danger zones with a high probability of occurrence of high magnitude floods related to artificial dam failure. Five of these danger zones are described in this paper. The most dangerous area extends across the lower Nobi alluvial plain, which concentrates a population of 2 million people. In the event of the Ontake volcano producing a large-scale pyroclastic flow or debris avalanche resulting in the failures of 18 artificial dams along the Kiso river, a torrential flood flow is predicted to cover the alluvial plain almost entirely, causing 10" to 105 human casualties in the process. The Yamba dam (with a capacity of 108-109 m3) along the Agatsuma river will be completed in 2006. The active Asama and Kusatsu-Shirane volcanoes are located in the headwater region of the drainage basin. Ten million people in Tokyo and the surrounding area live in the lower alluvial plain. Methods for monitoring community needs and perceptions, and research schemes for disaster prevention related to artificial dam failure should be urgently implemented.