学会誌「自然災害科学」

火山山麓における災害ポテンシャルの評価 (八ヶ岳南麓切掛沢扇状地を事例として)

Assessment of Hazard Potential at the Foot of Volcanoes - A Case Study: the Kikkakezawa Fan on the Southern Foot of Mt.Yatsugatake, Japan -

奥西 一夫*・諏訪 浩*

Kazuo OKUNISHI* and Hiroshi SUWA*

Abstract

Alluvial fans and cones on the foot of volcanoes have high hazard potential because of frequent inundation of debris flows and floods accompanying marked topographic changes. However, the social needs for the development of such lands are ever increasing, because of their high demand for recreational sites. Assessment of hazard potential and regulation of land use are thus urgent problems. This paper proposes fundamental principles for the assessment of hazard potential on the basis of a case study carried out at the Kikkakezawa Fan on the southern foot of Mt.Yatsugatake, central Japan. History of land use is summarized in Fig. 3. Existing villages are located below a major spring zone in the alluvial fan, which is fed by the groundwater in the volcanic body. Construction of a new road (PR) stimulated land developments along it and further upstream part of the fan. According to Kosaka (1992) the debris-flow deposits cover the fan in three geologically distinct ages (Fig. 5). The ages of the deposition of the debris flows in the new and middle ages are estimated according to a close examination of aerial photographs (Fig. 6) and a reconnaissance study of the microtopography (Fig. 7). It then enables an estimation of the frequency of the debris flow inundation in the future. The site-by-site assessment of hazard potential and risk of disasters is usually difficult because the past occurrences of inundation are not always recorded and a reliable estimation of its future possibility is hardly available. It is, therefore, essential to regulate land use for individual areas on the basis of the assessment of the hazard potential from the regional viewpoint using the data about the geomorphic processes of a large time scale. In Fig. 3, for example, it is obvious that the areas above the prefectural road (PR) constitute a zone of high hazard potential and high risk, among which the area of the planned extension of the golf courses (Gp) are extremely dangerous.

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