Fan Formation by the Sediment Runoff Due to River Bed Erosion

Abstract

We had a localized torrential downpour near the Fukui-Gifu prefectural border in September, 1965 (we call this Okuetsu Rainstorm). Especially, Nishitani village in Fukui prefecture (after this disaster, residents in this village moved to another place and now this district is included in Ohno city) suffered crushing damage from this storm. Namely, in Nakajima district which was the main part of the Nishitani village, before the disaster the number of households was 154, but after the disaster, 58 households were swept away and 86 were buried. A distinguishing feature of this disaster was that the bulk of the houses in the main part of the Nakajima district situated on the debris fan which had been formed by the sediment runoff from the Kamata river was buried second floor deep in sediment but not swept away. Judging from the fact-finding survey and data analysis, these houses were buried by the sediment which outflowed and deposited from the Kamata river taking the type of the sediment flow rather than the debris flow.In this paper, in order to reproduce the burying process of the Nakajima district by the sediment flow, sediment hydrograph of the Kamata river is evaluated by the calculation of the bed level variation at first, then by using this sediment hydrograph as a input boundary condition, simulation of the horizontally twodimensional bed level variation in Nakajima district is carried out, and it is found that the calculated results are comparatively in good agreement with actual phenomena.