Geomorphological analysis on flood and sediment hazards in the Omoto River resulting from Typhoon Lionrock in 2016

Abstract

In late August 2016, Typhoon Lionrock took an unusual course across Japan and induced heavy rainfall over the Tohoku Region, causing a serious disaster in the Omoto River basin, which left 20 dead and 1 missing in Iwaizumi Town. The disaster included slope failures and debris flows in upper mountainous parts, sediment flooding with driftwoods along tributary channels, and a widespread flood with abundant sediment and countless driftwoods along the main channel in the downstream. This paper firstly overviews the disaster situation and societal characteristics of the Iwaizumi area, and secondly discusses the relationship between its topographical characteristics and damaged areas along the main and tributary channels by analyzing aerial photographs and 1:25,000 topographic maps as well as the results from field investigations. In addition, temporal changes of house distribution and land use since 1947 at three downstream sites were analyzed to investigate relationship between houses and their damage. Our investigation has revealed that the occurrence of flood expansion, overflow and sediment f looding are related to sudden change of local topography, such as valley width, channel sinuosity, and longitudinal profiles from steep to mild. Although most of the houses in the area are historically settled at some distance from the river in the downstream, some of the houses built after 1977 have been found to stand where channel bars used to be and to have suffered severe damage in this flood disaster. It has also found that the f lood expanded over the valley in the downstream and inflicted some damage on most of the houses there, which suggests that this flood disaster was unprecedented in scale and unpreventable even for the residents who had been practicing traditional land use adjusted to local land conditions.