[HDS15-P06] Examination and systematization of predispositions to floods in the Heavy Rain in July 2018
Keywords:the Oda River, raised bed river, alluvial ridge, crevasse splay, channel slope
Though predispositions to floods have been mentioned in the studies of river engineering, geography and geomorphology, the relationship between flood and topography / geology is dealt with as self-evident in most of them. Hence, this study focuses on the predispositions to floods.
The floods in the Heavy Rain in July 2018, especially those of the Oda River (the Takahashi River System) are the subjects of this study. The purposes of this study are (1) to create a database on the floods, (2) to examine predispositions for each space scale, taking into account the influence of artificiality, (3) to acquire knowledge contributing to high accuracy of flood risk assessment.
Fields & Methods
In the Chugoku district, 3 denudation surfaces are seen. And, the plains on the Setouchi side have been formed by the rivers flowing down from the Chugoku mountains. These rivers are characterized by weathering granitoids distributed upstream.
The Oda River is a first-class river which joins the Takahashi River in Kurashiki city. Since the Oda River has a gentler river bed than that of the Takahashi River, it is affected by the backwater from the Takahashi River.
The Association of Japanese Geographers has released the map of the flood points caused by the heavy rain on the Web (http://ajg-disaster.blogspot.com/). The characteristics of the river channels on the flood points on this map (mainly those of the Oda River) were investigated and compiled into a matrix.
On 9 dike break points of 4 rivers, the aerial photos were interpreted, and the classification maps of flood landforms were created.
Besides, the downstream change graph of the river bed height, the bed slope…etc. of the Oda River were created.
Many dike break points lie in the administrative sections of the prefectures, and raised beds are seen on 9 of the 10 points in the tributaries to the Oda River. There are sluice gates on 2 dike break points and 5 slope failure points reported in the Council for Social Infrastructure (2018). At the outer bank of the large river curve in Fukuhara, where the overflows and the slope failures were occurred this time, dike breaks were also occurred in the past.
Although alluvial ridges have been regarded as safe against floods, the largest dike breaks occurred on the alluvial ridge.
The downstream change graphs show that floods didn’t occur in a downstream section of a weir with a relatively steep river bed but occur in a further downstream section where a river bed becomes gentle.
On the point scale, a sluice gates and a raised bed section is one of the predispositions to floods. From comparing dike break points and non-dike break points, it is suggested that steep slope gradient affects dike break, too. And it is necessary to update the flood risk assessment on alluvial ridges.
On the section scale, compared the downstream section of a weir with a steep river bed, the further downstream section with a gentle bed is dangerous.
On the regional scale, it is considered that the bed slope difference of the Oda River and the Takahashi River is affected by the denudation surfaces. It is also one of the predispositions that granitoids is widely distributed in Chugoku district.
Regarding artificiality, it turned out that the river maintenance has not kept up. However, there is the more serious issue that the Mabi-cho area, where many floods occurred this time, has become a bedroom town. Therefore, it is necessary to reconsider how to use land.
Council for Social Infrastructure, The Ministry of Land, Infrastructure, Transport and Tourism (2018), About the Heavy Rain in July 2018
Izumida, A. (2018), Crevasse splay formation and contribution to the development of alluvial ridges in the lower Kinu River, central Japan, Master's thesis of Graduate School of Frontier Sciences, The University of Tokyo
Uchida, K. (2011), Activities of the flood prevention unions in the Oda River basin in Okayama prefecture, Water Utilization Science No.320, pp.40-55