Research groups at the National Institute of Advanced Industrial Science and Technology, Tsukuba University, and South Mississippi University conducted an urgent survey of the flood deposits of the Kinugawa River, which was flooded by the heavy rains in the Kanto and Tohoku regions in September 2015.The characteristics of flood inundation and flood sediments were elucidated.
Natural disasters such as huge tsunamis and large-scale floods are rare events, so there are few records and disaster prevention measures are difficult.However, since the 2011 Tohoku-oki Earthquake, the importance of research and recording of catastrophes has been emphasized.For that purpose, it is necessary to investigate the sediments immediately after the formation of floods and tsunamis before the current situation changes due to disaster relief and restoration, but there are very few such investigations in the world.
The research group entered the site immediately after the flooding of the Kinugawa River due to the "September 2015 Kanto / Tohoku heavy rain".Around the breached area of the embankment (Misaka district, Joso city), the direction and depth of the inundation flow were measured and the traces of erosion and sedimentation were observed.
As a result, the flood sediments were classified into the base, the lower part, and the upper part, and each part was formed according to the flooding process of the river.The base is the sediment of the turbid water stage in the early stage of flooding when the river water crosses the embankment, and the initial flooding flow seems to have flowed relatively south of the flooded area.The lower part was deposited in the process of strengthening the flow until the levee broke after flooding, and it spread relatively far from the breach site.The upper part was deposited in the process where the strong current gradually subsided after the levee broke, and it is distributed only near the breach site.The survey confirmed changes in the flow of flood sediments from the spread of flood deposits.
By continuing and accumulating surveys and research in the event of a disaster like this, it will be possible to accurately identify sediments caused by past floods and tsunamis.It is expected that this will improve the accuracy of past geological analysis, leading to accurate risk assessment of large-scale natural disasters, future lessons learned, and raising awareness of disaster prevention.
