STATISTICAL ANALYSIS OF RAINFALL IN OKAYAMA PREFECTURE AND INVESTIGATION OF FLOOD DAMAGES IN THE ASAHI RIVER BASIN ATTRIBUTABLE TO THE HEAVY RAIN EVENT OF JULY 2018

Abstract

 This study investigated the rainfall characteristics in Okayama prefecture and the related flood damages in the Asahi River basin because of the Heavy Rain Event of July 2018, using statistical analyses, field observations, and numerical simulations. Results demonstrated the following findings. The return period of the severe rainfall event is over 50 years for the rainfall duration of over 24 hours. The massive flood inundations were caused by the riverbank failures because of the overbank and seepage flows observed at the prefecture-managed river sections in the Asahi River basin. The improvement project completed almost at the bifurcation point in the lower Asahi River presumably prevented the Okayama city from suffering flood damages, judged from the numerical results of the 2018 flooding and previous records of damages attributable to flooding of the same discharge.