Quantifying disaster casualties centered on flooding in the chikugo river middle basin in the past 400 years to determine the historical context of the July 2017 northern Kyushu torrential rainfall

In this study, we collated the number of deaths caused by disasters that took place in the Chikugo River middle basin on the island of Kyushu, Japan, from the 1600s to 2017. The compilation of quantitative statistics of floods in Japan began in the modern era, while the statuses of previous disasters are described by vague indices and are thus difficult to quantify. In this study, we geographically restricted our investigation to the Chikugo River middle basin. We quantified the scale of each disaster by using the number of deaths recorded in historical documents. We then compared the disasters that took place in the area since the 1600s to identify the potential disaster risks harbored by our study area. During the period examined, the great famine from 1732 to 1733 was the greatest disaster and caused the most deaths. However, this is the only recorded famine for which fatalities were documented. Meanwhile, floods occurred frequently through this period, 26 of which resulted in fatalities: they had a total death toll of 292. Thus, famines occur infrequently but cause severe damage, whereas floods occur frequently but cause relatively minor damage. During the approximately 400 years examined, there were four floods with death tolls exceeding 30 people. Three of these occurred after 1868, when the modern era of Japan began. Meanwhile, there have been almost no small-scale floods during and after the modern era. By quantitatively assessing the disasters’ scales, we were able to establish that the 2017 northern Kyushu torrential rainfall was the fourth gravest water-related disaster, in terms of its death, toll since the 1600s. If only the north bank of the Chikugo River middle basin is considered, it was the greatest flood disaster to occur in this period.