In 2018, Japan not only had an abnormally hot summer, but also experienced successive disasters, including the Northern Osaka Earthquake, the Western Japan Heavy Rain, Typhoon No. 21, the Hokkaido Mid Iburi Earthquake, and Typhoon No. 24. In particular, the record-breaking heavy rains continued in a large area of Western Japan from June 28 to July 4, due to the storm front and Typhoon No. 7. The amount of rainfall totaled over 1,800 mm in the Shikoku Region and 1,200 mm in the Tokai Region. The quantity of rain that fell within 48 and 72 hours in both the Chugoku and Kinki Regions, as well as many other areas, was the highest rainfall ever recorded. A special warning regarding heavy rain was issued in 10 prefectures and every kind of disaster that Japan had experienced recently occurred in various locations. As of August 21, a report from Disaster Management Section, Cabinet Office indicated 221 deaths, 9 missing persons, 68 severely injured persons, 319 slightly injured persons, 3 persons with an unknown level of injury, 6,206 destroyed homes, 9,764 severely-damaged homes, 3,765 partially-destroyed homes, 9,006 homes with flooding above the first-floor level, and 20,086 houses with flooding below the first-floor level.
During this large-scale disaster, which was named the Western Japan Heavy Rain, the Disaster Relief Act was applied to 110 municipalities and JDR decided to issue a special edition to address issues pertinent to this specific disaster event. Paper submissions were requested that not only comprised demonstrative researches on hazard and damage characteristics, methods of evacuation, and features of disaster response, but also included introductions of best practices, which were conducted in various fields and prompted diverse collaboration to develop and establish measures to mediate the effect of the future Nankai Trough Earthquake, as well as problems and solutions to successfully realize diverse collaboration. In response to the call for papers for the special issue, nine researches were submitted and six were accepted following a strict review process. To address the category of hazard characteristics analyses, two papers analyzing the characteristics of the flooding resulting from the Western Japan Heavy Rain and one paper comprising an analysis of landslide disasters were accepted. In the category of disaster response, one paper focusing on the use of SNS and two papers regarding the elderly were accepted. It would be our sincere pleasure if this special issue could contribute to future reductions in damage resulting from natural disasters.
The heavy rain that hit Western Japan in July 2018 triggered the worst rain-related disaster in the Heisei era, with the total of dead and missing persons exceeding 230, mainly in the Hiroshima and Okayama Prefectures. At several locations along Oda River (of the Takahashi river system) and its tributaries, dikes were breached due to large-scale flood, leaving 51 persons dead. This paper aims to shed light on the scale of inundation along Oda River and its tributaries and identify the characteristics of and critical factors for human damage. Field surveys were conducted to measure flood marks in flooded areas and river channels, and gauge the extent of damage to people and property. The surveys found that a large area was inundated on the north side of Oda River, with an inundation depth exceeding 5 m for 1 km in the south-north direction and 3.5 km in the east-west direction, which made vertical evacuation of residents difficult. The findings that about 80% of the dead were found on the first floor of their houses, with those who had lived in a one-story house and those who had lived in a two-story house accounting for 50% each of the deceased, indicate how difficult even vertical evacuation was. The findings appear to be related to the considerable inundation depth and high rate of water level increase, along with the fact that the majority of the deceased were elderly people.
This study reveals the characteristics of flood flow with active sediment transport that caused the Sozu River flood disaster in July 2018. The results of field surveys revealed that the basin contains steep mountains and flat areas, facilitating the rapid transport of sediment produced in the mountains to downstream areas during the disaster. Two-dimensional numerical simulations of flood flow with sediment transport are conducted to realize the flood flow characteristics. As a result of the simulation, a huge amount of sediment deposition in the channel near the top of the alluvial fan increases the inundation damage at downstream areas. The results indicate that within two flood peaks during the disaster, the inundation in the second flood peak is more severe than that in the first peak, whereas the discharge of the second peak is much less than that in the first peak, caused by the reduction in the capacity of the original channel owing to the sand deposition during the first and second flood peaks.
Record-breaking heavy rainfall in July 2018 caused an extremely large number of slope movements over a broad area of western Japan. We mapped the distribution of slope movements in the southern part of Hiroshima Prefecture through an interpretation of aerial photographs that were acquired after the rainfall by the Geospatial Information Authority of Japan, and counted a total of 8,497 slope-movement starting points. The widespread distribution of slope movements – from Etajima City of Hiroshima Prefecture to Kasaoka City of Okayama Prefecture – suggests that the heavy rain affected a very large area. The starting points of debris flow during this disaster were commonly close to the crest of mountain ranges. We compared the distribution of slope movements to the 24-hr rainfall accumulation during the heaviest rainfall event to clarify the factors that caused regional difference in slope-movement distribution. We found the area of highest density of the slope movements was consistent with the area receiving a cumulative rainfall of >250 mm. This observation indicated that the position of slope-movement starting points was not related to differences in geology.
Western Japan was hit by heavy rain from June 8 to July 28, 2018. Record-breaking rain caused nearly all rivers to flood in Hiroshima and other areas. Over 200 people died following this disaster. Authorities attempted to understand why evacuation was not conducted swiftly enough to stop these deaths. They mentioned that normalcy bias and cognitive dissonance are two primary causes of significant damage . Moreover, an effective alert system is necessary to ensure that evacuation behaviors and procedures are incited at the appropriate time. To understand the factors that influence people’s behavior, we estimated the probability of irregular behavior by unit changes in external condition. We chose 500 m mesh as a unit of analysis to consider individual singularity and classified 3 classes of mesh to identify abnormal behavior. We verified that as the number of residents in each mesh increases, the likelihood of a person in that region to exhibit normalcy bias increases as well. Owing to data, the accuracy of this method is somewhat low. However, several implications may still be drawn from our results, such as the demand for an adequate alert system. Using the results of people’s mobility and disaster risk information, approaches to dangerous situations such as the examined case may be improved in the future.
The July 2018 torrential rainfall caused tremendous damage to western Japan, claiming the lives of 237 people. This research analyzed the mortality due to the disaster by age group and municipality and compared it with historical records of past wind and flood disasters. The analysis confirmed that the total death toll of 237 including 115 deaths in Hiroshima Prefecture alone was the second highest in 41 years since 1978, following 427 deaths including 294 in Nagasaki Prefecture in the Nagasaki Great Flood in 1982. The analysis also found that the mortality rate in the age group 70 years and more was extremely high in Mabi Town of Kurashiki City, Okayama Prefecture and Saka Town, Hiroshima Prefecture, compared with that recorded in past disasters. Moreover, the paper discusses practical target setting for future mortality reduction by comparing the mortality rates in past disasters.
The heavy rain disaster in July 2018 caused significant damages such as leaks and inundation above floor level at many facilities for the elderly. In the present study, we surveyed facilities for the elderly near Oda River, Okayama Prefecture, and those near Hiji River, Ehime Prefecture, to study the necessity of a facility-specific criterion to start evacuation on the basis of the characteristics of the facilities used by people who need care. The results of the survey indicated that evaluation information released by the local government might not ensure sufficient evacuation time. An example of a criterion to start evacuation based on the water level of a nearby river or the amount of dam discharge was shown as one of the ways that each facility voluntarily makes decisions on fast evacuation.