NATURAL DISASTERS IN JAPAN DUE TO INTENSIFIED STORMS AND HUMAN IMPACTS ON LAND

Japan is a disaster-prone country with frequent heavy rainfall, earthquakes, tsunamis, and volcanic activities. Increased sea surface temperature in the Pacific due to global warming and the resultant moisture supply have increased the probability of intensified storms over Japan. This change has led to hazardous consequences, including increased local flooding in highly urbanized areas. In such places, artificial land modification occasionally produces locally closed topographic depressions, and paved roads act as drainage lines during heavy rainfall. The drained water accumulates in the depressions, resulting in inundation if the ability of sewage systems and pumps is insufficient. To predict such flooding in the heart of Tokyo, machine learning using high-resolution road and topographic data was effective. Another increasing type of disaster is levee breakage along a major river. Many Japanese lowland rivers have been embanked to utilize floodplains for various purposes. The levees were designed to cope with extreme rainfall, and dams upstream regulate river flow during heavy rain. However, rainfall of previously unexpected levels happened recently, resulting in levee breakage and extensive lowland inundation. Although local governments provided detailed flood hazard maps with predicted inundation depth based on high-resolution elevation data, residents did not consider the prediction realistic. Therefore, they could not take effective action upon flooding. If levee breakage along a river happens in major coastal cities, including Tokyo, Osaka, and Nagoya, the consequences will be catastrophic because of the broad land below sea level. Such land was produced due to excessive industrial groundwater pumping in the early to middle 20th century. Although the government later restricted pumping and land subsidence almost stopped, some land is now 3 to 4 m below sea level. The intensified storms over Japan also increase the possibility of landslides and debris flows in hilly and mountainous regions. A recent debris-flow disaster in central Japan also reflected inappropriate artificial land transformation with landfilling in a valley head. Attention to human-induced changes in climate and land susceptibility is essential for today’s disaster risk reduction in Japan and other countries.