Due to the effects of climate change, flash floods are becoming increasingly frequent in Japan, and in tropical Asia, intense, localized downpours—known as “squalls”—are growing in scale and occurring more frequently. In major cities, roads are almost invariably flooded after a squall, causing damage to homes and businesses; recently, however, the extent of this flooding has expanded, and the duration has lengthened.

Many cities in Asia, including Japan, have prioritized drainage systems—focusing on how to remove floodwater and rainwater from within the city—since the early stages of their development. Despite the implementation of various flood control measures, the problem of urban flooding has not been resolved but has instead increased. It is now believed that this is due not only to the intensification of rainfall patterns but also to the fact that urbanization has led to the paving of roads and land (known as artificial surface coverage), reducing the amount of water that can infiltrate the ground, and that the drainage channels constructed to address this issue are failing to perform at their designed capacity.

As part of its disaster environment research, the National Institute for Environmental Studies has been conducting initiatives both domestically and internationally to examine preventive measures for mitigating flood damage during disasters, propose rapid flood waste disposal schemes for early recovery, and establish resilient wastewater and waste management systems. In particular, regarding the mitigation and prevention of flood damage in cities across tropical Asia, the institute has demonstrated that effective measures require not only “hard” infrastructure solutions—such as the construction of flood control facilities like dams and underground drainage channels—but also “soft” measures, such as the proper management of daily operations like drainage channel maintenance and garbage collection.