Risk Reduction and mitigation of water-related disasters, Japan

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Risk Reduction and mitigation of water-related disasters



Focus Areas

Geographic Scope


Government of Japan



Background and Significance

The main challenge is risk reduction and mitigation of water-related disasters in Japan. Given Japan's unstable geology, mountainous topography and overall amount of inhabitable land, 50% of the population and 75% of its physical assets are concentrated on the alluvial plains, which account for only 10% of Japan's total land area and are often the site of substantial flood damage, accentuated by the fact that Greater Tokyo suffers from severe weather conditions of the Asia monsoon period. These changes in land use, and periodic increases in rainfall have enhanced the danger of floods in recent years, and have exacerbated flood damage. The 275 deaths and $17 billion of damage caused by floods in Japan in 2004 is a case in point, of the high risk of water-related disasters in the country.

In addition, groundwater withdrawal is still causing land subsidence. Furthermore, due to intense urbanization, the quality of water in Greater Tokyo has deteriorated. Action was taken to reduce the discharge load, such as drainage regulation and sewage maintenance, and the quality of water started to improve in major rivers. However, the concentration level is still high in some tributaries, lakes and marshes, and new Environmental Endocrine Disruptors have been problematic. In addition, the increase of imported non-native species of fish and plants is becoming a serious ecological problem.

The Experience: Challenges and Solutions

In order to mitigate against future disasters of this kind and to move towards a system of risk reduction, the national programme for comprehensive flood disaster control measures was created. The programme promotes the holistic integration of structural measures such as river improvement schemes (e.g. levees, channel improvement, dredging, floodway construction, etc.) and non-structural measures, such as controlling basin land development, the creation of an early warning system, the establishment of a community flood fighting corps and the dissemination of flood hazard maps. However, it was after such actions that the 2004 floods caused such catastrophic damage.

Consequently, in order to combat environmental degradation caused by human activities new legislation to regulate the use and discharge of effluent has been brought into action. The Flood Fighting Act was revised in 2005, to expand the scope of flood forecasting activities to include smaller rivers in order to promote a more responsive and timely emergency evacuation. The amended act also aims to enhance and adopt extensive flood information and communication systems for an additional set of rivers, while improving already existing flood forecast systems to disseminate information necessary for ensuring a smooth evacuation operation.

In addition, the Sediment-Related Disaster Prevention Law was enacted in 2000 with the aim of implementing comprehensive non-structural measures that would protect people from sediment-related disasters. Measures included raising public awareness on high-risk areas prone to sediment-related disaster, the development of a warning and evacuation system, the restriction of new land development for housing and other purposes and the promotion of relocation for some existing houses. After the revision of the Law in 2005, new regulations to prevent housing development in hazardous areas were introduced and the preparation and dissemination of hazard maps for smooth evacuation mechanisms made obligatory.

Forecasting and warning systems in Japan were also advanced, and backed by the dense network of rain gauges and water-level telemeters. These observation points, in combination with twenty-six radar systems, provide high precision information concerning the spatial and temporal distribution of rainfall.

To ensure the sustainability of water resources, comprehensive water resource development plans have been put into action and the efficient use of water resources have been promoted in all sectors, and in the sanitation sector, the government is attempting to expand the coverage of the public sewer system.

Results and Impact

This case study presents the example of river basins that serve one of the world’s most populous areas, a region of 27 million people. In addition to its extremely high density, Tokyo metropolitan is subject to seasonal floods and other hazards such as droughts and earthquakes. However, because it is a rich and industrialized country, Japan has the means – and the skills – to manage these risks using infrastructure such as dams, levees and underground floodways. There is also great emphasis placed on public awareness raising and disaster preparedness. The authorities have developed early warning systems that rely on the Internet, Geographic Information Systems (GIS) and hazard mapping, and there are shelters where people can take refuge. Such continuous efforts have ensured that one of the world’s largest economic developments has been able to safeguard its population in the high-risks region. Other concerns include a degraded natural environment and pollution of groundwater, and many efforts, such as river restoration works, are being implemented with wide public participation.

Risk reduction and the mitigation of water-related disasters is considered as one of Japan’s main challenges. Accordingly, the Government has revised and amended disaster-related legislation, and continues to strive to overcome current and future water challenges.

To enable comprehensive future assessment, indicators have been established relating to the following six criteria: relevance, cost, comprehensibility, clarity, continuity and social benefit. It is recommended that such indicators be created for future assessment of all related projects.

Lessons for Replication

To be successful water-related decisions, actions and policies to reduce the risk of water-related disasters must consider the needs of modern life and integrate them into a wellfunctioning healthy ecosystem, so as to increase public prosperity and long-term environmental sustainability simultaneously.

Structural and non-structural flood control measures alone were insufficient in preventing widespread flood damage as the 2004 floods showcased. Disaster-related legislation had to be amended and changed for significant results. Activities that aimed to raise public awareness were also crucial.

Testimonies and Stakeholder Perceptions


The Experience at a Glance

Implementing Agency(ies)


See also

Additional case studies in Japan
  1. Risk Reduction and mitigation of water-related diseases, Japan/Map

External Resources

Case Study summary

Full case study report


 Japan.pdf  Japanfull.pdf

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