Author(s)

K. Sato, K. Masuhara, S. Mochida, T. Yamamoto, H. Gotoh & M. Takezawa

Abstract

The Tokyo Metropolitan Government (TMG) has conducted a variety of river projects to protect citizens and their property from damage due to flooding, storm surges, and landslides. Projects have also been undertaken to improve and create riversides that support nature, and to protect and restore the natural environment. The hardening of urban and inner city areas with high population densities and developed areas have relatively poor water retention and flood prevention properties. Consequently, these areas have sustained extensive water damage during times of heavy rainfall and flooding. Indeed, extensive flood damage was observed in downtown areas as recently as September 2005 and August 2008, demonstrating the need to implement additional flood mitigation measures to prevent flood damage in the metropolitan area. The number of rivers currently under the direct management of the TMG is 61 class A rivers and two class B rivers, which together measure 494.82 km. In addition, the city office of the TMG also controls 33 class A rivers and 13 class B rivers, which together equal 215.91 km. The main projects undertaken by the TMG is small river development, reinforce of lowland, improvement of the river environment for ecology, improvement of sediment disaster control facilities, development of software programs, river enlightenment activities and improved shoreline protection measures. In 2010, the total costs of river projects amounted to approximately JPY 65 billion. Small river projects are primarily concerned with the construction of several regulation reservoirs and river diversion channels. Conversely, the development of high tide breakwater structures and the reinforcement of rivers in lowland areas using super levees

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