Connecting Delta Cities

Tokyo

Tokyo: Climate change adaptation

Climate change adaptation

Apart from conventional measures like flood walls against river floods and ponds against storm water floods several innovative adaptive measures are used to cope with climate change. Examples of such measures are the super levee and multipurpose storage basin, impermeable pavements and a mobile evacuation system. The first two will be elaborated here.

Super levees

As climate change is projected to increase peak river discharge and peak frequencies, extreme floods are bound to occur on a more frequent basis in the future. The concept of a super levee is designed especially for extreme events in dense urban areas, such as Tokyo Metropolis. A super levee is a broad river embankment which can withstand even overflow, so that destruction can be prevented. Main difference to a conventional dike is the width; a super levee with a height of 10 meters will have a width of about 300 meters. A super levee is resistant to overflow, seepage and earthquakes. Super levee projects are always implemented in conjunction with urban redevelopment, and enable multifunctional structures which incorporate both the needs of flood control and the interests of the inhabitants. Super levees can be found along the Ara River and Sumida River in Tokyo. The former combines a very broad dike with a park and a small amount of high rise whereas the latter combines a broad dike – flood wall with a promenade and a large amount of high rise.

Multipurpose water storage facility

The Tsurumi multipurpose detention basin is designed to ensure a safety from a 150-year flood. The Tsurumi River is an urban river with a length of 42.5 km and a catchment area of 235 km2 in the central part of Tokyo Metropolis. In 40 years, the urbanized percentage of the basin increased from 20% to 80%. As storm water run-off accelerated due to the increase of paved areas, the peak discharge nearly doubled since the early 1960s.

To combat this problem, the Tokyo Metropolitan Government and the city of Yokohama produced a Master Plan for an overflow location with multifunctional facilities. The basin is excavated deeper than the surrounding grounds and has a storage capacity of 3.9 million m3. It contains a sports venue, a recreation area and a park for local residents. The International Stadium Yokohama, where 70.000 spectators watched the 2002 World Cup final, is the most prominent building in the basin. The primary roads inside the basin are constructed along embankments or are elevated. The Tsurumi multipurpose detention basis demonstrates that combining water storage and urban activities is a feasible solution in urban areas to cope with increasing river levels and increasing frequencies of peak discharges. It is therefore a promising solution to cope with climate change in flood endangered urbanized regions such as Tokyo metropolis.

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