Asian Network of Major Cities 21

Flood and Storm Surge Control MeasuresTokyo

■ Overview

• 1) Urbanized regions and inner-city areas with considerable population and property concentrations have experienced decreased water retention and flood prevention functions, and have sustained water damage during times of heavy rainfall due to increased runoff to rivers.
  Therefore, the Tokyo Metropolitan Government is engaged in river improvement projects so as to enable the rivers to safely drain a flood of an hourly rainfall of 50mm by expanding the river width or digging down the riverbed. As it takes a considerable amount of time to complete river width expansion for rivers flowing in highly urbanized areas, rivers or the portions of rivers that often suffer flood damage need some makeshift measures to quickly ensure safety against flooding. Such actions currently underway include regulating reservoir to store part of the flood water or diversion channel to let down part of the flood.
  A representative development example is the Kanda River/Loop Road No.7 Underground Regulating Reservoir.
• 2) The eastern lowlands of the Tokyo Metropolitan Area are composed of weak ground, and the ground elevation of most regions is below sea level owing to land subsidence caused by post-Meiji period groundwater pumping. These areas are thus especially susceptible to damage caused by such natural disasters as storm surges and large earthquakes. Therefore, the Tokyo Metropolitan Area eastern lowlands are being bolstered against water damage through lowland river improvement projects, the representative example being the Super Levee Project.


Photo: Kanda River/Loop Road No.7 Underground Regulating Reservoir　
Photo: Sumida river Super Levee

　　

■ Period of Implementation

• 1) Kanda River/Loop Road No.7 Underground Regulating Reservoir: 1988-2008
• 2) Super Levee Project: 1985-

■ Features

• 1) Kanda River/Loop Road No.7 Underground Regulating Reservoir
  A tunnel (length: 4.5km; internal diameter: 12.5m) under Loop Road No. 7 installed to store 540,000m3 of flood water from Kanda River, Zenpukuji River, and Myoshoji River in order to quickly improve safety of the midstream reach of Kanda River, which is prone to flood damage.
• 2) Super Levee Project
  A super levee is an especially wide embankment built in cooperation with riverside urban redevelopment projects. It assists in effective land usage, the strengthening of earthquake countermeasures, the development and maintenance of a healthy river environment, and the improvement of the urban environment.

■ Outcomes / Achievements

• 1) Kanda River/Loop Road No.7 Underground Regulating Reservoir
  When Typhoon Melor (Typhoon No. 18) struck Tokyo in October 2009, a record 505,000m3 of water was stored, equal to 94% of the total storage capacity, thus preventing flood damage.
• 2) Super Levee Project
  Approximately 14.5km was completed by FY2009

■ Expenses

• ・Kanda River/Loop Road No.7 Underground Regulating Reservoir
  Approximately JPY100 billion

■ Inquiries

• Planning Section, River Department, Bureau of Construction,
  Tokyo Metropolitan Government
  (Telephone)+81-3-5320-5411
  (E-mail) S0000384@section.metro.tokyo.jp

■ Related links

• (Japanese only)
  http://www.kensetsu.metro.tokyo.jp/river.html