To protect the lives and livelihoods of Tokyo residents from floods due to typhoons or torrential rains, the Bureau has been carrying out improvements on small and medium-sized rivers with the aim of withstanding rainfall up to 50 mm per hour, in areas including urban areas, where people and assets are concentrated.
In recent years, however, heavy rains and downpours exceeding that amount are increasing, causing flood damages.
In response to this, the rainfall criteria were raised to 75 mm per hour for the Kandagawa, Shakujiigawa and other rivers in the ward area and 65 mm per hour for the Nogawa and other rivers in the Tama area. These are the levels of rainfall with an annual exceedance probability of 5 percent. Based on the new criteria, the Bureau is stepping up its flood control measures according to the priority of each of its projects.
To deal with rainfall up to 50 mm, the Bureau basically seeks to release flood waters safely by carrying out river channel improvements. It also takes other approaches, such as constructing regulating reservoirs, to implement effective measures depending on the situation of each area.
River Channel Improvement
River channel improvements, such as channel widening and deepening, are being implemented on the length of 324 km of 46 rivers in Tokyo, including the Kandagawa, Shakujiigawa and Shirakogawa rivers flowing through the ward area, as well as the Karaborigawa, Tsurumigawa and Yajigawa rivers in the Tama area.
In addition to flood control purposes, the river channel improvements are also designed to make rivers more accessible to people and more favorable to plants and animals. For example, some trails for maintenance are intended to also accommodate recreationists and allow them to enjoy lush greenery, and, for rivers with sufficient space, revetments are constructed to slope gently.
(Kandagawa River in Suginami Ward)
(Misawagawa River in Inagi City)
Constructing Regulating Reservoirs and Diversion Channels
In areas where there are many buildings and houses along a river, making it particularly time consuming to carry out channel widening or other forms of channel improvements, the Bureau has been constructing regulating reservoirs to hold excess flood waters and diversion channels to divert a portion of flood waters, so it can quickly enhance the areas’ safety against flood damages.
As of the end of fiscal 2013, 25 regulating reservoirs for 11 rivers had been completed, with a total capacity of some 2,000,000 cubic meters (including those that were only partially in use), and eight diversion channels for five rivers had been built, with a total length of approximately 12 km.
Of those structures, the Kandagawa River Ring Road No. 7 Underground Regulating Reservoir made a particularly significant contribution. Completed in March 2008, it is a 4.5 km-long tunnel with inner diameter of 12.5 meters located beneath Ring Road No. 7, and can store 540,000 cubic meters of overflow from the Kandagawa, Zenpukujigawa and Myoshojigawa rivers. When torrential rains hit the area in September 2013, the reservoir held a record 540,000 cubic meters of water, successfully mitigating flood damages.
Currently, five regulating reservoirs are under construction, including ones for the Shirakogawa River and Furukawa River.
As for rainfall exceeding 50 mm per hour, the Bureau, in principle, controls floods with regulating reservoirs. It will proceed with the construction of new regulating reservoirs, starting with the eight high priority areas such as those along the Kandagawa and Nogawa rivers. The plan includes a regional regulating reservoir, which consists of interconnected reservoirs that can mutually handle runoff from the others.
As for areas along the Kandagawa, Shakujiigawa and Shirakogawa rivers, a regional reservoir project is now under consideration, which would interconnect the Kandagawa River Ring Road No.7 Underground Regulating Reservoir and Shirakogawa River Underground Regulating Reservoir and enable both reservoirs to mutually handle overflow from those rivers.