Kawasaki City begins operation of the Gotanda River Spillway to reduce flood damage downstream due to heavy rain; ceremony to commemorate completion on 27th: Tokyo Shimbun TOKYO Web

The Gotanda River Discharge Channel, a large-scale flood control project that Kawasaki City began constructing in fiscal 1992, began operation at the end of March. If the water level rises due to heavy rain, the water will be discharged directly into the Tama River through an underground tunnel, thereby reducing flood damage downstream. A ceremony to commemorate the completion will be held on the 27th at Inada Tamagawa Park in Tama Ward. (Koko Hojo)

The Gotanda River has its source in Hosoyama, Aso Ward, and is approximately 4.7 kilometers long. It joins the Nikaryo Honkawa River near Mukogaoka Yuen Station on the Odakyu Line in Tama Ward. The area has often caused flood damage in the past, and local residents have requested early flood control measures.

When the water level of the Gotanda River exceeds 2.7 meters, the main river gate is closed at the diversion section built in Ikuta, Tama Ward, and up to 150 tons of river water per second is taken into the spillway. The water will flow into a 2,025-meter tunnel built using the shield method approximately 50 meters underground. Approximately 130,000 tons of water accumulates inside the tunnel and flows into the Tama River from the outlet in Noborito Shinmachi, the ward.

The project cost of approximately 30 billion yen will be borne by the national, prefectural and city governments. Construction began in 1997 and was scheduled to be completed in about 10 years, but the project took a long time due to difficulties in acquiring land and interruptions due to tunnel bolts breaking due to a shield machine operation error.

In December 1997, it was discovered that the city had fraudulently received subsidies from the national and prefectural governments by falsely claiming that it had acquired unpurchased land. Twenty-eight senior officials, including then Mayor Kiyoshi Takahashi, were punished. In 2009, bidding for construction of a shaft at the outlet was canceled due to suspicions of bid rigging. Last August, a male worker who was working on a bank protection project died due to rising waters caused by rapid rain.

A rainfall of 90 millimeters per hour, which is expected to occur once every 30 years, was expected to flood 381 hectares in Tama Ward. The city’s River Division says, “With the start of operation of the spillway, the area expected to be flooded will decrease by 225 hectares.”

For many years, the city has explained that the effect of constructing the spillway is that the Gotanda River and Nikaryo Honkawa River will be able to withstand rainfall of up to 90 mm per hour. It is necessary to conduct a maintenance project to connect the former Misawa River, which flows into the upper reaches of the Keryo Honkawa River, to the Misawa River, and to take measures to address the eight bridges that obstruct the flow downstream of the Nikeryo Honkawa River.

Currently, work is underway to replace the Ikuta No. 8 Bridge, which was destroyed as part of the diversion work, and to work on the banks of the Gotanda River upstream and downstream, and the entire spillway development project is expected to be completed by the end of this year. The city is also considering a project to improve the Old Misawa River this year, with the aim of responding to the 50mm of rainfall per hour that is expected to occur once every three years.