Water Resources Agency/Flooding Test at Nanma Dam (Kanuma City, Tochigi Prefecture), a project completed by Taisei Corporation in March 2025
November 11, 2024
The steel gate has been closed and the test flooding has begun (Photo taken on the 8th)
Overview of the dam from upstream (courtesy of the Japan Water Resources Agency)
Test flooding began on the 8th at Nanma Dam, built by the Japan Water Agency in Kanuma City, Tochigi Prefecture. The reservoir has a total water storage capacity of about 51 million cubic meters, and functions to supplement water in the event of a drought in the surrounding rivers and regulate flooding downstream. Taisei Corporation will construct the dam itself, aiming to complete the work by March 2025. Test flooding is expected to continue until fiscal 2026, with the gates closed to prevent water from the Tone River and Nanma River systems from flowing into the dam.
The Nanma Dam is being built as part of the Omokawa Development Project and has a dike height of 86.5 meters and a dike volume of approximately 2.4 million cubic meters. To shorten the construction period, a concrete rockfill dam (CFRD) will be used to reduce the inflow of water into the Minami River during rains. At the same time, it will contribute to drought countermeasures by sharing water with the Minami River and the surrounding Oashi and Kuro Rivers.
During the flood test, a steel gate (4.2 meters wide and 5.1 meters high) was closed to the temporary drainage tunnel built to divert the flow of the Nanma River into the dam. The safety of the dam will be confirmed by filling it with water to the highest flood level, which is about 80 meters above the bottom of the dam. The test period is expected to last until fiscal 2026. Related construction work, including pipes to divert water from the Kuro River and the Oashi River, is scheduled to be completed in March 2027.
About 100 people attended the meeting that day, including staff from the Japan Water Resources Agency and Taisei Corporation. After about 10 minutes of closing the gate, the participants gave three cheers in unison to celebrate the successful completion of the work. Tomohisa Hasemi, director of the organization's River Development and Construction Office, expressed his gratitude and said, "We were able to receive the test flood thanks to the efforts of everyone involved, including water users and the Taisei Corporation, which was responsible for the construction. .''
The Omokawa development project began in 1969. The total cost of the project is expected to be around 210 billion yen. About 80 households (300 people) will have to be relocated due to the dam's construction. The Nanma Dam is the last new dam the agency will build.
