An environmentally friendly flood control dam that does not store water

Asakawa Dam Project, Nagano City

The Asakawa Dam was constructed on the Asakawa River of the Shinano River system, a Class A river with a length of about 20 kilometers originating from Mt. Iizuna in northern Nagano City. The Asakawa River flows through a newly developed urban area of Nagano City, joining with the Chikuma River. Although it is a medium to small sized river, the flow is steep, and floods have been frequent. For this reason, the dam was completed in 2017 as a dedicated flood control dam operated by Nagano Prefecture. It is a 53-meter-high flow-through concrete gravity dam.

　

However, there were twists and turns in the events leading up to the construction of the Asakawa Dam. In February 2001, the then-governor of Nagano Prefecture, Yasuo Tanaka, launched a declaration of “No More Dams.” Subsequently, in June 2002, the prefectural assembly announced that they would be suspending construction of the Asakawa Dam. They conducted a study that targeted not only the Asakawa Dam, but also the Kiyokawa Flood Control Dam, the Kakuma Dam, and the Kurosawa Dam in the Shinano River system. All of them—except the Asakawa Dam—have been suspended.

　

Although the administration studied flood control measures for the Asakawa River that do not rely on dams, including the use of in the river channel, they ultimately changed direction and put forth a policy that included the construction of dams as a flood control measure. After that, a flood control plan that combined the use of a dedicated flood control dam and river improvements was adopted, and construction of the Asakawa Dam began at last in May 2010. This time, CTI Engineering was responsible for a wide range of fields, including geological analysis of the surrounding area, reservoir landslide countermeasures, environmental surveys, schematic and detailed designs of the dam itself, and construction planning.

　

Since water is not normally stored in the dam, it does not interfere with the flow of sediment or fish runs, and there is no deterioration of water quality due to . The opening in the dam, called the “principal spillway,” is equipped with a stair-like fish ladder for fish to run up. For this reason, the Asakawa Dam is sometimes referred to as a flow-through dam or “perforated dam.” On the other hand, in the event of a flood, it will start storing water and prevent damage downstream. Because of these features, the Asakawa Dam is said to be an “environmentally friendly dam.”