On June 12 and 13, 2015, I attended a two-day event, organized by Ecology and Civil Engineering Society, in Kumamoto City. A symposium was held on the afternoon of the 12th and an all-day field trip was arranged for the 13th, for discussing sedimentation in water flows from the river to the sea. The presentations focused on many initiatives on Kuma River, which has received international attention for being the first river in Japan with a major dam removal project — the Arase Dam.
The Friday symposium was kicked off by Sumi Tetsuya (Kyoto University), who presented on the problem of dams and sedimentation. He talked about the problems caused by sedimentation, such as changes to the downstream environment, and some examples of solutions, such as bypass channels and dredging, in Japan and around the world. Professor Sumi focused on dam removal as one of these solutions to the problem of sedimentation, and discussed the change in sedimentation, water quality, and environmental changes after a dam is removed.
This was followed by Horiuchi Shinji, who formerly headed the Arase Dam removal team of Kumamoto Prefecture. He explained the history of Arase Dam, from its construction in the immediate postwar period when the hydroelectric plant supplied power to the area, to the process of building general consensus to dismantle the dam. With great detail he explained the six-stage dismantling process, still underway, which was planned through talks with the sweetfish (ayu) fishing cooperatives so as to halt any work during the fish’s spawning season. He also discussed the environmental monitoring efforts, whose results thus far indicate that the dam removal is having, overall, a positive effect on the environment, such as increased vegetation, more variety of sediment size, and the formation of rapids and pools in the river.
Asazaki Katsuyuki presented on the effort to release sediment built up behind dams on Mimikawa River in Oita Prefecture. As the vice head of the Mimikawa River Hydropower Management Office of Kyushu Electric, Asazaki explained that environmental concerns as well as safety concerns were the reasons for adding water gates to two dams so as to prevent sediment build up. He presented on results of environmental monitoring, which showed evidence of growing variability in habitats for various river creatures, some of which are registered as endangered species.
The next presenter, Onikura Norio (Kyushu University), presented on his finding from an on-going research project to measure the effect of the sedimentation flowing due to the Arase Dam removal in the delta of the Kuma River. A biologist who specializes in brackish and freshwaters of river estuaries, he is part of a “working group” of biologists and civil engineers who are overseeing a few nature restoration projects along Kuma River. He noted that the dam removal seems to have resulted in increased variety of sedimentation particle size and elevation — two key factors in providing a wider range of habitats the area.
The next presentation was by Takigawa Kiyoshi a professor emeritus of Kumamoto University who has headed and is still involved in environmental monitoring projects for Ariake and Yatsushiro Seas. For this occasion he gave an overview of the two seas and then focused on the Yatsushiro Sea, into which Kuma River flows. Prof. Takigawa explained how these seas are environmentally degrading because not enough water is flowing into the sea. Coastal land reclamation have further deteriorated the tidal mudflats for many creatures in these seas. He explained a few of the initiatives that are in the works to restore these valuable marine environments.
Lastly Shimatani Yukihiro (Kyushu University), who presented on the role of sedimentation management in river restoration efforts. He foregrounded his discussion with a general overview of how different components of the river provide habitat for living creatures. Then he discussed the specific effects of dams on sedimentation in rivers, such as the reduction of larger particles downstream and the lowering of the river bed, for example. He then introduced the Hachinoji Wier Restoration Project on Kuma River, which incorporate the revival of a river engineering technique from the early Tokugawa period.
The final event was a panel discussion in which the speakers summarized the presentations and answered a few questions from the audience. The discussion moved to the difficulties in evaluating these nature restoration projects because of the lack of concrete targets when it comes to recreating a suitable habitat for wildlife. Another issue raised was the general sense that this association has done more work in river restoration compared to the sea, perhaps because of the complexity of the ocean compared to that of rivers.
The second day was a field trip to Kuma River. About 80 participants hopped on two buses from Kumamoto City. It rained most of the time but the trip participants managed to visit the major restoration projects along the river.
The group stopped by the following spots:
- Yōhai Weir – Hachinoji Weir
- Arase Dam Removal Site
- Nakakita Area Reed Field Restoration Site
- Minami River Tidal Flat – Biosurvey of fish and crabs
- Yatsushiro Port Seashore Restoration Experiment Site
At each site, we were given explanation of the project by the key members involved in these projects. We unfortunately couldn’t see much of the Hachinoji Weir, because of the rain and the rise in the water level. But we were able to walk on the remaining parts of the Arase Dam removal site — parts that are scheduled to be dismantled later this year. The tidal flats areas downstream were fascinating, as some of us were able to walk far onto the flats. Graduate students, who was spending the morning in the mud, showed us many species of fish and crab that are on the IUCN endangered species list. It was a great opportunity to hear about these projects from those directly involved in them.
For lunch we went to a restaurant that specializes local cuisine, including boar meat and shaku ayu — large sweetfish that grow to be as large as 30cm. During lunch we heard from Shoko Tsuru, who heads a local NGO involved in the environmental restoration of Kuma River.
All in all, the two-day event provided a unique perspective on the state of river engineering in Japan today, especially as it pertains to the work of nature restoration.