Setagawa weir
For the second part of the excursion at the Kyoto municipality, we went to the Setagawa dam in the Seta river, about 20 km south-east of Kyoto. First, we were shown two videos about the water system in the area of the dam. Then we could visit the dam itself, followed by a short visit in the museum.
The Biwa lake is with a capacity of 27.5 billion m3 the largest lake of Japan, currently serving about 13 million people in the surroundings of Kyoto and Osaka. The whole lake has only one outlet: the Seta river, which becomes the Yoda river further downstream and then ends in the Osaka bay. In the past, the villages around the lake have seen both the lake’s advantages in terms of fishery and good conditions for rice fields, but on the other hand the disadvantages in terms of floods. After a long period of negotiations about water use, the national and local governments signed an agreement in 1972 about who could use water and to what extent. This agreement consists of three pillars: flood control, water utilization and conservation.
For flood safety and control, the Setagawa dam was constructed about 50 years ago. The old dam had a capacity of around 50 m3/s, the current dam increased the capacity to 800 m3/s. In the future, the capacity should be further increased to 1500 m3/s. This will be done by dredging and redesigning the total system. With the Setagawa dam, together with the Amagase dam (which will be visited during the next excursion) and the Yoda river weir, the water system of lake Biwa and the Yodo river is controlled in order to prevent flooding and to maintain the (drink)water supply in the area throughout the year. The aim is to keep the water level within 1.5 m from the standard level.
The museum clearly shows the dam’s function in a small-scale model. There were two basins, the first one has an undeep river as outlet, resembling the original situation. The second one has a deeper river combined with a dam. Then some heavy rainfall is simulated in both areas. In the original situation, the villages around the lake are all flooding. However, by lowering the dam’s height in the new situation, the outflow rate increased which prevents the area from flooding. In a dryer period, the higher dam decreases the outflow again, which makes a proper water level control throughout the year possible.
By gaining some knowledge about the background of the dam and its surroundings beforehand, the site visit was very interesting. All participants did immediately understand what could be seen outside. However, parts of the shown videos were rather tourist advertisements for the lake than an informative video. Afterwards there were many opportunities to ask questions, which did compensate this lack of information. In the Netherlands, we don’t have a comparable lake to the Biwa lake. I do however think that the policy of the water use, for both nature and society, is a good example of how the Dutch system should also work.