At the Shonai River around Nagoya, Japan, several flood related problems occur. These problems occur at different locations, each with its own problems or limitations. The desired safety level as requested by the government is that the river should be able to have a discharge which has a probability of failure of once in 200 years. At many locations, the current probability of failure is lower than once in 50 years. This has led to the following research question:
How can the discharge capacity of the Shonai River be improved to modern standards?
The report has been divided in four phases. The first phase is used to formulate the final research question. This phase focuses on which part of the Nagoya urban area is most prone to flooding. Three kinds of flooding were examined: by peak river discharge, impact by tsunamis and impact by storm surge. The area is already well protected against tsunamis due to the natural shape of the bay Nagoya is situated to. The coastline is well protected against storm surges in the second half of the 20th century. At the river banks however, flood safety is still below the desired level. In the area, risks are relatively high along the Shonai River. Therefore, it has been decided to focus on the threats around the Shonai River. As already described above, there are several locations along the river with safety risks.
In phase 2, several locations and solutions are described to increase the capacity of the river. One major problem is the bottleneck around the Biwajima bridges, where four bridges are narrowing the river. At this location, the desired safety level of a flood discharge occurring once in 200 years is still far away. After discussion with officials of the Shonai River office, it was found that this problem was most urgent. Therefore, it had been chosen to elaborate further on this option in phase 3 and 4.
In phase 3, several options are described to remove the bottleneck. The first option is to replace the bridges with more clearance and larger spans. The second option is to remove the bridges and replace them for tunnels. The third and last option is to construct a bypass along the river, with flow through a tunnel. The Shonai River office is already working on a plan to raise the bridges, thus the first option has been dropped. The second option required large amounts of space for the tunnels. Therefore, it had been decided to work out the third option of creating a bypass. To minimise the impact of such a bypass and its construction on the surrounding area, the decision is made to construct a bored tunnel.
In the fourth and final phase the solution is verified. This is done concerning structural, hydraulic and construction method aspects. All the aspects are found to be possible. In hydraulic aspect, it is found that the required discharge capacity to reach a once in 200 year safety level equals 4250 m3/s. The current discharge capacity is found to be 2850 m3/s. Therefore, the bypass should have a minimal discharge capacity of 1400 m3/s. When using a bored tunnel, it is found that two tunnels with the maximal internal diameter of 16 m should be applied. On top of that, because the concrete surface is to rough in the best circumstances, it was decided to apply an epoxy layer on the surface. Using such a layer, it is found that the total discharge capacity becomes 1490m3/s.
The construction of the tunnel would take 137 weeks to complete. The total costs are estimated to be in the order of ¥60,000,000,000, or 60 billion yen, equivalent to 463 million euro. In comparison, the plan of the river office to replace the bridges would cost ¥68,400,000,000. It can be concluded that this option is a reasonable alternative for the current plan.