The Ministry of Land, Infrastructure, Transport and Tourism has planned a large-scale tunnel spillway with internal diameter of 11.5m and total length of approximately 450m in order to drastically strengthen the flood discharge capacity of the Kanogawa Dam. This tunnel spillway will have the following four unique technical features. Firstly, because the gate and operating room will be installed near the outlet of the tunnel spillway, the tunnel will be a pressure tunnel acted on throughout its length by maximum external water pressure of about 0.9MPadependent on the ground water level of the natural ground and by maximum internal water pressure of about 0.4MPa dependent on the reservoir water level. Secondly, its maximum discharge flow rate will be 1,000m~3/s, and its maximum flow velocity will be high velocity flow exceeding 10m/s. Thirdly, in order to minimize the impact on the downstream river course, the energy dissipater work will be a contractive type stepped energy dissipater. Fourthly, in order to install an inflow channel and vertical shaft inside the reservoir of the Kanogawa Dam and to excavate the tunnel in the natural rock on the right bank of the reservoir, measures to prevent leaking from the reservoir will be important.
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