Miwa Dam (2)
I have just walked over the embankment of the Miwa Dam to the west end.
At the end of the dam is a tunnel, which is off-limits.
So I will turn back here.
The embankment of Miwa Dam. It is 367 meters long and 69.1 meters high.
Looking down, the Miwa Power Station can be seen.
In the previous article, I mentioned that the dam was completed in 1959, but the power station has been in operation since February 1958.
But now, the power station is undergoing renovations, and power generation has been suspended since February 2022.
They plan to reopen in April 2025. After the renovation, a space for visitors will be provided in the building.
Near the dam gate.
Looking at the power plant again.
Although not shown in this photo, on the other side of the bridge, on the left bank, there is what looks like the drainage outlet I mentioned in the previous article.
(Re-posting the previous photo)
I finished my visit to the Miwa Dam without knowing the role of this drain-like structure, but I found out its function later.
Sediment accumulates in dams. The Miwa Dam has been storing more sediment than the planned sediment volume in the dam lake since it started operation. As sediment accumulates, the flood control and water utilization functions of the dam are affected.
Therefore, it was decided to build a sediment bypass facility to prevent sediment accumulation.
(This map has north on the left.)
The sediment bypass tunnel, diversion weir, sediment stock yard, and sediment trap weir facilities were built as part of the sediment control work.
Rough sediment is stopped by the sediment trap weir. Sediment that exceeds the dam during flooding is stopped by a diversion weir and guided into the sediment bypass tunnel.
The fine sediment that accumulates in the dam lake is dredged when the water level is not high and transferred to the sediment stock yard. The sediment stock yard is 220 m long, 40.5 m wide, and 5.5 m high, and can hold 30,000 cubic meters of sediment.
Sediment accumulated in the stock yard is discharged downstream of the dam in a bypass tunnel along with water during flooding.
This is said to reduce sedimentation in the dam lake.
The sediment trap weir, diversion weir, and sediment bypass tunnel were completed in 2005.
The stock yard was completed in 2021.
What I thought was a drainage outlet was the sediment bypass tunnel outlet.
Unfortunately, I did not visit the stock yard that day because I did not know about the sediment control work. I will go back to check it out at the next opportunity.
I will link to the Mibugawa Comprehensive Development Construction Office’s website, where they have posted the test operation of the stockyard facility.
'Test Operation of Stockyard Facility Underway' (June 2, 2023) (This is a pdf file written in Japanese.)
Note that the Mibugawa Comprehensive Development Construction Office was closed at the end of March 2024, as the redevelopment work has come to a close. The building will be used as a branch office of the Tenryu River Upper River Office.
[Reference]
Mibugawa River Comprehensive Development Construction Office, Chubu Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism, website (written in Japanese)