The Safety Valve of Onbara Dam
The Heisakubara Power Station Onbara Reservoir Dam Spillway (平作原発電所恩原貯水池堰堤余水路) is a reinforced concrete spillway built in 1928 on the right bank of Onbara Dam in Kamisaibara, Kagamino Town, Okayama Prefecture, and registered as a Registered Tangible Cultural Property on 29 November 2006.
It runs 132 metres with a floor about 4 metres wide. When the reservoir is close to full, surplus water leaves at the right end of the crest and is carried down to the Onbara River below the dam, keeping the level in check without water pouring over the dam itself.
Onbara is a buttress dam, a light structure built of a thin wall and a row of supports, so a channel that leads overflow away from the dam body and down the hillside is essential.
Two Gentle Bends to Tame the Water
The Heisakubara Power Station Onbara Reservoir Dam Spillway was registered as a structure that contributes to the historic scenery of the country.
According to the Agency for Cultural Affairs, its concrete walls rise from a slope of roughly one in four that has been reinforced with stone retaining walls. The gradient is gentle enough for the water to be walked down the hillside instead of dropped.
The clever part is in plan. The channel bends gently near its top and again near its foot, turning the water as it gathers speed and bleeding off its energy. A straight chute would have fired the flow into the river at full speed; two curves calm it first.
Concrete channel walls working with stone retaining walls is a pairing that recurs throughout Heisakubara Power Station.
What to Look For
Trace the line of the channel. It leaves the right end of the crest and bends twice on its way down, adjusting its course over 132 metres. From above, you can see how the curves follow the lie of the land.
The entrance is on the right-bank side of Onbara Dam. A visit reported by the regional site Okayama Style confirms the spillway can be seen on that side, so head there first when you come to see the dam.
After snowmelt or heavy rain, water may run down a channel that is usually dry. Catch it in use and the purpose of the structure becomes obvious.
Visiting the Onbara Reservoir Dam Spillway
The Heisakubara Power Station Onbara Reservoir Dam Spillway belongs to a working power plant run by Chugoku Electric Power, which says the station's exteriors can be viewed at any time. The channel itself, the road down to the downstream side and any gated or fenced areas are closed.
Water can surge through a spillway without warning during releases, so stay away from its edges. The area lies on the Onbara Plateau at about 700 metres, where winter snow can exceed 1 metre.
There is no admission fee. Chugoku Electric's page gives no restriction on photographing exteriors, but closed areas cannot be entered for pictures.
- 📍 Plan Your Visit to Okayama
- Find tours, activities & transport passes near Heisakubara Power Station Onbara Reservoir Dam Spillway
- [ Kurashiki Bikan District & Korakuen Garden Tours ]
Q&A
- What is the Heisakubara Power Station Onbara Reservoir Dam Spillway?
- It is a reinforced concrete channel built in 1928 to carry surplus water from Onbara Dam's right bank into the Onbara River. It is 132 metres long and became a Registered Tangible Cultural Property in 2006.
- What is distinctive about the Heisakubara Power Station Onbara Reservoir Dam Spillway?
- Its concrete walls sit on a stone-reinforced slope of about one in four, and the channel bends gently at its top and bottom to slow the water.
- Where can I see the Heisakubara Power Station Onbara Reservoir Dam Spillway?
- Its entrance is on the right-bank side of Onbara Dam. The road to the downstream side is closed, so view it from accessible places.
- Is there a fee to see the Heisakubara Power Station Onbara Reservoir Dam Spillway?
- No. Viewing the exterior is free, but the channel and operational areas are off limits.
- How large is the Heisakubara Power Station Onbara Reservoir Dam Spillway?
- It is 132 metres long, with a floor about 4 metres wide.
Basic Information
| Name | Heisakubara Power Station Onbara Reservoir Dam Spillway |
|---|---|
| Location | Onbarayama area, Kamisaibara, Kagamino Town, Tomata District, Okayama Prefecture |
| Designation | Registered Tangible Cultural Property (Structure) |
| Year Designated | Registered 2006/11/29 (Heisei 18) |
| Quantity | 1 site |
| Dimensions | Length 132 metres, floor width about 4 metres |
| Owner | Chugoku Electric Power Co., Inc. |
| Public Access | Exterior viewing only; operational areas closed |
References
- Agency for Cultural Affairs, Database of National Cultural Properties: Heisakubara Power Station Onbara Reservoir Dam Spillway
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00005990
- Cultural Heritage Online: Heisakubara Power Station Onbara Reservoir Dam Spillway (Japanese)
- https://bunka.nii.ac.jp/heritages/detail/152595
- Agency for Cultural Affairs, Database of National Cultural Properties: Heisakubara Power Station Onbara Reservoir Dam
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00005989
- Chugoku Electric Power: History of Heisakubara Power Station (Japanese)
- https://www.energia.co.jp/isan/heisakubara_history.html
- Chugoku Electric Power: Heisakubara Power Station, Historic Electric Power Heritage (Japanese)
- https://www.energia.co.jp/isan/heisakubara.html
- Okayama Style: Onbara Dam (Japanese)
- https://okayamastyle.com/onbaradam/
- Kagamino Town Tourism: Lake Onbara (Japanese)
- https://www.kagamino.holiday/spot/entry-322.html
Last verified: 2026.09.25
