Hirasakuhara Power Plant Water Tank

Hirasakubara Power Plant Head Tank: A 1928 Concrete Industrial Heritage in the Yoshii River Headwaters

Tucked deep into the wooded mountains of northern Okayama Prefecture, where the Yoshii River begins its long journey to the Seto Inland Sea, stands a quiet rectangular concrete structure that has been performing the same vital task for nearly a century. The Hirasakubara Power Plant Head Tank (平作原発電所水槽, Heisakubara Hatsudensho Suisou) is a reinforced concrete reservoir built in 1928 to regulate the flow of water that drives one of Chugoku Electric Power Company's earliest hydroelectric stations. Registered as a National Tangible Cultural Property in 2006, it is one of ten heritage components that together tell the story of how electricity first reached the towns and villages of central western Japan.

For visitors interested in industrial heritage, early Showa-era civil engineering, or simply the beauty of the forested Chugoku Mountains, this modest but historically significant structure is a quiet highlight of the Kamisaibara area of Kagamino Town.

Historical Background

In the mid-1920s, electricity demand in Okayama Prefecture was rising sharply as factories expanded and households began to convert from oil lamps to electric lighting. To meet that demand, engineers turned to the upper reaches of the Yoshii River, the largest river system in eastern Okayama, where steep terrain and abundant rainfall offered ideal conditions for hydroelectric generation.

Construction of the Hirasakubara Power Plant began with the building of the Onbara Dam (恩原ダム), located near the border with Tottori Prefecture, where the river could be impounded to form a stable reservoir. The plant began operations in 1928 (Showa 3), making it the second-oldest dam-and-waterway hydroelectric facility still operating within Chugoku Electric Power Company's portfolio, after the Nishikigawa Daiichi Power Plant in Yamaguchi Prefecture, which had started service in 1924.

What makes the Hirasakubara project especially noteworthy is how ambitious it was for its mountainous setting. The remote site made transportation of construction materials extremely difficult, so the designers chose a buttress-style reinforced concrete dam for the Onbara Reservoir. This design used considerably less concrete than a conventional gravity dam, reducing the volume of cement that had to be hauled up into the mountains, lowering costs, and shortening the construction schedule. The plant's broader system, including the head tank described here, was built with the same pragmatic, engineering-led mindset.

What Is a Head Tank, and Why Does This One Matter?

In a run-of-river or dam-and-waterway hydroelectric system, water travels from the intake or reservoir through a long, gently sloping waterway before plunging down a steep penstock to the turbines in the powerhouse far below. Between the waterway and the penstock sits a critical regulator known as a head tank (上水槽 or ヘッドタンク). Its job is to absorb sudden changes in flow when the power plant's output rises or falls, to allow any excess water to overflow safely, and to settle out any remaining sediment before the water enters the high-pressure pipe.

The Hirasakubara Head Tank performs all of these roles. Located approximately 2.3 kilometres downstream of the main waterway sedimentation basin and about 330 metres upstream from the powerhouse, the tank is a roughly rectangular reinforced concrete structure measuring approximately 25 metres on its long side and 10 metres on its short side, with a total area of about 355 square metres. A discharge gate for flushing accumulated sand sits at the centre of the north wall, which runs parallel to the spillway channel that carries away surplus water.

What might look like a simple concrete basin is, in fact, the result of careful 1920s civil engineering, sized precisely to match the flow capacity of the Hirasakubara plant and to keep the downstream turbines running smoothly through demand swings.

Why It Was Designated a Cultural Property

On 29 November 2006, the Hirasakubara Head Tank was registered as a National Tangible Cultural Property (Building) by the Agency for Cultural Affairs, together with nine other components of the Hirasakubara and neighbouring Okutsu and Kamisaibara hydroelectric systems. The collective designation recognises this group of structures as a rare and largely intact ensemble of early-Showa-era hydroelectric civil engineering.

Several specific values underpin the designation. First, the head tank dates from 1928 and is built almost entirely of reinforced concrete using high-quality cement produced by the Asano and Onoda cement companies, both pioneers of Japan's domestic cement industry. Second, the tank is one piece of a fully preserved water-management chain that begins at remote intake weirs on small tributaries, passes through sedimentation basins and the Onbara Reservoir with its rare buttress dam, and finally arrives at this head tank before the water drops to the powerhouse. Almost the entire system remains in active service, which makes it especially valuable for understanding how 1920s engineers approached large-scale hydroelectric projects.

The structure also contributes to the historic landscape of the upper Yoshii River, harmonising visually with the older Onbara Dam upstream and the powerhouse building downstream, both of which are also registered cultural properties. Together they form a coherent industrial landscape that has shaped the upper reaches of Kagamino Town for almost a century.

Architectural and Engineering Highlights

Although the head tank is functional rather than ornamental, careful observation reveals the qualities that earned it cultural status.

Honest Concrete Construction

The walls are built of cast-in-place reinforced concrete, with no decorative cladding. Yet nearly a century after construction, the concrete remains in remarkable condition, a testament to the use of well-sourced cement from Asano and Onoda, both of which were prized for their consistency in the early twentieth century. The aging of the surfaces, with light staining and moss in places, gives the structure a quiet patina that blends naturally with the surrounding forest.

Rectangular Plan and Functional Geometry

The basin is essentially a long rectangle, roughly 25 by 10 metres. This is not an arbitrary shape: it provides the dwell time required for the last fine particles to settle out of the water before it enters the penstock, while also giving the operating staff enough room to manage gates and screens. A sand discharge gate in the middle of the north wall allows accumulated sediment to be flushed periodically into the parallel spillway.

Integration With Terrain

Like most components of the Hirasakubara system, the head tank is set carefully into the slope. Because flat ground is scarce in this part of the Chugoku Mountains, the designers used the topography itself as part of the structure, with retaining walls and the spillway carefully aligned to the contour of the hillside. The result is a structure that feels at home in its forested setting.

Part of a Living System

Unlike many industrial monuments, the Hirasakubara Head Tank is not a museum exhibit. It is still doing its job, holding water that, moments later, will fall through the penstock and turn turbines down at the powerhouse. The plant is normally unmanned and is monitored remotely from the Tsuyama Control Centre, with staff visiting roughly twice a month for routine inspection.

Visiting the Site

A few important points should be kept in mind by anyone considering a visit.

The Hirasakubara Power Plant Head Tank is the working property of Chugoku Electric Power Company and is not open to the public. There are no on-site tours, visitor facilities, or interpretive signs aimed at general tourists. The plant area is restricted, and visitors should not attempt to enter operational zones, climb fences, or approach the water surface of the tank.

However, the structure can be glimpsed from public roads and trails in the Kamisaibara area, and the surrounding cultural landscape, including the Onbara Dam upstream and the powerhouse building downstream, can be appreciated from outside the fence line. The best approach for travelers interested in the heritage is to combine a drive along the upper Yoshii River valley with a visit to the Onbara Highland recreation area and the Okutsu Onsen hot-spring district, which together provide a fuller picture of how this remote corner of Okayama has been shaped by water, electricity, and tourism.

For photography enthusiasts, the autumn colours from late October to mid-November are especially rewarding, as are the fresh greens of late May and June. Snow can fall heavily here in winter, and roads may be closed or require chains.

What to See Nearby

The Kamisaibara and Okutsu districts of Kagamino Town offer a rich combination of natural beauty, hot springs, and additional industrial heritage that complement a visit to the Hirasakubara facility.

Onbara Dam and Onbara Highland

Upstream from the head tank, the Onbara Dam impounds the reservoir that feeds the entire Hirasakubara system. As a rare example of a buttress-style reinforced concrete dam, it has been designated both as a National Tangible Cultural Property and as a modern civil engineering heritage site by the Ministry of Land, Infrastructure, Transport and Tourism. Around the reservoir, the Onbara Highland sits at about 700 metres elevation on the border with Tottori, with birch and larch forests, a campsite, and good views of Mount Mikuni mirrored in the lake's surface.

Okutsu Gorge

About a half-hour drive downstream, the Okutsu Gorge (奥津渓) follows the Yoshii River for roughly three kilometres of granite cliffs, pools, and rapids. Designated as a National Place of Scenic Beauty in 1932, it is one of Okayama's most celebrated autumn destinations, with the November Momiji Festival drawing visitors from across the country. The gorge is also home to a remarkable group of pothole formations carved into the granite riverbed over hundreds of thousands of years.

Okutsu Onsen and the Kagamino Three Hot Springs

Okutsu Onsen, one of the so-called Mimasaka Three Hot Springs alongside Yunogo and Yubara, is a small hot-spring village with several historic inns. Together with Kamisaibara Onsen and Notoro Onsen, all three of Kagamino Town's hot springs are alkaline simple springs known for their gentle effect on the skin.

Okayama Prefectural Forest Park

Spanning more than 300 hectares across the Okutsu and Kamisaibara districts, the Okayama Prefectural Forest Park is a varied wilderness of larch plantations, wetland flora, and alpine plants, with well-maintained walking paths that are particularly rewarding in late summer and autumn.