Hirasakuhara Power Station Main Waterway Settling Basin

Heisakubara Power Station Trunk Waterway Sediment Basin: A Hidden Witness to Early Showa Hydropower Engineering

Tucked into the highland forests of northern Okayama Prefecture, near the border with Tottori, sits a quiet rectangular structure of weathered concrete. Few travellers who come to admire the autumn colours of Onbara Lake ever notice it, yet this modest pool — the Heisakubara Power Station Trunk Waterway Sediment Basin (Heisakubara Hatsudensho Kansen-suiro Chinsachi) — is officially recognised by the Japanese government as a Registered Tangible Cultural Property. Completed in 1928, it forms one indispensable link in a chain of ten heritage structures that together tell the story of how electricity first came to rural Okayama nearly a century ago.

For visitors with an interest in industrial heritage, early reinforced-concrete engineering, or the quieter corners of Japan's mountain landscape, this is a destination that rewards a slower, more attentive kind of travel.

What Is the Trunk Waterway Sediment Basin?

A sediment basin (chinsachi) is a piece of waterworks infrastructure designed to slow the flow of water so that sand, gravel, and other debris can settle out before the water enters a power station's penstock and turbines. Without such basins, sediment would rapidly wear down the impellers of hydroelectric generators, shortening the life of expensive equipment.

The Heisakubara Power Station Trunk Waterway Sediment Basin is located approximately 270 metres downstream from the Onbara Reservoir Dam. It is a roughly rectangular reinforced-concrete structure with a surface area of about 85 square metres. On its western and southern sides, where the mountain slope rises steeply against the structure, reinforced-concrete slabs have been cantilevered outward to roof over the top of the basin. This was a clever piece of design: it prevents soil, rocks, and other debris from the hillside from washing into the basin during rain or snowmelt, which would otherwise undermine its function.

The water that passes through this basin has begun its journey high in the mountains, gathered behind the Onbara Dam, and flows onward through a 2.5-kilometre conduit toward another sediment basin, then a head tank, then steel penstocks, and finally the generator hall of the Heisakubara Power Station itself.

Historical Background: Bringing Electricity to Rural Okayama

In the 1920s, Okayama Prefecture was experiencing a surge in demand for electric power. To meet this need, the Chugoku Godo Denki company (a predecessor of today's Chugoku Electric Power Co.) planned a hydroelectric project at the very headwaters of the Yoshii River, near the prefectural border with Tottori. The plan called for a dam to impound water at an elevation of roughly 700 metres, with a network of intakes, conduits, sediment basins, and a head tank delivering that water to a generating station several hundred metres lower in elevation.

The whole system began operating in February 1928, with a maximum output of 2,900 kilowatts. Today, this still places Heisakubara among the oldest dam-and-conduit hydroelectric installations operated by Chugoku Electric Power, second only to the Nishikigawa Daiichi Power Station in Yamaguchi Prefecture, which opened in 1924.

The choice of construction techniques reflected the practical challenges of building deep in the mountains. The Onbara Dam itself was built as a buttress dam — an unusual choice in Japan — because this type of dam uses far less concrete than a conventional gravity dam, an enormous advantage when every bag of cement had to be hauled by horse, oxcart, or rudimentary tramway through the rugged Chugoku highlands. The sediment basins, conduits, and other ancillary structures were built of reinforced concrete using high-quality cement supplied by the Asano and Onoda Cement Companies, two leading producers of the era.

Why Was It Designated a Cultural Property?

On November 29, 2006, ten structures connected with the Heisakubara Power Station were registered together as Tangible Cultural Properties under Japan's Cultural Properties Protection Law. The sediment basin was given registration number 33-0136. The collective designation recognises several layers of historical and engineering significance.

First, the Heisakubara complex is one of the most complete surviving examples of an early Showa-era dam-conduit hydroelectric system in western Japan. Many comparable facilities elsewhere have been modernised, replaced, or demolished, but here a near-complete original ensemble survives largely intact and still operating.

Second, the Onbara Dam at the head of the system is the oldest hydroelectric buttress dam still in existence in Japan. (An earlier one, the Koyasan Dam in Niigata, was dismantled during redevelopment.) The Onbara Dam is also recognised by the Ministry of Land, Infrastructure, Transport and Tourism as a Modern Civil Engineering Heritage site.

Third, the cluster of related structures — intake weirs, sediment basins, conduits, the head tank, retaining walls, and the powerhouse itself — together illustrate how early-twentieth-century engineers solved an entire chain of practical problems using the then-new technology of reinforced concrete. The Trunk Waterway Sediment Basin, modest though it appears, is a vital piece of that integrated heritage system.

Highlights and Points of Interest

Although small in scale, the sediment basin embodies several features that engineering enthusiasts and heritage travellers will appreciate. Its cantilevered roof slabs on the slope side are a quietly elegant response to a difficult site, sparing the structure from the constant threat of debris flow. The concrete itself, almost a century old, demonstrates the durability of high-grade pre-war Japanese cement and the careful workmanship of an age before mechanised construction.

Visitors who walk the area around Onbara Dam can appreciate the basin in its broader context: a system of works that quietly conducts mountain water across hillsides, through tunnels, and down a steep gradient to generate clean electricity. It is a piece of living industrial archaeology — still doing the same job, in the same way, that it did when first switched on in the spring of 1928.

The setting itself is one of the region's quiet treasures. The structure sits at the edge of the Onbara Highland, a plateau of birch and larch forests at an elevation of roughly 700 metres, ringed by mountains that turn brilliant gold and crimson in late October.

Visitor Information

The Trunk Waterway Sediment Basin is an active piece of operating power infrastructure owned by Chugoku Electric Power Co., Ltd. It is not open for interior inspection or guided tours, and guests are asked not to enter restricted areas marked by the company. However, the general area around the Onbara Dam is freely accessible, and the basin and other registered heritage structures can be observed from publicly accessible roadsides and viewpoints.

The simplest approach is to combine a visit to the Trunk Waterway Sediment Basin with a tour of the Onbara Dam, which lies only about 270 metres upstream. Together they offer an excellent introduction to the engineering of early Japanese hydroelectric power.

The site is at an elevation where winters are deeply snowy, so the most comfortable seasons for visiting are late spring through late autumn. Late October to early November offers spectacular foliage; midsummer is cool and welcoming for highland walking.

Surrounding Attractions

The Onbara Highland area offers a rich variety of attractions within easy reach of the sediment basin. Onbara Lake, the reservoir created by the dam, has a 4.5-kilometre shoreline and is one of northern Okayama's premier autumn-foliage spots; in deep winter its frozen surface can produce mysterious circular ice patterns known as hyomon, sometimes called the "Kagamino mystery circles." Adjacent to the lake are the Onbara Highland Auto Campground and the Onbara Highland Ski Area.

A short drive south brings visitors to Okutsu Onsen, one of the three famed hot-spring resorts of the historic Mimasaka region, with riverside open-air baths that have welcomed travellers since the Edo period. Nearby Okutsukei Gorge is a designated national place of scenic beauty, particularly stunning in autumn. The Yousei no Mori Glass Museum in Kamisaibara, the Atom Science Museum, and the historic Kijishi no Yakata (woodturner's house) round out the local cultural offerings.