Heisakubara Power Station

A Power Station Spread Across a Mountain Valley

Heisakubara Power Station (平作原発電所, Heisakubara Hatsudensho) is a dam-and-conduit hydroelectric plant in Kamisaibara, Kagamino Town, Okayama Prefecture, that began generating in February 1928; ten of its structures, including the Onbara Reservoir Dam and the main building, were listed as Registered Tangible Cultural Properties on 29 November 2006.

The plant sits in the headwaters of the Yoshii River, close to the border with Tottori Prefecture. It was planned by Chūgoku Gōdō Denki, a regional utility that developed power sources along the Yoshii River system between 1926 and 1941, to meet rising electricity demand in Okayama. The station opened with an output of 1,800 kilowatts and gained another 1,100 kilowatts in 1930, reaching its present maximum of 2,900 kilowatts.

Wartime consolidation of the electricity industry passed the plant through several hands. It went to Sanyō Haiden in 1941 and to the national generating company Nippon Hassōden in 1942, then joined Chugoku Electric Power when that company was founded in 1951. Among the dam-and-conduit stations Chugoku Electric still operates, only Nishikigawa No. 1 in Yamaguchi Prefecture, running since 1924, is older.

Nobody works at the site day to day. According to the company, the station is run remotely from its Tsuyama control office, with staff visiting twice a month on inspection rounds.

Following the Water Through Ten Structures

A dam-and-conduit plant combines two sources of head: the height a dam creates and the drop gained by carrying water along a channel. The ten listed structures of Heisakubara Power Station are scattered along that route, and together they cover every stage of the system: intake, storage, sediment removal, conveyance and generation.

The route begins in a valley south of the Onbara Reservoir. Weirs and settling basins were built on two streams, the Endō River and the Endōbara River, and the Agency for Cultural Affairs describes the Endō River weir as the uppermost of the Yoshii River hydropower facilities. The Endōbara channel joins the Endō channel. Chugoku Electric lists the Yoshii, Endō and Endōbara rivers as the plant's water sources, and water from the two streams is understood to be led by channel into the reservoir.

The Onbara Reservoir Dam stands about 1.7 kilometres north of the Endō River weir, with a spillway on its right bank that drops surplus water into the Onbara River. Some 270 metres downstream lies the settling basin of the main waterway, and the water then runs about 2.3 kilometres to the head tank. The main building of Heisakubara Power Station sits about 330 metres below the tank, and granite retaining walls hold the slope beside the penstock and around the building.

Trace the ten points on a map and you start to see how the engineers read the terrain before they poured any concrete.

Why These Structures Were Registered

Chugoku Electric says the ten structures were registered because they are valuable evidence for the history of hydropower in Japan and contribute to the historic landscape. In the Agency for Cultural Affairs criteria, the Onbara Reservoir Dam is listed as a structure that would be difficult to reproduce, and the other nine as structures that contribute to the historic scenery of the country.

The dam is the headline piece. It is a reinforced concrete buttress dam, a thin watertight wall braced from behind by a row of buttresses. Chugoku Electric describes it as one of six surviving buttress dams in Japan and the oldest built for power generation. A Japan Society of Civil Engineers report adds that it was the first dam designed with the seismic theory for buttress dams developed after the 1923 Great Kantō Earthquake by Mononobe Nagaho, head of the Home Ministry's public works research institute. Engineers first planned a gravity dam, but the fairly weak foundation, the difficulty of hauling materials into the mountains and a tight schedule pushed them toward a design that used far less concrete.

That haul was hard going. Materials came by rail as far as Innoshō Station on the Kishin Line, then by truck to a point 8 kilometres short of the dam site; after that, the same report says, only horse carts could get through.

Some traits run through the whole group. In its note on the head tank, the Agency for Cultural Affairs records that, like the other structures, it was built with high-grade cement from the Asano and Onoda companies. Many of the settling basins and retaining walls face the river or slope with granite laid in the Tanizumi pattern. Each site tackles the same problem, silt carried by fast mountain streams, in a slightly different way, which makes the structures rewarding to compare.

Before You Visit Heisakubara Power Station

Every structure of Heisakubara Power Station is part of a working power plant, and none of the interiors are open. Chugoku Electric says the exteriors can be viewed at any time, but there is no reception or visitor center, and gated or fenced operational areas are off limits. A visit means looking at the structures from public roads and other places where access is allowed.

Plan on driving. Chugoku Electric gives about 70 minutes by car for the station address (Heisakubara 1878, Kamisaibara) via the Innoshō Interchange on the Chūgoku Expressway, and about 75 minutes via JR Innoshō Station. Kagamino Town's tourism site puts Lake Onbara about 50 minutes out of the Innoshō Interchange and about an hour by car out of JR Tsuyama Station, so estimates vary by source. The ten structures are spread over several kilometres; decide which ones you want to see before you set out.

The area lies on the Onbara Plateau at about 700 metres above sea level, and the town notes that snow can pile up more than 1 metre deep in winter. Autumn color usually peaks in late October and early November. If you plan to drive the mountain roads in winter, check conditions and carry snow equipment.