Heisakubara Power Station Onbara Reservoir Dam

Japan's Oldest Surviving Buttress Dam Built for Power

The Heisakubara Power Station Onbara Reservoir Dam (平作原発電所恩原貯水池堰堤, better known as Onbara Dam) is a reinforced concrete buttress dam built in 1928 in Kamisaibara, Kagamino Town, Okayama Prefecture, and registered as a Registered Tangible Cultural Property on 29 November 2006.

It is 94 metres long and 24 metres high. Instead of holding back water with sheer mass, a buttress dam uses a thin watertight wall braced from behind by a row of buttresses. The lake behind it, Lake Onbara, is the reservoir that feeds Heisakubara Power Station; Chugoku Electric gives its total storage as 1,853,300 cubic metres from a catchment of 9.25 square kilometres.

The dam was raised in 1952. In 1965, according to the Japan Society of Civil Engineers, frost damage led to repairs on the watertight wall and to 15-centimetre concrete jackets on the buttresses and horizontal beams.

An Earthquake Theory Put to Work

Of the ten registered structures at Heisakubara, only the Heisakubara Power Station Onbara Reservoir Dam was listed under the criterion for structures that would be difficult to reproduce. The Agency for Cultural Affairs calls it a rare type that quickly adopted Mononobe Nagaho's research on earthquake-resistant reservoir walls, cutting both the weight of the structure and the materials needed.

Mononobe was a civil engineer who headed the Home Ministry's public works research institute and taught at Tokyo Imperial University. The civil engineers' society credits Onbara as the first dam built with the seismic theory for buttress dams that he developed after the Great Kantō Earthquake of 1923.

Chugoku Electric describes it as one of six surviving buttress dams in Japan and the oldest built for power generation. It was first planned as a gravity dam. A fairly weak foundation, a remote site where materials were hard to haul and a tight schedule tipped the choice toward the buttress type, which needs far less concrete. Actual construction took about 10 months.

Supplies came by rail to Innoshō Station on the Kishin Line; for the final 8 kilometres, horse carts did the hauling. Here the design's promise of lightness and economy was a practical necessity.

What to Look For at Onbara Dam

The real character of a buttress dam shows on the downstream side, where buttresses stand at regular intervals behind the thin wall, tied together by horizontal beams. That grid of piers and beams looks nothing like the solid face of a gravity dam.

You will not get the classic view from below, though. The road down to the downstream side is closed, and a visit reported by the regional site Okayama Style notes that the dam cannot be seen head-on. Expect to look from the lake side and from accessible points near the crest.

Sources differ slightly on the numbers: the Agency for Cultural Affairs lists 94 metres long and 24 metres high, while the civil engineers' report gives 24.03 metres high and 93.64 metres long.

Turn toward the water and Lake Onbara spreads out in a ring of birch and larch roughly 4.5 kilometres around. Autumn color reflects on its surface; in winter it freezes over. Knowing how the dam works, it is striking to think that this whole lake has been held since 1928 by a single slim wall of concrete.

Visiting the Onbara Reservoir Dam

The Heisakubara Power Station Onbara Reservoir Dam is a working power dam, and Chugoku Electric says its exterior can be viewed at any time. Maintenance roads, the downstream side and any gated or fenced areas are closed to visitors.

Lake Onbara lies on the Onbara Plateau at about 700 metres. Kagamino Town's tourism site puts it about 50 minutes by car from the Innoshō Interchange on the Chūgoku Expressway and about an hour by car from JR Tsuyama Station. Snow can top 1 metre in winter, so bring proper winter gear if you come then.

There is no admission fee. Chugoku Electric's page gives no restriction on photographing the exterior, but you cannot enter closed areas to take pictures.