What a buttress dam is
A buttress dam holds back water with a thin reinforced-concrete watertight wall supported on its downstream side by buttresses set at regular intervals. Records describe the type as a buttress reinforced-concrete dam. The buttresses take the water pressure on the wall at an angle and carry it down into the ground.
Most dams are gravity dams, which resist water pressure by their own weight. A gravity dam is simple and robust but needs a great deal of material, since its whole body is filled with concrete or stone. A buttress dam carries the load with the wall and buttresses alone, greatly reducing the quantity of material and the weight of the structure. On the other hand, its thin reinforced members must be built accurately, and construction is more complex.
In Japan the type was used in the early Showa period for reservoir dams serving hydroelectric plants. The Marunuma Dam in Gunma Prefecture, designed by Mononobe Nagaho, head of the Home Ministry's civil engineering laboratory, was begun in 1928 and completed in 1931. The Agency for Cultural Affairs describes it as the representative Japanese example of a buttress dam, built to reduce material and lighten the structure in the face of a harsh site and difficult ground.
Reading the records and the dam
Records give the type and size in the structure line. The Marunuma Dam, designated an Important Cultural Property, is a buttress reinforced-concrete dam 88.2 metres long and 32.1 metres high. The Onbara reservoir dam of the Heisakubara power station in Okayama is a dam of the same type, 94 metres long and 24 metres high, with its watertight wall supported by buttresses; its record says that it was quick to adopt Mononobe's research on earthquake resistance and reduced the weight and material of the structure, a type with few parallels.
On site, view the dam from downstream. A gravity dam presents a continuous sloping face. A buttress dam shows a row of roughly triangular buttresses like the teeth of a comb, with the back of the watertight wall visible between them. A walkway and railing along the crest often tie the tops of the buttresses together.
Gravity dams are recorded as gravity concrete dams or gravity mortared-masonry dams, so the type name in the record distinguishes them.
References
- 丸沼堰堤｜文化遺産オンライン
- https://online.bunka.go.jp/heritages/detail/149021
- 平作原発電所恩原貯水池堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00005989
Last verified: 2026.09.26