What an apron is
An apron, mizutataki in Japanese, is the section of riverbed immediately downstream of a dam that is covered with stone paving or concrete to take the water falling over the crest. Falling water strikes the bed hard and, unprotected, would wash out the soil and expose the foundations of the dam. The apron absorbs the impact and prevents scour. Records also write the term in a shortened form.
It works together with the other parts of a dam. Water dropping through the spillway notch of the main dam loses its force on the apron and then passes over the sub-dam at its downstream end. Revetments on both banks keep the sides of the apron from being eroded. An apron between main dam and sub-dam is the typical arrangement, and the record for Dam No. 4 on the Mannai River describes the sub-dam in front linked to the main dam by a stone-paved apron and a stone revetment.
Materials vary with date and size. Early modern erosion-control dams often have aprons of large paving stones, and concrete became more common later.
Reading the records and the site
Records describe the material and construction of the apron. The record for the seventh Minamisawa stone dam on the Shohoji River says that stones about 50 centimetres square are laid in front of the dam to form the apron, and the record for the sixth describes an apron and revetments in front of the dam that help stabilise the banks. The record for Dam No. 11 on the Mannai River lists a solidly stone-paved apron and a wide notch among its measures against scour.
Aprons and sub-dams do not always come together. At Dams Nos. 7 and 8 on the Mannai River, one dam has an apron and revetments downstream while the other has a sub-dam, the combination chosen to suit the ground.
On site, check whether the bed just below the dam is paved flat. At low water the joints of the paving and stones worn smooth by the flow can often be seen. A step at the downstream end of the apron is either a sub-dam or a sill that holds the apron's edge in place.
References
- 万内川四号堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00003214
- 万内川十一号堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00003218
- 正法寺川南沢第六号石堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00004889
- 正法寺川南沢第七号石堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00004890
Last verified: 2026.09.26