What a spillway notch is
A spillway notch, mizutoshi in Japanese, is the section of the crest of an erosion-control dam that is cut down below the rest, so that water from upstream gathers there and passes downstream. It is usually trapezoidal in section; the width of its floor is the notch width and its sloping sides are called its shoulders.
Its first purpose is to concentrate flow at a fixed point. If water spilled over the whole crest, it would scour the banks at either end and could undermine the dam from the side. A notch at the centre drops the flow into the middle of the channel and keeps it off the banks. The falling water lands on the apron below and passes over the sub-dam downstream.
The width and depth of the notch are set according to the flood flow expected. A wider notch spreads the falling water and reduces its power to dig into the bed, and records often note measures of this kind for limiting scour.
Reading the records and the dam
Records give the position of the notch, its width and the slope of its shoulders. The record for Dam No. 11 on the Mannai River notes a notch at the centre of the dam giving an almost symmetrical form, with a stone-paved apron and a generous notch width among the measures against scour. The record for Dam No. 14 gives shoulders sloping one in one and a notch 15 metres wide.
The notch also shapes the landscape. The record for Dam No. 6 on the Mannai River observes that the notch is cut down by as much as the height of the dam itself and, with shoulders sloping one in one over a length of 15 metres, keeps the view upstream open. The record for the seventh Minamisawa stone dam on the Shohoji River notes that the corners of its notch are rounded, attention to detail even in the stonework.
On site, look at the dam face-on from downstream and find the lowered section in the middle of the crest. In normal flow the water usually runs only there, leaving the rest of the crest dry. Wear on the stones of the notch floor and shoulders shows where the water strikes hardest.
References
- 万内川六号堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00003215
- 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
- 万内川十四号堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00003221
- 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