Circular diversion works
A facility for dividing agricultural water among several irrigation channels accurately in fixed proportions. Water is made to well up from the center of a cylinder, and the water that spills over the cylinder's outer rim is apportioned by the ratio of the angles into which the circumference is partitioned. Because the ratio of division does not change whether the flow increases or decreases, and because the division can be checked by eye, these were built throughout Japan as devices for settling disputes over water. The design was conceived in the Taisho era, spread across the country in the Showa era, and more than 100 facilities are still in operation today※1.
How it works
Water brought from upstream is sent to the center of the cylinder, by means such as an inverted siphon, and made to rise from below. The water that wells up overflows the upper edge of the cylinder around its whole circumference and falls to the outside. If this circumference is partitioned in proportion to the irrigated area of each recipient, the fallen water divides itself automatically in that proportion.
The essential point of this method is that the ratio of division does not depend on the volume of water. With the conventional method of dividing water by weirs and sluices, the division changed whenever the volume changed, and the more severe the drought, the more the division went wrong, sowing the seeds of quarrels. With a circular divider, as long as the water overflows isotropically from the center, it flows with the same depth in every direction, so the geometric ratio of the lengths into which the circumference is partitioned is the only thing that determines the division.
A device for settling water disputes
For rice growing, securing irrigation water is a matter of life and death, and how to share limited water was a long-standing cause of conflict between districts. Circular dividers were built across the country not so much for reasons of technical efficiency as to resolve this conflict.
The Kuji circular diversion works in Kawasaki is a typical example. The Nikaryo irrigation canal originally divided its water among four channels with a facility called the "Kuji measuring sluice", but it could not divide accurately and quarrels over the volume never ceased. The circular diversion works was built in 1941 (Showa 16) to solve this. It has a double structure of nested reinforced-concrete cylinders of 8 meters and 18 meters in diameter, and by partitioning the outer circumference into lengths proportional to the irrigated area of each channel it made it possible to divide the water in a constant ratio however the flow changed. It was registered as a national Registered Tangible Cultural Property in 1998 (Heisei 10).
The essential value of this device can be said to lie in the fact that all the parties can see on the spot, with their own eyes, that the water is "being divided correctly". It embeds the fairness of the division in the mechanism itself and makes it visible; though a civil engineering structure, it is also a tool for building consensus.
History and surviving examples
The design is said to have been conceived by Kanichi Kachi, a civil engineer in Gifu Prefecture, and the first facility in Japan was built in 1914 (Taisho 3) in the village of Koizumi, Gifu Prefecture※2, but it does not survive. The oldest surviving example is said to be the Iboiwa circular diversion works in Zao, Miyagi Prefecture, completed in 1931 (Showa 6).
They spread across the country through the Showa era, and many remain in the Tohoku region, particularly in Miyagi Prefecture. Because of their distinctive geometric form and the sight of water welling up in a circle and overflowing without end, in recent years some people travel around visiting them as civil engineering structures to be appreciated.
To verify
- The variation among the names "円筒分水工", "円筒分水槽", "円形分水工" and others, and whether they are used with any distinction
References
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