Yanagidani No. 7 Sabo Dam

Where the Staircase Method Was First Tried

Yanagidani No. 7 Sabo Dam is an erosion-control dam of 1928 at the lowest point of the Yanagidani ravine, in the headwaters of the Tedori River at Hakumine, Hakusan City, Ishikawa Prefecture, listed on 23 February 2012 as a Registered Tangible Cultural Property. It measures 60.5 metres across and 8.0 metres high, and it was among the first dams built once the Home Ministry took the scheme over.

Its place in the history of Japanese sabo engineering is particular. The Kanazawa River and National Highway Office describes Yanagidani No. 7 Sabo Dam as the first stepped dam in the country: an experiment in how to build where no bedrock offers a footing, using an existing dam as the base on which to raise the next.

In the Jinnosuke Valley, a tributary of the same river, eleven stepped dams stand in a line built on exactly that principle. The sequence began here.

Year One of the National Scheme

Sabo work on Hakusan began in 1910, when the governor of Ishikawa looked at the stripped slopes of this very ravine. The prefecture started building in the Jinnosuke and Yanagidani valleys in 1912, and torrential rain in 1914 and 1919 destroyed nearly all of it. After the Erosion Control Act was amended in 1924, Ishikawa petitioned for national help, and in 1927 the work became a directly managed national project. This dam followed the next year.

The designer was Masao Akagi, first director of the Home Ministry's Hakusan sabo works office and the man Japan remembers as the father of sabo engineering. The problem he faced was a streambed with no exposed bedrock anywhere. The staircase method was his answer, and Yanagidani No. 7 Sabo Dam is where he tried it.

Registration is a notification-based form of protection, separate from the permit-based designation system. This dam qualified as a structure contributing to the historic landscape of the country. Fifteen sabo structures on Hakusan were listed on the same day, thirteen of them in the Jinnosuke Valley.

Mortared Stone with an Overhanging Lip

The body is mortared rubble, stone quarried nearby and bedded with mortar in the joints. The spillway at the centre is cut as an inverted trapezoid opening upward, and the coping along its horizontal lip projects forward. That overhang is a mark of the earliest years of the national works on Hakusan, and this dam is among the first places it appears.

At 60.5 metres by 8.0 metres, Yanagidani No. 7 Sabo Dam is a heavier structure than anything standing in the Jinnosuke Valley, all of which is lower. The national record judges that it retains the character of the earliest Home Ministry work in good condition.

Visiting

There are no opening hours and no admission fee. Yanagidani No. 7 Sabo Dam is a working structure in the mountains; it cannot be walked on, nothing has been built beside it for visitors, and no public route leads to it.

The Yanagidani ravine is badly degraded and steep, with a high risk of debris flows, and the left bank is liable to collapse and rockfall. According to Ishikawa Prefecture, remote-controlled construction methods have been introduced here in stages since 1997. When the engineers themselves work the site by remote control, it is not a valley to wander into.

The Hakusan Sabo Science Museum at Hakumine covers the history and the structures. It opens from 9 a.m. to 5 p.m., admission is free, and it closes on Thursdays, or the following day when a Thursday is a public holiday, and from 29 December to 3 January.