Jinnosuke Valley Sabo Works

Thirteen Dams in One Mountain Valley

Jinnosuke Valley Sabo Works (甚之助谷砂防施設群) is a chain of thirteen erosion-control structures standing in the Jinnosuke Valley on Mount Hakusan, at Hakumine in Hakusan City, Ishikawa Prefecture, built between 1915 and 1938 and listed on 23 February 2012 as Registered Tangible Cultural Properties. They are strung along the valley between roughly 1,500 and 2,100 metres above sea level, the smallest 13.3 metres across, the largest 40 metres.

The thirteen fall into three sets. One is the No. 2 Check Dam of 1915. Eleven are the stepped dams of the middle reaches, raised between 1932 and 1938. The last is the Upstream No. 16 Sabo Dam of 1933, set highest in the working section of the stream. The Kanazawa River and National Highway Office of Japan's transport ministry calls the eleven middle-reach dams the Jinnosuke Valley Sabo Dam Group. Fifteen sabo structures on Hakusan were listed on that same day in 2012; the two not in this valley stand in the Yanagidani ravine and at Ozo.

Every one of them belongs to Ishikawa Prefecture, and every one is still doing its job. Jinnosuke Valley Sabo Works is not a preserved ruin but a working defence against the debris that the mountain sends down.

From a Prefectural Scheme to a National One

Sabo work on Hakusan began with a governor's visit. In 1910 the governor of Ishikawa looked at the stripped slopes of the Yanagidani ravine and concluded that they could not be left alone. Survey work started the following year, and in 1912 the prefecture began building slope works and check dams in the Jinnosuke and Yanagidani valleys. Torrential rain in 1914 and again in 1919 destroyed almost all of it. The engineers changed tack, shifting the emphasis to stopping surface erosion and protecting the foot of the slopes.

An amendment to the Erosion Control Act in 1924 opened a route for the national government to take over works that were too difficult or too costly for a prefecture. Ishikawa petitioned the Home Minister in 1926, and in 1927 the scheme became a directly managed national project. A sabo works yard was built near Bettodeai that year, and its first director was Masao Akagi, remembered in Japan as the father of sabo engineering. Most of the dams in Jinnosuke Valley Sabo Works were built in the years immediately after that handover.

On 11 July 1934, meltwater from an unusually deep snowpack met the rains of the seasonal front. The flood that came down the Tedori River killed 97 people, left 15 missing and injured 35. At Ichinose the riverbed rose by 12 metres. The Jinnosuke and Yanagidani valleys, where the sabo works stood, did not collapse. That contrast settled the argument about whether the money had been well spent. In 1943 the dams were transferred to Ishikawa Prefecture, which owns them still.

A landslide control programme followed in 1961. Engineers had noticed that the No. 5 dam, the keystone structure of the valley, had shifted about 10.5 metres between 1927 and 1960. The whole slope, it turned out, was moving. The unstable mass here is estimated at more than 40 million cubic metres, one of the largest landslides in the country. The dams of Jinnosuke Valley Sabo Works sit on ground that will not hold still.

Building on What the Last Dam Caught

Akagi described the method in his memoir Sabo Ichiro. Nowhere in this stream bed does bedrock come to the surface, so a tall dam had no footing. The answer was to build a low one, around 8 metres high, far down the valley, wait for it to fill with gravel, and the next season build another 8-metre dam on top of the deposit the first had caught. Repeat, and the valley fills with a staircase that holds the slopes on either side. It had not been tried in Japan before.

Look at the crest of any of these dams and you will find, or find the scar of, a stone coping that juts forward like a small eave. The detail belongs only to the earliest years of the national works on Hakusan and does not appear on later structures. The dam bodies are mortared rubble, stone quarried on the spot and bedded with mortar in the joints. The No. 2 Check Dam of 1915 is the exception: it is dry-laid, stone cut and fitted without mortar at 2,100 metres, where every tool had to be carried up.

Everything was carried by hand. Photographs from about 1928 show men moving aggregate up the slope on their backs. In 2004 the Japan Society of Civil Engineers certified the middle-reach dams as a Civil Engineering Heritage site, describing them as among the oldest stepped masonry dam groups in Japan. It was the first such certification anywhere in Ishikawa Prefecture.

Reading the Valley from the Bottom Up

The thirteen run in sequence, low to high. The No. 11 Sabo Dam sits lowest; at 40 metres it is the longest of the group and acts as the base of the staircase above it. Above it come No. 12, No. 13, No. 14, No. 16, No. 18, No. 19, No. 20, No. 21, No. 22 and No. 23. The gaps in the numbering are simply because not every dam built in this valley was put on the register.

Higher again stands the Upstream No. 16 Sabo Dam, and highest of all, at 2,100 metres, the No. 2 Check Dam. That one dates from 1915 and the prefectural years, more than seventeen years older than any of the twelve below it. The history of the valley, in other words, reads downhill.

The Sabo Shindo, the main climbing route from the Bettodeai trailhead to the Murodo huts below the summit, works its way up the flank of the Jinnosuke Valley to the Jinnosuke emergency hut at about 1,970 metres. The stepped dams of the middle reaches lie at roughly the same heights as the stretch of trail between the Nakaiba rest point and that hut. The No. 2 Check Dam is up near the Nanryu path, which branches off towards the Nanryugababa basin.

Visiting

There are no opening hours and no admission fee, because Jinnosuke Valley Sabo Works is not a visitor site. No interpretive panels or viewing platforms have been built beside the dams. The only way the public can approach the valley is on the mountain trail.

Start at Bettodeai. By car, leave National Route 157 for Prefectural Route 33, the Hakusan Park Line, and follow it past Ichinose to the car park at Bettodeai, which holds about 200 vehicles and is free. During the peak climbing season private cars are barred from the 6 kilometres between Ichinose and Bettodeai; drivers leave their cars in the Ichinose car park, which holds about 560, and take the paid shuttle bus. The restricted dates are fixed each year, so check the Ishikawa Prefecture page before setting out. Seasonal buses run to Ichinose from JR Kanazawa and JR Mattō stations, and a climbing taxi service connects Komatsu Airport with Bettodeai. In the snow months the prefectural road is closed and the valley is out of reach altogether.

Anyone climbing Hakusan is required by prefectural ordinance to file a climbing plan, either in the boxes at the trailhead or online. Bettodeai to the Jinnosuke hut is about two and a half hours of walking and some 700 metres of ascent, so this is not a place to drop in on lightly. Nothing prohibits photography, but the dams stand on moving ground and repair work is more or less continuous, so keep clear of any working site.

For the background, the Hakusan Sabo Science Museum at Hakumine, down in the valley, is the place to start. 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. Its displays on the floods of the Tedori basin explain what the dams of Jinnosuke Valley Sabo Works were built to hold back.