Bannai River Sabo Facilities

What Are the Bannai River Sabo Facilities?

The Bannai River Sabo Facilities (万内川砂防施設, Bannaigawa Sabō Shisetsu) are a group of sabo dams, ground sills and a stone chute built between 1921 and about 1934 on the Bannai River and its tributaries, the Hikagesawa and Kuzuresawa, in Nishinoya, Myoko City, Niigata Prefecture; 14 of them were registered as Registered Tangible Cultural Properties in 2003.

Sabo is the Japanese term for erosion and sediment control in mountain streams. A sabo dam is a check dam that traps sediment and slows debris flows; a ground sill is a low structure laid across a stream bed to fix its level. These were the first sabo works carried out in Niigata, and the Bannai valley is known as the birthplace of sabo in the prefecture.

The registration came in two stages. On March 18, 2003, eleven sabo dams on the Bannai River (ten entries) were registered, and on July 1 the same year three groups of ground sills on the Hikagesawa and the stone chute on the Kuzuresawa followed. All belong to Niigata Prefecture and are managed by its Myoko Sabo Office. Every one of them still works as a disaster-prevention structure.

The Bannai rises on the north side of Mount Awadachi (粟立山, 1,194 meters), in the hills west of central Myoko, and runs about 7 kilometers within the Sekikawa river system. The Hikagesawa joins it from the right bank, and the small Kuzuresawa in turn enters the middle reach of the Hikagesawa from its right. In 2017 the Japan Society of Civil Engineers selected the whole group as Civil Engineering Heritage.

The 1902 Collapse and Niigata's First Sabo Project

On May 19, 1902, Mount Awadachi gave way with a great roar and buried the Bannai and the Hikagesawa in debris. Mixed with snowmelt, the material became a debris flow that struck the village of Nishinoya downstream. Thirty of its 53 houses were carried away and most of the fields were lost, but villagers who noticed the mountain moving fled quickly, and the prefecture records a single death.

Sediment kept coming down long after the collapse. In 1921, prompted by debris washing out of the Hikagesawa, Niigata Prefecture began its first sabo project here, a prefectural scheme backed by national subsidies. The prefecture's explorer map explains that sabo engineers of the day could either build one tall, massive dam or a series of low dams set in steps, and that the Bannai received the stepped approach, which had European roots, as the most up-to-date technique in Japan at the time.

No machinery was used. According to the prefecture, stonemasons split rocks on the riverbed with chisels to make facing stones, and the gravel for the core was broken from river stones with hammers. Cement and sand came by horse cart from Naoetsu, and on site villagers carried loads in pairs slung from rope nets. The explorer map adds that women carried two 50-kilogram bags of cement at a time, and that the works office was set up in a rented farmhouse in Nishinoya, with staff lodging in villagers' homes.

The work did not end in the Taisho era. After a flood in 1933, dams No. 13 and No. 14 were built above the earlier series around 1934, and many of the existing dams were remodeled at about the same time. The Hikagesawa ground sills received additions around 1937.

Counts differ between sources. The prefecture says 18 sabo dams have been built on the Bannai since 1921 and 49 ground sills on the Hikagesawa, of which 11 dams and 41 sills and related works were registered. A Japan Society of Civil Engineers nomination document gives 30 dams on the Bannai and 47 sills on the Hikagesawa.

The 14 Registered Structures, Downstream to Upstream

From Bannai River Sabo Park at the lower end of the valley, the 14 registered structures follow in this order. The numbers rise going upstream but do not always match the order of construction: No. 12 is about a year older than No. 10 and No. 11.

• Bannai River No. 1 Sabo Dam (around 1921; 43 meters long, 5.0 meters high). The lowest registered dam and an early example of rubble concrete.
• Bannai River No. 3 Sabo Dam (around 1922; 2.0 meters high). A low dry-stone dam later strengthened with mortar.
• Bannai River No. 4 Sabo Dam (around 1922; 26 meters long). Drops water in two steps over a stone apron.
• Bannai River No. 6 Sabo Dam (around 1923; 2.0 meters high). A deeply cut notch opens the view upstream.
• Bannai River No. 7 and No. 8 Sabo Dams (around 1926; two dams). Also serve as an irrigation intake.
• Bannai River No. 10 Sabo Dam (around 1926; 3.5 meters high). Spaced about 120 meters from its neighbors.
• Bannai River No. 11 Sabo Dam (around 1926). Built in a gorge, with measures against scour.
• Bannai River No. 12 Sabo Dam (around 1925). Laid entirely in fine tanizumi masonry, with a stone nameplate.
• Bannai River No. 13 Sabo Dam (around 1934; 5.9 meters high). Built after the 1933 flood; the tallest registered dam.
• Bannai River No. 14 Sabo Dam (around 1934). The uppermost registered dam, with four arched drain openings.
• Hikagesawa No. 1 Ground Sill (late Taisho era; 30 meters long, 5.0 meters high). The lowest and largest of the Hikagesawa sills.
• Hikagesawa Middle Reach Ground Sills (late Taisho era, added to around 1937). Thirty-two sills curving up a slope of about 30%.
• Kuzuresawa Stone Chute (late Taisho era). A stone-paved channel at a 25% gradient built together with a ground sill.
• Hikagesawa Upper Reach Ground Sills (late Taisho era, added to around 1937). Five cut-stone sills on a slope of about 40%.

On the Bannai, construction moves from Taisho-era dry stone to mortared masonry and concrete in the Showa years. On the Hikagesawa, a far steeper valley, low stone sills are packed closely up the slope. Seeing how one project adapted its methods to two very different streams is what makes this group rewarding.

How to Visit

The Bannai dams sit in and above Bannai River Sabo Park, which occupies the lower part of the valley, with a riverside walking path leading upstream. The park was laid out as a recreation area between 1989 and 1997, and after the registration, footpaths and rest areas were added around the dams between 2004 and 2008. It is open from April to November, admission is free, and there is a parking lot and toilets; inquiries go to the Myoko City Construction Division.

The prefecture publishes an explorer map of dams No. 1 to No. 4 on its website. At Viewpoint 1 you can compare a photograph of No. 1 just after completion with the present view, and from Viewpoint 2 dams No. 6, No. 7 and No. 8 appear stacked like steps.

By car, turn off National Route 18 at the Nagamori intersection toward ARAI ski resort, then turn right at the T-junction in Nishinoya-Shinden; the park is about five minutes further on. Public transport is thin. The Yashiro Line of the Myoko municipal bus (Myoko Meguririn) runs toward Nishinoya from Arai Bus Terminal, close to Arai Station on the Echigo TOKImeki Railway Myoko Haneuma Line, but there are only a few services a day and some do not run on weekends or holidays, so check the city timetable first.

The Hikagesawa and Kuzuresawa structures are a different matter. The prefecture notes that there is not even a maintenance road up the Hikagesawa, which makes it hard for its own staff to check for damage, and no visitor route could be confirmed. The ravine is steep; do not try to scramble up to these structures, and rely on the prefecture's published material to see them.

The dams are outdoor structures, and neither the prefecture nor the city lists any restriction on photography. They are working sabo facilities: stay off the crests and spillway notches, and keep back from the water after heavy rain.