Tsukahara Dam

Tsukabaru Dam: A Towering Monument to Pre-War Engineering

Deep in the mountainous heart of Miyazaki Prefecture, where the Mimi River carves through the rugged terrain of the Kyushu mountain range, stands Tsukabaru Dam — a gravity concrete dam that was once the tallest in all of Japan and East Asia. Completed in 1938, this 87-meter-high structure is far more than a hydroelectric facility; it is a living testament to the ambition and ingenuity of early 20th-century Japanese civil engineering. Registered as a national Tangible Cultural Property and designated as a Modern Industrial Heritage site, Tsukabaru Dam draws visitors who appreciate the intersection of industrial history, architectural beauty, and breathtaking natural scenery.

Historical Background

The development of hydroelectric power along the Mimi River system began in the early decades of the Showa era. Kyushu Soden Corporation (later absorbed into Kyushu Electric Power Company) undertook a series of dam projects on the river, starting with Saigo Dam and Tashiro Power Station in 1929, followed by Yamasuhara Dam in 1931. Construction of Tsukabaru Dam commenced in 1935, and the dam was completed three years later in 1938.

At the time of its completion, the 87-meter-high dam was the tallest concrete dam in Japan and was hailed as the tallest in East Asia. The project drew upon the most advanced international construction technology available, notably techniques developed for the Boulder Dam (now Hoover Dam) in the United States. However, all construction materials were sourced domestically, making the project a remarkable showcase of Japan's growing industrial capabilities.

The massive construction effort had a profound impact on the local community. The influx of workers to the remote village of Morotsuka swelled the population to a peak of 8,930 in 1937 — a record that has never been surpassed. Tragically, the project claimed the lives of 44 workers, whose sacrifice is commemorated by a memorial with a poignant relief sculpture on the dam's right bank.

Why Tsukabaru Dam Is a Registered Cultural Property

Tsukabaru Dam was registered as a national Tangible Cultural Property (Building) by the Agency for Cultural Affairs in March 2004. In 2007, it was further recognized as a Modern Industrial Heritage site by the Ministry of Economy, Trade and Industry. The Japan Society of Civil Engineers has also awarded it the highest A-rank designation among modern civil engineering heritage structures.

The dam's cultural significance stems from several pioneering achievements. It was the first dam in Japan to employ hard-mixed concrete — a technique that reduces the water-to-cement ratio to produce stronger, more durable concrete. It was also the first Japanese dam to use a movable cable crane for concrete placement, and it featured an on-site batch plant for producing uniform-quality concrete. These innovations collectively represented a leap forward in mechanized dam construction in Japan, making Tsukabaru Dam an important milestone in the nation's civil engineering history.

Architectural Highlights and Design Features

Tsukabaru Dam is a gravity-type concrete dam with a crest length of 215 meters. Its most striking visual feature is the row of battlement-style turrets along the dam crest, which give the structure an appearance reminiscent of the Great Wall of China or a European medieval castle. This ornamental design, unusual for a utilitarian structure, reflects the pride and craftsmanship that the builders invested in the project.

The dam's central section houses eight Tainter (radial) gates for flood discharge. The water impounded by the dam is conveyed through approximately 2.2 kilometers of conduit to the Tsukabaru Power Station downstream, where it generates a maximum output of 62,600 kilowatts of electricity. The reservoir has a total storage capacity of approximately 34.33 million cubic meters, with a lake surface area of 122 hectares.

The dam was designed by Itaru Yamamoto, constructed under the technical guidance of Professor Tokujirou Yoshida, and built by the Hazama Corporation (Hazama-gumi). The original batch plant used during construction still remains at the site, serving as a rare surviving piece of industrial archaeology.

Resilience Through Natural Disaster

In September 2005, Typhoon No. 14 brought unprecedented rainfall to the mountains of Miyazaki Prefecture, with cumulative precipitation exceeding 1,300 millimeters. A catastrophic landslide downstream created a natural dam across the Mimi River, causing floodwaters to back up and submerge the downstream face of Tsukabaru Dam to a depth of approximately 62 meters — nearly reaching the gate piers. Despite this extraordinary stress, the dam structure itself withstood the disaster without failure, a remarkable testimony to the quality of its original construction.

Visiting Tsukabaru Dam

Tsukabaru Dam is located in the village of Morotsuka along National Route 327 in northern Miyazaki Prefecture. The dam can be viewed from the roadside, where parking spaces are available both before and after the tunnel adjacent to the dam. A section of the chain-link fence near the viewing area has been specially cut open by Kyushu Electric Power to allow visitors to photograph the dam without obstruction — a thoughtful touch that has become a minor attraction in its own right on social media.

Access to the dam crest and management facilities requires permission from the site office, as the dam is an active power generation facility. The nearest bus stop is "Furuzono Entei-mae" on the Miyazaki Kotsu bus line running between Hyuga City and Shiiba Village. The local name for the dam is "Furuzono Entei" (Furuzono Weir), derived from the nearby place name.

Visitors should be aware that road conditions in the area may be affected by construction work or weather-related closures. It is advisable to check the Miyazaki Prefecture website for current road conditions before planning a visit.

Surrounding Attractions

The Morotsuka area offers a wealth of natural and cultural experiences for visitors exploring beyond the dam. The village is renowned for its forestry heritage and was recognized as part of a Globally Important Agricultural Heritage System (GIAHS) for its traditional forest management practices. Key nearby attractions include:

• Kamishiiba Dam — Located upstream on the Mimi River, this arch dam was the first of its type to be completed in Japan and is itself a significant engineering landmark.
• Akimasa Observatory — A panoramic viewpoint offering 360-degree views of the Kyushu mountain range, the Sobo-Katamuki National Park, and on clear days, the Hyuga Sea.
• Shiitake no Yakata 21 — A local facility celebrating Morotsuka's famous shiitake mushroom culture, with a restaurant serving mountain cuisine.
• Yoshinomiya Shrine — A historic mountain shrine associated with local legends and known for blessings related to business prosperity and eye health.
• Mount Kurogatake — The highest peak in Morotsuka Village, home to a pristine beech forest and rare botanical species including the southernmost colony of Japanese adonis.