Hirasakuhara Power Station Onbara Reservoir Dam Spillway

What is the Onbara Reservoir Dam Spillway?

A spillway is a structure designed to safely discharge water that exceeds a reservoir's storage capacity, preventing damage to the dam itself during heavy rainfall or snowmelt. The Onbara spillway carries excess water from the right bank of the Onbara Dam crest down a slope of roughly 40 percent gradient and returns it to the Onbara River, a tributary of the Yoshii River system, below the dam.

What makes this spillway distinctive is its careful engineering. The channel measures approximately 132 meters in length and 4 meters in bed width. Reinforced concrete walls rise from a slope reinforced with stone retaining walls, and the upper and lower portions of the channel are gently curved to slow and dissipate the force of the falling water. This was a thoughtful solution to the challenge of safely releasing large volumes of water down a steep mountainside without eroding the surrounding terrain.

A Pioneering Hydroelectric Project

To understand the value of the spillway, one must first understand the broader Heisakubara Power Station project. In 1928, Chugoku Godo Electric Company completed construction of the Onbara Dam in the upper reaches of the Yoshii River to meet the rising electricity demand of Okayama Prefecture. Water stored behind the dam was channeled to the Heisakubara Power Station downstream, where it generated up to 2,900 kilowatts of electricity. The station, now operated by Chugoku Electric Power Co., is one of the oldest dam-and-waterway-style hydroelectric plants still in operation under that company.

Because the construction site was deep in the mountains and far from any railway, transporting heavy materials such as cement was extraordinarily difficult. To minimize concrete usage, the engineers chose the buttress dam form: a thin reinforced concrete water-blocking wall supported by a series of buttresses. This design required less concrete than conventional gravity dams, lowering both cost and construction time. The result was the Onbara Dam, which today is the oldest surviving hydroelectric buttress dam in Japan.

Why Was It Designated a Cultural Property?

In November 2006, ten facilities of the Heisakubara Power Station, including the spillway, were collectively registered as Tangible Cultural Properties by the Japanese government. The Agency for Cultural Affairs recognized them as valuable structures for understanding the history of hydroelectric power in Japan and as features that contribute to the historical landscape of the country.

The spillway was registered under number 33-0135. Although it is a utilitarian structure, its careful design—the elegant curvature, the integration of stone and reinforced concrete, and the close relationship with the buttress dam it protects—makes it a representative early-Showa-era piece of civil engineering. Together with the dam, intake weirs, sand basins, water tank, and powerhouse, it forms a nearly complete and largely intact hydroelectric system from the late 1920s, a rarity in modern Japan.

The Onbara Dam itself has additionally been designated as part of the Ministry of Land, Infrastructure, Transport and Tourism's selection of modern civil engineering heritage, underlining the broader significance of the entire complex.

Highlights and Things to See

Visitors who make the journey up to the Onbara Highlands will find the spillway most striking when viewed from the dam crest. Looking down to the right bank of the dam, you can trace the long concrete channel as it sweeps down the wooded slope toward the river below. Although the surrounding forest can obscure parts of the structure, the geometry of the curved walls and the contrast between the gray concrete and the natural greenery make for a quietly beautiful scene.

Adjacent features worth observing include the buttress wall of the Onbara Dam itself, a series of commemorative monuments at the dam site dating from May 1928, and the broader engineered landscape of intake weirs and sand basins along the Onbara and Endo rivers. Please note that the area around the dam structure is fenced off for safety; visitors should view the spillway only from publicly accessible vantage points and never enter restricted zones.

Around the Onbara Highlands

The spillway is set within the Onbara Highlands, a beautiful upland area at an elevation of around 700 meters surrounded by white birch and larch forests. Onbara Lake, the reservoir created by the dam, is the centerpiece of the area and is celebrated for its stunning autumn foliage and, in winter, the rare ice patterns that form on its frozen surface.

The area offers year-round outdoor recreation. Onbara Kogen Ski Resort is one of the largest ski areas in Okayama Prefecture, with slopes for beginners through advanced skiers and a free play zone for sledding. The Onbara Kogen Auto Camping Ground is popular in the warmer months for its quiet sites and proximity to nearby hot springs. The Takasimizu Trail, a roughly 8.7-kilometer trekking course straddling the prefectural border between Okayama and Tottori, also begins nearby and offers expansive ridge-line views.

For those interested in modern engineering heritage, a visit to the Onbara Dam complex pairs naturally with a stop at the nearby Tomata Dam, a large gravity-style dam completed in 2005, allowing visitors to compare the construction philosophies of different eras of Japanese dam engineering.