A Tour of Dams in East and Oku-Mikawa: Why Four Dams with Different Purposes Are Concentrated in the Same Region
1. Introduction
On my way back to my parents' home in Aichi, I took a slight detour to visit four dams.
Ure Dam, Shin-Toyone Dam, Kuroda Dam, and Yahagi Dam.
Although I traveled by car rather than by motorcycle this time, the benefit of collecting dam cards remains the same in that I never get lost on my way to my destination.
However, this combination was too significant to end as a mere collection record.
When you line up these four dams and look at them, their types are different, their administrators are different, and even the“purpose for which they were built”is different. Why are dams with such different characteristics concentrated in a relatively small area in the mountainous region of Mikawa? I would like to use this sense of incongruity as a starting point to organize the origins of each of the four dams.
2. Basic Data on the Four Dams
When lined up like this, it becomes clear that even though they are all grouped under the single word “dam,” they are actually four completely different types of structures.
Looking at their locations, Ure Dam is in East Mikawa, Shin-Toyone Dam is in Oku-Mikawa, and Kuroda Dam and Yahagi Dam are located in Toyota City in West Mikawa, meaning the itinerary spanned the geographical divisions of East, Oku, and West Mikawa.
3. Decoding by Purpose
Ure Dam as a water utilization dam, and water intake across river systems
The starting point for Ure Dam lies in the national “Toyokawa Irrigation” project, which began shortly after the war in 1949.
At that time, East Mikawa, especially the Atsumi Peninsula, was a region suffering from chronic drought.
Ure Dam was completed in 1958 as a reservoir for that water, and since then it has continued to supply irrigation, tap water, and industrial water to the Atsumi Peninsula.
What cannot be overlooked here is that Ure Dam is not just a facility that stores water from the Toyo River.
As the Japan Water Agency itself cites as a technical feature, Ure Dam incorporates a watershed diversion that draws water from the Tenryu River system (Onyu River and Ochi-se River) into the Toyo River system (Ure River).
In other words, this dam is not a facility that is completed within a single river system, but a facility that makes the Toyokawa Irrigation project possible by mobilizing water from adjacent river systems as well.
The very idea of diverting water across administrative river system boundaries to make water utilization possible in a region with little precipitation is one answer to geographical constraints.
Furthermore, in relation to it being a water utilization dam not intended for flood control, Ure Dam has another characteristic piece of equipment.
It is a massive 6-stage surface water intake facility with a height of 48 meters, which is a cold-water countermeasure facility that allows the selection of intake levels while considering water temperature, rather than just letting the water from the dam lake flow downstream as is. You can really understand its scale when looking up from the crest.
Shin-Toyone Dam, whose purpose was added later
Among the four, the most structurally interesting is Shin-Toyone Dam.
Initially, Electric Power Development Co., Ltd. (J-POWER) planned it as a power-generation-only dam.
Surveys began in 1962, construction started in 1968, and the concept was to perform pumped-storage power generation with Sakuma Dam.
However, during construction in 1968 and 1969, the Tenryu River basin was hit by successive major floods.
Large-scale damage occurred, including the collapse of levees on the tributary Ochi-se River and the washing away of the Japanese National Railways Iida Line railway bridge.
Following this disaster, the Ministry of Construction (now the Ministry of Land, Infrastructure, Transport and Tourism) solidified a policy for Shin-Toyone Dam to participate in flood control, and after raising the dam height by 4 meters from the initial plan, it was completed in 1973 as a multi-purpose dam for flood control and power generation.
The system of joint management by the Ministry of Land, Infrastructure, Transport and Tourism and J-POWER continues to this day.
It can be seen as a case of policy purpose change and addition, where flood control functions were incorporated into the initial power-generation-only plan, triggered by a major flood during construction.
Kuroda Dam and Yahagi Dam as upper, middle, and lower regulating reservoirs
Kuroda Dam and Yahagi Dam have a relationship that cannot be discussed individually.
Both are part of Chubu Electric Power's pumped-storage hydroelectric power system, which combines two pumped-storage power plants, Oku-Yahagi No. 1 and No. 2, with Kuroda Dam as the upper regulating reservoir, Tominaga Dam as the middle regulating reservoir, and Yahagi Dam as the lower regulating reservoir.
The total output is 1,095,000 kW, making it the largest hydroelectric power plant in the Yahagi River system.
The origin of Kuroda Dam is old; the first dam was built in 1932 after Yahagi Hydroelectric Power started a plan to dam the Kuroda River in 1930.
Later, based on the old Kuroda Dam built in 1932, redevelopment proceeded in the 1970s, and the current Kuroda Dam was completed in 1980.
In September of the same year, Unit 1 of the Oku-Yahagi No. 1 and No. 2 power plants began operation.
On the other hand, Yahagi Dam is a multi-purpose dam managed by the Ministry of Land, Infrastructure, Transport and Tourism, and it serves diverse purposes including flood control, maintenance of normal water flow, irrigation, tap water, industrial water, and power generation (this combination of purposes is often represented by the symbols F, N, A, W, I, and P).
Even though they are in the same river system, one is a power-generation-only pumped-storage reservoir managed by an electric power company, and the other is a multi-purpose dam managed by the national government; two dams with different management entities and purposes function as a single power generation system in practice.Unless you read the explanations in the Dam Handbook or on dam enthusiast websites, it is difficult to notice this “set structure.”
4. Why are infrastructures from such different eras piled up in the same region?
Based on the organization so far, a deeper question emerges beyond simply “the four dams have different purposes.”
The question is: why are water and power policies from such different eras piled up in the same mountainous region of Mikawa?
First, there are the constraints of topography and river systems.
The mountainous areas of East, Oku, and West Mikawa have many steep rivers, making water flow unstable, while flat areas like the Atsumi Peninsula have lacked water sources and have suffered from chronic water shortages.
This geographical constraint necessitated water utilization projects that cross river systems, like Ure Dam.
Second, there is the accumulation of policy demands from each era.
Kuroda Dam began as a private power project before the war, Ure Dam as a national water utilization project during the post-war reconstruction period, Shin-Toyone Dam shouldered both power demand and flood control measures during the high economic growth period, and Yahagi Dam was planned as a post-war comprehensive development project, each under different policy demands of different eras. Even in the same region, if the era in which the dam was built is different, the role that the era demanded of the dam is also different.
Third, facilities built by different entities have resulted in being linked as a single regional infrastructure.
Four entities operating with different purposes and logic—the Ministry of Land, Infrastructure, Transport and Tourism, J-POWER, the Japan Water Agency, and Chubu Electric Power—built dams at different times for different reasons.
However, as a result, cases have emerged where facilities with different management entities function as one in practice, such as the Kuroda-Tominaga-Yahagi power generation system.
It can be said that the dam groups in this region have responded to the water resource and power demands of the entire region by combining multiple facilities with different purposes, rather than having a single massive infrastructure handle everything.
Ure Dam became a facility that diverts water across river systems, Shin-Toyone Dam changed from power-generation-only to a multi-purpose dam for flood control and power generation, and Kuroda Dam changed from a pre-war power generation pond to an upper regulating reservoir for pumped-storage power generation; all have reached the present while changing their roles from their original forms.
Looking at the four dams side by side, I felt once again that the purpose of infrastructure is not fixed once decided, but is something that is rewritten later in response to the constraints of the terrain and the demands of the times.
5. Future Tasks: Connecting to the History of Submergence and Compensation
This time, I organized the article focusing on the technical aspects and history of the dam types and purposes, but the other axis I was initially interested in—the history of submergence, relocation, and compensation of local residents accompanying dam construction—was not sufficiently touched upon in this article due to a lack of data collection on-site.
What kind of villages were on the shores of the Ure Dam lake? What kind of compensation negotiations took place during the redevelopment of Kuroda Dam?
These are parts that cannot be seen without consulting local museums or local history materials, and they are themes I would like to add as a sequel after my next visit or after conducting separate literature research.
6. Conclusion
Touring a single region is also about tracing the modern history of that region in a condensed form.
Post-war drought countermeasures, flood and power demand during the high economic growth period, the division of roles between power companies and the government—I had not imagined before I started collecting dam cards that such different logic from such different eras would be piled up in the mountainous region from East Mikawa to Oku-Mikawa and West Mikawa.
Next, I would like to follow the river system itself as a single line.
Should I look at the Yahagi River from the upper reaches to the mouth, or trace the entire Toyo River system? In any case, dams are not objects to be “visited and finished,” but rather an entrance to decoding the policies and regional history leading up to them, and it seems like a theme that will keep me interested for a while.