The Jyosai-gokuchi Irrigation System is around 12 km length of an irrigation canal in Toyama City, Toyama Prefecture and runs parallel to the Joganji River which is one of the world’s most rapids rivers. The canal is extremely important for irrigation and supports local life because it not only irrigates around 3,300 ha of agricultural land but also provides domestic and industrial water supplies as well as hydroelectric power.

History

Until the end of the 19^th century, the Joganji River had many irrigation intakes on both sides of its banks. Intakes on the left bank exposed to comparatively larger currents were destroyed or buried under sediment after every flooding. This caused significant damage to local agriculture. To prevent repeated flooding, a Dutch civil engineer, Johannis de Rijke, invited by the Ministry of Home Affairs in 1891, advocated a project for the Joganji River to create a unified intake (called ‘gokuchi’ in Japanese) for irrigation canals at a safer location upstream by closing all 12 irrigation intakes on the left side of the river together with a flood control project. The construction work began the following year.

At that time, the irrigated area was around 5,000 ha, and it was the first intake unification project of such magnitude in Japan. The irrigation project came with the main canal 12 km long, two tunnels, settling basins, and many more works that proved extremely difficult, but with over 10,000 workers mobilized from the prefecture and elsewhere, the canal was completed in just two years. This project broke new ground in Japan’s modern agricultural engineering history in terms of the design and scale. Later, several intake unification projects flourished in the prefecture, which helped eliminate water disputes and stabilize the supply of water. The rice production increased throughout the prefecture and resulted in Toyama Prefecture becoming one of Japan’s leading agricultural regions as well as accelerating similar intake unification projects in Japan.

Moreover, to deal with the sediment taken downstream of the canal due to repeated flooding, the Shinjo Anti-sediment Sluice was constructed in 1900. Entirely brick-built, it has been called the “Shinjo Red Gate” for many years and is loved by the neighbourhood. The 16th century “Sassa Levee” was built to contain the Joganji River and prevent flooding during the Warring States period and remains visible to this day in the bed of the current canal, revealing glimpses of its original appearance. Finally, this canal was one of the first nationwide to be developed as a calming waterfront. Its historical and scenic value make it a key facility for our prefectural citizens.

Water Heritage

The Jyosai-gokuchi Irrigation System represents a milestone / turning points in development of irrigated agriculture and should bear an exceptional testimony to development of agriculture and increase in food production along with the improvement of economic condition of farmers

The Meiji government of Japan (1868-1912) invited many foreign experts from multiple foreign countries to learn about modern construction methods. Johannis de Rijke was one of the most important of these invited foreign experts, who came to Toyama Prefecture on August 6, 1891, and proposed a flood-control plan for the Joganji River, known for its repeated flooding. Based on this plan, the following year, the first large-scale national intake unification project was undertaken and in 1893, the Josai Unified Intake Irrigation Canal was completed.

This intake unification project spurred other similar projects around the prefecture, which eliminated water disputes and enabled the stable distribution of water, helping boost rice production and paving the way for Toyama Prefecture to become one of Japan’s leading agricultural regions.

The Jyosai-gokuchi Irrigation System was ahead of its times in terms of project formulation, engineering design, construction techniques, dimensions of the structure itself, quantum of water diverted, and size of the command.

With a large irrigation area of around 5,000 ha and the unification of 12 complex irrigation canal systems, this project was the first of its kind nationwide and the most advanced in terms of the scale or the facilities and the area served. Furthermore, focusing on the large volumes of sediment from the Joganji River, a large settling basin upstream to remove sediment (photos 2 and 3) and an anti sediment sluice (photos 4 and 5) were constructed as part of advanced anti-sediment measures for this project. Starting from this project, intake unification projects nationwide were undertaken alongside riverbed repairs, making the Josai Unified Intake Irrigation Canal a trailblazing project that transformed water use in Japan.

The Jyosai-gokuchi Irrigation System made an outstanding contribution to enhancing food production, livelihood opportunities, rural prosperity, and poverty alleviation in a region.

The Joganji River is one of the steepest rivers nationwide. Known as Japan’s most overflowing river, the river would flood almost annually with strange debris flow (photos 6 and 7). The 1891 floods were particularly bad and with around 7 km of levees being destroyed, part of the flooded area was submerged for around a month, with survivors fleeing the damage one after the other (Fig. 5). Large volumes of sediment penetrated the system and prevented the adequate distribution of water, which had a great negative impact on local agriculture.

Completing the intake unification project helped agriculture develop significantly by becoming a force to prevent disasters and protect people’s lives and livelihoods, as well as improving food production productivity by ensuring a stable water supply.

The Jyosai-gokuchi Irrigation System was innovative in its ideas at the time of its construction.

De Rijke conducted in-depth surveys (photo 8) and found that one of the reasons for the Joganji River’s flooding was a problem with the intake of irrigation water. To solve this, de Rijke proposed an “intake unification,” and this large-scale project was completed in a short time (photos 9, 10, 11 and 12). Consequently, a large area of agricultural land gained a stable irrigation system and the centralization of its management, a revolutionary concept, spawned new agricultural water utilization systems.

The Jyosai-gokuchi Irrigation System contributed to the evolution of efficient and contemporary engineering theories and practices

Mentioned in paragraph (ⅱ).

The Jyosai-gokuchi Irrigation System is an example of attention to environmental aspects in its design and construction

Once the intakes were unified, floods and sediment influxes fell sharply and a stable water supply was ensured, which allowed the creation of a farmland ecosystem network.

The Jyosai-gokuchi Irrigation System was an example of engineering marvel or excellence at the time of its construction

This canal was the first endeavour of its kind nationwide. Questions were raised about whether or not the project could ensure that high volumes of water were delivered on a stable basis and whether or not such a delivery could be completed timely and consistently, but the facilities were completed exceptionally swiftly and the project was finished in around two years.

The project built upon the accumulation of detailed surveys and records from previous disasters before the project started, as well as extensive and precise water utilization statements. Ever since the Hietsu Earthquake in 1858, technologies for surveys, flood control and disaster relief for the Joganji River had been nurtured in the region and many highly precise planar and discharge surveys of various irrigation networks including the Joganji River had been commissioned. Our predecessors used these documents as the basis to create extremely precise water utilization calculations which were unheard of at the time and their effective and fair irrigation water distribution plan meant they could create a more stable irrigation system. Consolidating these irrigation networks was only possible due to the excellent surveys and planning technologies used by our predecessors.

The Jyosai-gokuchi Irrigation System was unique in some positive and constructive way.

Taking into account the fact that floods had caused water intakes to collapse and be destroyed and buried, the first intake tunnel was created by boring through the Takadomari Cliff, a tough rock wall plunging into the river and chosen for its capacity to withstand the Joganji River’s large floods. To deal with the rising river bed or water level changes during flooding as well as blockages, the rock wall was pierced in multiple places to provide a stable water supply. All this, including the fact that all was completed with manpower, came thanks to the tireless efforts and marvellous hard work of our predecessors.

The Jyosai-gokuchi Irrigation System bears the stamp of a cultural tradition or a civilization of past the past.

We note that in 1580, the warrior Sassa Narimasa created a long open levee by gathering and assembling massive rocks to prevent flooding caused by the Joganji River. The “Sassa Levee” is still visible to this day in the bed of the current irrigation canal and reminds us of the original’s shape.

Later, in 1769, Maeda Toshitomo, lord of Toyama Castle, planted a pine forest of around 6 ha near the irrigation canal to prevent sediment flows during the flooding of the Joganji River. This protective forest was called the Tonosamabayashi (Lord’s Woods). Over 100 trees remain and continue to protect the area from flooding damage. These pieces of heritage show vestiges of the civilization around the Joganji River and underline the high historical and technological value of the Josai Unified Intake Irrigation Canal.

Engineering utility vis-à-vis designed utility

Built with manpower alone following de Rijke’s plan, the irrigation system for the Joganji River’s left bank comprised dirt and masonry channels and provided a stable irrigation water supply for over 50 years.

Starting in around 1900, sediment control dam projects were commissioned in the Joganji River area and after half a century of the river bed becoming lower and lower, water intake from the Joganji River became increasingly difficult. Under these conditions, to solve water shortages due to the riverbed level changes and to reduce maintenance costs related to water management, the national Joganji River Agricultural Water Use Plan was created and work on a unified intake combining both the left and right banks of the Joganji River began.

This intake unification helped further stabilize the supply of water to the Josai Unified Intake Irrigation Canal, which saw peak intake water volume go from 33 m³/s (at the time of construction) to the current 35 m³/s. Additionally, the creation of the first water lovers’ promenade in an area around the irrigation canal in the country as well as taking care of the historical remains and sights have made this facility well known and popular with residents. This project became an opportunity for the country to create “Water Environment Establishment Projects” and similar facilities were created around the country.

Present State of Conservation

Construction on this irrigation canal began in 1892 when 16 towns and villages in the Kamidaki town area created an irrigation association. Following its construction in 1893, this association operated the completed system and defined distribution volumes to ensure water would be distributed equally upstream and downstream. The association was then renamed the Josai Irrigation Land Improvement Area in 1952, managing the irrigation canals and the land improvement area. Each branch line was managed by the local agricultural organizations and have done so for many years, following the rules and keeping order.

Presently, as the region has become increasingly urbanized and crowded, as well as a declining population and the ageing farming population, maintaining land improvement facilities has become more and more difficult. The association has been planning to restructure the irrigation management system more efficiently to ensure rational use and labour-saving using remote-controlled facilities, camera surveillance and management, measuring and recording of precipitation, water level and current data.

Three hydroelectric power plants using the irrigation canal were also constructed in 1961 to ensure the co-existence and co-prosperity of agricultural irrigation and hydroelectric power generation, meaning that the electric power company and the land improvement area association jointly manage and maintain a single sector. Furthermore, the land improvement area has installed a small-scale hydroelectric power generator using the irrigation canal and sales of the electricity generated go toward maintaining the agricultural water use facilities to lower costs. They have also partnered with Toyama City to install a micro-hydroelectric power generator using the irrigation canal, helping promote environmental awareness and the production of a low CO₂ green energy-reducing environmental loads. As of December 2019, Toyama Prefecture had 30 small-scale hydroelectric power plants using irrigation canals, in most of the nation. Small-scale hydroelectric power generation initiatives on the Joganji River were pioneering projects, not just in Toyama Prefecture, but nationwide.

Finally, the area led the way nationwide in 1984 in creating a waterfront area and facilities using the irrigation canal, which was chosen as one of Japan’s top 100 canals in 2006. This irrigation canal combines historical views such as the previously mentioned Sassa Levee and the Lords’s Woods with a beautiful waterfront space adorned with cherry trees, enjoyed by not just locals but also visitors from all over the prefecture.