The Bizenkyo Irrigation System is an irrigation canal around 23 km long. It draws water from the midstream area of the Tone River, which has the largest basin area in Japan and has irrigated approximately 1,400 ha of agricultural land along the right bank of the Tone River in northern Saitama Pref. since the early 17^th century. Before the canal was constructed, residents in this area relied on rainwater, spring water, reservoirs and other water resources to cultivate rice and agricultural management was quite unstable. In 1604, Daikangashira Ina Bizen-no-kami Tadatsugu (Governor of Bizen Province, hereinafter referred to as “Ina”), was ordered to build a canal by the central government called ‘Edo Shogunate’ and did so within a year. The canal is one of the oldest canals in Saitama Pref. and has long been nicknamed “Bizen moat” after the official title of Ina.

After investigating the amount of available water and the topography of the Karasu River, a tributary of the Tone River, Ina constructed a water intake and excavated the irrigation canal.

This project involved constructing the Yajima weir, a reservoir system using part of the Oyama River, to distribute water through the main and branch canals. This water distribution system was later called "Kantouryu (Ina Style)" and was used to irrigate the right bank of the Tone River, which previously had no irrigation canal. This agricultural development to boost food production improved the economic situation considerably. Also, the downstream flow into the Kitagawara and Hanyuryo canals, located in the uppermost stream of the Nakagawa River System, contributed as a key water resource to irrigate paddy fields in south-eastern Saitama Pref., lacking any water resources from the mountains.

Furthermore, many watercourses exist, even now, many years after its construction and it is considered a historically valuable irrigation system, helping replenish groundwater, preserve the landscape, conserve ecosystems and prevent flooding.

Also, in 2006, the Bizenkyo Irrigation System was registered as one of “Japan’s fine 100 canals” by the Ministry of Agriculture, Forestry and Fisheries, in recognition of its long history and excellent landscapes. The traditional “floating lanterns on the river” were revived by residents in 2009 when developing the amenity embankments, using natural stones.

To raise awareness among future generations, the Bizenkyo Irrigation System is being introduced as an important component of regional infrastructure, alongside Ina’s accomplishments, in supplementary reading material used in the social studies classes in city-wide elementary schools.

Water Heritage

The Bizenkyo 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;

Although the land formed by flooding was fertile, it was too high to draw water, which forced the residents to rely on rainwater, spring water and reservoirs, etc. for rice production. Accordingly, Ina excavated an irrigation canal of about 23 km by installing an intake port in the river. The Canal stabilized the supply of agricultural water, which considerably boosted agricultural development and food production as well as helping the farmers prosper.

The Bizenkyo 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;

In 1604, Ina surveyed the water volume and topography, formulated a plan and finished excavating the irrigation canal of about 23 km in one year.

When excavating the irrigation channel, the “Kanto Style” (Ina Style) irrigation technique was used, drawing on natural methods such as the “Tamei method,” whereby the Karasu River was used as the water source to temporarily fill the Koyama River, the Yajima Weir installed downstream to ensure a heightened intake water level and water flowing down the Koyama River channel was stored to realize a stable water intake.

Its downstream flows into the Kitagawara and Hanyuryo Canals of the Nakagawa River System, helping it contribute as a key water source for paddy fields in southeastern Saitama, which lacks any mountain water sources.

The intake port, relocated to the Kuguu area in Honjo City in 1887, was the first brick-built watergate in Saitama Pref. designed by Mulder, a Dutch civil engineer and incorporated the latest technology at that time.

Bizenkyo Irrigation System made an outstanding contribution to enhancing food production, livelihood opportunities, rural prosperity, and poverty alleviation in a region;

Rice production, which relied on rainwater, spring water and reservoirs, etc., was unstable. However, after the Canal was excavated, the food production yield for a vast area covering over 140 villages increased to 10,500 tons and stabilized to help farmers prosper. The Canal is a structure that was key to revitalizing the local economy.

The Bizenkyo Irrigation System was innovative in its ideas at the time of its construction;

It was a groundbreaking project that involved surveying the water volume and topography, formulating a structured plan and completing the work in one year, so that the river could be used for irrigation rather than relying on rainwater, spring water and reservoirs, etc.

The Canal has secured its water intake through overfall river weirs to stably accept river water. The water volume and water level are also secured by installing Yajima Weir, in the section where channels of the Koyama River are used, to stabilize the irrigation water supply. The surplus water is directed to the Fuku River, which contributes as a valuable water source for paddy fields in southeastern Saitama.

The Bizenkyo Irrigation System contributed to the evolution of efficient and contemporary engineering theories and practices;

Ina developed the “Kanto Style (Ina Style)” irrigation technique as an excellent civil engineer. The technique, which is an efficient irrigation plan, utilizes the natural topography, where water is taken from a river and Yajima Weir that employs the reservoir system utilizing the Koyama River is installed, which also enables the use of drainage water in the weir’s upstream area.

The Bizenkyo Irrigation System is an example of attention to environmental aspects in its design and construction;

The area is a fertile agricultural promotion area representative of Saitama Pref., combining paddy and upland field areas and producing the “Fukaya green onion” as the main crop.

In the upland field areas, unlined canals are also crucial to replenish groundwater as well as helping maintain and conserve the aquatic ecosystem and natural environment. The Bizenkyo Irrigation System Land Improvement District maintains and manages the facilities, while farmers and residents weed and pick up trash around the Canal, as activities that help underpin the rural communities.

The Bizenkyo Irrigation System was an example of engineering marvel or excellence at the time of its construction;

Surveys, investigation, planning and excavation work were carried out and the work period was shortened by partially using the Koyama River. To secure adequate water, overfall weirs were installed in the river to maintain the intake water level. Since its current flow paths are the same as those of the time of excavation, the plan can be evaluated as optimum based on appropriate surveys.

The Bizenkyo Irrigation System was unique in some positive and constructive way;

To stabilize the intake volume, the water level is secured by overfall weirs. In 1828, the first sluiceway was installed upstream of the intake port and the second sluiceway was installed downstream to solve a conflict between villages over floods and water shortage measures. This double sluiceway system was used for 131 years until 1959.

Currently, the Canal channels water for 2.2 km through the riverbed in the foreland after taking in water and the water enters the protected inland via the third sluiceway at the Tone River embankment in the Kuguu area, Honjo City. This is a unique form of irrigation, whereby water channels are installed in the foreland and the sluiceways are left as they are when moving the intake port to upstream of the Tone River.

Diversion weirs installed in the main canal divert the water to branch canals by raising the water level (a reservoir function), so that each weir maintains its characteristic form such as the inclination of a weir with respect to the centre of the stream and weir structure, etc. At Suzumenomiya Weir installed in the Ehara area, Fukaya City, a weir is installed diagonally to the centre of the stream and the centre of the stream is directed toward the water intake port of the branch canals.

Bears the stamp of a cultural tradition or a civilization of past the past;

In the Joshikimen area, Fukaya City, a railway dedicated to transporting bricks from Japan Brick Manufacturing Company was constructed in 1895 and the Bizenkyo iron bridge was built. The plate girder bridge is designated as an important national cultural property along with the adjoining brickwork arch bridge. Eiichi Shibusawa helped establish the company and was honoured with a stone monument in the Harago area, Fukaya City, which conveys the history of how the paddy fields expanded by digging raw soil for bricks from upland fields as mentioned by Shibusawa to the present. The Canal was registered as one of “Japan’s 100 fine canals” by the Ministry of Agriculture, Forestry and Fisheries in 2006 and events such as the “floating lanterns” were revived in 2009 in the Arai area, Fukaya City when embankments using natural stones were developed.

Engineering utility vis-à-vis designed utility

The Canal remains unlined to this day and embankments, which are constructed around bridges, weirs and vulnerable parts of slopes, also use natural stones with the natural environment and landscape in mind.

The Yajima Weir and diversion weirs have been reconstructed into sturdy structures such as gates made of reinforced concrete or steel, to ensure they can adjust the intake volume and ensure the safe flow of floods.

Present State of Conservation

Even at present, the Bizenkyo Irrigation System serves as an agricultural irrigation canal supporting regional agriculture and a key structure supplying water to a vast agricultural land area of about 1,400 ha along the right bank of the Tone River in northern Saitama Pref.

The current flow paths are the same as those of the time of excavation, even after 400 or so years and many sections remain unlined, which makes the Canal also historically valuable in conveying the appearance of the time of excavation to the present.

The Canal has been maintained unlined, which has helped it continue to fill multiple functions, including replenishing groundwater, maintaining rural landscapes, conserving ecosystems and preventing floods. Environmental conservation activities, for example, cooperation on the part of residents in weeding and cleaning activities by the Land Improvement District for maintenance and management purposes, are also carried out.

The Canal has been fondly referred to as “Bizen moat”. To improve the living environment and maintain the landscape, flowers such as cosmos and hydrangea as well as cherry trees are planted along the Canal by residents and corporations, enhancing its water amenity function and allowing it to be utilized as a local resource.

As part of school education, alongside Ina’s accomplishments, the Bizenkyo Irrigation System is introduced as a supplementary social studies component in elementary schools as a key local structure. Moreover, efforts are made to raise awareness among future generations, e.g., by holding surveys on living creatures in collaboration with elementary schools.