Mannou-ike Reservoir boasts a bank height of 32.0m, bank length of 155.8m, a water storage volume of 15,400,000 cubic meters, with an irrigation area of 3,003ha, which is Japan~s largest and one of the oldest agricultural reservoir. The reservoir is said to have originally been constructed in the year 701 when rulers actively promoted the cultivation of new paddy fields and development of irrigation system for expanding production capacity of the newly formed nation, and it was constructed as a large-scale source of water for the region.
In the year 818, the reservoir was damaged in severe flooding. The restoration of the reservoir became an urgent matter, leading to the famous high priest Kukai, who studied the latest civil engineering methods in Tang Dynasty China, was appointed to the reconstruction. This great work was completed using new construction methods including arched banks, spillways, and specialized revetments which helped prevent both erosion and landslides in 821.
The repeatedly damaged Mannou-ike Reservoir was again reconstructed in 1631. A major feature of the reconstruction is the installation of 5 intake holes on a vertical pipe making for a structure that allowed water intake from the top of the reservoir according to the water level (Illus.-1). This method for collecting water closer to the surface, in other words closer to warm water, is considered more suitable for paddy farming. This is still used as a model for reservoirs today, and is highly esteemed as an excellent practical technology.
However, the wooden pipes would quickly decay, with the bottom pipes being replaced approximately 6 times and the vertical pipes being replaced approximately 12 times over approximately 230 years. A plan was proposed to tunnel irrigation holes into the large rock located on the western corner of the bank to replace the wooden bottom irrigation pipes, which had required massive amounts of heavy labor to periodically replace, in order to lessen the burden on farmers, and the construction was completed in 1870. The bottom irrigation pipes, which were constructed using this excellent idea and technique, greatly improved the efficiency and efficacy of maintenance and management as these irrigation holes have not yet needed to be repaired to this day.
Since its construction in 701, the reservoir has collapsed as a result of flooding 3 times, and our forerunners utilized their collected knowledge to reconstruct the reservoir each time it collapsed. The bank heights were increased 3 times since 1900, continually improving the reservoir to its modern form which boasts one of the largest water reservoir volumes in Japan.
In addition, large numbers of tourists gather to view the water flowing forcefully from the irrigation outlets during the |yurunuki| pulling out of reservoir plugs to release water. It has earned the area a reputation as a memorable and peaceful place. In this way, Mannou-ike Reservoir has continued to preserve civil engineering feats over a period of 1300 years, when our forerunners battled against the ferocity of nature, and can be said to possess a truly universal heritage value in historical, cultural, traditional, technical, and maintenance aspects as one of the world~s preeminent agricultural reservoirs.