The Genius of the Echigo Plain
The Ohkouzu Diversion Channel is man-made river constructed by excavating ground for 10 km from the head of Ohkouzu in BunsuiĦĦtown, where the Shinano River gets closest to the Japan Sea, to Teradomari beach. Its purpose is to protect the Echigo Plain from flood damage of the Shinano River. During time of rain, the main weir is closed and the total discharge of flood water from the upper part of the Shinano River is directed to the Japan Sea through the movable weir and the fixed weir, at the side of the diversion channel.
A long time ago the Echigo Plain was a bay. Soil and sand carried by the Shinano River had been accumulating little by little and a marshy low land was created there. In such a marshy low land, floods often occured and continuously deprived people's lives and possessions.
Picture showing the curtailing work for the Ohkouzu Diversion Channel
Paddy field before the completion of the Ohkouzu Diversion Channel
The Ohkouzu Diversion Channel as a long-cherished desire of citizens
To prevent such flood disasters, people of Echigo began planning to build a diversion channel. It seemed like an eternity passed until the construction of this diversion channel was realized. Starting with a petition to the Edo Shogunate during the Kyoho Era, petitions were made repeatedly for over 200 years and at last, in 1910, a petition was admitted and full-dress construction started. Although the big civil engineering project had many difficulties, in 1936, the people's long-cherished desire came true.
The width of river at the Ohkouzu Diversion Channel watershed is 720 meters. The narrower the widthĦĦbecomes, the closer it reaches to the Sea of Japan. It is only 180 meters around its entry to the sea. The cross slope of the river gets steeper, closer to the watershed. This is caused by using the contours of the mountain side and reducing the cost of construction. At the mouth, the second fixed bed was constructed to protect the riverbed from erosion caused by the steep slope. After construction, the riverbed at the lower part of the second fixed bed began eroding. To protect it from this condition, a sub-weir bank was constructed 250 meters downstream. Moreover an energy dissipater (baffle pier) was constructed to strengthen the effect of energy dissipating through the second fixed bed.
This widely attributed to the development of the Echigo Plain
Before the construction of the Ohkouzu Diversion Channel, damage by flood occurred once every three or four years. After it was constructed however, damage by flood decreased and furthermore a marsh field called Ashinuma changed into dry land, where the volume of harvest increased. On the habitual flood areas the Shinkansen express trains can now pass and transportation by expressway flourished. It led to major developments in the cities, towns and villages located in the basin of the Shinano River.