Ohkohzu Diversion Flowing High

This picture, taken on 24 April, shows high water in Ohkohzu Diversion Channel, just above where the channel flows out into the Japan Sea. Ohkohzu Bunsui is a massive diversion channel about 10 km in length that was completed in 1924 to divert flood flows in the Shinano River and reduce the frequent flooding of the Niigata Plain. The Shinano River is the longest in Japan (367 km) with a catchment area of 11,900 km2, but more importantly it has the greatest volume of flow with a mean annual discharge of about 500 m3/s. Peak flows, however, can be much larger due to melting snows and in particular heavy rains. During these flood conditions most of the flow in the Shinano River is diverted down this channel and into the Japan Sea. Spring snowmelt and heavy rains Saturday combined to give this high water, as shown in this month's stage hydrograph below taken from the Ministry of Land, Infrastructure and Transport (MLIT) homepage (http://www1.river.go.jp/):

We can see that the peak on 24 April is the largest this month, at about 8.3 m, and it represents a flow of about 2100 m3/s. Compare this with the all-time record high stage of 10.94 m during a major rainstorm on 20 July 2006, when the flow reached over 7250 m3/s!

If all this water had gone down the Shinano River, there would have been a disastrous flood on the Niigata Plain. There are of course negative impacts on the environment resulting from the contruction of the Ohkohzu Diversion Channel, but it does a very good job in keeping dangerous flood waters away from the many cities and paddy fields of the Niigata Plain, which has an area of about 1,400 km2.