Abstract
The Shibetsu River originally had a number of meanders, and a wetland extended at its lower reaches. Cut-off works to straighten the meandering river course began in 1953. As a result, safety against flooding was improved, the wetland disappeared and farmland development in the river basin progressed. In recent years, however, there have been increasing demands from local residents for the creation of a better river environment. Restoration of the natural river environment has therefore been promoted since 2000. Since it was considered to be difficult to ensure safety against flooding if only the old meandering river course was restored, it was ultimately decided that the old river would be restored while maintaining the present straightened river course. Water was reflowed into the restored meandering river course on March 18, 2002. The meandering river experienced six floods during which annual maximum daily discharge exceeded the average annual maximum daily discharge rate. Eight field studies were conducted to visually observe changes in the channel and flow conditions. And to quantitatively understand changes in landform, five cross-section surveys were performed. These investigations showed that bank erosion, other than the expected bank erosion at the bifurcations, occurred in many places because of meander bends and flow deviation due to sandbars. The erosion resistance of banks differed markedly between the pre-improved channel and the excavated channel. Therefore, it is essential to take this behavior into consideration when predicting channel migration. It is also important to gain a thorough understanding of the areas to be affected by the plan through the examination of relevant data, including the centerline of the channel, bed configuration and flow conditions.
