Investigation of recovery techniques of ecology potential downstream from a dam using bed form improvement

After dam construction, the flow and sediment regime change downstream from the dam. For example, the armor coat progresses because of sediment trapping by the dam. To solve these problems, 'sediment replenishment' has been executed recently. Nevertheless, it is difficult to maintain effects of sediment replenishment over the long term because the restored sediments are washed away easily in steep rivers, especially in slippery bedrock sections. Therefore, the authors have proposed techniques that promote gravel sedimentation by improving riverbed forms. The effects of proposed techniques such as the river widening and multistep masonry weirs were investigated using a two-dimensional numerical model. Results showed that the proposed techniques are useful to reduce tractive force in steep rivers. They can create a desirable bed state for river ecosystems with various gravel sizes.