Numerical simulation of low-flow channel evolution due to sediment augmentation

For the conservation and restoration of river environment, a sediment replenishment technique, which conveys a part of the sediments excavated and/or dredged from reservoirs to the river below dams is developed and has been implemented tentatively in several dams. Sediments placed as replenishment can be flushed out and transported downstream by floodwater or dam releases. The flushed sediments are expected to contribute to the control of degradation and the variation of the low-flow channel. However, this technique is in the development stage because there are many unknown factors. Therefore, systematic investigations are necessary for practical management of the technique. In this study, the effects of the location of replenishment sediment on sediment flushing and on control of degradation were investigated by means of flume experiments. A two-dimensional numerical model was also developed to further investigate the effects of sediment augmentation on river restoration. The numerical model treats bank erosion and sediment transport over fixed beds. The simulation results were verified against the experimental results. The flushing process of replenishment sediment was investigated first, and then its effect as a countermeasure for river bed degradation was analyzed. Results are summarized as follows: (1) Augmentation at upper riffles is effective for flushing of replenishment sediment and variation of low-flow channel. (2) Amelioration of degradation can be found in the cases of two types of placement. The amelioration effect of upper riffle placement was larger than that of lower riffle placement (3) Aggradation rate of the bed near the replenishment site in the fixed bed is large in the lower riffle placement compared with the upper riffle placement. (4) The numerical model was found to be generally successful as a predictive tool.