289 / 2024-02-29 08:28:50

Removal of fixed weirs and its influence on river-bed evolution

Removal of weir,River-bed evolution,Flume experiment,Numerical simulaiton

Since fixed weirs have often negative impacts on the sediment management, removal works of weirs have been promoted in the world. Therefore, it is important to know the response of river-bed evolution due to removable of the weir. In particular, the river-bed evolution in case of multiple weirs may not only be influenced by the distance between them but also may be depended on removal method, that is, simultaneous removal or individual removal. Investigations on these situations have not been always conducted enough, so there are many unknown factors. In this study, we investigate the effects of removal of a single weir and two weirs on river-bed evolution in the upper and lower reaches of the removed weir(s) by means of the flume experiments and numerical simulations. The difference of rive-bed evolution between the simultaneous removal and individual removal of weirs is also investigated.



Experiments were conducted in a straight open channel with a length of 10 m and a width of 0.5 m. Two kinds of weirs was applied in the channel. The uniform and non-uniform sediments as river-beds were used in the experiments. As for the hydraulic condition, the ratio of the shear stress to critical shear stress was set at approximately 1.5 in the experiment. This condition corresponds with the mean annual maximum flow rate of the target River in this study. The river-bed evolution process after the weir removal was investigated in the experiment. The numerical simulations were conducted not only to reproduce the flume experiments but also to further investigate a removal method of weir. The calculation conditions were same as the experiments.



The results are summarized as follows:

1. The upper and lower reaches of the weir mark were gradually degraded and aggraded, respectively. This phenomenon found to be proceeded fast in early stage of the experiment.

2. As for the two weirs experiment, the aggradation area downstream of the lower weir mark due to the simultaneous removal of weirs was longer than that due to the individual removal of them. Inversely, the degradation area upstream of the upper weir mark was vice versa.

3. The degradation and aggradation areas of the non-uniform sediment bed were relatively small compared with the uniform sediment bed.

4. The calculation results could reproduce the features of the experiment results. These results and methods can apply to the other removal work of weirs.