| Traverse measurements with ADCP mounted on a tethered platform were conducted in actual rivers with/without the fluctuation of river bed elevation during flooding to investigate the change of Manning’s roughness coefficient. Based on the result of this measurement, roughness coefficient showed remarkable change with the fluctuation of river bed elevation during flooding. Especially, with the sudden degradation of river bed, the coefficient increased rapidly. This phenomenon is considered to be same as the increase of flow resistance with the passage of sand waves which caused flow exfoliation at the trough part of the sand waves. The coefficient in rivers without the fluctuation of river bed elevation did not change during flooding. There seems to be a threshold about the relation between the dimensionless tractive force and the roughness coefficient. It seems that the threshold of the dimensionless tractive force is 0.1 and the tendency of the roughness coefficient in zone where the dimensionless force is larger than 0.1 is different from the tendency in the other zone. |
