THREE DIMENSIONAL FLOW STRUCTURES AROUND A LARGE-SCALE DEEP SCOUR IN THE KISO RIVER

The existence of a vertical vortex (vortex having vertical axis) is considered to be the cause of development and sustention of large-scale local scouring in the Kiso River. In order to make clear the 3D flow structures in this deep local scour, we conducted a field observation and model experiments. From the result of ADCP set on the bottom of the scour hole, the velocity in the hole was increasing and the vertical vortex was developing, with an increase of river discharge. It is indicated that considerable magnitude of velocity was generated even near the bottom by the action of vertical vortex. This vortex was recognized in the model experiments, but the vortex structure was rather unstable. Consequently, the vertical vortex contributes to develop and maintain the large-scale local scouring.