Abstract: To study the three-dimensional structure of the flows in a narrow, deep curved main channel in a river reach, we conduct acoustic Doppler velocimetry (ADV) measurements of the 3D flows in a sine-generated meandering laboratory channel, using 12160 gauge points located at 40 cross sections. The results show that in the channel, the curved bank boundaries cause changes in flow structure, and near the cross sections of zero curvature occurs a transition zone where the dynamic axis of flow swings between two banks. In near-bank regions, streamwise velocity takes fluctuating variations along the banks, with low velocity zones occurring near the concave bank; fluctuations along the left and right sides feature the same wave length but a phase difference of 180o. At all the cross sections, a main circulation near the bottom of the channel center and small reverse circulations near the water surface of the near-bank regions can be observed; the main circulation takes the lowest intensity around each bend vertex. At certain cross sections, small circulations rotating in the same direction as the main one also occur near the water surface of the near-bank regions. The experimental data can provide validation for numerical simulation with continuous multiple curves.

Key words: Sine-generated meandering channel, longitudinal velocity, hydrodynamic axis, secondary flow structure, secondary flow circulation strength