Abstract: To investigate the flow characteristics in meandering channels under backwater conditions caused by overflow dams, a meandering channel model with an overflow dam at its outlet is designed and constructed. Velocity and water level data in thirteen cross sections are measured using an Acoustic Doppler Velocimeter (ADV) and a digital water level altimeter. The experimental results demonstrate that the absolute transverse water surface gradients in curved segments decrease as the relative dam height increases. The longitudinal water surface gradients between a bending apex and the inlet of the downstream adjacent crossover area exhibit positive values, while the longitudinal water surface gradients between the inlet of a crossover area and the downstream adjacent bending apex display negative values. Nevertheless, the magnitudes of these gradients diminish with the increasing relative dam height. Moreover, secondary flow vortex blobs are observed in the crossover areas of the meandering channel, rotating in the same direction as those in the upstream adjacent curved segment. With an increase in relative dam height, the depth-averaged longitudinal velocities along the meandering channel decrease to varying extents. Likewise, the maximum turbulent kinetic energy in any cross section decreases as the relative dam height increases. Regarding the distribution of turbulent kinetic energy, the longitudinal velocity fluctuation contributes the most, followed by the transverse velocity fluctuation, while the vertical velocity fluctuation contributes the least. The Reynolds stress (R_uv) can be used as an approximate indicator of the directions of cross-sectional transverse flow movements at the inlets and outlets of the crossover areas. If the Reynolds stress value is positive in a specific region, the fluid will flow from left to right; otherwise, the fluid will flow from right to left.

Key words: overflow dam, backwater condition, meandering channel, flow motion, turbulent characteristics