Transition from permeable to impermeable beds and vice versa in open channels: Effects on the velocity distribution of turbulent flow

This study investigates the impact of permeable to impermeable (and vice versa) bed transition on the velocity distribution of turbulent flow in an open channel. This experimental investigation is based on the 2D Particle Image Velocimetry (PIV) method, which allows data acquisition at fine spatial-temporal resolution. The experiments were conducted in a horizontal channel of 6.5 m length, 7.5 cm width, and 25 cm height. A grass-like vegetation of 2 cm height was used to represent a permeable bed, since these conditions are typical of flows encountered in sediment transport problems. In total, 36 experiments were carried out. The velocity is measured above the vegetation for the permeable bed and above the impermeable bed at three different discharges (0.735, 0.845 and 0.970 lt/s) and three different flow depths (4, 7 and 10 cm). The experiments were performed in four different locations of the channel (over the permeable bed, in the transition point from permeable to impermeable bed, over the impermeable bed, and in the transition point from impermeable bed to permeable bed). Results show that the velocity distribution in channels with transited permeable-impermeable beds (and vice versa) is different to distributions of velocity in solely permeable or impermeable channel beds. In particular, results show high sensitivity to the magnitude of discharge and total flow depth.