Velocity profile of turbulent open-channel flows over rough and permeable beds

Key words: 
• velocity distribution /
• log law /
• rough and permeable bed /
• open-channel flow

Abstract: The time-mean velocity profile of turbulent open-channel flows over rough and permeable beds is analyzed both experimentally and theoretically in this study. The velocity profile can be derived from a slip velocity at the bed surface and an origin displacement, a modified logarithmic law is thus obtained. The experiment is conducted in a glass-sided flume where the channel bed is prepared with uniform marbles 1 cm in diameter. The velocity is measured using a laser doppler velocimeter. The profile of the measured velocity is used to validate the theoretically derived profile. The result shows that the modified logarithmic law is able to well describe the measured velocity profile. Under the same flow condition, the shear velocity over the permeable bed would be greater than that over the impermeable bed. The value of the flow at the permeable interface is about 0.35 to 0.45 times that of the average stream flow.