Negative surges and unsteady turbulent mixing induced by rapid gate opening in a channel

• A series of detailed unsteady measurements was conducted in negative surges.
• Three types of bed roughness were tested including smooth and very rough inverts.
• Instantaneous water elevations and velocities were recorded systematically.
• Large free-surface elevation fluctuations were measured beneath the surge.
• Large velocity and Reynolds stress fluctuations were observed.

A negative surge is an unsteady open channel flow characterised by a drop in water surface elevation. In this study, a negative surge generated by the rapid gate opening was investigated experimentally with three types of bed roughness. Both instantaneous free-surface and velocity measurements were performed and the results were ensemble-averaged. The experimental data showed a rapid flow acceleration beneath the negative surge, and large and rapid fluctuations in all instantaneous velocity components were observed during the passage of the negative surge leading edge. The Reynolds stress data showed large ensemble-average and fluctuation levels, significantly larger than in the initially steady flow, occurring slightly after the passage of the surge leading edge. The time difference between the maximum Reynolds stress and surge leading edge was observed to increase with increasing distance from the gate, and it was comparable to the time delay for the occurrence of maximum free-surface fluctuations. The findings suggested that the unsteady flow properties were little affected by the bed roughness, despite the broad range of equivalent sand roughness heights tested herein.