In open channels and water supply canals, the brusque operation of control valves and gates may induce large unsteady flow motions called surges. To date, the literature has focused on the propagation of surges in smooth canals, ignoring the effects of large roughness and debris. Herein, a physical study was conducted under controlled flow conditions to study the turbulent mixing in the close vicinity of a large circular bed roughness element during the upstream propagation of positive surges. Detailed free-surface and instantaneous velocity measurements were conducted with and without the large and flat cylindrical element. For a number of tests, the experiments were repeated 25 times, and the results were ensemble averaged. The data suggested that the positive surge propagation was associated with large instantaneous free-surface fluctuations for all investigated flow conditions. The velocity measurements showed large variations in longitudinal velocity during the surge generation, as well as large fluctuations of all velocity components. The presence of the large bed element modified the velocity fluctuations and unsteady Reynolds stresses in the vicinity of the element. The present results implied the potential for bed scour around the element during surge propagation.
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