Author(s): Violaine Dugue; Koen Blanckaert; Qiuwen Chen; Anton J. Schleiss
Keywords: Bubble screen; Erosion; Morphodynamics; Open-channel bend
Abstract: The typical bar-pool bed topography which develops in open-channel bends due to complex interaction between streamwise flow, curvature-induced secondary flow, sediment transport and bed morphology leads to adverse impacts such as bed erosion near the outer bank that can endanger bank stability, and sediment deposition near the inner bank that can reduce the navigable width of the river. Previous experiments have shown that a bubble screen placed near the outer bank can produce a bubble-induced secondary flow that is able to redistribute the base flow pattern and consequently the bed morphology. The feasibility of the so-called bubble-screen technique is investigated in a sharply curved laboratory flume under different conditions of bed and sediment transport. Results of experiments performed on a fixed horizontal bed and on a mobile bed under both clear-water scour and live-bed conditions are compared. The bubble screen is found to be efficient if the maximum inward transverse velocities at the water surface produced by the bubble screen are higher than the maximal outward transverse velocities induced by the topographic steering and curvature-induced secondary flow. Moreover, this condition has to be satisfied all around the bend. Based on this criteria, a straightforward method to evaluate the efficiency of a bubble screen to redistribute the flow patterns and the morphology is proposed, and illustrated by means of an application case. The minimum air discharge required to counteract the maximal transverse velocities induced by the curvature is relatively low as compared to other types of environmental applications of the bubble screens.