An air-bubble screen used as a countermeasure to reduce erosion in open-channel bends

In open-channel bends, interactions between streamwise flow, curvature-induced secondary flow and sediment transport lead to a typical bar-pool bed topography. Erosion occurs near the outer bank that can endanger structures, whereas deposition occurs near the inner bank that can reduce the navigable width. Laboratory experiments have been performed in a sharp open-channel bend to investigate how a bubble screen can influence the bend hydrodynamics and morphological evolution. A porous tube placed near the outer bank and connected at both ends to a pressurized-air system generated a bubble screen strong enough to counteract the descending velocities of the curvature-induced secondary flow. Foregoing clear-water scour experiments with and without the bubble screen have shown that the bubble screen modifies the velocity distribution and acts directly on the bend morphology by shifting the region of maximum scour further away from the outer bank. In the reported study, similar experiments have been performed under live-bed conditions. Measurements of the threedimensional flow field and topography were compared and allowed estimating the beneficial effect of the bubble screen. In the upstream part of the bend, the redistribution of velocity pattern is not sufficient to strongly modify the morphology whereas at the downstream part of the bend, the scour hole is reduced and shifted from the outer bank to the middle of the cross-section.