Reduction of bend scour with an air-bubble screen - morphology and flow patterns

The interplay between streamwise flow, curvature-induced secondary flow, sediment transport and bed morphology leads to the formation of a typical bar-pool bed morphology in open-channel bends. The associated scour at the outer bank and deposition at the inner bank may endanger the outer bank's stability or reduce the navigable width of the channel. Previous preliminary laboratory experiments in a sharply curved flume with a fixed horizontal bed have shown that a bubble screen located near the outer bank can generate an additional secondary flow located between the outer bank and the curvature-induced secondary flow and with a sense of rotation opposite to the latter. This bubble-induced secondary flow redistributes velocities and bed shear stresses. The reported study investigates the implications of a bubble screen on the flow and the morphology in configurations with mobile bed. Velocity measurements show that the bubble-induced secondary flow shifts the curvature-induced secondary flow in inwards direction and reduces its strength. The bubble screen considerably reduces morphological gradients. Maximum bend scour is reduced by about 50% and occurs further away from the outer bank where it does not endanger the bank stability anymore. The location of maximum scour coincides with the junction of the curvature-induced and bubble-induced secondary flows. At this same location, the maximum streamwise velocities and maximum vertical velocities impinging on the bed also occur, which indicates their importance with respect to the formation of bend scour. The bubble screen also substantially reduced deposition at the inner bank. These preliminary experiments show the potential of a bubble screen to influence and modify the bed morphology.