Influencing bend morphodynamics with an air bubble screen

Interactions between curvature-induced secondary flow, streamwise flow and bed morphology in open-channel bends lead to the development of a typical bar-pool bed morphology, shaped by scour at the outer bank and deposition at the inner bank. This typical bar-pool bed morphology may endanger the stability of the outer bank or reduce the navigable width of the channel. Preliminary laboratory experiments in a sharply curved channel with a fixed horizontal bottom (Blanckaert et al. 2008) have shown that a bubble screen located near the outer bank can generate secondary flow with a sense of rotation opposite to the curvature-induced secondary flow.

The reported study investigates the application of bubble screens in configurations with mobile-bed morphology under live-bed and clear-water scour conditions. Velocity measurements show that the bubble-induced secondary flow decreases the strength of the curvature-induced secondary flow, and shifts it in inwards direction. Maximum scour occurs where the curvature-induced and bubble-induced secondary flow cells meet. At this same location, the maximum streamwise velocities and maximum vertical velocities impinging on the bed occur, which indicates their importance with respect to the formation of bend scour. The application of the bubble screen resulted in a reduction of the maximum bend scour by about 50%. Moreover, the location of maximum scour is shifted away from the outer bank and does not endanger its stability anymore. These preliminary experiments show the potential of a bubble screen to influence and modify the bed morphology.