Violaine Dugué

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 banks 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.

Influencing Flow Patterns and Bed Morphology in Open Channels and Rivers by Means of an Air-Bubble Screen

The ability of a bubble screen to redistribute the flow field and bed morphology in shallow rivers and open channels has been investigated in laboratory experiments. Rising air bubbles generated by a pressurized porous tube situated on the bed induced secondary flow perpendicular to the porous tube. The secondary flow redistributed the longitudinal velocity, which caused also morphological redistribution under mobile-bed conditions. The strength and size of the bubble-induced secondary flow were independent of the base flow velocity and increased with water depth. The size of the secondary flow cell ranged from 3× (immobile bed) to 7× (mobile bed) the water depth. Similar sizes of bubble-induced secondary flow cells have been reported in literature for water depths ranging from 0.1 to 5 m, indicating that the laboratory experiments are relevant for natural rivers and open channels.Amutually strengthening interplay occurred between the bubble screen, the bubble-induced secondary flow, and the morphology. The bubble-induced secondary flowconsiderably increased the rising velocity of the air bubbles, which on its turn strengthened the secondary flow. The morphological redistribution increased the flow depth in the region covered by the secondary flow cell, which on its turn increased the size and strength of the secondary flow cell, and its effect on the morphological redistribution. This coupled hydrodynamic-morphologic behavior explains the larger size and strength of the secondary flow over a mobile bed than over a flat immobile bed. The results demonstrate the potential of the bubble screen as a technique to modify the morphology in a variety of applications in shallowrivers and open channels.