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.