Flow patterns induced by a bubble screen in a sharply curved flume based on Acoustic Doppler Velocity Profiler measurements

Open-channel bends are characterized by a bed morphology where erosion occurs near the outer bank and a point bar develops at the inner bend. This morphology is induced by complex interactions between the streamwise flow, the curvature-induced secondary flow and the bed topography. Several techniques already exist to counteract the development of bend scour, which can endanger foundations of structures. However, existing techniques mostly imply substantial construction works in the river. Preliminary laboratory experiments have shown that a porous tube placed near the outer bank can generate a bubble screen that modifies the flow patterns and leads to a substantial reduction of the bend scour. A better understanding of the hydrodynamic mechanisms induced by the bubble screen and involved in the morphological development will allow determining the range of application of the bubble-screen technique. Experiments performed in a sharply curved open-channel bend under live-bed conditions show that the bubble screen is able to redistribute the velocity patterns in the bend. The bed morphology is then partially modified, specially at the bend exit. This paper illustrates Acoustic Doppler Velocity Profiler measurements of the bubble-induced secondary flow and its interaction with the channel base flow in the most influenced cross-section.

Bend scour reduction induced by an air-bubble screen under live-bed conditions

Koen Blanckaert, Violaine Dugué, Anton Schleiss

Open-channel bends are characterized by complex interactions between streamwise flow, curvature-induced secondary flow and bed morphology. These bend effects cause erosion near the outer bank, and deposition near the inner bank. Previous laboratory experiments in a sharply curved flume with a mobile bed under clear-water scour conditions have shown that an air-bubble screen generated with a porous tube on the bed can counteract the curvature-induced secondary flow and lead to a shift of the maximum scour away from the outer bank. The here reported study provides experimental results on the optimization of the bubble screen under live-bed conditions. The air-bubble screen considerably reduces erosion near the outer bank for all investigated configurations of the bubble screen. Comparison of three different locations of the porous tube shows that the bubble screen is most efficient when placed very near to the outer bank.

Taylor & Francis Group, CRC Press2012

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

Koen Blanckaert, Violaine Dugué, Anton Schleiss

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.

2012

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

Koen Blanckaert, Violaine Dugué, Anton Schleiss

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.

Irtces2013