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

Abstract

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.

Wall-Roughness Effects on Flow and Scouring in Curved Channels with Gravel Beds

“…For the rectangular channel, Blanckaert et al (2010) suggest that scouring close to the wall could be reduced with increasing wall roughness, whereas for the trapezoidal channel, depth-averaged downstream velocity over the bank toe is similar for all experiments regardless of the outer-bank roughness. Recently, in the very same laboratory installation, Dugué et al (2013) proposed a solution based on the installation of a bubble screen to reduce bend scour.…”

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

“…Blanckaert et al (2008) have shown that the bubble-induced secondary flow causes redistribution of the longitudinal velocity and of the boundary shear stress, which suggests that the bubble screen would also lead to morphological redistribution in configurations with mobile riverbed. Dugué et al (2013) have performed experiments under one hydraulic condition in the same curved laboratory flume as Blanckaert et al (2008). Instead of fixing a horizontal bed, Dugué et al (2013) performed experiments under clear-water scour conditions.…”

“…Dugué et al (2013) have performed experiments under one hydraulic condition in the same curved laboratory flume as Blanckaert et al (2008). Instead of fixing a horizontal bed, Dugué et al (2013) performed experiments under clear-water scour conditions. Under clear-water scour conditions, no sediment is fed to the flume and the critical shear stress necessary for transporting sediment is not exceeded in the straight inflow reach of the flume.…”

“…Because no sediment is fed to the flume, sediment transport ultimately vanishes and a stable topography is formed. Dugué et al (2013) have shown that the bubble screen considerably modifies the ultimate bed topography under clear-water scour conditions. These results suggest that bubble screens could represent a useful tool for influencing morphodynamics in shallow rivers.…”

“…The objective of the present paper is to extend the previous investigations by Blanckaert et al (2008) and Dugué et al (2013). The parameter space will be extended by investigating various conditions of shallowness and base flow velocity, and experiments will be performed with live-bed sediment transport.…”