Influencing river morphodynamics by means of a bubble screen - application to open-channel bends: Communication 54 (Laboratoire de constructions hydrauliques, Ecole polytechnique fédérale de Lausanne)
Curved reaches and bends are a characteristic feature of rivers and channels. Their morphology is heterogeneous and typically includes a point bar at the inner side of bends, and bend scour at the outer side. Curvature-induced secondary flow, also called spiral flow or helical flow, is known to play an important role in the generation of this morphological heterogeneity. This secondary flow is outward directed in the upper part of the water column, and causes an increase of velocities and erosive capacity at the outer side of the channel. Near the outer bank, the secondary flow has an important vertical component that impinges on the channel bed and enhances bend scour. The eroded sediment is then transported in inward direction by the near-bed component of the secondary flow, and contributes to the development of the point bar at the inner side of the channel. The morphological heterogeneity has adverse effects: scour at the outer side of the bend threatens the stability of the bank and structures like bridge abutments or piers, and deposition of sediment at the inner side of the bend reduces the navigable width. Therefore, river engineers have always tried to attenuate this morphological heterogeneity, for example by artificially roughening the outer bank in order to reduce near-bank velocities, or deflecting the flow away from the outer bank by means of groynes or bottom vanes. All these techniques involved permanent fixed structures. Dr. Violaine Dugué has investigated an innovative technique that is reversible and does not involve fixed structures. It consists in generating an air-bubble screen with a porous tube that is positioned parallel to the outer bank. The rising air-bubbles generate a secondary flow that is opposed to the curvature-induced secondary flow, and thereby reduces the morphological heterogeneity. Dr. Violaine Dugué has performed a series of laboratory experiments with increasing degree of complexity, including straight and curved flows under various conditions of sediment transport. These experiments provided insight in the mutual interactions between the main flow, the curvature-induced flow components, the bubble-screen induced flow components, the sediment transport and the morphology. Based on this novel insight, Dr. Violaine Dugué has developed a methodology that allows quantifying the application range of the bubble-screen technique. It relates the required air discharge of the bubble screen to the geometric and hydraulic characteristics of the open-channel bend. The methodology is illustrated by means of two case studies.