Project

open-channel bend; meander; bubble screen; local scour; morphology; bubble plume; deposition; recirculating cell; ADVP; bubble-induced recirculating cell

Lead
Lay summary
Secondary flow in open-channel bends redistributes the velocities and the boundary shear stress and thereby creates variations in the bed topography. Outer banks are vulnerable to scouring whereas deposition near the bank may reduce the navigable width of the channel.Laboratory experiments in a sharply curved channel show that a secondary flow cell opposed to the curvature-induced secondary flow cell can be created near the outer bank by means of an air-bubble screen. The air bubbles coming from a porous tube installed on the bed rise to the water surface and thereby generate upwards flow, which drives the secondary flow cell.With respect to "hard" engineering techniques, bubble screens have the advantage of being controllable, ecological (oxygenation), reversible and non-permanent. They have already been applied in industrial, material, chemical, mechanical, and environmental applications, and abundantly investigated. According to literature, the application of bubble screens in river morphodynamics has not yet been investigated or applied in a systematic way.Comparison of reference experiments without bubble screen to experiments with bubble screen under similar conditions of water and sediment discharge may permit to estimate the influence of bubble screen on the bend morphology.

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Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

The interaction between velocity, boundary shear stress, sediment transport, sediment deposition and erosion and morphology in geophysical flows is often designated by the term morphodynamics. This proposal is part of a project that investigates how river morphodynamics can be influenced by means of bubble screens, which may be beneficial, for example, to reduce scour in open-channel bends and thereby increase the navigable width, reduce local scour around structures like bridge piers or abutment in open-channels, control reservoir sedimentation, prevent harbour silting.Bend scour, local scour and sedimentation can be attributed to vertically descending velocities impinging on the channel bed. Basically, the rising air-bubbles of the bubble screen counteract these descending velocities. With respect to "hard" engineering techniques, bubble screens have the advantage of being controllable, ecological (oxygenation), reversible and non-permanent. Bubble plumes and screens have been applied in industrial, material, chemical, mechanical, and environmental applications, and abundantly investigated. To our knowledge, the application of bubble screens in river morphodynamics has not yet been investigated or applied in a systematic way. Particularities of bubble screens in river morphodynamics are the shallowness of the flow and the interaction with the mobile bed.Preliminary experimental and numerical research, triggered by HKVconsultants and the RIZA department of the Dutch Ministry of Transport, Public Works and Water Management, has shown the feasibility of this bubble-screen technique to reduce scour in open-channel bends and increase the navigable width (see Annexe 3). The present project will carry out generic fundamental research on the use of bubble screens and subsequently focus on the application in open-channel bends. It will now be embedded in the framework of an ongoing joint research effort that exploits synergies between laboratory experiments at EPFL, field experiments at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB, Germany) and numerical modelling at Delft University of Technology (TUD, The Netherlands) (see chapter 4.1). It will benefit from this ongoing programme by adopting the same methodology (see chapter 4.1), using the same experimental and numerical facilities (see chapter 4.2.2), using available experimental data (see chapter 4.3), and being supervised by the same group. The present proposal concerning laboratory experiments at EPFL is self-contained, but will benefit from ongoing complementary numerical research at TUD and planned field experiments at IGB.Objectives of the project are:-Gaining insight in the processes underlying the bubble screen and the entrainment of flow in shallow configurations.-Defining guidelines for the numerical modelling of these processes.-Gaining insight in the interaction between bubble screen, flow, sediment transport and morphology in shallow free-surface flows, and in particular in open-channel bends.-Providing high-quality data that is appropriate for model validation.-Optimising the design of the planned pilot project in a natural river.Experiments without and with bubble screen will be carried out following a stepwise approach:-Experiments in shallow still water with fixed bed.-Experiments in shallow straight open channel flow with fixed horizontal bed and mobile bed.-Experiments in shallow curved open channel flow with fixed horizontal bed and mobile bed.