The effect of migrating dune forms on the flow field of an alluvial river

The bed of an alluvial river is highly susceptible to changes during the course of its existence. Besides variations of the large scale topography and plan form of the river, smaller scale dune forms can be observed. These recurring dune forms migrate on top of the large scale topography and can yield local yet important variations in the flow field. In order to study the effect of migrating dune forms on the flow characteristics and consequently the erosive capacity of an alluvial river, an experiment with mobile bed has been carried out in a laboratory flume representing a sharp meander bend. In this experiment, changes to an initially flat, slightly sloped river bed under a steady flow and sediment discharge were observed until a recurring pattern of migrating dune forms could be seen on top of the characteristic pool-bar topography of meander bends. Once the dune forms were established, an Acoustic Doppler Velocity Profiler (ADVP) was placed in several positions alongside the river bend and used to measure the flow depth and flow characteristics under the influence of the passing dunes. Several times during the experiment, the topography was mapped using laser altimetry on a grid of large spatial resolution in order to isolate the dune forms from the large scale topography and determine the dune characteristics and the dune celerity. In this paper the large scale topography and dune characteristics will be shown and the effect of the migrating dune forms on the flow field and the erosive capacity will be discussed in detail

Experimental study on a widening tributary channel and its influence on the confluence morphology

River morphodynamics and sediment transportRiver morphology and morphodynamic

Scalar dispersion in strongly curved open-channel flows

River hydrodynamicsTurbulent open channel flow and transport phenomen

The effect of migrating dune forms on the flow field of an alluviral river

The bed of an alluvial river is highly susceptible to changes during the course of its existence. Besides variations of the large scale topography and plan form of the river, smaller scale dune forms can be observed. These recurring dune forms migrate on top of the large scale topography and can yield local yet important variations in the flow field. In order to study the effect of migrating dune forms on the flow characteristics and consequently the erosive capacity of an alluvial river, an experiment with mobile bed has been carried out in a laboratory flume representing a sharp meander bend. In this experiment, changes to an initially flat, slightly sloped river bed under a steady flow and sediment discharge were observed until a recurring pattern of migrating dune forms could be seen on top of the characteristic pool-bar topography of meander bends. Once the dune forms were established, an Acoustic Doppler Velocity Profiler (ADVP) was placed in several positions alongside the river bend and used to measure the flow depth and flow characteristics under the influence of the passing dunes. Several times during the experiment, the topography was mapped using laser altimetry on a grid of large spatial resolution in order to isolate the dune forms from the large scale topography and determine the dune characteristics and the dune celerity. In this paper the large scale topography and dune characteristics will be shown and the effect of the migrating dune forms on the flow field and the erosive capacity will be discussed in detail

Influencing bend morphodynamics by means of an air-bubble screen-Topography and velocity field

Interactions between curvature-induced secondary flow, streamwise flow and bed morphology in open-channel bends lead to the development of a typical bar-pool bed morphology, shaped by scour at the outer bank and deposition at the inner bank. This typical bar-pool bed morphology may endanger the stability of the outer bank or reduce the navigable width of the channel. Preliminary laboratory experiments in a sharply curved channel with a fixed horizontal bottom (Blanckaert et al. 2008) have shown that a bubble screen located near the outer bank can generate secondary flow with a sense of rotation opposite to the curvature-induced secondary flow. The reported study investigates the application of bubble screens in configurations with mobile-bed morphology under live-bed and clear-water scour conditions. Velocity measurements show that the bubble-induced secondary flow decreases the strength of the curvature-induced secondary flow, and shifts it in inwards direction. Maximum scour occurs where the curvature-induced and bubble-induced secondary flow cells meet. At this same location, the maximum streamwise velocities and maximum vertical velocities impinging on the bed occur, which indicates their importance with respect to the formation of bend scour. The application of the bubble screen resulted in a reduction of the maximum bend scour by about 50%. Moreover, the location of maximum scour is shifted away from the outer bank and does not endanger its stability anymore. These preliminary experiments show the potential of a bubble screen to influence and modify the bed morphology

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

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

Influence of shallowness, bank inclination and bank roughness on the variability of flow patterns and boundary shear stress due to secondary currents in straight openchannels

Boundary shear stress and flow variability due to its interaction with main flow and secondary currents were investigated under conditions that extend previous research on trapezoidal channels. Secondary currents that scale with the flow depth were found over the entire width in all experiments. These findings contradict the widespread perception that secondary currents die out at a distance of 2.5 times the flow depth from the bank, a perception which is largely based on experiments with smooth boundaries. The reported results indicate that a stable pattern of secondary currents over the entire channel width can only be sustained over a fixed horizontal bed if the bed's roughness is sufficient to provide the required transverse oscillations in the turbulent shear stresses. Contrary to laboratory flumes, alluvial river bed always provide sufficient roughness. The required external forcing of this hydrodynamic instability mechanism is provided by the turbulence-generated near-bank secondary currents. The pattern of near-bank secondary currents depends on the inclination and the roughness of the bank. In all configurations, secondary currents result in a reduction of the bed shear stress in the vicinity of the bank and a heterogeneous bank shear stress that reaches a maximum close to the toe of the bank. Moreover, these currents cause transverse variability of 10–15% for the streamwise velocities and 0.2u* 2–0.3u* 2 for the bed shear stress. These variations are insufficient to provide the flow variability required in river restoration projects, but nevertheless must be accounted for in the design of stable channels