Influencing river morphodynamics by means of a bubble screen - application to open-channel bends

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.

Intermittent motion and sediment transport -- experimental and theoretical insight

Christophe Ancey

We investigate the entrainment, deposition, and motion of coarse spherical particles within a turbulent shallow water stream down a steep slope. This is an idealization of bed-load transport in mountain streams. Earlier investigations have described this kind of sediment transport using empirical correlations or concepts borrowed from continuum mechanics. The intermittent character of particle transport at low water discharges led us to consider it as a random process. Sediment transport in this regime results from the imbalance between entrainment and deposition of particles rather than from momentum balance between water and particles. We develop a birth-death immigration-emigration Markov process to describe the particle exchanges between the bed and the water stream. A key feature of the model is its long autocorrelation times and wide, frequent fluctuations in the solid discharge, a phenomenon never previously explained despite its ubiquity in both nature and laboratory experiments. We present experimental data obtained using a nearly two-dimensional channel and glass beads as a substitute for sediment. Entrainment, trajectories, and deposition were monitored using a high-speed digital camera. The empirical probability distributions of the solid discharge and deposition frequency were properly described by the theoretical model. Experiments confirmed the existence of wide and frequent fluctuations of the solid discharge, and revealed the existence of long autocorrelation time, but theory overestimates the autocorrelation times by a factor of around three. Particle velocity was weakly dependent on the fluid velocity contrary to the predictions of the theoretical model, which performs well when a single particle is moving. For our experiments, the dependence of the solid discharge on the fluid velocity is entirely controlled by the number of moving particles rather than by their velocity. We also noted significant changes in the behavior of particle transport when the bed slope or the water discharge was increased. The more vigorous the stream was, the more continuous the solid discharge became. Moreover, while 90% of the energy supplied by gravity to the stream is dissipated by turbulence for slopes lower than 10%, particles dissipate more and more energy when the bed slope is increased, but surprisingly, the dissipation rate is nearly independent of fluid velocity.

2007

Entrainment and motion of coarse particles in a shallow water stream down a steep slope

Christophe Ancey, Anthony Christopher Davison, Peter Frey

We investigate the entrainment, deposition and motion of coarse spherical particles within a turbulent shallow water stream down a steep slope. This is an idealization of bed-load transport in mountain streams. Earlier investigations have described this kind of sediment transport using empirical correlations or concepts borrowed from continuum mechanics. The intermittent character of particle transport at low-water discharges led us to consider it as a random process. Sediment transport in this regime results from the imbalance between entrainment and deposition of particles rather than from momentum balance between water and particles. We develop a birth–death immigration–emigration Markov process to describe the particle exchanges between the bed and the water stream. A key feature of the model is its long autocorrelation times and wide, frequent fluctuations in the solid discharge, a phenomenon never previously explained despite its ubiquity in both nature and laboratory experiments. We present experimental data obtained using a nearly two-dimensional channel and glass beads as a substitute for sediment. Entrainment, trajectories, and deposition were monitored using a high-speed digital camera. The empirical probability distributions of the solid discharge and deposition frequency were properly described by the theoretical model. Experiments confirmed the existence of wide and frequent fluctuations of the solid discharge, and revealed the existence of long autocorrelation time, but theory overestimates the autocorrelation times by a factor of around three. Particle velocity was weakly dependent on the fluid velocity contrary to the predictions of the theoretical model, which performs well when a single particle is moving. For our experiments, the dependence of the solid discharge on the fluid velocity is entirely controlled by the number of moving particles rather than by their velocity. We also noted significant changes in the behaviour of particle transport when the bed slope or the water discharge was increased. The more vigorous the stream was, the more continuous the solid discharge became. Moreover, although 90% of the energy supplied by gravity to the stream is dissipated by turbulence for slopes lower than 10 %, particles dissipate more and more energy when the bed slope is increased, but surprisingly, the dissipation rate is nearly independent of fluid velocity. A movie is available with the online version of the paper.

Cambridge University Press2008

Measuring Bedload Sediment Transport with an Acoustic Doppler Velocity Profiler

Koen Blanckaert, Joris Heyman

Acoustic Doppler velocity profilers (ADVP) measure the velocity simultaneously in a linear array of bins. They have been successfully used in the past to measure three-dimensional turbulent flow and the dynamics of suspended sediment. The capability of ADVP systems to measure bedload sediment flux remains uncertain. The main outstanding question relates to the physical meaning of the velocity measured in the region where bedload sediment transport occurs. The main hypothesis of the paper, that the ADVP measures the velocity of the moving bedload particles, is validated in laboratory experiments that range from weak to intense bedload transport. First, a detailed analysis of the raw return signals recorded by the ADVP reveals a clear footprint of the bedload sediment particles, demonstrating that these are the main scattering sources. Second, time-averaged and temporal fluctuations of bedload transport derived from high-speed videography are in good agreement with ADVP estimates. Third, ADVP based estimates of bedload velocity and thickness of the bedload layer comply with semi-theoretical expressions based on previous results. An ADVP configuration optimized for bedload measurements is found to perform only marginally better than the standard configuration for flow measurements, indicating that the standard ADVP configuration can be used for sediment flux investigations. Data treatment procedures are developed that identify the immobile-bed surface, the layers of rolling/sliding and saltating bedload particles, and the thickness of the bedload layer. Combining ADVP measurements of the bedload velocity with measurements of particle concentration provided by existing technology would provide the sediment flux. (C) 2017 American Society of Civil Engineers.