Experimental study on bedload transport, fluctuations and coupling with bedforms in steep turbulent streams.

Sediment transport is one of the main morphodynamic processes in mountain streams. Understanding bedload transport is fundamental to prevent and mitigate hydrogeological hazards and for the resources management. The classic bedload discharge predictions are usually expressed by empirical formulae as a one-to-one relationship between sediment flux and water discharge. Unfortunately, this framework is often not sufficient to describe the variability of bedload discharge. In fact, large fluctuations in bedload transport rates are reported, even for steady flow conditions, over different time scales. Especially in case of mountain streams, the predicted and measured bedload transport rates may reflect differences of several orders of magnitude. Different processes, acting at different spatial and temporal scales, have been proposed as causes of the observed fluctuations, such as the intrinsic variability of particle paths, the migration of bedforms or the unsteadiness in sediment supply. During the second half of the twentieth century, some scientists started to study the bedload transport as a stochastic process involving the grains transported by a stream. The purpose of my research is to assess and improve the performance of a stochastic model in conditions representative of mountain streams and in presence of bedforms. To address this challenge, experiments will be performed in a narrow 2D flume, with steady flow conditions and stationary sediment feeding, for slope angles varying between 1 and 3°. The sediment feeding rate and the bedload tranport rate at the outlet will be measured using high-speed cameras and image processing techniques. Another high-speed camera will acquire side images of the flume for monitoring the evolution of the bed topography and for tracking the moving particles inside a control window. The analyses will be focused on activity and velocity distribution of the particles inside the control window. The collected data will allow the comparison of model outputs with the observed fluctuations of bedload discharge. The dependence of this fluctuations on different variables (shear stress, slope, antidune geometry, antidune migration celerity, etc.) will be investigated.

Experimental Study on Bedload Transport and Bedforms: Behaviour and Interplay in Steep Turbulent Streams

Ivan Pascal

In mountain regions, steep streams play an important role in water and sediment connectivity. In these highly dynamic systems, water flow features, sediment fluxes and stream morphologies are tightly interlinked over a broad range of temporal and spatial scales. Understanding their interactions is key to forecast, prevent and mitigate the impacts of stream-related hazards on human population and facilities as well as to manage the resources connected to the rivers. Sediment clasts at rest on the bed surface may be eroded, transported and deposited by the water flow. Grains that move in rolling and saltation regimes close to the bed compose the 'bedload', which is considered the main mode of sediment transport when studying morphodynamics of mountain rivers. The fate of these particles is of primary interest for river scientists and engineers as bedload predominantly drives the river morphology evolution and responds to it in a complex manner. Therefore, measuring and predicting bedload transport rates are fundamental concerns for many works of research and applications. Bedload transport rates may exhibit large non-Gaussian fluctuations, even under steady-state conditions, especially when bedload transport intensity is weak. These fluctuations may originate from different phenomena and affect the accuracy of bedload transport rate estimates. Particles transported by the water flow can destabilise other particles at rest on the bed surface. Under weak bedload transport conditions, this positive feedback can lead to large bedload pulses. When grains settle, they often organise in small clusters or larger bedforms. Bedform migration has been identified as one of the potential causes of bedload transport fluctuations. Trains of upstream-migrating antidunes may develop in steep coarse-bedded streams when the water discharge is sufficiently high to ensure fully supercritical flow conditions. In this dissertation, I present an experimental investigation on the behaviour of bedload transport fluctuations and antidunes in steep supercritical flows. The experiments were conducted in narrow flumes under well-controlled conditions with high-resolution image-based bedload transport and bed topography monitoring. The dependence of mean bedload transport rate uncertainty on the averaging time interval was analysed. Furthermore, I performed experimental runs with increasing transport intensity and a nearly constant mean bed slope angle to study how antidune sequences composed by bedforms of variable shape typically responded to the mean bedload flux imposed under steady-state conditions. Spectral analysis of the bed elevation perturbation in time and space allowed us to quantify the variability ranges of antidune shape and celerity. Scaling the spectra with opportune reference scales made it possible to identify a consistent dimensionless relationship between antidune geometry and migration celerity, which includes the dependencies on flow and sediment supply forcings. I studied how the bedload transport rate fluctuates in presence of migrating bedforms. The effects on the bedload transport rate periodicity due to non-uniformities in the antidune migration period and to deviations from the steady-state transport conditions were also investigated. This work is a further step on the way towards a complete understanding of the interplay between bedload transport fluctuations and bedforms in mountain streams.

EPFL2022

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

Bed load fluctuations in a steep channel

Tamara Ghilardi, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

Abstract: Bedload transport rate fluctuations have been observed over time in steep rivers and flumes with wide grain size distributions even under constant sediment feeding and water discharge. The observed bedload transport rate pulses are periodic and a consequence of grain sorting. Moreover, the presence of large, relatively immobile boulders, such as erratic stones, which are often present in mountain streams, has an impact on flow conditions. The detailed analysis of a 13 hours laboratory experiment is presented in this paper. Boulders were randomly placed in a flume with a steep slope (6.7%), and water and sediment were constantly supplied to the flume. Along with the sediment transport and bulk mean flow velocity, the boulder protrusion, boulder surface, and number of hydraulic jumps, which are indicators of the channel morphology, were measured regularly during the experiment. Periodic bedload transport rate pulses are clearly visible in the data collected during this longduration experiment, along with correlated fluctuations in the flow velocity and bed morphology. The links among the bulk velocity, the time evolution of the morphology variables, and the bedload transport rate are analyzed via correlational analysis, showing that the fluctuations are strongly related. A phase analysis of all observed variables is performed, and the average shapes of the time cycles of the fluctuations are shown. Observations indicate that the detected periodic fluctuations correspond to different bed states. Furthermore, the grain size distribution through the channel, which varies in time and space, clearly influences these bedload transport rate pulses. Finally, known bedload transport rate formulae are tested, showing that only the application of a drag shear stress allows a correct estimation of the time fluctuations.