Bedload transport and bedforms migration under sand supply limitation
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Bedform
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Hyperconcentrated flow
The saltating mode of sediment transport is the dominant mechanism of bedload transport in a fluvial stream, especially in mountainous rivers. In most simulation studies, the actual shapes of sediment particles are simplified considering the spherical shape of particles, which can affect the accuracy of the estimation of bedload transport rate. In this study, a model of sediment particle saltation by streamflow is presented, simulating the wall-shear turbulent flow over a sediment bed by large-eddy simulation and calculating the forces acting on the saltating particles by Newton's second law of motion. The model was calibrated by using the experimental data from previous studies on particle saltation. To study the impact of particle shapes, the computational results of saltation lengths, heights, and velocities of different-shaped particles were analyzed. Then, three key parameters were used to estimate bedload transport rates: bedload concentration, saltation height, and particle velocity. By summarizing the above results, this work presents a formula of bedload transport rate that improves upon a previous well-celebrated formula from the literature. The proposed formula introduces shape factors of natural sediment particles.
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Acoustic observations of bedform migration and suspended sediment transport at the New Jersey LEO-15 site revealed that bedload and near-bottom suspended load could be the dominant sediment transport mode over wave-formed ripples on a sandy bottom. Bedforms observed during storms using a rotary sidescan sonar were found to be wave orbital scale ripples which migrated in the onshore direction, forced by asymmetrical wave orbital velocities. Estimates of suspended sand transport were calculated from observations of acoustic backscattering and water velocity profiles. Suspended sand transport was also forced by asymmetrical wave velocities, and was found to occur primarily during the weaker offshore phase of wave motion. This net offshore suspended sediment transport was an order of magnitude less than the flux associated with onshore ripple migration. Thus it is hypothesized that ripple migration is most likely forced by unobserved bedload and near-bottom suspended transport. Spatially resolving bedload transport from the stationary bed is difficult since the motion occurs within a few grain diameters of the bed. Acoustic Doppler-based techniques, which are ideal for this type of measurement since the sediment velocity can be resolved from the stationary bed in the frequency domain, are being developed.
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In this study, the large eddy simulation (LES) under the Eulerian method is used to solve the Navier-Stokes equations for turbulent flow simulation. The Lagrangian point-particle model is applied to track particle trajectories and to calculate the forces exerted by the flow on the particles, and the particle–wall and particle–particle collisions are also accounted for. Nine simulations cases were carried out along the line of previous experiments that considered different bedform regimes, namely, ripples and dunes. The resulting bedload intensity parameter and the simulated bedforms for all the cases agree with the results obtained from the existing classical formulas. The three-dimensionality of sediment transport randomly occurs due to the turbulent flow. Coherent structures are formed as the near-bed low-speed fluid streaks entrain into the mainstream over the stoss-side of the ripples, and the high-speed fluid streaks from the mainstream rush toward the bed over the leeside. As a result, kolk–boil and hairpin vortices develop nearby. Ejection and sweep prevail near the bed, where the particles transport. The phenomenon disappears as the flow intensity increases. The presence of bedload particles also modifies the propagation angle and range of velocity fluctuation, especially in the streamwise direction. To conclude, a logistic regression formula for bedload intensity parameters, accounting for the fluid rotation, deformation, and translation terms that signify the fluid vortical motions, is obtained. It reveals that as long as these three terms are accurately quantified, the bed shear stress and bedload transport rate can be effectively estimated.
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Abstract Bedload transport is an important mechanism for sediment flux in the nearshore. Yet few studies examine the relationship between bedform evolution and net sediment transport. Our work contributes concurrent observations of bedform mobility and bedload transport in response to wave dominant, current dominant, and combined wave‐current flows in the nearshore. Bedload sediment flux from migrating bedforms during combined wave‐current conditions accounted for at least 20% more bedload transport when compared with wave dominant flows and at least 80% more than current‐dominant flows. Bedforms were observed to transport the most sediment during periods with strong currents, with high‐energy skewed waves, and while bedform orientation and transport direction were aligned. Regardless of flow type, bedform migration rates were directly proportional to the total kinetic energy contained in the flow field. Eleven bedload transport models formulated to be used in combined flows (both shear and energetics based) were compared with sediment flux estimated from measured bedform migration. An energetics based sediment transport model was most representative for our data.
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