Bed-load transport in obliquely dune-covered riverbeds

Sieben, J. W.; Talmon, A.M.

doi:10.1080/00221686.2011.566252

Cited by 6 publications

(8 citation statements)

References 15 publications

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“…Compared to the ripple regime, this means a relative increase in downslope sediment transport with increasing sediment mobility that linearly depends on dune height. Sieben and Talmon () found that the increase in downslope sediment transport on lee sides of dunes resulted in lower transverse slopes, which is caused by the fact that avalanching on the dune slip face is in downward direction rather than perpendicular to the bed or in the direction of dune migration. In our experiments, long dunes, and thus fewer dunes, were observed at intermediate sediment mobilities, where also the maximum slope factor was observed during experiments with coarse sediments (Figure d).…”

Section: Discussionmentioning

confidence: 99%

“…Existing predictors based on an empirical fit through experimental data mainly focused on the effect of different bed states, which in the case of Wiesemann et al () lead to a different trend for a bed with ripples or with dunes, since they observed that downslope sediment transport decreased when dunes were present and became independent of sediment mobility. In contrast, Sieben and Talmon () used artificial dunes to show that the slope effect is enhanced when oblique dunes are present, due to avalanching at the lee sides of the dunes. Talmon et al () manually prepared dunes based on earlier experiments, since the development of natural dunes required the same time as the duration of their bed leveling experiments.…”

Section: Existing Transverse Slope Predictorsmentioning

confidence: 99%

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Sediment Transport of Fine Sand to Fine Gravel on Transverse Bed Slopes in Rotating Annular Flume Experiments

Baar

Smit

Uijttewaal

et al. 2018

Water Resources Research

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Large‐scale morphology, in particular meander bend depth, bar dimensions, and bifurcation dynamics, are greatly affected by the deflection of sediment transport on transverse bed slopes due to gravity and by secondary flows. Overestimating the transverse bed slope effect in morphodynamic models leads to flattening of the morphology, while underestimating leads to unrealistically steep bars and banks and a higher braiding index downstream. However, existing transverse bed slope predictors are based on a small set of experiments with a minor range of flow conditions and sediment sizes, and in practice models are calibrated on measured morphology. The objective of this research is to experimentally quantify the transverse bed slope effect for a large range of near‐bed flow conditions with varying secondary flow intensity, sediment sizes (0.17–4 mm), sediment transport mode, and bed state to test existing predictors. We conducted over 200 experiments in a rotating annular flume with counterrotating floor, which allows control of the secondary flow intensity separate from the streamwise flow velocity. Flow velocity vectors were determined with a calibrated analytical model accounting for rough bed conditions. We isolated separate effects of all important parameters on the transverse slope. Resulting equilibrium transverse slopes show a clear trend with varying sediment mobilities and secondary flow intensities that deviate from known predictors depending on Shields number, and strongly depend on bed state and sediment transport mode. Fitted functions are provided for application in morphodynamic modeling.

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Section: Discussionmentioning

confidence: 99%

Section: Existing Transverse Slope Predictorsmentioning

confidence: 99%

Sediment Transport of Fine Sand to Fine Gravel on Transverse Bed Slopes in Rotating Annular Flume Experiments

Baar

Smit

Uijttewaal

et al. 2018

Water Resources Research

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“…Furthermore, dunes deform when migrating through bends, which affect the above processes and strongly modifies the near‐bed flow direction, potentially with near‐bed flow parallel to the dune troughs [ Dietrich and Smith , ; Kisling‐Moller , ]. In such cases, dunes enhance helical flow, increasing transport capacity toward the inner bend [ Sieben and Talmon , ], but also add local upward slopes to the bed, reducing transport capacity toward the inner bend. The balance of these two deflections remains poorly understood and is at present implicitly calibrated as a subgrid process.…”

Section: Discussionmentioning

confidence: 99%

Physics-based modeling of large braided sand-bed rivers: Bar pattern formation, dynamics, and sensitivity

Schuurman

Marra

Kleinhans

2013

J. Geophys. Res. Earth Surf.

184

240

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.[1] Braided rivers have complicated and dynamic bar patterns, which are challenging to fully understand and to predict both qualitatively and quantitatively. Linear theory ignores nonlinear processes that dominate fully developed bars, whereas natural river patterns are determined by the combined effects of boundary conditions, initial conditions such as planimetric forcing by fixed banks and the physical processes. Here we determine the capability of a state-of-the-art physics-based morphological model to reproduce morphology and dynamics characteristic of braided rivers and determine the model sensitivity to generally used constitutive relations for flow and sediment transport. We use the 2-D depth-averaged morphodynamic model Delft3D, which includes the necessary spiral flow and bed slope effects on morphology. We present idealized scenarios with the smallest possible number of enforced details in the planform and boundary conditions in order to allow free development of bars driven by the physical processes in the model. We analyze bar and channel shapes and dynamics quantified by a number of complementary metrics and compare these with imagery, field data captured in empirical relations, flume experiments, and predictions by linear analyses. The results show that the chosen set of boundary conditions and physics in the numerical model is sufficient to produce many morphological characteristics and dynamics of a braided river but insufficient for long-term modeling. Initially, braiding intensity with low-amplitude bars is high in agreement with linear analysis. In a second stage when bars merge, split, and increase amplitude up to the water surface, the shape, size, and dynamics of individual bars compare well to those in natural rivers. However, long-term modeling results in a reduction of bar and channel dynamics and formation of exaggerated bar height and length. This suggests that additional processes, such as physics-based bank erosion, or enforced fluctuations in boundary conditions, such as spatial-temporal discharge variation, are necessary for the simulation of a dynamic equilibrium river. The most important outcome is that the modeled pattern of bars and channels is highly sensitive to the constitutive relation for bed slope effects that is used in many morphological models. Regardless of this sensitivity and present model limitations of many models, this study shows that physics-based modeling of sand-bed braided improves our understanding and prediction of morphological patterns and dynamics in sand-bed braided rivers.Citation: Schuurman, F., W. A. Marra, and M. G. Kleinhans (2013), Physics-based modeling of large braided sand-bed rivers: Bar pattern formation, dynamics, and sensitivity,

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“…Consequently, near-bed flow velocities exhibit significant variations in local magnitude and direction in the presence of dunes (Figure 6a) that are almost entirely absent in the model results generated using the Smoothed DEM (Figure 6b). Such flow structures in the leesides of oblique, or 3-D dunes, have been well documented for individual dunes (Allen, 1968;Omidyeganeh & Piomelli, 2013a, 2013bSieben & Talmon, 2011;Talmon et al, 1995;Walker, 1999;Walker & Shugar, 2013) but until now have proven difficult to measure or simulate across multiple bedforms. 10.1029/2020JF005571…”

Section: Effect Of Dunes On Near-bed Flow Structurementioning

confidence: 99%

Influence of Dunes on Channel‐Scale Flow and Sediment Transport in a Sand Bed Braided River

et al. 2020

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Current understanding of the role that dunes play in controlling bar and channel-scale processes and river morphodynamics is incomplete. We present results from a combined numerical modeling and field monitoring study that isolates the impact of dunes on depth-averaged and near-bed flow structure, with implications for morphodynamic modeling. Numerical simulations were conducted using the three-dimensional computational fluid dynamics code OpenFOAM to quantify the time-averaged flow structure within a 400 m × 100 m channel using digital elevation models (DEMs) for which (i) dunes and bars were present within the model and (ii) only bar-scale topographic features were resolved (dunes were removed). Comparison of these two simulations shows that dunes enhance lateral flows and reduce velocities over bar tops by as much as 30%. Dunes influence the direction of modeled sediment transport at spatial scales larger than individual bedforms due to their effect on topographic steering of the near-bed flow structure. We show that dunes can amplify, dampen, or even reverse the deflection of sediment down lateral bar slopes, and this is closely associated with 3-D and obliquely orientated dunes. Sediment transport patterns calculated using theory implemented in depth-averaged morphodynamic models suggest that gravitational deflection of sediment is still controlled by bar-scale topography, even in the presence of dunes. However, improved parameterizations of flow and sediment transport in depth-averaged morphodynamic models are needed that account for the effects of both dune-and bar-scale morphology on near-bed flow and sediment transport.

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Bed-load transport in obliquely dune-covered riverbeds

Cited by 6 publications

References 15 publications

Sediment Transport of Fine Sand to Fine Gravel on Transverse Bed Slopes in Rotating Annular Flume Experiments

Sediment Transport of Fine Sand to Fine Gravel on Transverse Bed Slopes in Rotating Annular Flume Experiments

Physics-based modeling of large braided sand-bed rivers: Bar pattern formation, dynamics, and sensitivity

Influence of Dunes on Channel‐Scale Flow and Sediment Transport in a Sand Bed Braided River

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