Why do large, deep rivers have low-angle dune beds?
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Abstract Dunes are the most common bedform in sand-bedded rivers. Small, high-angle dunes (HADs) dominate in shallow (<2.5 m) flows and have lee sides with steep downstream-facing slipfaces (>24°) and reversed upslope-oriented flow in the separation vortex. In contrast, large, deep (>2.5 m) rivers have low-angle dunes (LADs) with gentle lee-side slipfaces (<24°; often <10°), little or no flow separation, and more-pronounced downslope currents. Avalanches driven primarily by particle interaction maintain slipfaces of HADs. On LADs, excess pore pressures that occur during the failure of thick, loosely packed deposits can produce liquefied avalanches that flow and stop on gentle gradients. As lee-side angles decrease over LADs, downslope currents increase in strength, accelerating avalanches, transporting bedload, and creating smaller, superimposed dunes and uniform concave slopes on the lee side. Nearly a century of research on dunes in shallow laboratory flows has assumed that dune morphodynamics are scale invariant, which is not true.Keywords:
Beach morphodynamics
Bedform
Sand dune stabilization
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Experimental sediment transport and river morphologic studies in laboratory flumes can use two sediment-supply methods: an imposed feed at the upstream end, or a recirculation of sediment from the downstream end to the upstream end. These methods generally produce similar equilibrium bed morphology, but temporal evolution can differ. The adjustment of natural rivers may be reproduced by both modes. Nevertheless, computer models of river morphodynamics typically use a sediment-feed boundary condition, which can impact the simulated evolution of transient features such as bedforms. The effect of sediment transport boundary conditions on bedform dynamics was analyzed through numerical experiments using a two-dimensional, depth-averaged sediment transport model. Two different boundary conditions were imposed at the inlet (constant sediment feed and sediment recirculated from the outlet) for two bedform scales (dunes and bars). The type of sediment transport boundary condition greatly influenced dune development. The sediment-recirculating condition produced a more dynamic bed morphology with dunes of higher amplitude. The associated zones of higher shear stress had a direct impact on the hydrodynamics and patterns of sediment transport. In the case of the bar bed morphology, the simulated bars had similar mean length and height for both sediment boundary conditions. However, the sediment-recirculating case produced a more dynamic bed, in which the dominant bar length varied over time. Finally, the simulated bed morphology with bars, agreed much better with that observed when using a calibrated sediment transport equation to match sediment discharges instead of the standard empirical sediment transport equations available in literature.
Beach morphodynamics
Bedform
Sedimentary budget
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Bedform
Flume
Hyperconcentrated flow
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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.
Bedform
Suspended load
Acoustic Doppler velocimetry
Ripple marks
Surf zone
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Bedform
Beach morphodynamics
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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.
Bedform
Energetics
Energy flux
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While the ecological significance of hyporheic exchange and fine particle transport in rivers is well established, these processes are generally considered irrelevant to riverbed morphodynamics. We show that coupling between hyporheic exchange, suspended sediment deposition, and sand bedform motion strongly modulates morphodynamics and sorts bed sediments. Hyporheic exchange focuses fine-particle deposition within and below mobile bedforms, which suppresses bed mobility. However, deposited fines are also remobilized by bedform motion, providing a mechanism for segregating coarse and fine particles in the bed. Surprisingly, two distinct end states emerge from the competing interplay of bed stabilization and remobilization: a locked state in which fine particle deposition completely stabilizes the bed, and a dynamic equilibrium in which frequent remobilization sorts the bed and restores mobility. These findings demonstrate the significance of hyporheic exchange to riverbed morphodynamics and clarify how dynamic interactions between coarse and fine particles produce sedimentary patterns commonly found in rivers.
Beach morphodynamics
Bedform
Deposition
Hyporheic Zone
Particle (ecology)
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Bedload transport on Spratt Sand is a result of migrating bedforms and a lack of sediment clearly limits the growth and development of these bedforms. Maximum bedload transport based on dune-tracking is observed for a combination of waves and currents, probably due to an increase in migration rates rather than an increase in ripple dimensions.
Bedform
Ripple marks
Suspended load
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Citations (2)