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Seminara, G.

doi:10.1023/a:1004225516566

Cited by 69 publications

(11 citation statements)

References 63 publications

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“…Over time, the planform of meandering rivers typically changes as bends migrate and evolve. The process of planform evolution involves interactions among extant channel morphology, hydrologic variability, turbulent flow structure, sediment transport, and bank erosion (Pizzuto, 1994;Hooke, 1995;Seminara, 1998Seminara, , 2006Parker et al, 2011;Engel and Rhoads, 2012). Increases in the sinuosity and curvature of meanders over decadal timescales can produce elongate bends with multiple curvature maxima known as compound bends or loops (Frothingham and Rhoads, 2003).…”

Section: Introductionmentioning

confidence: 99%

Three‐dimensional flow structure and patterns of bed shear stress in an evolving compound meander bend

Engel

Rhoads

2016

Earth Surf Processes Landf

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Compound meander bends with multiple lobes of maximum curvature are common in actively evolving lowland rivers. Interaction among spatial patterns of mean flow, turbulence, bed morphology, bank failures and channel migration in compound bends is poorly understood. In this paper, acoustic Doppler current profiler (ADCP) measurements of the three‐dimensional (3D) flow velocities in a compound bend are examined to evaluate the influence of channel curvature and hydrologic variability on the structure of flow within the bend. Flow structure at various flow stages is related to changes in bed morphology over the study timeframe. Increases in local curvature within the upstream lobe of the bend reduce outer bank velocities at morphologically significant flows, creating a region that protects the bank from high momentum flow and high bed shear stresses. The dimensionless radius of curvature in the upstream lobe is one‐third less than that of the downstream lobe, with average bank erosion rates less than half of the erosion rates for the downstream lobe. Higher bank erosion rates within the downstream lobe correspond to the shift in a core of high velocity and bed shear stresses toward the outer bank as flow moves through the two lobes. These erosion patterns provide a mechanism for continued migration of the downstream lobe in the near future. Bed material size distributions within the bend correspond to spatial patterns of bed shear stress magnitudes, indicating that bed material sorting within the bend is governed by bed shear stress. Results suggest that patterns of flow, sediment entrainment, and planform evolution in compound meander bends are more complex than in simple meander bends. Moreover, interactions among local influences on the flow, such as woody debris, local topographic steering, and locally high curvature, tend to cause compound bends to evolve toward increasing planform complexity over time rather than stable configurations. Copyright © 2016 John Wiley & Sons, Ltd.

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Section: Introductionmentioning

confidence: 99%

Three‐dimensional flow structure and patterns of bed shear stress in an evolving compound meander bend

Engel

Rhoads

2016

Earth Surf Processes Landf

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“…These features develop from interactions between coarse and fine grains [Whiting et al, 1988;Dietrich et al, 1989;Tsujimoto, 1990;Seminara et al, 1996;Whiting, 1996]. Following the terminology proposed for bar types [e.g., Seminara, 1998], we refer to these end-member types of patches as ''forced patches'' (spatially persistent associated with strong topographic controls), ''fixed patches'' (spatially persistent because of weak topographic influences and coarsening), and ''free patches'' (migrating patches, typically bed load sheets in gravel bed rivers) . The occurrence, distribution and dynamics of these patch types have not been systematically mapped.…”

Section: Introductionmentioning

confidence: 99%

Response of bed surface patchiness to reductions in sediment supply

et al. 2009

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[1] River beds are often arranged into patches of similar grain size and sorting. Patches can be distinguished into ''free patches,'' which are zones of sorted material that move freely, such as bed load sheets; ''forced patches,'' which are areas of sorting forced by topographic controls; and ''fixed patches'' of bed material rendered immobile through localized coarsening that remain fairly persistent through time. Two sets of flume experiments (one using bimodal, sand-rich sediment and the other using unimodal, sand-free sediment) are used to explore how fixed and free patches respond to stepwise reductions in sediment supply. At high sediment supply, migrating bed load sheets formed even in unimodal, sand-free sediment, yet grain interactions visibly played a central role in their formation. In both sets of experiments, reductions in supply led to the development of fixed coarse patches, which expanded at the expense of finer, more mobile patches, narrowing the zone of active bed load transport and leading to the eventual disappearance of migrating bed load sheets. Reductions in sediment supply decreased the migration rate of bed load sheets and increased the spacing between successive sheets. One-dimensional morphodynamic models of river channel beds generally are not designed to capture the observed variability, but should be capable of capturing the time-averaged character of the channel. When applied to our experiments, a 1-D morphodynamic model (RTe-bookAgDegNormGravMixPW.xls) predicted the bed load flux well, but overpredicted slope changes and was unable to predict the substantial variability in bed load flux (and load grain size) because of the migration of mobile patches. Our results suggest that (1) the distribution of free and fixed patches is primarily a function of sediment supply, (2) the dynamics of bed load sheets are primarily scaled by sediment supply, (3) channels with reduced sediment supply may inherently be unable to transport sediment uniformly across their width, and (4) cross-stream variability in shear stress and grain size can produce potentially large errors in width-averaged sediment flux calculations.

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“…A succession of unsteady flows have been reproduced in tests performed in a laboratory flume on a steep, uniform and mixed sediment slope. The experiments have shown moving layer depths increasing from upstream to downstream, so confirming the tendency of the bottom to reach the morphodynamical equilibrium for the varying liquid discharge (Seminara, 1998). This study has highlighted how difficult it is to know for certain the equilibrium achievement, not only for the choice of the equilibrium expression, but also for the estimation of the parameters involved in it (e.g.…”

Section: Discussionmentioning

confidence: 52%

“…Morphological processes in unsteady conditions tend to become equilibrium shapes, correlated to the instantaneous water discharge (Seminara, 1998). We have verified this assessment by comparing non-dimensional moving layer measures h s /d(s j ,t i ) to equilibrium curves computed with Egashira equation (Egashira and Ashida, 1992) (plotted for representative mean slopes and friction angles).…”

Section: Discussionmentioning

confidence: 69%

“…Unfortunately, there are only few works regarding the unsteady non-uniform sediment transport, and unsteady flows are usually treated with expressions that have been developed for a uniform flow condition (Papa et al, 2004;Egashira et al, 2001;Franzi, 2001;Correira et al, 1992;Takahashi, 1991) that is often described as the "equilibrium condition" (Seminara, 1998). This approach is incorrect for purely unsteady flows, for which the immediate achievement of transport capacity, which actually occurs in equilibrium conditions, is not ensured (Armanini and Di Silvio, 1988).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

An expression for the water-sediment moving layer in unsteady flows valid for open channels and embankments

Berta

Bianco

2010

Nat. Hazards Earth Syst. Sci.

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Abstract. During the floods, the effects of sediment transport in river beds are particulary significant and can be studied through the evolution of the water-sediment layer which moves in the lower part of a flow, named "moving layer". Moving layer variations along rivers lead to depositions and erosions and are typically unsteady, but are often tackled with expressions developed for steady (equilibrium) conditions. In this paper, we develop an expression for the moving layer in unsteady conditions and calibrate it with experimental data. During laboratory tests, we have in fact reproduced a rapidly changing unsteady flow by the erosion of a granular steep slope. Results have shown a clear tendency of the moving layer, for fixed discharges, toward equilibrium conditions. Knowing the equilibrium achievement has presented many difficulties, being influenced by the choice of the equilibrium expression and moreover by the estimation of the parameters involved (for example friction angle). Since we used only data relevant to hyper-concentrated mono-dimensional flows for the calibration -occurring for slope gradients in the range 0.03-0.20 -our model can be applied both on open channels and on embankments/dams, providing that the flows can be modelled as mono-dimensional, and that slopes and applied shear stress levels fall within the considered ranges.

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Cited by 69 publications

References 63 publications

Three‐dimensional flow structure and patterns of bed shear stress in an evolving compound meander bend

Three‐dimensional flow structure and patterns of bed shear stress in an evolving compound meander bend

Response of bed surface patchiness to reductions in sediment supply

An expression for the water-sediment moving layer in unsteady flows valid for open channels and embankments

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