Investigation of Tan Chau Reach in Lower Mekong Using Field Data and Numerical Simulation

Luu, Loc X.; Egashira, Shinji; Takebayashi, Hiroshi

doi:10.2208/prohe.48.1057

Cited by 20 publications

(18 citation statements)

References 9 publications

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“…Rather than using the standard Exner formulation for sediment conservation, we introduce a modified form due to Luu et al [], which includes sediment stored in the bed load layer:

\frac{\partial V_{b}}{\partial t} + ((), 1 - λ) P_{c} \frac{\partial η_{a}}{\partial t} + \frac{\partial q_{b}}{\partial x} = 0 .

Here V b is the volume of sediment in bed load transport per unit area, t is time, λ is alluvial porosity, η a is the thickness of the alluvial layer, q b is the bed load transport rate, and x is the longitudinal axis. In rivers with sufficiently thick alluvial layers, ∂ V b /∂ t is often treated as negligible, because V b is small relative to η a ; this approximation leads to the standard form of the Exner equation.…”

Section: Discussionmentioning

confidence: 99%

“…However, in a bedrock river with only partial alluvial cover, the bed load transport rate q b may be less than the bed load transport capacity q bc . Luu et al [] expressed the bed load transport rate q b in terms of the following equation:

q_{b} = ({, \begin{array}{c} center & ((), \frac{V_{b}}{V_{bc}}) q_{bc} & normalfor 0 \leq \frac{V_{b}}{V_{normalbc}} < 1 \\ center & q_{bc} & normalfor \frac{V_{b}}{V_{normalbc}} = 1 \end{array})

where V bc is the saturation volume of bed load per unit area. We determine V bc by the following relationship:

V_{bc} = \frac{q_{normalbc}}{u_{s}}

where u s is the saltation velocity, which is calculated using the following empirical relation based on Sklar and Dietrich []:

\frac{u_{s}}{\sqrt{R_{b} g d}} = 1.56 {(τ_{*} / τ_{* c} - 1)}^{0.56} .

…”

Section: Discussionmentioning

confidence: 99%

“…Luu et al [] applied to the cases with and without an alluvial layer separately. For the case without any alluvial layer (i.e., the case for which the bed is completely exposed bedrock), ∂ η a /∂ t is 0 and V b is 0 or larger but less than V bc ; its time rate of change is computed from with η a = 0.…”

Section: Discussionmentioning

confidence: 99%

“…Rather than using the standard Exner formulation for sediment conservation, we introduce a modified form due to Luu et al [2004], which includes sediment stored in the bed load layer:…”

Section: 1002/2014jf003133mentioning

confidence: 99%

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Interaction among alluvial cover, bed roughness, and incision rate in purely bedrock and alluvial‐bedrock channel

et al. 2014

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A major control on bedrock incision is the interaction between alluvial cover and erosive mobile grains. The extent of alluvial cover is typically predicted as a function of relative sediment flux (sediment supply rate over bed load transport capacity, q bs /q bc ), yet little is known about how the bed roughness affects the alluvial cover. We performed field experiments with various flow discharges, sediment supply rates, grain sizes, and bed surface topographies. We then developed physically based models for estimating the threshold of sediment movement and the extent of alluvial cover, so as to include the effect of roughness change. The results for the threshold of sediment movement and the extent of alluvial cover obtained from our models show reasonable agreement with the results of the field experiments. We explored the sensitivity of the models to variations in sediment supply and bedrock relative roughness (bedrock hydraulic roughness height over grain size, k sb /d). The results suggest the following: (1) a larger relative roughness yields a greater dimensionless critical shear stress required for initial sediment motion; (2) at a given sediment supply rate, the extent of alluvial cover is larger when the relative roughness is larger; (3) when the sediment supply rate and the relative roughness are small, throughput bed load moves over (and can abrade) a purely bedrock channel with no alluvial cover; and (4) the critical value of sediment supply rate below which throughput bed load transport occurs increases with decreasing relative roughness. The experimental results and analysis provide a framework for treating the (a) incisional morphodynamics of purely bedrock rivers by throughput bed load with no alluvial cover, (b) incisional/alluvial morphodynamics of mixed bedrock-alluvial rivers, and (c) purely alluvial morphodynamics, as well as the transition between these states.

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“…Rather than using the standard Exner formulation for sediment conservation, we introduce a modified form due to Luu et al [], which includes sediment stored in the bed load layer:

\frac{\partial V_{b}}{\partial t} + ((), 1 - λ) P_{c} \frac{\partial η_{a}}{\partial t} + \frac{\partial q_{b}}{\partial x} = 0 .

Section: Discussionmentioning

confidence: 99%

q_{b} = ({, \begin{array}{c} center & ((), \frac{V_{b}}{V_{bc}}) q_{bc} & normalfor 0 \leq \frac{V_{b}}{V_{normalbc}} < 1 \\ center & q_{bc} & normalfor \frac{V_{b}}{V_{normalbc}} = 1 \end{array})

where V bc is the saturation volume of bed load per unit area. We determine V bc by the following relationship:

V_{bc} = \frac{q_{normalbc}}{u_{s}}

where u s is the saltation velocity, which is calculated using the following empirical relation based on Sklar and Dietrich []:

\frac{u_{s}}{\sqrt{R_{b} g d}} = 1.56 {(τ_{*} / τ_{* c} - 1)}^{0.56} .

…”

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

“…Rather than using the standard Exner formulation for sediment conservation, we introduce a modified form due to Luu et al [2004], which includes sediment stored in the bed load layer:…”

Section: 1002/2014jf003133mentioning

confidence: 99%

See 2 more Smart Citations

Interaction among alluvial cover, bed roughness, and incision rate in purely bedrock and alluvial‐bedrock channel

et al. 2014

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show abstract

“…To calculate the alluvial layer thickness, we use a form of sediment conservation that specifically includes sediment stored in the bedload layer itself, in addition to sediment deposited on the bed (Luu et al , ; Inoue et al , , 2016)

\frac{\partial V_{b}}{\partial t} + ((),, 1 - λ) \frac{\partial η_{a}}{\partial t} + ((),, \frac{\partial q_{bx}}{\partial x} + \frac{\partial q_{by}}{\partial y}) = 0

…”

Section: Model Developmentmentioning

confidence: 99%

Morphodynamics of a bedrock‐alluvial meander bend that incises as it migrates outward: approximate solution of permanent form

Inoue

Parker

Stark

2017

Earth Surf Processes Landf

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In meandering rivers cut into bedrock, erosion across a channel cross‐section can be strongly asymmetric. At a meander apex, deep undercutting of the outer bank can result in the formation of a hanging cliff (which may drive hillslope failure), whereas the inner bank adjoins a slip‐off slope that connects to the hillslope itself. Here we propose a physically‐based model for predicting channel planform migration and incision, point bar and slip‐off slope formation, bedrock abrasion, the spatial distribution of alluvial cover, and adaptation of channel width in a mixed bedrock‐alluvial channel. We simplify the analysis by considering a numerical model of steady, uniform bend flow satisfying cyclic boundary conditions. Thus in our analysis, ‘sediment supply’, i.e. the total volume of alluvium in the system, is conserved. In our numerical simulations, the migration rate of the outer bank is a specified parameter. Our simulations demonstrate the existence of an approximate state of dynamic equilibrium corresponding to a near‐solution of permanent form in which a bend of constant curvature, width, cross‐sectional shape and alluvial cover distribution migrates diagonally downward at constant speed, leaving a bedrock equivalent of a point bar on the inside of the bend. Channel width is set internally by the processes of migration and incision. We find that equilibrium width increases with increasing sediment supply, but is insensitive to outer bank migration rate. The slope of the bedrock point bar varies inversely with both outer bank migration rate and sediment supply. Although the migration rate of the outer bank is externally imposed here, we discuss a model modification that would allow lateral side‐wall abrasion to be treated in a manner similar to the process of bedrock incision. Copyright © 2016 John Wiley & Sons, Ltd.

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Advances in computational morphodynamics using the International River Interface Cooperative (iRIC) software

Shimizu

Nelson

Ferrel

et al. 2019

Earth Surf Processes Landf

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Results from computational morphodynamics modeling of coupled flow-bed-sediment systems are described for 10 applications as a review of recent advances in the field. Each of these applications is drawn from solvers included in the publicdomain International River Interface Cooperative (iRIC) software package. For mesoscale river features such as bars, predictions of alternate and higher mode river bars are shown for flows with equilibrium sediment supply and for a single case of oversupplied sediment. For microscale bed features such as bedforms, computational results are shown for the development and evolution of two-dimensional bedforms using a simple closure-based two-dimensional model, for two-and three-dimensional ripples and dunes using a three-dimensional large-eddy simulation flow model coupled to a physics-based particle transport model, and for the development of bed streaks using a three-dimensional unsteady Reynolds-averaged Navier-Stokes solver with a simple sedimenttransport treatment. Finally, macroscale or channel evolution treatments are used to examine the temporal development of meandering channels, a failure model for cantilevered banks, the effect of bank vegetation on channel width, the development of channel networks in tidal systems, and the evolution of bedrock channels. In all examples, computational morphodynamics results from iRIC solvers compare well to observations of natural bed morphology. For each of the three scales investigated here, brief suggestions for future work and potential research directions are offered.

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Investigation of Tan Chau Reach in Lower Mekong Using Field Data and Numerical Simulation

Cited by 20 publications

References 9 publications

Interaction among alluvial cover, bed roughness, and incision rate in purely bedrock and alluvial‐bedrock channel

Interaction among alluvial cover, bed roughness, and incision rate in purely bedrock and alluvial‐bedrock channel

Morphodynamics of a bedrock‐alluvial meander bend that incises as it migrates outward: approximate solution of permanent form

Advances in computational morphodynamics using the International River Interface Cooperative (iRIC) software

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