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Rhine, Netherlands, flood 4 Nov 1998 (Wilbers & Ten Brinke, 2003)The Impact ofVariability in Dune Dimensions onSediment Sorting and Morphodynamics
Astrid Blom
University of Twente US National Center for Earth-surface Dynamics (NCED)
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Blom et al., WRR, 2003
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Developments Sediment continuity framework by Parker, Paola & Leclair (2000): active bed described by PDF of bed surface elevationsExtension to dunes (Blom, 2003, Blom & Parker, 2004)Present research: extension to aggradational / degradational casesContinuity of non-uniform sediment
Existing bed layer models (e.g., Hirano, 1971)Useful, butInadequate description of sediment fluxesEllipticity of set of equationsProblematic definition of active layer
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Three sediment continuity models:The Hirano active layer modelThe sorting evolution model with regular dunesThe sorting evolution model with irregular dunes
Flow described using formulation backwater curveSimple power-based and surface-based sub-model of sediment transport (Ribberink, 1987)Case study The Ribberink (1987) aggradation experiment
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cbsediment concentration within the bed (cb= 1 - porosity)Fmivolume fraction content of size fraction i in the active layerFaivolume fraction content of size fraction i in the transported sedimentFIivolume fraction content of size fraction i at the interfaceqasediment transport ratethickness of active layerIelevation of interface between active layer and substrateSediment continuity of active layer:The Hirano active layer model (1971)
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Ciconcentration of size fraction i at level z (Ci= cb Ps Fi)Psprobability that the bed level is higher than zFivolume fraction content of size fraction i at level zDei Eei deposition and entrainment densities of size fraction i at level zcbsediment concentration within the bed (cb= 1 - porosity)The sorting evolution model (Blom, 2003) based on Parker-Paola-Leclair (2000) framework
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Apply the Einstein step length formulation (Einstein, 1950) to the stoss face:deterministic step lengthsAt the lee face entrainment neglected. Lee face deposition determined by a lee sorting function.Only bed load transport.Sediment fluxes through dune migrationThe sorting evolution model
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Sediment fluxes through dune migration variability in bedform dimensionsThe sorting evolution model(Leclair, 2002) e.g. E(z) = Estoss(z) pb db
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distributed over depth according to exposure to flowcomposition of fluxes uniform over bed elevationsSediment fluxes through net aggradation / degradationThe sorting evolution model
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Experiment E8-E9 by Ribberink (1987)2 sand fractions (0.78 mm, 1.29 mm)DunesStart from exp E8Over 30h, feed of fines was decreased to 0Total feed rate was maintained steadyThe Ribberink aggradation experiment
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Active part of the bed
Mean dune height and PDF troughs assumed steadyHydraulic roughness assumed steady (Ribberink, 1987)Mean dune height = 3cmActive layer thickness = 1/2
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Results: Net aggradation or degradation
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Results: Composition of active part of bed
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Results:Composition of bed load transportat downstream end of flume
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Results: Vertical sorting
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Note: no parameter tuningTaking into account the variability in dune dimensionsallows sediment to be stored at lower elevationsimproves the predicted adaptation time scalesimproves the predicted sorting profileReduction to a simplified modelApplicable to alternate bars? (data Lanzoni, Miwa)Model PDF trough elevations (Van der Mark et al., 2005)New flume experiments at VTCHLDiscussion and conclusions
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The research project is supported by:The Netherlands Organization for Scientific Research (NWO-STW)University of Twente (Civil Engineering)US National Center for Earth-surface Dynamics (NCED)The following persons have contributed to the work through discussion:Gary Parker, Jan Ribberink, Maarten Kleinhans, Suzanne Hulscher, Suzanne Leclair, Bert Jagers, Kees Sloff, Rolien van der Mark, and David Mohrig.
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Case study 1: Results, composition active bed
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Apply the Einstein step length formulation (Einstein, 1950) to the stoss face:deterministic step lengthsThe sorting evolution model (Blom, 2003)At the lee face neglect entrainment. Lee face deposition determined by a newly-developed lee sorting function i.Only bed load transport.
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Lee sorting function i
Simple power-based sub-model of sediment transport (Ribberink, 1987), surface-based (Fsur,i)Model A
Models B and C
Numerical schemes calculation backwater curve:predictor-corrector scheme calculation bed evolution:Euler explicit + upwind scheme (upw coeff 0.6) Time stepsModel Afluxes due to net aggrad/degrad: 5minModels B and Cfluxes due to dune migration:20 sec fluxes due to net aggrad/degrad:2 minMeasured data, and Models B and C:
Model A:
Time steps are small and computational times long:Model A, comp time = 10 minModel B, comp time = 160 min (multiplication factor 16)Model C, comp time = 600 min (multiplication factor 60!)