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)

Blom et al., WRR, 2003

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

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

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)

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

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

Sediment fluxes through dune migration variability in bedform dimensionsThe sorting evolution model(Leclair, 2002) e.g. E(z) = Estoss(z) pb db

distributed over depth according to exposure to flowcomposition of fluxes uniform over bed elevationsSediment fluxes through net aggradation / degradationThe sorting evolution model

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

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

Results: Net aggradation or degradation

Results: Composition of active part of bed

Results:Composition of bed load transportat downstream end of flume

Results: Vertical sorting

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

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.

Case study 1: Results, composition active bed

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.

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!)