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DSD-INT 2018 Critical dependence of morphodynamic models on transverse slope effects - Baar

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Presentation by Anne Baar, University of Utrecht, The Netherlands, at the Delft3D - User Days (Day 3: Sediment transport and morphology), during Delft Software Days - Edition 2018. Wednesday, 14 November 2018, Delft.

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DSD-INT 2018 Critical dependence of morphodynamic models on transverse slope effects - Baar

  1. 1. Faculty of Geosciences River and delta morphodynamics Critical dependence of morphodynamic models on transverse slope effects Anne Baar Marcio Boechat Albernaz, Wout van Dijk, Maarten Kleinhans
  2. 2. gravity Secondary flow Transverse slope effect 2
  3. 3. Transverse slope effect Major influence large-scale morphology Flow direction default value strong (physically correct) (Baar et al, 2018 WRR) weak ? X 100 3
  4. 4. tan ψ = 1 Ash θ 𝛿𝑧 𝛿𝑦 qn = qs αbn θ 𝑐 θ 𝛿𝑧 𝛿𝑦 Slope effect in Delft3D Ikeda (Bagnold) Koch & Flokstra ψ 4
  5. 5. tan ψ = 1 Ash θ 𝛿𝑧 𝛿𝑦 qn = qs αbn θ 𝑐 θ 𝛿𝑧 𝛿𝑦 Slope effect in Delft3D αbn Ash Ikeda (Bagnold) Koch & Flokstra ψ 5
  6. 6. Different effect on morphology Ikeda (αbn) Koch & Flokstra (Ash) X 15 Engelund- Hansen Van Rijn X 100 6
  7. 7. Calibrating with slope parameter ➢ Sediment transport predictor determines tendency to incise ➢ Slope parameterization influences bar/channel stability 7
  8. 8. Incision vs transverse slope effect What controls severe incision in models with default slope effect? flow Sediment transport W decaytransverse sediment transport 8
  9. 9. growth Incision vs transverse slope effect What controls severe incision in models with default slope effect? flow Sediment transport W transverse sediment transport 9
  10. 10. Van Rijn 2004 Growth of perturbation ➢ W/h = 21 ➢ Default slope effects (abn = 1.5) Straight channel of 5 grid cells Engelund-Hansen Initial channel 10
  11. 11. Van Rijn 2004 Growth of perturbation ➢ W/h = 21 ➢ Increased slope effects (abn = 30) Straight channel of 5 grid cells Engelund-Hansen Initial channel 11
  12. 12. Suspended sediment Slope effect only acts on bedload Van Rijn Van Rijn, suspended load treated as bedload 12
  13. 13. Grid size-dependent incision αbn= 1.5 Engelund- Hansen 13
  14. 14. Grid size-dependent incision Engelund- Hansen αbn= 3 αbn= 1.5 14
  15. 15. Grid size-dependent incision Engelund- Hansen αbn= 100 αbn= 3 αbn= 25Van Rijn 15
  16. 16. Incision vs transverse slope effect Rate of incision determined by:  Transport predictor  Amount of suspended sediment  Grid size Determines magnitude slope effect 16
  17. 17. Role of environment Large-scale morphology Erosive Balance Depositional Van Rijn Engelund-Hansen Van Rijn EH X 15 17
  18. 18. Literature? 18
  19. 19. time Van Rijn EHVan Rijn EH αbn= 5 αbn= 15 αbn= 1 Channel migration 19 A B A B A B αbn= 5 αbn= 15 αbn= 1
  20. 20. Channel migration Van Rijn, αbn=3 EH, αbn=3 20
  21. 21. Channel migration Van Rijn, αbn=3 EH, αbn=3 Location of channel centers time αbn= 25 21
  22. 22. Large-scale morphology Channels can migrate when incision is balanced by increasing slope effect Environment / objective determines if this is necessary 22
  23. 23. Implications calibrated models Calibrating on sediment transport: different morphology Calibrating on morphology: sediment transport? 23
  24. 24. 24 Western Scheldt estuary Ikeda (αbn = 30) Koch & Flokstra (Ash= 0.05) (Van Dijk et al., under review) sea sea land land
  25. 25. Western Scheldt estuary Difference in sediment transport vector ~ 1 million m3/year difference in dredged volume 25
  26. 26. Optimizing model design Combination depends on research objective Good Bad Dynamic/erosional Sediment transport Depositional αI αK αI αKαI αK αI αK Morphology VR EH Sediment transport Channel dynamics VR EH Bed slopesSediment transport Bar/bend shape Network 26
  27. 27. Conclusions Default slope parameters physically correct, but leads to severe incision Incision: Grid size Suspended sediment > diffusion? Channel migrate when incision is balanced by increasing slope effect Calibrating with slope effects: Either sediment fluxes or bed slopes calibrated → Change morphology & time scales 27

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