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DSD-INT 2018 Adjusting bed slope effects to improve beach dune behavior - Roelvink

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Presentation by Dano Roelvink, IHE Delft/Deltares, at the XBeach User Day 2018, during Delft Software Days - Edition 2018. Thursday, 15 November 2018, Delft.

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DSD-INT 2018 Adjusting bed slope effects to improve beach dune behavior - Roelvink

  1. 1. 1 Dano Roelvink IHE Delft & DELTARES, Netherlands Adjusting bed slope effects to improve beach/dune behaviour Dano Roelvink IHE Delft Institute for Water Education & Deltares
  2. 2. 2 Contents • Improving beach and berm behaviour • Example applications • Praia de Faro • La Faute sur Mer • Houtribdijk • Coupling with aeolian model Duna • Long-term application • Conclusions
  3. 3. 3 XBeach modes • Stationary • Surfbeat • Non- hydrostatic
  4. 4. 4 XBeach • Originally developed for modelling storm impacts on sandy coasts • Waves, currents, sediment transport and morphology change in 1D or 2DH • Now three modes of application: • stationary wave model, fast for longer-term evolution; includes term for longshore transport gradient • surfbeat mode, resolving variation on scale of wave groups, infragravity waves • nonhydrostatic mode, short-wave resolving; also usable for tsunami propagation
  5. 5. 5 Typical behaviour of Atlantic beaches in S & SW Portugal Successive observed profiles at Praia de Faro, Portugal, over a two-year period, collected as part of the CROP Project (Almeida et al., 2010)
  6. 6. 6 Typical behaviour of XBeach... Simulated profile evolution over 4 tidal periods. sensitivity to Hm0 wave height, for period Tp= 10 s.. sensitivity to wave period Tp, for wave height Hm0 = 3 m.
  7. 7. 7 Possible solutions • Detailed, intra-wave approach • Resolve complex swash processes • Many processes uncertain and computation very time- consuming • Still with many coefficients • Heuristic approach • Estimate equilibrium slope of swash zone from observations: typically steeper for coarser grain size • Introduce an up-slope term in the swash zone that nudges the profile toward the desired bermslope
  8. 8. 8 Bermslope effect • Only where H/h>1 b b BS bermslope z z q q x x a ¶ ¶æ ö = -ç ÷¶ ¶è ø Simulated ratio of wave height/water depth ratio H/h through four tidal cycles; the darkest red indicates where H/h>1, the área where the bermslope effect is applied.
  9. 9. 9 No bermslope
  10. 10. 10 Bermslope = 0.1
  11. 11. 11 Bermslope = 0.2
  12. 12. 12 Effect of a longshore gradient Accreting profile Eroding profile
  13. 13. 13 • A simple approach is helpful in resolving the treatment of steep beach face • It can be tuned to existing situations or based on knowledge on grain size dependence • The method does not fixate the coastline as in earlier approaches, but leaves the profile free to accrete or erode • This makes it a useful model to couple with a dune growth model Conclusions
  14. 14. 14 New validation against large-scale data Oregon Large Wave Tank • Excellent agreement using new ‘WTI’ settings • But: slope of swash zone much steeper in reality 1:7 1:15
  15. 15. 15 La Faute sur Mer – large-scale C.H. Lashley, X. Bertin, D. Roelvink, Coastlab2018
  16. 16. 16 La Faute sur Mer – small-scale C.H. Lashley, X. Bertin, D. Roelvink, Coastlab2018 AfterBefore
  17. 17. 17 Houtribdijk, NL Robert McCall, Marlies van der Lugt
  18. 18. 18 New XBeach parameters Keyword Meaning bermslopetransport Turn on or off bermslope swash transport model bermslopebed Turn on or off bermslope swash transport model for bed load bermslopesus Turn on or off bermslope swash transport model for suspended load bermslope Swash zone slope for (semi-) reflective beaches bermslopefac Bed slope transport factor for bermslope model bermslopegamma Wave height - water depth ratio to turn on bermslope model in surf-beat bermslopedepth Water depth to turn on on bermslope model in stationary and nonh
  19. 19. 19 Coupling beach and dune Profile storm waves moderate waves wind tsunami H, # waves vwind, dirwind , duration Hm0, Tp, dirw, zs, duration Hm0, Tp, dirw, zs, duration
  20. 20. 20 Coupled model approach Processes • Storm erosion and moderate conditions • Aeolian transport and vegetation growth Models • 1D XBeach instationary or stationary • 1D Duna model inspired by Kroy, Herrmann, Durán a.o.
  21. 21. 21 Duna model Roelvink & Costas (subm.) • Wind model Kroy et al. (2002) • Advection equation sand with pickup according to Bowen – implicit scheme • Adapted critical shear stress for moisture and slope • Vegetation growth and reduced shear stress because of it • Armouring • Morphological updating • Matlab implementation Wind model With vegetation Without vegetation
  22. 22. 22 • 2 yr simulation • Wind • Storms • Moderate • Random timeseries per season • Realistic seasonal variation • Large fluctuations, little net change Application: Praia de Faro, 2009-2011 Roelvink and Costas (subm)
  23. 23. 23 • In 1D simulations, longshore transport gradient can be included • Assumption: transport gradient proportional to transport • Coefficient lsgrad (~ 1 / length scale of transport change) • Allows profiles to accrete or erode Longshore transport gradient ( )1 0yb x y y Sz S p t x y S lsgrad S y ¶¶ ¶ - + + = ¶ ¶ ¶ ¶ = ¶
  24. 24. 24 10-year simulation with repeated 2-year cycle Dune height vs progradation rate
  25. 25. 25 40-year simulation with strong accretion due to longshore transport gradient
  26. 26. 26 • Simple heuristic adjustment in a few cells in swash zone • Drastically improves behaviour of intertidal beach • Allows for berm building and dune erosion • Opens the way for long-term simulations with XBeach only • Longshore transport gradient allows for structural erosion or accretion Conclusions
  27. 27. 27 • Re-calibrate testbed including bermslope effect • Find dependence on grain size (and other factors?) • Next challenge: reproduce longshore bars! Next steps
  28. 28. 28 Long-term modelling strategies • Hybrid model approach, e.g. • Delft3D FM <-> ShorelineS <-> XBeach-Duna

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