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DSD-INT 2019 ShorelineS and future coastline modelling - Roelvink

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Presentation by Dano Roelvink, IHE Delft Institute for Water Education, The Netherlands, at the Delft3D and XBeach User Day: Coastal morphodynamics, during Delft Software Days - Edition 2019. Wednesday, 13 November 2019, Delft.

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DSD-INT 2019 ShorelineS and future coastline modelling - Roelvink

  1. 1. 1 Dano Roelvink IHE Delft & DELTARES, Netherlands ShorelineS and future coastline modelling Dano Roelvink1,2,3, Bas Huisman2,3 , Ahmed Elghandour1,4,5 , Johan Reyns1,3 and Mohamed Ghonim1 1IHE Delft Institute for Water Education, Delft, The Netherlands 2Deltares, Delft, The Netherlands 3Delft University of Technology, Delft, The Netherlands 4Department of Civil Engineering, Faculty of Engineering, Port Said University, Port Said, Egypt 5 CIMA, University of the Algarve, Faro, Portugal
  2. 2. 2 Motivation • In complex large-scale systems, resolving the surf zones is prohibitively expensive on longer timescales • Coastline modelling has long been a cheap alternative, but existing models have severe limitations. • Can we make a better, more flexible coastline model (and couple it with a 2DH tidal morphology model?)
  3. 3. 3 Existing model capabilities: ‘standard’ models (UNIBEST LT/CL, Litpack, Genesis, CosMoS-COAST) • Capabilities: • Wave-driven longshore transport gradients • Sand mining and sediment delivery by rivers • Cross-shore transport contributions • Headlands and structures • Wide appication in engineering studies • Limitations: • Only relatively small changes relative to reference coastline possible • No fun processes like spits, islands, migrating inlets, barrier rollover
  4. 4. 4 Szmytkiewicz et al, Coastal Engineering 2000 Intercomparison of coastline models
  5. 5. 5 Vitousek et al, JGR-ESP 2017 coastline model of southern California
  6. 6. 6 Existing models: Ashton, Murray & Arnault (2001) and offspring • Mixture of grid and coastline • Simple transport formulas • Powerful in describing many types of features • High-angle instability important mechanism
  7. 7. 7 Pros and cons • Standard models: • much experience in engineering • too inflexible • not enough processes represented • CEM and similar models • beautiful processes represented in elegant way • relatively complex logic and structure • only used for schematic system studies • New, flexible engineering model needed
  8. 8. 8 Approach • Classical coastline model approach: fixed reference line Example: IJmuiden
  9. 9. 9 New approach s n Qs,i-1 Qs,i xi,yi • Coastline is like a ‘string’ of points • Flexible due to regular regridding 1 1 tan s i c c n Q c RSLR q t D s D   = − − +   
  10. 10. 10 • n is the cross-shore coordinate • s the longshore coordinate • Dc the active profile height • c a coefficient • the profile slope • RSLR the relative sea level rise • qi source/sink terms (m3/m/yr) due to cross-shore transport, overwashing, nourishments, sand mining and exchanges with rivers and tidal inlets New approach 1 1 tan s i c c n Q c RSLR q t D s D   = − − +    tan𝛽
  11. 11. 11 Basic equation s n Qi-1 Qi xi,yi xi+1,yi+1 xi-1,yi-1
  12. 12. 12 • Simple transport formulas • Based on deep water wave conditions or conditions at breaker line • Similar behaviour as function of deep water wave angle Transport formulations Author Notation Formula USACE, 1984 (simplified) CERC1 5/2 0 sin 2( )s S locQ bH = Ashton & Murray, 2006 CERC2 12 1 6 5 5 5 2 0 cos ( )sin( )s S loc locQ K H T  = USACE, 1984 CERC3 5/2 1 sin2( )s sb locbQ b H = Kamphuis, 1992 KAMP 2 1.5 0.75 0.25 0.6 502.33 sin (2 )s sb b locbQ H T m D − = Where: b : calibration coefficient CERC1 1 / , ~ 0.1 0.2 16( )(1 )s k g k b k p    = − − − ( ) sin( ) arctan 2 cos( ) j j c w loc i c w i       − =   −  1 5 2 1( ) 2 g K K   = , 1/2 1 0.4 /K m s 0sH : offshore significant wave height sbH : significant breaking wave height T : peak wave period 50D : median grain diameter [m] bm : mean bed slope (beach slope in the breaking zone) b : breaking wave angle Author Notation Formula USACE, 1984 (simplified) CERC1 5/2 0 sin 2( )s S locQ bH = Ashton & Murray, 2006 CERC2 12 1 6 5 5 5 2 0 cos ( )sin( )s S loc locQ K H T  = USACE, 1984 CERC3 5/2 1 sin2( )s sb locbQ b H = Kamphuis, 1992 KAMP 2 1.5 0.75 0.25 0.6 502.33 sin (2 )s sb b locbQ H T m D − = Where: b : calibration coefficient CERC1 1 / , ~ 0.1 0.2 16( )(1 ) k g k b k p    = − − − Transport is a function of wave height and wave direction Maximum at around 45o relative to coast
  13. 13. 13 Representation of coastline and structures • Coastline is represented as an arbitrary number of free-form polylines that can be open or closed (islands) 0 2000 4000 6000 8000 10000 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Easting [m] Northing[m] Add coastline (LMB); Next segment (RMB); Exit (q) 0 2000 4000 6000 8000 10000 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Easting [m] Northing[m] Add structure (LMB); Next structure (RMB); Exit (q)
  14. 14. 14 Wave shadowing • Waves can be shielded • by other parts of the same or other coast sections or • by structures, also represented by polylines • Sediment transport along the coastlines is driven by a CERC- like transport formula (based on deep water conditions) -4000 -2000 0 2000 4000 6000 8000 0 2000 4000 6000 8000 10000 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 0 1000 2000 3000 4000 5000 6000 7000
  15. 15. 15 Shadowing
  16. 16. 16 • Central scheme becomes unstable when angle >45º • Upwind scheme creates propagating front as in spit Upwind correction Central scheme Upwind scheme
  17. 17. 17 Other features • Coastline changes are computed based on the transport gradients, with modifications to deal with high-angle instabilities. • Regridding takes place continuously to allow the growing of spits and other forms.
  18. 18. 18 Overwashing, merging, splitting up • A set of routines is called every time step to check whether overwashing takes place, spits get too narrow and break up, or sections merge.
  19. 19. 19 • No spreading • Spreading 90o • Spreading 180o Island merging tests 0 yr 1 yr 2 yr 4 yr3 yr 5 yr 6 yr
  20. 20. 20 Van Rijn’s bestiary of coastal forms
  21. 21. 21 Sensitivity to resolution
  22. 22. 22 Innovative mega nourishment Sand Motor
  23. 23. 23 Sand Motor modelled with ShorelineS
  24. 24. 24 Sand Motor 2011-2016
  25. 25. 25 Error metrics A B C D A B C D E E
  26. 26. 26 St Louis, Senegal • Small breach to reduce lagoon water levels in 2003 • Grew to 5 km width
  27. 27. 27 • Coastlines extracted from LANDSAT imagery • Long period of southward migration • Until artificial opening in 2003 • Schematized wave climate • Mean direction 310 deg N • Hs = 1.4 m • Spreading +/- 30 deg • Simplest CERC formula • Calibration factor adjusted in calibration • Problem: inlet closes Calibration 1984-2003
  28. 28. 28 • Two options: • Implement two-way coupling with a 2DH morphological model that describes the effect of river flow and tide • Moving coastline modifies the 2D bathymetry • Erosion/sedimentation near the coast affects the coastline • Implement a heuristic model that • Maintains width of the channel (could be function of discharge or tidal prism) • Pushes back when the coastline moves the channel centre line • Distributes the retreat of the narrowest parts towards neighbouring points Simulating inlet/river mouth behaviour Let’s try this first
  29. 29. 29 Interaction with river mouth/inlet Green line: original coast line; red line : rough input sketch of river axis ; blue line : automatically refined and moved to exact center line ; black line : adjusted coastlines based on an equilibrium width of 500 m and adjustment factor of 0.1.
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  44. 44. 44 Spit locations and width of opening
  45. 45. 45 St Louis validation 2003-2018 • After artificial breaching • Initial losses to new flood delta mimicked in heuristic flood_delta function • Explains the initial stalling of the migration of the northern spit
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  63. 63. 63 • Presence of large-scale features (headlands, shoals, canyons) can significantly affect shoreline orientation. • ShorelineS can use a ‘lookup table’ of nearshore wave conditions defined on a 2D grid • This can be generated by an offline model such as SWAn or with an inline wave propagation model • Example: coastline near Port Bouet, Ivory Coast, which is influenced by the Trou Sans Fond canyon Effect of large-scale refraction
  64. 64. 64 Port Bouet, Ivory Coast
  65. 65. 65 Port Bouet, Ivory Coast x coordinate (km)→ ycoordinate(km)→ bed level (m) 00-Jan-0000 00:07:10 375 380 385 390 395 400 405 560 565 570 575 580 585 -600 -400 -200 0 200 400 600 x coordinate (km)→ ycoordinate(km)→ bed level (m) 00-Jan-0000 00:03:10 375 380 385 390 395 400 405 560 565 570 575 580 585 120 140 160 180 200 220 240 260
  66. 66. 66 Effect of large-scale refraction on coastline evolution With refraction Without refraction
  67. 67. 67 Lobitho Spit, Angola MSc Casper Mudde No large-scale refraction With large-scale refraction
  68. 68. 68 Improved modelling of groyne bypassing MSc Mohamed Ghonim • Accurate positioning of groynes • Partial bypassing and transmission
  69. 69. 69 Field test: Al Gamil beach, Egypt Shoreline 2011 Shoreline 2013 Shoreline 2015 Shoreline 2017 Shoreline 2018 1 2 3 4 5 6 7 8 9 10 11 12 13 14 100 m 300m200 m0 m
  70. 70. 70 Conclusions ShorelineS • Fun new model • Prototype for next generation coastline model for engineering purposes • Is available in open source Matlab code • Great for explaining coastal processes • Easy to add more processes • Needs very little input that is readily available: • Initial coastline • Rocky parts or structures • Wave climate
  71. 71. 71 • Coupling with dune foot (Mohamed) • Barrier overwashing and rollover (Ahmed) • Tidal inlet migration (Dano, Ahmed) • Wave diffraction (Ahmed) • Large-scale wave refraction (Bas, Casper, Dano) • Ensemble Kalman Filter (Shadrack, Johan, Sean) • Case studies in Portugal, Alaska, Senegal, Angola, NL Current research
  72. 72. 72 • What would you like to add • What cases could you bring • What could be the ambition • How would you like to use it • … Over to you…
  73. 73. 73 • New approach to coastline modelling • Available at shorelines.nl • Can run in interactive mode Conclusions

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