Performance of Delft3D Flexible Mesh in a complex intertidal bay

1. Performance of Delft3D
Flexible Mesh in a complex
intertidal bay
Comparison with Delft3D, Mike21
Bart-Jan van der Spek
Sander Post
3 November 2015

2. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Introduction
 Background
 Project in the Western Port Bay, Australia
 Objective
“Model comparison between a structured grid model (Delft3D) and two unstructured
grid models Delft3D FM and MIKE to determine the most appropriate model for
further research”
 Selection of preferred modelling software based on:
 (Preliminary) model calibration
 Run times
 Representation of complex features
 Wetting and drying of intertidal mudflats
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3. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Geographical setting Western Port Bay
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 Estuary of irregular shape
 40km east-west
 40km north-south
 680km² in total
 270km² intertidal area
 Two islands
 Two entrances

4. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model packages
 Delft3D Flow (version 4.01.00)
 Developed by Deltares
 Structured grid using rectilinear or curvilinear cells
 For spatial discretization of the equations a cell-centred finite difference method is used
 For time integration an implicit scheme is used
 MIKE 21 FM HD (Service Pack 2, 2014)
 Developed by DHI
 Unstructured Mesh using triangles and quadrilateral elements
 For spatial discretization of the equations a cell-centred finite volume method is used
 For time integration an explicit scheme is used
 Delft3D Flexible Mesh (version 1.1.125)
 Developed by Deltares
 Unstructured Mesh using linear (1D) and polygonal cells with at most 6 sides
 For spatial discretization of the equations a staggered grid finite volume approach is used
 For time integration an implicit scheme is used
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5. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model set up
 Hydrodynamic modelling
 Depth-averaged 2D
 Detailed bathymetry input for all models:
 LiDAR survey
 Multi beam echo sounder survey
 Water level boundary conditions (in combination with Neumann)
 0.125º Global Tidal Model by DTU Space
 Comparable model resolutions.
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6. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D model
 Two domains to allow resolution difference
 Three DD boundaries
 Due to grid development restrictions
 Curvilinear in main channel branches
 Rectilinear grid in complex bathymetry
(Upper North Arm)
 87,800 computational elements
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7. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
MIKE 21
 Only triangular elements
 Resolution varies
 Approximately 100,000 computational elements
 More elements than Delft3D model :
 To achieve comparable resolution in key
areas with curvilinear of rectilinear cells
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8. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D FM
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 Unstructured model
 Capable to combine benefits of curvilinear cells and triangles
 Curvilinear in deeper channels with uniform flow direction
 Highly variable resolution
 80,000 computational elements

9. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D
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Key area:
Lower North Arm

10. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
MIKE 21
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Key area:
Lower North Arm

11. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D FM
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Key area:
Lower North Arm

12. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D
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Key area:
Upper North Arm

13. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
MIKE 21
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Key area:
Upper North Arm

14. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D FM
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Key area:
Upper North Arm

15. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D
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Key area:
Western Entrance

16. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
MIKE 21
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Key Area:
Western Entrance

17. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D FM
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Key area:
Western Entrance

18. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison
 Spatially constant bed roughness
 Manning’s n coefficient of 0.025 for all models
 Assessment of the performance
 Model calibration
 Run times
 Representation of complex features
 Wetting and drying of intertidal flats
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Location of measured data

19. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison - calibration
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 (very) Comparable results

20. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison - calibration
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 Comparable results
 No single model performs better
Statistical Measure (modelled – observed)
Model
MIKE 21 Delft3D D-Flow
Mean peak ebb speed difference (m/s) 0.00 0.00 -0.01
Mean peak flood speed difference (m/s) 0.02 -0.01 0.01
Root-mean-square of ebb speed difference 0.03 0.04 0.03
Root-mean-square of flood speed difference 0.06 0.06 0.05
Mean ebb % difference relative to maximum observed speed -0.2 -0.3 -0.6
Mean flood % difference relative to maximum observed
speed 3.1
-1.9 1.5
Mean peak ebb direction difference (°) -1 -3 -2
Mean peak flood direction difference (°) 0 -2 -2

21. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison – run times
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 All models were optimized in terms of model efficiency to reduce run times
 Delft3D – implicit numerical scheme with fixed time step
 MIKE and Delft3D FM – explicit numerical scheme with dynamic time step
 Delft3D: approx. 88,000 computational elements
 MIKE21: approx. 100,000 computational elements
 D-flow FM: approx. 80,000 computational elements
 Due to DD domains only 2 cores could be used for Delft3D
Core Usage
Model run time for 31 day period (hours)
MIKE 21 Delft3D D-Flow
Single Core 60 24 18
Multiple Cores 12 (8 cores) 12 (2 cores) 11 (8 cores)

22. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison – complex features
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 Example: Upper North Arm
 Tendency for flow to be concentrated in subtidal channel in Deltares models
 Expected to be a result of differences in numerical scheme
 Current speed differences
Point 4

23. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison – complex features
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 Example: Upper North Arm
 Delft3D
Peak Flood Peak Ebb

24. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison – complex features
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 Example: Upper North Arm
 MIKE
Peak Flood Peak Ebb

25. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison – complex features
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 Example: Upper North Arm
 Delft3D FM
Peak Flood Peak Ebb

26. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison – Wetting and drying
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 Despite the different wetting and drying schemes the models show similar
representation of wetting and drying
 Delft3D

27. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Conclusions
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 Overall similar results
 With MIKE having tendency for spreading channelised flow compared to Deltares models
 All models achieve (very) reasonable and similar level of calibration
 Using a single processor, DFM has substantially faster run times
 Small variability in run times using multiple cores
 Similar representation of wetting and drying
 Unstructured mesh provides much more flexibility when constructing a complex model
domain compared to a structured grid using rectangular cells and domain decomposition.

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Thank you for your attention!
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29. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
BONUS material
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 MIKE tendency for spreading channelised flow over wider area
 MIKE

30. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
BONUS material
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 MIKE tendency for spreading channelised flow over wider area
 MIKE

31. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
BONUS material
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 MIKE tendency for spreading channelised flow over wider area
 Delft3D including grid (bathy data cell centred and grid cell averaged, function DP module)

32. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
BONUS material
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 MIKE tendency for spreading channelised flow over wider area
 Delft3D including grid (bathy data cell centred and grid cell averaged, function DP module)

33. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
BONUS material
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 MIKE tendency for spreading channelised flow over wider area
 MIKE grid

34. Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
BONUS material
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 MIKE tendency for spreading channelised flow over wider area
 Delft3D grid