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Performance of Delft3D
Flexible Mesh in a complex
intertidal bay
Comparison with Delft3D, Mike21
Bart-Jan van der Spek
Sander Post
3 November 2015
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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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
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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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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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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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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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
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D
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Key area:
Lower North Arm
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
MIKE 21
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Key area:
Lower North Arm
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D FM
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Key area:
Lower North Arm
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D
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Key area:
Upper North Arm
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
MIKE 21
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Key area:
Upper North Arm
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D FM
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Key area:
Upper North Arm
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D
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Key area:
Western Entrance
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
MIKE 21
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Key Area:
Western Entrance
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Delft3D FM
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Key area:
Western Entrance
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
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Model Comparison - calibration
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(very) Comparable results
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
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)
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
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
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
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
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
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.
Performance of Delft3D Flexible Mesh in a complex intertidal bay | 3 November 2015
Thank you for your attention!
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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
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
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)
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)
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
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