dsd-int 2015 - performance of delft3 d flexible mesh in a complex intertidal bay-v2

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

2


Geographical setting Western Port Bay

3

Estuary of irregular shape

40km east-west

40km north-south

680km² in total

270km² intertidal area

Two islands

Two entrances


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

4


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.

5


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

6


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

7


Delft3D FM

8

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


Delft3D

9

Key area:

Lower North Arm


MIKE 21

10

Key area:

Lower North Arm


Delft3D FM

11

Key area:

Lower North Arm


Delft3D

12

Key area:

Upper North Arm


MIKE 21

13

Key area:

Upper North Arm


Delft3D FM

14

Key area:

Upper North Arm


Delft3D

15

Key area:

Western Entrance


MIKE 21

16

Key Area:

Western Entrance


Delft3D FM

17

Key area:

Western Entrance


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

18

Location of measured data


Model Comparison - calibration

19

(very) Comparable results


Model Comparison - calibration

20

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


Model Comparison – run times

21

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)


Model Comparison – complex features

22

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


Delft3D

Peak Flood Peak Ebb



24


MIKE

Peak Flood Peak Ebb



25


Delft3D FM

Peak Flood Peak Ebb


Model Comparison – Wetting and drying

26

Despite the different wetting and drying schemes the models show similar

representation of wetting and drying

Delft3D


Conclusions

27

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.


Thank you for your attention!

28


BONUS material

29

MIKE tendency for spreading channelised flow over wider area

MIKE


BONUS material

30


MIKE


BONUS material

31


Delft3D including grid (bathy data cell centred and grid cell averaged, function DP module)


BONUS material

32


Delft3D including grid (bathy data cell centred and grid cell averaged, function DP module)


BONUS material

33


MIKE grid


BONUS material

34


Delft3D grid

dsd-int 2015 - performance of delft3 d flexible mesh in a complex intertidal bay-v2

Software