Engineering Task

2019/5/9 1

Coastal/Ocean

Modelling

Numerical Models

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Processes and

science knowledge

Projects & engineering

applications Numerical models

training

Continuous parameter is described in a number of discrete points

Accuracy of description depends on the number of points, i.e. the resolution

of the numerical grid

MIKE Powered by DHI

◼ https://www.mikepoweredbydhi.com/

◼ MIKE Powered by DHI’s software products have been used in water

environments all over the world to solve tough and complex challenges in

areas such as oceans and coastlines, rivers and reservoirs, ecology,

groundwater, water distribution, wastewater and many more.

◼ Its data management, decision support and operational forecasting

software suite traverses all the areas of applications.

◼ Software MIKE 21 can be downloaded at https://www.mikepoweredbydhi.com/download/mike-

2017?utm_source=landingpage&utm_medium=website&utm_campaign=mpbd-rel-2017

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WATER RESOURCES

◼ MIKE FLOOD

◼ MIKE HYDRO River

◼ MIKE SHE

◼ MIKE HYDRO Basin

◼ MIKE 21C

◼ MIKE OPERATIONS

COAST AND SEA

◼ MIKE 21

◼ MIKE 3

◼ LITPACK

◼ MIKE FLOOD

◼ MIKE Animator Plus

◼ ABM Lab

◼ MIKE ECO Lab

◼ MIKE C-MAP

◼ MIKE OPERATIONS 42019/5/9

CITIES

◼ MIKE URBAN

◼ WEST

◼ MIKE FLOOD

◼ MIKE OPERATIONS

GROUNDWATER AND

POROUS MEDIA

◼ FEFLOW

Port of Brisbane Pty Ltd (PBPL), Australia, considered expanding its navigational channel to

enable deeper draft vessels to pass through its waters. In order to carry out the planning

effectively, the client implemented the Port Expansion Solution, which uses an integrated

approach to reduce costs significantly, while ensuring safe working conditions at the port.

This solution has paved the way to providing cost-efficient port expansion planning in the

future. 52019/5/9

https://www.mikepoweredbydhi.com/global/references/apac/overview/port-of-brisbane

MIKE 21 SW - SPECTRAL WAVES

◼ Wave modelling is indispensable in a large number of contexts

related to activities offshore and in coastal regions. In the design of

structures, accurate assessment of wave conditions is of major

importance. Sediment transport is also to a large extent caused by

wave induced currents. Thus, knowledge of wave climate is

necessary in order to design solutions to challenges, such as coastal

erosion or harbour sedimentation.

◼ The spectral wave module, MIKE 21 SW, is a state-of-the-art

spectral wind-wave model. The module enables you to simulate

growth, decay and transformation of wind-generated waves and

swells in offshore and coastal areas. The model works in flexible

meshes, which makes it particularly well-suited to handle variable

spatial resolution in the model domain. The model includes the main

physical phenomena, for example wave-wave interaction, white-

capping, dissipation, refraction and shoaling.

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Key equation to solve:

◼ The evolution of the wave spectrum is described

by the spectral action balance equation:

• coordinates: longitude, λ; latitude, ϕ; wave direction, θ;

• relative angular frequency, σ=2pf;

• action density spectrum, N(σ, θ);

• propagation velocities in λ, ϕ, θ and σ space, cλ, cϕ, cθ and cσ;

• S - source term in terms of energy density representing effects of

generation, dissipation and nonlinear wave-wave interactions.

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 



 SNcNcNcNc

t

N =



 +



 +



 +



 +



 −1)(cos

Example: Fetch-limited Wave Growth in a Lake

The purpose of this simple application is to study fetch-limited wind-

wave growth in a 40 km long and 40 km wide lake having a constant

water depth of 15 m. The fully spectral formulation is used. The results

can readily be compared to well-known fetch-limited growth

relationships in the literature.

The wind is blowing from West (270 °N) for 15 hours. The wind speed

is constant U10= 13 m/s. Estimate the wave charateristic parameters.

8

westly wind U10

How to set up the model

using MIKE 21 SW?

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The On-line Help can be activated in several

ways, depending on the user’s requirement:

◼ F1-key seeking help on a specific activated dialog:

To access the help associated with a specific dialog

page, press the F1-key on the keyboard after opening

the editor and activating the specific property page.

◼ Open the On-line Help system for browsing manually

after a specific help page: Open the On-line Help

system by selecting “Help Topics” in the main menu bar.

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Project Oriented

◼ This template provides a folder structure that helps you

organise your model data. You may modify the

template as desired, for instance by deleting folders or

adding new folders.

◼ Open application “MIKE Zero”

◼ Generate a new project folder:

“File”->”New”->””->”Project” …

◼ General template

 External Data

 Model

 Project Documents

 Final Report

 Results

Step 1: Create a new project.

◼ Give the project a name, for example “Lake”, a folder named “Lake”

will be generated and a file called “Lake.mzp” will be created as well.

Lake

Step 2: Create a new model setup

file - Specification file (xxx.sw) ◼ Generate a new specification file to set up a model:

“File”->”New”->”MIKE 21”->”Spectral Waves FM”

◼ Save as “filename.sw”, e.g. “Lake.sw”

Step 3: Set up the specification file

– Basic Parameters

3.1 Set up domain

3.2 Select mesh (bathymetry) file

Details on mesh generation can be found at “MIKE Zero Mesh Generator,

Step-by-step training guide” uploaded at learning@griffith

3.3 Define boundary names

Details on mesh generation can be found at “MIKE Zero Mesh Generator,

Step-by-step training guide” uploaded at learning@griffith

The Universal Transverse Mercator System

UTM Zone conversion

◼ The Universal Transverse Mercator Coordinate (UTM) system provides

coordinates on a world wide flat grid for easy computation. The

Universal Transverse Mercator Coordinate system divides the World into

60 zones, each being 6 degrees longitude wide, and extending from 80

degrees south latitude to 84 degrees north latitude. The polar regions are

excluded. The first zone starts at the International Date Line (longitude

180 degrees) proceeding eastward.

◼ To find the grid zone for any longitude:

◼ Treat west longitude as negative and east as positive.

◼ Add 180 degrees; this converts the longitude to a number between zero

and 360 degrees.

◼ Divide by 6 and round up to the next higher number.

𝑈𝑇𝑀 = 𝑖𝑛𝑡𝑒𝑔𝑒𝑟 𝑙𝑜𝑛𝑔𝑡𝑖𝑡𝑢𝑑𝑒+180

6 +1

http://www.uwgb.edu/dutchs/FieldMethods/UTMSystem.htm

http://www.ngs.noaa.gov/TOOLS/utm.shtml

Bathymetry

◼ Bathymetry raw data and information

data in

“Bathy_MoretonSouth_data.txt” and

“Bathy_MoretonSouth_information.tx

t”.

◼ Use Mikezero to create a bathymetry

data file for modelling

◼ (153.283E, 27.667S)

Southern Moreton Bay: 13km by 14km

Displaying 2D results with Google

Earth

◼ The MIKE to Google Earth is a visualisation tool to be used in

conjunctionwith the Google Earth viewer to display contour maps of geo-

referenced dfs2 files, e.g. inundation flood maps or water surface maps, on

top of satellite images. To use this plug-in Google Earth must be installed

(http://earth.google.com/).

◼ Start -> Programs -> MIKE BY DHI 2016 -> MIKE Zero -> Tools -> MIKE

to Google Earth

3.3 Set up Simulation Period

Number of time step, n

Time interval, Dt

If the wind or water level

conditions are constant, the

time is used to avoid the step

function effect.

It is better to be less than 5.

CFL condition.

Total simulation time = Dt x n (s)

Number of time step, n

Time interval, Dt

Total simulation time = Dt x n (s)

Courant number

Closed boundary

◼ An offshore boundary where no wave information is

available - most often treated as an absorbing (land)

boundary. No waves can enter the model domain

from this type of boundary and waves propagating

out of the domain are fully absorbed.

Open Boundary

◼ If offshore wave data is available (e.g. wave measurements or

data derived from a MIKE 21 SW simulation), an essential

boundary can be chosen.

◼ The boundary conditions can be described through either a

parameterized formulation (Hm0, Tp, etc.) or a wave

spectrum. The wave boundary conditions can be variable in

time and space.

Step 4: Set up the specification file

– Spectral Wave Module

4.1 Set up – Basic Equations

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directional-frequency wave action

spectrum is the dependent variable

the zeroth and first moment of the wave action

spectrum as dependent variables

a steady state solution is calculated at

each time step

wave studies involving wave growth

4.2 Set up – Spectral

Discretization

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Two types of discretisation are available; logarithmic and equidistant distribution. It is recommended to always use the

logarithmic distribution of frequencies, which is given by

𝑓𝑛 = 𝑓0𝑐 𝑛 𝑛 = 1, 2, …

where fn is the frequency, f0 minimum frequency and c the frequency factor (= 1.1 as default).

The frequency range should cover wave frequencies expected to occur in the computational domain. For typical

offshore applications wave periods from 4 s to 25 s (i.e. frequencies from 0.25 Hz to 0.04 Hz) are found. In enclosed

waters wave periods of 2-3 s (i.e. frequencies from 0.33 Hz to 0.5 Hz) may also be of interest and should be resolved.

4.3 Set up – Solution

Techniques

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4.4 Set up – Water Level

Conditions

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4.5 Set up – Current

Conditions

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4.6 Set up – Wind conditions

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WINDS blowing across the ocean

surface

◼ How is the wind force on the ocean defined?

 Wind "stress" is the quantity that is modifying the

model's velocity field.

◼ What are the units?

 Units are Newtons / m2

 1 Pascal = 1 N/m2

1 dyne/cm2 = 0.1 N/m2

Why is it important to use realistic, daily

varying winds?

◼ Much of the ocean's variability, especially in the

top layers are wind driven.

◼ Realistic simulations REQUIRE realistic winds to

be used.

Wind conditions in MIKE 21

𝑈10= 𝑈𝑧 ln(

10

𝑧0 )

ln( 𝑧

𝑧0 )

U10 – wind speed at 10 m height

Uz - wind speed at z

Z0 – surface roughness, e.g. 0.0002 m

Wind roses show the frequency of

occurrence of wind speed and

direction

Convert the wind speed at a height of z to the

speed at the height of 10 m

Where do the winds for ocean model

forcing come from?

◼ Meteorological Centers  Nowcast and short-term forecast fields, both globally

and regionally

 Reanalysed fields (errors corrected), consistent data

sets

◼ Satellites  NASA Scatterometer (Active Radar instrument)

 ERS - ESA (European) satellites

What resolution do you need for

wind data?

◼ Ideally, the spatial resolution of the wind fields

should match that of the wave model.

◼ Practically speaking, global ocean models are

usually forced with wind fields of 1 - 1.5o

resolution. We are only now beginning to assess

the impact of fine scale wind forcing on coastal

ocean models.

4.7 Set up – Diffraction

Diffraction is included using the phase-decoupled refraction-diffraction approximation proposed by

Holthuijsen et al. (2003). For more details see the scientific manual.

The approximation is based on the mild-slope equation for refraction and diffraction, omitting phase

information. It does therefore not permit coherent wave fields in the computational domain as in

deterministic phase-resolving models such as Boussinesq type models.

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4.8 Set up – Energy Transfer

2D data, eg.

Surface

elevation

3D data, eg.

u, v, w etc

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4.9 Set up – Wave Breaking

2D data, eg.

Surface

elevation

3D data, eg.

u, v, w etc

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4.10 Set up – Bottom Friction

2D data, eg.

Surface

elevation

3D data, eg.

u, v, w etc

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4.11 Set up – Whitecapping

2D data, eg.

Surface

elevation

3D data, eg.

u, v, w etc

Specify white capping

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4.12 Set up – Outputs

2D data, eg.

Surface

elevation

3D data, eg.

u, v, w etc

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4.13 Set up – Wave parameters for all

Outputs

2D data, eg.

Surface

elevation

3D data, eg.

u, v, w etc

What ?

When, and how often?

Where?

File to save

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5 Run simulations

2D data, eg.

Surface

elevation

3D data, eg.

u, v, w etc

Run the simulation when menu bars get all green ticks

Points: time series .dfs0

Lines: time series for a line .dfs1

Areas: for a domain .dfsu or .dfs2

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Simulation completed

Run completed successfully

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Results analysis and

presentation

Example: Fetch-limited Wave Growth in a Lake

The purpose of this simple application is to study fetch-limited wind-

wave growth in a 40 km long and 40 km wide lake having a constant

water depth of 15 m. The fully spectral formulation is used. The results

can readily be compared to well-known fetch-limited growth

relationships in the literature.

◼ The wind is blowing from West (270 °N) for 15 hours. The wind

speed is constant U10= 13 m/s.

◼ The wind is blowing from West (270 °N) in 6 hours. Then the wind

direction turns to a south direction (180 °N) from where it blows for 8

hours. The wind speed is constant U10= 13 m/s.

Estimate the wave charateristic parameters.

51

◼ Note: When using and interpreting

model results, results near a boundary

may be questionable - if available, use a

model where the area of interest is in

the interior of the model domain, well

away from any boundary.