Undertaking Modelling of Flooding due to Wave Overtopping using the MIKE by DHI Software Suite - Dr Suzie Clarke (DHI) This presentation outlines the basis for one of the methodologies that can be followed in order to simulate the flooding of coastal areas due to overtopping of coastal defences by extreme or storm wave conditions. It is not expected that the slides are exhaustive in detail, nor present the only approach, but are provided to give basic guidance for all experience levels. Care is advised when following this methodology and all results should be subjected to reasonable checking. Read the full Executive Summary here - http://s3.amazonaws.com/dhiuk_blog_storage/UGM_2014/Overtopping-with-BW-Guidance-Executive-Summary.pdf

1. Flooding Due to Wave Overtopping of Coastal
Defence Structures using the MIKE 21 Suite
Suzie Clarke and Matt Easton

2. © DHI
1. Concept Introduction
2. Project Example
3. Step-by-Step Guide Walkthrough
1) Planning and Data Analysis
2) MIKE 21 FMHD and SW Model Construction
3) MIKE 21 BW 1DH Model Construction
4) MIKE 21 BW 1DH Application Runs
5) MIKE 21 FMHD Flooding due to Wave Overtopping Runs
4. Questions?
Agenda

3. 01.
Concept Introduction
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4. 01. Concept Introduction
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Aim of Study
• Screening
− Indicator of extent of flooding from
wave overtopping
− Use Overtopping Table approach
with mean overtopping rates for a
selection of water levels and wave
characteristics
• Individual Storm
− Instantaneous overtopping volumes
from phase-resolved wave
environment providing a refined hazard
assessment
− Use Instantaneous Overtopping
Time Series approach using
instantaneous overtopping rates from
MIKE 21 BW 1DH runs

5. 01. Concept Introduction
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How:
Incident
Environmental
Conditions
MIKE 21 FM HD
MIKE 21 SW
Wave
Transformation and
Overtopping
Overland Flows
MIKE 21 BW 1DH MIKE 21 FM HD

6. 01. Concept Introduction
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Why use BW?
• Allows you to maintain the spectral characteristics of your wave climate – you can use
a user-defined spectrum (output from SW) to generate your water level input to BW
• You can use your actual structure!
• You do not need to know the location of the toe of the structure for identifying relevant
water depth and wave height, waves are transformed from offshore

7. 02.
Project Example
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8. © DHI
• Site of a proposed new infrastructure development thought to be
susceptible to flooding
• Potential for waves to overtop natural defences and contribute to
inundation of low-lying areas
• Develop a series of conceptual pathway mechanisms to determine
overtopping volumes and inform inundation modelling
Problem definition and overview

9. Problem definition and overview
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10. Problem definition and overview
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11. General approach
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Source Pathway Receptor
• Storm waves
• Storm surge
• Tidal water level
• Overtopping of
defences
• Breach of natural
defences
• Proposed new
infrastructure &
personnel
• Coastal morphology
MIKE21 FM HD
MIKE21 FM SW
MIKE 21 BW MIKE21 FM HD

12. The Source
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• Wave conditions
− From MIKE21 FM SW model
− Storm events at various return periods (1, 10, 50, 100 years)
• Storm surge
− From MIKE21 FM HD model
− Water level rise (surge) (1, 10, 50, 100 years)
• Tidal water level
− From MIKE21 FM HD model
− Local tide gauge

13. Source – transformation of wave boundaries
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100 year storm event
Input boundary from regional SW model
• D > 20 m
• Hm0 = 6.4 m
• Tp = 10.9 s
• MWD = 79°
Output point A from local SW model
• D = 6 m
• Hm0 = 2.3 m
• Tp = 6.6 s
• MWD = 95°
A

14. 1. Ratio of max. water depth to deep
water wavelength: Hmax/L0 < 0.5
𝐿0 =
𝑔𝑇 𝑚𝑖𝑛
2
2𝜋
 𝑇 𝑚𝑖𝑛 =
4𝜋𝐻 𝑚𝑎𝑥
𝑔
for Hmax = 6.0m, Tmin ≈ 2.8 s
Tmin << TP  Criterion met 
Pathway: check MIKE21 BW criteria
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2. Resolve ~10 characteristic wave-
lengths in BW model
𝐿 = 𝑇𝑃
𝑔𝐿 tanh(2𝜋𝐻 𝑚𝑎𝑥/𝐿)
2𝜋
Iterative solution gives Lmax ≈ 46m
10Lmax<< profile length
 Criterion met 

15. Pathway: 1D profile
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1. Extract profile from MIKE21 SW mesh 2. Adjust profile for water level condition
(note sign conventions!)
e.g. SWL = MSL – HAT correction - surge level
SWL = MSL – 1.1 – 0.6

16. Pathway: 1D profile
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3. Create unstructured mesh (MIKE21 toolbox)
− Fixed resolution of 40 nodes per wavelength
− Dependent on characteristic wave period (peak wave period)

17. Defining the pathway – wave boundary
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• The model is forced by waves generated inside the model domain.
• The internal wave generation of waves allows you to absorb all waves leaving the
model domain (radiation type boundaries).
• Time series’ of water elevations synthesised from random wave generator
(MIKE21 toolbox)
− Input derived from MIKE21 FM SW model

18. Defining the pathway – running BW
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− Sponge Layer (included)
− Wave breaking (included, default parameters)
− Filter (included)
− Moving shoreline (included, increased slot width)
− Porosity layer (not included)

19. Pathway – model results
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20. Pathway – model results (breach case)
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21. Scenario modelling
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• Storm wave return periods (1, 10, 50, 100 years)
• Storm surge return periods (1, 10, 50, 100 years)
• Tidal water level variations over semidiurnal tidal cycle
• Climate change scenarios (projected sea level rise)
• Modification to existing natural defences (breach scenarios)
Each combination of scenarios requires modification of model
inputs (bathymetry profile, mesh, waves, etc.) and a review of the
BW model criteria.

22. Scenario modelling: varying water level
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Time Water level (mMSL) Mean OT flux
[m3/s/m]
Mean OT flux (breach
scenario [m3/s/m]
00:00 1.58 0.00199 0.00215
01:00 1.66 0.00249 0.00443
02:00 1.72 0.00419 0.01120
03:00 1.66 0.00249 0.00443
04:00 1.59 0.00199 0.00215

23. 03.
Step-by-Step Guide Walkthrough
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24. 03_1a. Planning and Data Analysis
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Example:
• Penzance Promenade
• Wave and Bathymetry Data
from Channel Coastal
Observatory
• Tide Data from DHI Global Tide
Model

25. 03_1b. Planning and Data Analysis
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• 27 December 2010
• Hs = 4m
• Tp = 9.5s
• Surge = 2.2m

26. 03_2a. MIKE 21 FMHD and SW Model Construction
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Points to Consider:
• Include arcs in the mesh to define where the defence structures are and add
breaklines on top of these to ensure any sharp changes in bathymetry / ground level
due to the presence of the defence structures is preserved
• You should output the defence arcs as xyz files so you can load them in later as the
defence locations in the dike structure in the HD model
• Cut your buildings out of the mesh or elevate them sufficiently to avoid rapid depth
changes if you don’t have building outlines

27. 03_2a. MIKE 21 FMHD and SW Model Construction
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Example mesh:

28. 03_2b. MIKE 21 FMHD and SW Model Construction
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Undertake preliminary HD and SW runs:
• To determine the tidal water levels and wave conditions in the region where the BW
profiles are expected to be…

29. 03_3a. MIKE 21 BW 1DH Model Construction
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Profiles – how to define them:
Calculate dominant wavelength L with
Dispersion Equation using Tp
10xL is first guess at the
profile length
Find approximate offshore total water
depth from preliminary HD runs
Use BW Set Up Planner
to calc Tmin
Is Tmin acceptable in
comparison to Tp?
Reduce profile length to
reduce offshore depth
Use original profile length but set
depths along it that are greater
than acceptable offshore depth to
acceptable offshore depth
No
Yes
L =
𝑔
2𝜋
𝑇2
𝑡𝑎𝑛ℎ
2𝜋ℎ
𝐿

30. 03_3a. MIKE 21 BW 1DH Model Construction
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Example
profiles’
locations:

31. 03_3a. MIKE 21 BW 1DH Model Construction
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Profile development process:
• Create baseline regular profile (to
vertical datum 0mODN)
• Create adjusted regular profile for
each water level (do not forget to
add surge to tidal level to create
total water level)
• Create unstructured (u/s) profile
from adjusted regular profile
• Repeat for all your profile / water
level combinations

32. 03_3b. MIKE 21 BW 1DH Model Construction
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Create offshore boundary conditions using:
• Total water depth at the Wave Generation Line (WGL) location (from profile – located
just inshore of the outer sponge layer)
• Tmin for each wave condition (from BW Set Up Planner)
• Hs & Tp for each wave condition (from the preliminary SW runs)

33. 03_4a. MIKE 21 BW 1DH Application Runs
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Important parameters for a successful run completion:
• Moving Shoreline
− DO: try increasing the slot friction coefficient and/or decreasing slot smoothing
parameter to help the model remain stable
− DO: increase your slot depth if you want to get away with larger “errors” before
crashing!
− DO NOT: change the slot width as this represents the porosity of the artificially
permeable structure and needs to be small in order to keep flows through the
structure as small as possible

34. 03_4a. MIKE 21 BW 1DH Application Runs
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Important parameters for a successful
run completion:
• Filter
− Choose a suitable depth to apply the filter
from and increase the value of the filter
rather than taking the filter layer further
offshore
• Porosity
− Try including a low porosity (for example,
0.98) for steep structures – apply the layer
to roughly one quarter of your most
energetic wavelength (L, calculated earlier)
offshore from the crest of your structure

35. 03_4b. MIKE 21 BW 1DH Application Runs
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And remember:
• Adjust Cell Locations
− The unstructured mesh location of the WGL (edge of the outer sponge layer) and
structure crest changes with each new water level and profile so update your cell
locations in WGL and Outputs
• Output Interval
− Output fluxes at a very small time step to ensure all instantaneous overtopping is
identified and captured (for example, 100x per Tp)
• Timing
− Add a few extra minutes to the start of the run to give the first waves time to reach the
structure

36. 03_4b. MIKE 21 BW 1DH Application Runs
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• Profile 01, TS 03 Movie

37. 03_5a. MIKE 21 FMHD Application Runs
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Wave overtopping rates are applied to dike structures:
• Table (for Screening Studies)
− Take average overtopping rates from the BW model outputs (or use EuroTop)
and create a table for each profile that considers the range of freeboards (crest
height – SWL) and wave conditions
− Create a time series for wave conditions or
use output from the preliminary SW run

38. 03_5a. MIKE 21 FMHD Application Runs
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Wave overtopping rates are applied to dike structures:
• Time Series (for Storm or Enhanced Studies)
− Remember to remove the overtopping rates calculated for the extra minutes added
at the start of each BW run to allow the waves to reach the structure
− Create one concatenated time series of instantaneous overtopping rates from the
various BW runs for each profile
− Add extra steps at the start of the overtopping time series with zero values to
account for spin up of the HD model
− Make sure the overtopping rates have the correct sign – positive fluxes are from
right to left when looking from the start of the dike structure to the end

39. 03_5a. MIKE 21 FMHD Application Runs
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Wave overtopping rates are applied to dike structures:
• Time Series (for Storm or Enhanced Studies)
− In order to avoid losing overtopping volume due to time step
difference between HD run and BW output frequency, use an HD
time step that is close to the output frequency of the BW results
and is less than Tp
− For larger HD time steps, take a moving average of the BW data
over a period equal to the time step of the HD model. For
example, output from BW is every 0.1s, HD time step is 10s so
take moving average of BW data over 100 BW time steps
(=10s). Use the Time Series Interpolation tool in the MZ Toolbox
to convert from 0.1s to 10s time steps in the dfs0 input file

40. 03_5b. MIKE 21 FMHD Application Runs
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Final Instantaneous Overtopping Rate Time Series:

41. 03_5b. MIKE 21 FMHD Application Runs
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Movie with buildings cut out - TS

42. 03_5b. MIKE 21 FMHD Application Runs
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Movie with buildings cut out - Table

43. 03_5b. MIKE 21 FMHD Application Runs
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• Instantaneous
• Table

44. 04.
Questions?
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