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

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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

01.

Concept Introduction

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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

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

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

02.

Project Example

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• 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

Problem definition and overview

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Problem definition and overview

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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

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

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

1. Ratio of max. water depth to deep

water wavelength: Hmax/L0 < 0.5

𝐿0 =𝑔𝑇𝑚𝑖𝑛

2

2𝜋 𝑇𝑚𝑖𝑛 =

4𝜋𝐻𝑚𝑎𝑥

𝑔

for Hmax = 6.0m, Tmin ≈ 2.8 s

Tmin << TP Criterion met

Pathway: check MIKE21 BW criteria

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Output point A from local SW model

• D = 6 m

• Hm0 = 2.3 m

• Tp = 6.6 s

• Distance from shoreline = 2000 m 2. Resolve ~10 characteristic wave-

lengths in BW model

𝐿 = 𝑇𝑃𝑔𝐿 tanh(2𝜋𝐻𝑚𝑎𝑥/𝐿)

2𝜋

Iterative solution gives Lmax ≈ 46m

10Lmax<< profile length

Criterion met

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

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)

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

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)

Pathway – model results

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Pathway – model results (breach case)

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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.

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

03.

Step-by-Step Guide Walkthrough

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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

03_1b. Planning and Data Analysis

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• 27 December 2010

• Hs = 4m

• Tp = 9.5s

• Surge = 2.2m

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

03_2a. MIKE 21 FMHD and SW Model Construction

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Example mesh:

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…

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

2𝜋𝑇2𝑡𝑎𝑛ℎ

2𝜋ℎ

𝐿

03_3a. MIKE 21 BW 1DH Model Construction

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Example

profiles’

locations:

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

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)

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

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

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

03_4b. MIKE 21 BW 1DH Application Runs

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• Profile 01, TS 03 Movie

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

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

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

03_5b. MIKE 21 FMHD Application Runs

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Final Instantaneous Overtopping Rate Time Series:

03_5b. MIKE 21 FMHD Application Runs

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Movie with buildings cut out - TS

03_5b. MIKE 21 FMHD Application Runs

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Movie with buildings cut out - Table

03_5b. MIKE 21 FMHD Application Runs

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• Instantaneous

• Table

04.

Questions?

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combined with our team’s expertise and the power of our technology,

hold the key to unlocking the right solution.

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