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Undertaking Modelling of Flooding due to Wave Overtopping using the MIKE by DHI Software Suite - Dr Suzie Clarke (DHI)

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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

Published in: Engineering, Technology
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Undertaking Modelling of Flooding due to Wave Overtopping using the MIKE by DHI Software Suite - Dr Suzie Clarke (DHI)

  1. 1. Flooding Due to Wave Overtopping of Coastal Defence Structures using the MIKE 21 Suite Suzie Clarke and Matt Easton
  2. 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. 3. 01. Concept Introduction © DHI
  4. 4. 01. Concept Introduction © DHI 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. 5. 01. Concept Introduction © DHI 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. 6. 01. Concept Introduction © DHI 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. 7. 02. Project Example © DHI
  8. 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. 9. Problem definition and overview © DHI
  10. 10. Problem definition and overview © DHI
  11. 11. General approach © DHI 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. 12. The Source © DHI • 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. 13. Source – transformation of wave boundaries © DHI 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. 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 © DHI 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. 15. Pathway: 1D profile © DHI 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. 16. Pathway: 1D profile © DHI 3. Create unstructured mesh (MIKE21 toolbox) − Fixed resolution of 40 nodes per wavelength − Dependent on characteristic wave period (peak wave period)
  17. 17. Defining the pathway – wave boundary © DHI • 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. 18. Defining the pathway – running BW © DHI − Sponge Layer (included) − Wave breaking (included, default parameters) − Filter (included) − Moving shoreline (included, increased slot width) − Porosity layer (not included)
  19. 19. Pathway – model results © DHI
  20. 20. Pathway – model results (breach case) © DHI
  21. 21. Scenario modelling © DHI • 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. 22. Scenario modelling: varying water level © DHI 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. 23. 03. Step-by-Step Guide Walkthrough © DHI
  24. 24. 03_1a. Planning and Data Analysis © DHI Example: • Penzance Promenade • Wave and Bathymetry Data from Channel Coastal Observatory • Tide Data from DHI Global Tide Model
  25. 25. 03_1b. Planning and Data Analysis © DHI • 27 December 2010 • Hs = 4m • Tp = 9.5s • Surge = 2.2m
  26. 26. 03_2a. MIKE 21 FMHD and SW Model Construction © DHI 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. 27. 03_2a. MIKE 21 FMHD and SW Model Construction © DHI Example mesh:
  28. 28. 03_2b. MIKE 21 FMHD and SW Model Construction © DHI 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. 29. 03_3a. MIKE 21 BW 1DH Model Construction © DHI 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. 30. 03_3a. MIKE 21 BW 1DH Model Construction © DHI Example profiles’ locations:
  31. 31. 03_3a. MIKE 21 BW 1DH Model Construction © DHI 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. 32. 03_3b. MIKE 21 BW 1DH Model Construction © DHI 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. 33. 03_4a. MIKE 21 BW 1DH Application Runs © DHI 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. 34. 03_4a. MIKE 21 BW 1DH Application Runs © DHI 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. 35. 03_4b. MIKE 21 BW 1DH Application Runs © DHI 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. 36. 03_4b. MIKE 21 BW 1DH Application Runs © DHI • Profile 01, TS 03 Movie
  37. 37. 03_5a. MIKE 21 FMHD Application Runs © DHI 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. 38. 03_5a. MIKE 21 FMHD Application Runs © DHI 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. 39. 03_5a. MIKE 21 FMHD Application Runs © DHI 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. 40. 03_5b. MIKE 21 FMHD Application Runs © DHI Final Instantaneous Overtopping Rate Time Series:
  41. 41. 03_5b. MIKE 21 FMHD Application Runs © DHI Movie with buildings cut out - TS
  42. 42. 03_5b. MIKE 21 FMHD Application Runs © DHI Movie with buildings cut out - Table
  43. 43. 03_5b. MIKE 21 FMHD Application Runs © DHI • Instantaneous • Table
  44. 44. 04. Questions? © DHI
  45. 45. DHI are the first people you should call when you have a tough challenge to solve in a water environment. In the world of water, our knowledge is second-to-none, and we strive to make it globally accessible to clients and partners. So whether you need to save water, share it fairly, improve its quality, quantify its impact or manage its flow, we can help. Our knowledge, combined with our team’s expertise and the power of our technology, hold the key to unlocking the right solution. © DHI About DHI
  46. 46. Thank you © DHI

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