Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines

Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines

1 Volker Roeber & Jeremy D. Bricker International Research Institute of Disaster Science (IRIDeS), Tohoku University, JAPAN Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines

Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines

http://cloudfront.mediamatters.org/static/uploader/image/2013/11/19/monster -typhoon-philippines-haiyan_73273_600x450.jpg Super Typhoon Haiyan, Philippines 2

Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines
3 Super Typhoon Haiyan, Philippines Social numbers:
  • Casualties: 7000
  • People affected: 13 million, 13% of Philippine population
  • People left homeless: 2 million
  • Economic loss: US$ 2.8 billion
  • Likelihood of reoccurrence: ? http://static.businessinsider.com/image/52823af0eab8eaba499f9e8f/image.jpg
Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines
4 Super Typhoon Haiyan, Philippines Technical numbers:
  • Date: November 08, 2013
  • Storm strength: Category 5, and the strongest tropical cyclone to ever make landfall!
  • Wind speed: Over 300 km/h at landfall, gusts near 400 km/h
  • Central Pressure: 895 hPa
  • Storm Tide: Over 6 m in Tacloban
  • Massive Waves: Hs ~ 20 m
Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines

5 Storm Surge and Storm Waves ! Permission granted by Plan International

Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines
6 Why did a Tsunami-like bore occur:
  • Seismic source: No record
  • Meteo-tsunami: Unlikely, water is too deep
  • Resonance Amplification over Reef: Likely! Often occurs during tsunami events! from: GRL paper by Roeber, Yamazaki, Cheung, 2010
Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines

7 Storm Surge vs. Storm Waves Strong variation of wave heights - Storm waves travel on top of surge - Storm surge is combined wind and pressure setup (on top of tides)

Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines

8 Field survey in Hernani

Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines

Hernani – Eastern Samar 500 m Hs offshore almost 20 m Wave processes over reef as driving factor for bore formation? Problem: Very limited bathymetry and topography data

Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines

Generation of DEM bathymetry/topography GPS and echo-sounder survey

Generation of DEM bathymetry/topography

Generation of DEM bathymetry/topography Reef No Reef - 5 m grid spacing - MSL+1.0m

  • 13 Model approach
  • Storm surge components are accounted for.
  • Limitation: Only phase-averaged processes are included. Spectral  wave   model   Storm  surge   model   Parametric   typhoon  model   Tide  model   Water level - Barom. Pressure - Wind forcing Wind Storm surge envelope Wave spectra coupling SWAN DELFT  3D   Holland  model   OTPS   coupling

14 Phase-averaged results from SWAN+Delft 3D At house Max Flow Speed [m/s] Max Water Level [m]

  • 15 Model approach
  • Storm surge components are accounted for.
  • Addition: Phase-resolving processes are included. Spectral  wave   model   Storm  surge   model   Parametric   typhoon  model   Tide  model   Water level - Barom. Pressure - Wind forcing Wind Phase-­‐resolving   wave  model   Storm surge envelope Wave spectra coupling SWAN DELFT  3D   Holland  model   OTPS   BOSZ   coupling

16 Connecting time-domain with frequency-domain Internal source allows for reflected waves to leave domain (open ocean conditions) - Decompose spectrum into series of linear waves - Summation of all components along line source - Change of mass in continuity equation € η x,y,t ( ) = i=1 M ω ∑ Dij j=1 M θ ∑ cos ki xcosθj + ysinθj ( )− ωit + φij [ ] SWAN spectrum BOSZ wavemaker Each component accounts for 1 wave All component are superimposed on each other with an initially locked random phase!

17 The Random Phase problem We do not know the phases from the spectral model. SWAN only provides an energy distribution. Assumption: Each wave spectral component has an initially assigned random phase Permutation of random seeds eliminates intensity of particular wave interaction.

  • 18 BOSZ - Boussinesq Ocean & Surf Zone model
  • Conservative form of Nwogu’s (1993) Boussinesq equations,
  • Finite Volume St. Venant equations as subset
  • Imbedded conservation laws for suband supercritical flows Computation of storm waves on top of storm surge (MSL+1.0 m) Input bathymetry SWAN output Wavemaker

BOSZ - Hernani, free surface

20 BOSZ - Hernani, flow depth

21 Bore front Reef edge

22 A closer look at the bore - Multiple bores of about 2 m flow depth - Fast Flow - Long duration Result from BOSZ

23 A closer look at the bore Result from BOSZ Result from DELFT 3D

24 Answer from different 1D phase-resolving models

25 What causes the bores? 2 previous studies: - Nakaza & Hino, 1991 - Nwogu & Demirbilek, 2010 Surf beat excites natural resonance of reef.

Tresonance = 4L (2n −1) gh Wave breaking location Quarter-wave oscillator

26 750 m Tresonance = 480s 4 m Does this work for Hernani ? 375 m 4 m Tresonance = 240s where µ0=m0 and µ2=m2-m12/m0. The spectral moments mr are given by Wave group return period: = 250 s from Longuet-Higgins, 1984

27 Tsunami-like bore due to wave groups T = 290 s Spectra for different reef configurations T = 480 s Tresonance = 480s reef Tresonance = 240s half reef Tresonance = 250s Wave group Natural Periods

28 Reef vs. No Reef All frequencies IG band Sea wall Wave group frequency Wave breaking zone Reef Resonance Broad surf zone dissipates energy!

29 Reefs do not always protect the coast! Haiyan intensity 100 % 80 % 60 % 40 % Increase in IG energy

  • Tsunami-like bore such as in Hernani can result from surf beat.
  • Abrupt wave-breaking at reef edge unbounds group-wave energy.
  • Contribution from resonance over reef can intensify problem.
  • Fringing reefs can exacerbate flood risk under extreme conditions.
  • Phenomenon is not accounted for in hazard management plans !
  • Phase-resolving models are suitable to identify locations at risk. Volker Roeber roeber@irides.tohoku.ac.jp Jeremy D. Bricker bricker@irides.tohoku.ac.jp We thank: - Shuichi Kure for supporting the field trip - Kwok Fai Cheung for providing computing power - Troy W. Heitmann for constructive criticism - Midori (Katie) Saito for logistic and moral support Conclusions
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