Amplification of Destructive Waves by Coral Reef during Typhoon Haiyan, Philippines - Volker Roeber & Jeremy D. Bricker
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Amplification of Destructive Waves by Coral
Reef during Typhoon Haiyan, Philippines
Volker Roeber & Jeremy D. Bricker
International Research Institute of Disaster Science (IRIDeS),
Tohoku University, JAPAN
1
Super Typhoon Haiyan, Philippines
http://cloudfront.mediamatters.org/static/uploader/image/2013/11/19/monster-typhoon-philippines-haiyan_73273_600x450.jpg
2
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
3
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
4
Storm Surge and Storm Waves
!
5
Permission granted by Plan International
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 6
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)
7
Field survey
in Hernani
8
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
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+1.0mModel approach
Wave Storm surge
spectra envelope
Water
Spectral
wave
Storm
surge
level
SWAN
DELFT
3D
Tide
OTPS
model
model
coupling model
Wind - Barom. Pressure
Parametric
- Wind forcing
Holland
model
typhoon
model
● Storm surge components are accounted for.
● Limitation: Only phase-averaged processes are included.
13Phase-averaged results from SWAN+Delft 3D
Max Water Level [m] Max Flow Speed [m/s]
At house
14Model approach
Phase-‐resolving
BOSZ
wave
model
Wave Storm surge
spectra envelope
Water
Spectral
wave
Storm
surge
level
SWAN
DELFT
3D
Tide
OTPS
model
model
coupling model
Wind - Barom. Pressure
Parametric
- Wind forcing
Holland
model
typhoon
model
● Storm surge components are accounted for.
● Addition: Phase-resolving processes are included.
15Connecting time-domain with frequency-domain
- Decompose spectrum into series of linear waves
- Summation of all components along line source
- Change of mass in continuity equation
Internal source allows for reflected Mω Mθ
waves to leave domain
η( x, y,t ) = ∑
i=1
∑D
j=1
ij [
cos ki ( x cosθ j + y sinθ j ) − ω i t + φ ij ]
(open ocean conditions)
€
SWAN spectrum BOSZ wavemaker
Each component accounts for 1 wave All component are superimposed on each other with
an initially locked random phase!
16The 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.
17BOSZ - Boussinesq Ocean & Surf Zone model
• Conservative form of Nwogu’s (1993) Boussinesq equations,
• Finite Volume St. Venant equations as subset
• Imbedded conservation laws for sub- and supercritical flows
Computation of storm waves on top of storm surge (MSL+1.0 m)
Input bathymetry
SWAN output
Wavemaker
18BOSZ - Hernani, free surface
BOSZ - Hernani, flow depth
20Bore front
Reef edge
21A closer look at the bore
Result from
BOSZ
- Multiple bores of about 2 m flow depth
- Fast Flow
- Long duration
22A closer look at the bore
Result from
BOSZ
Result from
DELFT 3D
23Answer from different 1D phase-resolving models
24What causes the bores?
2 previous studies:
- Nakaza & Hino, 1991
- Nwogu & Demirbilek, 2010
Surf beat excites natural resonance of reef.
4L
Tresonance = Quarter-wave oscillator
(2n −1) gh
Wave breaking
location
25Does this work for Hernani ?
750 m
4m
Tresonance = 480s
375 m
4m Tresonance = 240s
Wave group return period: = 250 s
1 where µ0=m0 and µ2=m2-m12/m0. The spectral moments mr are given by
!
!! = ! ! ! ! !"
from Longuet-Higgins, 1984 !
26Tsunami-like bore due to wave groups
Spectra for
different reef
configurations
T = 480 s T = 290 s
Natural Periods reef Tresonance = 480s
half reef Tresonance = 240s
Wave group Tresonance = 250s
27Reef vs. No Reef
All frequencies Sea wall
Wave breaking
zone
IG band
Wave group
frequency
Reef Resonance
Broad surf zone dissipates energy!
28Reefs do not always protect the coast!
Haiyan
intensity
100 %
80 %
Increase in
IG energy
60 %
40 %
29Conclusions
• 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.
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
Volker Roeber Jeremy D. Bricker 30
roeber@irides.tohoku.ac.jp bricker@irides.tohoku.ac.jpYou can also read
