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Interactions between Surface Swell and Tsunamis – A First Look Hoda El Safty and James M. Kaihatu Zachry Department of Civil Engineering Texas A&M University.

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Presentation on theme: "Interactions between Surface Swell and Tsunamis – A First Look Hoda El Safty and James M. Kaihatu Zachry Department of Civil Engineering Texas A&M University."— Presentation transcript:

1 Interactions between Surface Swell and Tsunamis – A First Look Hoda El Safty and James M. Kaihatu Zachry Department of Civil Engineering Texas A&M University College Station, TX, USA IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK

2 Motivation 2004 Indian Ocean Tsunami from Koh Jum Island IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK

3 Motivation Generally – default paradigm for tsunamis: solitary wave Used for lab studies of tsunami damage Madsen et al. (2008) Solitary wave paradigm flawed Cause order of magnitude errors in spatial and temporal evolution over a sloping bottom IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK

4 Interaction with the Swell Wave Field Hypothesis: – Overlying swell wavefield can have some subtle effect on underlying tsunami Front face steepness Wave-wave interaction Run long wave through random swell field Use transient analysis (wavelet, Hilbert- Huang) to investigate effects IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK

5 NEES Payload IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK One year project from NEES program, National Science Foundation Use NEES Tsunami facility at Oregon State University Tsunami Wave Basin: 48.8m x 26.5m x 2.1m 29-paddle multi-directional piston wavemaker 4 resistance gages and 2 ADVs on movable bridge

6 Test Conditions IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Test Number Hs (cm)Tp (s)kh  =a/h UrUr 15210.033 2540.450.0330.163 310210.067 41040.450.0670.331 Tsunami “height” ~30 cm Water depth 0.75 m Different runs with tsunami either in middle or end of swell record Gage 1 Gage 8 Gage 16 Gage 20

7 Experiments Tsunami Only IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Tsunami with Swell

8 Analysis Maximum Surface Elevation As proxy for waveheight: maximum surface elevation in record Maximum surface elevation reached earlier with combined conditions than with either alone. Different results with tsunami in middle or at end of swell record. IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Blue: Tsunami Black: Swell Red: Combined Blue: Tsunami Black: Swell Red: Combined With SwellWithout Swell Tsunami at End Tsunami in Middle

9 Wavelet Analysis IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Morlet Wavelet Transform Data from experiment – periodic signal interacting with a transient signal Standard Fourier Transform techniques not suitable Wavelet transform – time-dependent frequencies Spectral densities as a function of frequency and time The Morlet Wavelet

10 Wavelet Analysis IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Gage 1 Gage 8 Gage 16Gage 20 Tsunami Only

11 Wavelet Analysis IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Tsunami with Swell: Hs=5cm, Tp=2s Gage 1 Gage 8 Gage 16Gage 20

12 Wavelet Analysis IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Tsunami with Swell: Hs=5cm, Tp=4s Gage 1Gage 8 Gage 16 Gage 20

13 Wavelet Analysis IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Tsunami with Swell: Hs=10cm, Tp=2s Gage 1Gage 8 Gage 16 Gage 20

14 Wavelet Analysis IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Tsunami with Swell: Hs=10cm, Tp=4s Gage 1 Gage 8 Gage 16Gage 20

15 Hilbert-Huang Transform IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK Top: Signal. Middle: Signal, envelope and mean of envelope. Bottom: Original signal and signal minus mean of envelope Hilbert Transform plus Empirical Model Decomposition Take original nonperiodic signal Calculate envelope with Hilbert Transform, then calculate mean Subtract mean from original signal New signal Do this on each updated signal k times Result – mode c 1 Subtract mode c 1 from original data – obtain new data Perform sifting process again Advantage: No apriori basis function Modes usually have some physical basis

16 Conclusions and Future Work It was hypothesized that the overlying swell field might have some effect on a long tsunami (front face steepness, etc.) Experiments were run at the NEES Tsunami Facility at Oregon State University Some indication of swell affecting maximum surface amplitude of overall wave field Wavelet analysis: spectral structure of tsunami affected by swell Hilbert-Huang Transform may help break motion up into physically relevant scales. IAHR Europe Meeting: 4-6 May 2010 Edinburgh UK

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