2015 Fiscal Year Research-status Report
Boussinesq-type Modeling of Destructive Surf Beat over a Fringing Reef during Typhoon Haiyan
| Project/Area Number |
15K06224
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| Research Institution | Tohoku University |
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Principal Investigator |
Roeber Volker 東北大学, 災害科学国際研究所, 助教 (60725240)
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| Project Period (FY) |
2015-10-21 – 2018-03-31
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| Keywords | infragravity / tsunami waves / typhoon waves / fringing reef / numerical model |
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| Outline of Annual Research Achievements |
We have investigated the role of the reef near the village of Hernani, Philippines, with respect to the generation of destructive tsunami-like bores such as what happened during Typhoon Haiyan. We found that abrupt wave breaking of the storm waves during the peak of Typhoon Haiyan along the reef edge transferred energy from the gravity to the infragravity bands. The narrow wave breaking zone quickly dissipated energy from the storm waves but hardly affected dissipation of long wave energy.
The analyses have been carried out using the phase-resolving Boussinesq model BOSZ. Comparisons with the 3D-RANS solver OpenFOAM confirm that BOSZ is capable of resolving the wave and current processes characteristic to wave breaking over a fringing reef.
In addition, we analyzed the importance of the reef dimensions near Hernani. Had the surf beat been in resonance with the reef flat the wave would have been even larger. Results show that at this site, the reef protects the shoreline during moderate storms, but can exacerbate damage during strong storms like Haiyan. This phenomenon was successfully reproduced by a phase-resolving wave model. Since the outlined tsunami-type flooding originated mainly from the energetic storm waves, the potential of such a destructive wave is not limited to tropical and subtropical areas with coral reefs. Alternately, sites in the path of extra-tropical storms can be exposed to similar scenarios if the
local bathymetry favours the abrupt release of surf beat. We are using these findings to study storm waves along the Tohoku coast, in particular around Sendai.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
The study is progressing as planned.
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| Strategy for Future Research Activity |
With the findings from Typhoon Haiyan, Philippines, we will investigate wave conditions along the Tohoku coast, mainly around Sendai Harbor. The coastline near Sendai is different from the previously studies environment in the Philippines and lacks a fringing reef. However, surf beat and other nonlinear processes still occur. The question now is whether unexpected nonlinear wave amplification processes also occur along shelf coasts - especially near coastal structures such as harbors.
We will hindcast the offshore wave conditions from characteristic swell events near Sendai, Tohoku. First, typical storms such as typhoon Atsani from 2015, will be hindcasted by utilizing the SWAN/Delft-FLOW combined hydrodynamic and wave model with the boundary conditions of the reanalysis data from Japan Meteorological Agency. The output from the SWAN/Delft-FLOW suite defines the spectral wave boundary conditions for computation of phase-resolving surf-zone and inundation processes. Tidal variations are obtained from the OTPS tide model about 5 km offshore of the project site. Both spectral wave and tidal elevations define the input for the Boussinesq-type model, BOSZ.
Novel components in BOSZ include the wave generation mechanisms based on time- and space-varying spectra. Nonlinear wave processes might occur only after long computations. Therefore, we extended BOSZ to handle long wave input from tidal fluctuations to allow for the dynamics of both wave forcing and water level changes become important.
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| Causes of Carryover |
The grant was received 6 months delayed (10/2015). Most equipment purchases and travel expenses are therefore planned for the fiscal year 2016.
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| Expenditure Plan for Carryover Budget |
Research travel to Norway, France, Turkey, Germany, and USA to attend conferences and to carry out collaborative work (numerical studies and field work). Field survey equipment has been purchased after April 2016. Further purchases include a laptop computer, software, and cluster computing time.
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