Jian-Ming Liau

Simulating typhoon waves by SWAN wave model in coastal waters of Taiwan

The SWAN wave model is typically designed for wave simulations in the near-shore region and thus is selected for evaluating its applicability on typhoon waves in the coastal waters around Taiwan Island. Numerical calculations on processes of wave heights and periods during the passages of four representative typhoons are compared with measured data from field wave stations on both east and west coasts. The results have shown that waves due to typhoons of paths 2, 3 and 4 can be reasonably simulated on east coastal waters. However, discrepancies increase for the simulated results on west coastal waters because the islands central mountains partly damage the cyclonic structures of the passing-over typhoons. It is also found that the included nested grid scheme in SWAN could improve the accuracy of simulations in coastal waters to facilitate further engineering practices.

WWM Extended to Account for Wave Diffraction on a Current Over a Rapidly Varying Topography

The WWM (wind wave model) is extended to account for wave refraction-diffraction for wind waves propagating over a rapidly varying seabed in the presence of current. The wave diffraction effect is introduced into the wave action balance equation through the correction of wavenumber and propagation velocities using a diffraction corrected parameter. The approximation is based on the mild-slope equation for wave refraction-diffraction with current effect on a rapidly varying sea bottom. The relative importance of additional terms that influence the corrected diffraction parameter in the presence of currents was first introduced. The comparison of numerical results with other numerical models and experiments show that the validity of the model for describing wave propagating over a rapidly varying bottom with current effect is satisfactory. The implementation of this phase-decoupled refraction-diffraction approximation in WWM shows capability of the present model can be used in most practical engineering situations.© 2008 ASME

APPLICATION OF DATA ASSIMILATION FOR A SPECTRAL WAVE MODEL ON UNSTRUCTURED MESHES

The effect of the data assimilation of buoy data in the wind wave model (WWM) for wind wave simulations in both deep and shallow water regions developed by Hsu et al. [2005] is investigated. Following Lionello et al. [1992], the sequential method is implemented, where analyzed wave spectra and significant wave fields were assimilated by optimal interpolation (OI), then the analyzed values were used to reconstruct the wave spectrum. This paper examines the results of the assimilation of wave spectrum, significant wave height and significant wave period in a nearshore WWM model. The WWM model underestimates the wave period because it incorrectly applies past wave field data. The analysis has provided useful indications of the shortcomings of the WWM model. In summary, the OI approach is shown to be a reliable assimilation scheme in the WWM model.

Verification of a 3rd Generation FEM Spectral Wave Model for Shallow and Deep Water Applications

This paper shows some results of the work currently carried out to improve the wave forecasting and hindcasting in oceanic and coastal regions. A new spectral wave model with a flexible numerical scheme using triangular elements to describe the model domain was developed by Hsu et al. (2002). This new spectral wave model called WWM (Wind Wave Model) is feasible for the spectral wave modeling of irregular coastlines and complicated bathymetries because of its numerical scheme. The Wave Action Equation is solved with the aid of the Fractional Step Method (Yanenko, 1971). The Integration in the spatial space is carried out with the Taylor-Galerkin Method and the terms describing depth and current induced refraction are integrated with the aid of Leonard’s (1979) TVD Ultimate Quickest scheme, which was already introduced in the WWIII (H. Tolman, 1991) for the same purpose. In three applications the wave model was verified against in-situ spectral measurements of directional and non-directional wave buoys. The results show that the new spectral wave model is capable of hindcasting the wave climate with a comparable accuracy like the SWAN model (Ris et al., 1998), though with a better efficiency since fewer nodes are necessary to resolve the model domain and the boundary conditions adequately.Copyright