Wave modelling in the oceans and in the regional seas: A critical look and perspective

The state-of-the-art in wave modelling is reviewed, with particular attention on the problems arising by its application in the regional seas. After giving a compact description of a spectral wave model, the different scales of application are discussed, highlighting the practical implications. The most relevant uses are described, with a critical analysis of the physical and numerical parts of a model. Then the relevance of the accuracy of the input wind fields is considered, stressing its importance for the accuracy of the derived wave results. The problems connected to wave forecasting are considered. The conclusions indicate expectations for further developments in the near future.

Hydrodynamic Climate of Port Phillip Bay

This study is dedicated to the hydrodynamic climate of Port Phillip Bay (PPB)—a largest coastal lagoon system in Victoria, Australia. Novelty of the present study includes long-term hydrodynamic hindcast simulations integrated with a spectral wave model. Specifically, a coupled unstructured grid wave–current modelling system (SCHISM + WWM) was built upon a high resolution and advanced wave physics (ST6). This coupling system was thoroughly calibrated and validated against field observations prior to applying for 27-year hindcast and case scenarios. Data from these simulations were then used to investigate the hydrodynamic climate of PPB focusing on three main aspects: water levels, waves and currents. For sea levels, this study shows that tidal and extreme sea levels (storm tides) across a large part of PPB have a similar magnitude. The highest storm tide level is found along eastern coasts of the bay in line with the wind pattern. In the vicinity of the entrance, the extreme sea level slightly reduced, in line with wave decay due to coupling effects. This extreme level is lower than results reported by previous studies, which were not built on a wave–current coupled system. For the wave field, the mean wave direction inside PPB is strongly affected by seasonality, in line with wind patterns. The 100-year return significant wave height is above 2 m along the eastern coasts. At PPH, waves get refracted after passing the narrow entrance. For currents, this study shows that both mean variations and high percentile currents are not affected by seasonality. This highlights the fact that tidal currents dominate flow movements in PPB. However, in extreme conditions, the circulation in PPB is also driven by wind patterns, forming two gyre systems. Based on case scenarios simulations, the strongest magnitude of wind-driven currents is above 0.5 m/s and found in the confined shallow region in the southern portion of PPB.

A Method for Estimating the Risk of Fishery Ports Against Typhoon: A Case Study on Dongsha Fishery Port

After standard seawalls have been built successfully, fishery ports become the structures most easily damaged during a typhoon. Estimating the risk of fishery ports against typhoons would be useful for identifying weaknesses and implementing corrective measures to protect fishing boats from a typhoon. This study describes a versatile methodology for conducting this type of quantitative assessment at fishery ports. The Dongsha fishery port in Zhejiang Province was selected as a case study to test the results derived from a high-precision Hydrodynamic Flexible Mesh model coupled with the Spectral Wave model. First, typhoon characteristics were assessed based on historical typhoons in the study area, and then, the wind, tide, storm surge, and waves were modeled and tide-surge interactions were investigated. Through comparisons of the destructive parameters from the typhoon assessment with the design and structural parameters of the fishery port, the level of the Dongsha fishery port against typhoons was determined to be 12, and the main weaknesses of the port’s defenses were found to be located near feature points T2, T3, T8, and T15. The results obtained demonstrate that the proposed methodology can be used to acquire valuable information on the risk of fishery ports against typhoons.

The physics of ocean wave evolution within tropical cyclones

A series of numerical experiments with the WAVEWATCH III spectral wave model are used to investigate the physics of wave evolution in tropical cyclones. Buoy observations show that tropical cyclone wave spectra are directionally skewed with a continuum of energy between locally generated wind-sea and remotely generated waves. These systems are often separated by more than 900. The model spectra are consistent with the observed buoy data and are shown to be governed by nonlinear wave-wave interactions which result in a cascade of energy from the wind-sea to the remotely generated spectral peak. The peak waves act in a “parasitic” manner taking energy from the wind-sea to maintain their growth. The critical role of nonlinear processes explains why one-dimensional tropical cyclone spectra have characteristics very similar to fetch-limited waves, even though the generation system is far more complex. The results also provide strong validation of the critical role nonlinear interactions play in wind-wave evolution.

Evaluation of the resilience of fishery ports to typhoons: a case study on Dongsha fishery port

After standard seawalls have been built successfully, fishery ports become the structures most easily damaged during a typhoon. Assessments of the resilience of fishery ports to typhoon damage would be useful for identifying weaknesses and implementing corrective measures to protect fishing boats from a typhoon. This study describes a versatile methodology for conducting this type of quantitative assessment at fishery ports. The Dongsha fishery port in Zhejiang Province was selected as a case study to test the results derived from a high-precision Hydrodynamic Flexible Mesh model coupled with the Spectral Wave model. First, typhoon characteristics were assessed based on historical typhoons in the study area, and then, the wind, tide, storm surge, and waves were modeled and tide-surge interactions were investigated. Through comparisons of the destructive parameters from the typhoon assessment with the design and structural parameters of the fishery port, the resistance level of the Dongsha fishery port against typhoons was determined to be 12, and the main weaknesses of the port's defenses were found to be located near feature points T2, T3, T8, and T15. The results obtained demonstrate that the proposed methodology can be used to acquire valuable information on the resilience of fishery ports to typhoons.

Wave-current interactions representation by coupling spectral wave and coastal hydrodynamics models

<p>A parallelized unstructured coupled model is developed to investigate wave-current interactions in coastal waters at regional scales. This model links the spectral wave model Simulating Waves Nearshore (SWAN; Booij et al., 1999) with the coastal hydrodynamics shallow-water equation model <em>Thetis </em>(Kärnä et al., 2018). SWAN is based on the action density equations encompassing the various source-terms accounting for deep- and shallow-water phenomena. <em>Thetis</em> solves the non-conservative form of the depth-averaged shallow water equations implemented within Firedrake, an abstract framework for the solution of Finite Element Method (FEM) problems. In resolving wave-current interactions in the proposed model, <em>Thetis</em> predicts water elevation and current velocities which are communicated in SWAN, while the latter provides radiation stresses information for the former. The numerical domain is prescribed by an unstructured mesh allowing higher resolution to areas of interest, while maintaining a reasonable computational cost. As the models share the same mesh, interpolation errors and certain computational overheads can be contained, whereas the choice to employ a sub-mesh for SWAN model is being considered to reduce the overall cost.</p><p>The model is initially validated and its performance assessed by a slowly varying-bathymetry. Predictions are compared against the analytical solutions for the wave setup and significant wave height (Longuet-Higgins and Stewart, 1964). Comparisons also extend to results from a coupled 3-D hydrodynamics model with a spectral wave model (Roland et al., 2012). The results of the proposed coupled model exhibit good correlations with the analytical solutions showcasing the same level of efficiency as the 3-D coupled model.</p><p> </p><p>References</p><p>[1] Booij N, Ris RC, Holthuijsen LH. A third-generation wave model for coastal regions: 1. Model description and validation. Journal of geophysical research: Oceans 1999;104(C4):7649–7666.</p><p>[2] Kärnä T, Kramer SC, Mitchell L, Ham DA, Piggott MD, Baptista AM. Thetis coastal ocean model: discontinuous Galerkin discretization for the three-dimensional hydrostatic equations. Geoscientific Model Development 2018;11(11):4359–4382.</p><p>[3] Longuet-Higgins MS, Stewart R. Radiation stresses in water waves; a physical discussion, with applications. In: Deep sea research and oceanographic abstracts, vol. 11 Elsevier; 1964. p. 529–562.</p><p>[4] Roland A, Zhang YJ, Wang HV, Meng Y, Teng YC, Maderich V, et al. A fully coupled 3D wave-current interaction model on unstructured grids. Journal of Geophysical Research: Oceans 2012;117(C11).</p>