Narrow Results

A simulation of wind-wave activity for the 1995 Hurricane Luis has been performed using the WAM and SWAN wave models. This study tested the WAM/SWAN interface option present in the SWAN code to determine the effectiveness of applying SWAN in the finest WAM nest as a means to obtain more accurate nearshore wave predictions. The SWAN code was developed specifically for the nearshore and contains formulations for two physical processes not represented in the WAM code: depth-induced wave breaking and triad wave-interaction. SWAN was run with and without these processes to determine their effects on the results. The inclusion of triad wave-wave interaction did not significantly affect the results. In contrast, the inclusion of depth-induced wave breaking reduced the maximum wave height at the storm's peak by 31% at the test site located 900 m offshore in a water depth of 8 m. These results suggest that applying the SWAN code in the finest WAM nest can be an effective means of obtaining more accurate nearshore wave predictions.

The program SWAN (Simulating WAves Nearshore) is a third-generation wave model used to compute the spectra of random short-crested, wind-generated waves on Eulerian grids. Presently there is no release of the SWAN code to execute on parallel computers and computations are limited to a single processor. This paper shows how to easily create a version of the SWAN code that can execute on parallel platforms using a simple approximation. The approximation is to solve the transport equation as a stationary problem at every time level. This is referred to as a quasi time-accurate approximation to differentiate it from a truly time-accurate computation where the transport equation is solved as a nonstationary problem. Using this approximation, the individual integrations at different times are independent, and coarse-grain parallelism can be easily exploited using the Message Passing Interface (MPI) parallel programming system.

This study compares the quasi time-accurate method in Part I: Method, with the time-accurate version of SWAN for a simulation of 1995 Hurricane Luis and a Gulf of Mexico storm that occurred in September 2000. Using the quasi time-accurate method, significant reductions in wall times were achieved relative to time-accurate computations. Examination of the error norms show that the quasi time-accurate method agrees well with the time-accurate results. Thus, the MPI quasi time-accurate method, because of its efficiency, can play an important role for cases where the time-dependent forcing terms are the dominant influence on wave conditions.

Two wave transformation models, SWAN and CGWAVE, are used to simulate wave conditions at the Field Research Facility, Duck (North Carolina). The motivation is to examine how well these models reproduce observations and to determine the level of consistency between the two models. Stationary wave conditions pertaining to three different storm-induced bathymetric representations are modelled. It was found that SWAN and CGWAVE reproduced the observed wave behavior to a large extent, but CGWAVE results tended to be somewhat smaller than the SWAN results and the measurements. The differences were attributed to wave-wave interactions and breaking. Otherwise the models showed a high level of consistency. SWAN and CGWAVE were also used to explore other mechanisms reported in recent literature; the results were either consistent with some observations (in the case of the nonlinear mechanisms) or they shed more light on others (in case of the role of the research pier legs).

A simulation of wind-wave activity in the North Atlantic Basin, between June to August of 2002, has been performed using the SWAN model. The results of the simulations were compared with data obtained from the National Data Buoy Center and the results from NOAA's WAVEWATCH III (WW3) wave model. This study tested the applicability of the SWAN model at oceanic scales. The SWAN model was developed for coastal areas, but in version (40.11), spherical coordinates have been included in the code for oceanic applications. It was found that, in general, WW3 produces a somewhat better fit to the observations than SWAN. The same conclusion holds even when global scale WW3 results were used as boundary conditions in the SWAN simulations. Since the same wind forcing was used, we could conclude that this difference in significant wave height, Hs, could be due to the way wind energy is transferred to wave energy and its dissipation due to whitecapping in the models. The wind input parameterization in WW3 [Tolman and Chalikov, 1996] shows a better response to relatively fast changes in wind speed than the parameterization of Komen et al. [1984] used in SWAN. Although comparison between buoy observations and model output tends to favor WW3 over SWAN in deep water, we argue that the ease of use of SWAN, together with the simplification offered by just having to learn to use one model, makes the SWAN model a good option for simulations all the way from deep water up to the near shore.

Super Typhoon Meranti was one of the strongest typhoon that made landfall in the Philippines in 2016. Following the typhoon event, a post-disaster survey was conducted by Tajima et al. (2017), wherein inundation levels and wave runup heights were measured at 37 locations along the islands of Batan and Sabtang, Batanes. This paper discusses the results of the numerical modeling efforts performed to simulate the storm tide levels in Batanes Islands, and the comparisons of the simulations with the observed water levels. The numerical storm tide model for the study area was developed using the two-way coupled Delft3D-SWAN hydrodynamic and wave models. The observed water levels from the tide monitoring station in Basco Port in Batan Island indicated that the peak water level during the passage of Typhoon Meranti is only 0.87 m above MSL, while the simulated peak water level is 0.72 m above MSL. Comparing the results of the simulations to the findings of the post-disaster survey conducted by Tajima et al. (2017) showed that the simulated peak water levels at the survey locations were consistently and drastically lower than the measured water levels. The observed error in the simulated peak storm tide in Basco Port is less than 20 cm, and therefore does not account for the large differences between the measured inundation levels and simulated and measured inundation levels. The large differences are attributed to other nearshore processes (e.g. surge height amplification due to the fringing reefs) that are not considered in the current storm tide numerical model setup, and shall be investigated in a future study.

The parallel, unstructured-mesh SWAN model has been employed to study a tidal inlet with complex bathymetry in the Dutch Wadden Sea. The unstructured grid resolves the large-scale, O(1km) wave dynamics in the open sea whereas employing 15-20m grid-resolution over the tidal basins and flats to gain an insight of surf breaking, local wind-wave and wave-current interactions and to assess the performance of SWAN through hindcasts of storm events. Previous studies and the present study show that the default source term settings routinely underestimate the low-frequency peak energy in the wind-sea part of the spectrum and the finite-depth wave growth over nearly horizontal beds. High-resolution simulations have indicated that the accuracy is directly affected by parameterization of bottom friction, whitecapping dissipation and depth-induced wave breaking.

In the design of sand control facilities and in the assessment of beach erosion due to global climate change, long-term morphological changes should be estimated and evaluated. Wave and wind time series are important factors for the estimation of long-term shoreline change. This study proposes a method to model the long-term time series of wave and wind by using observation data, weather and wave prediction models in a physical sense and the Monte Carlo method in a statistical sense.