Wave Model SWAN Research Articles

Tide-surge and wave interaction in the Gulf of Maine during an extratropical storm

The fully coupled spectral wave and circulation model SWAN + ADCIRC was applied to investigate tide-surge and wave interaction in the Gulf of Maine during the extratropical storm on Patriot’s Day of 2007. Significant tide-surge and wave interaction was found over Georges Bank and in the coastal areas. Over Georges Bank, the wave-induced current reached 0.2 m/s at the storm peak, accounting for 17 % of the total depth-averaged current. In Saco Bay, the current was dominated by wave-induced current with a magnitude up to 1.0 m/s during the storm. Two clockwise circulation gyres were found to form and sustain over a period of 26 hours during the storm in the bay. They were driven by spatial variations of wave height, direction and the resulting wave radiation stress gradient. Wave setup reached 0.2 m at the storm peak along the coast of Saco Bay. In Saco Bay, wave energy dissipation was reduced and wave height increased due to the increased water depth at high tide and surge. Therefore, wave height was modulated by tide and surge accordingly along the coast. As a result, wave setup and wave-induced current in the bay were also modulated by tide and surge. During the tidal cycle at the storm peak, wave setup increased with tidal level and the maximum wave setup coincided with high tide.

Wind and wave characteristics in the Black Sea based on the SWAN wave model forced with the CFSR winds

Wind and wave characteristics and their long-term variability in the Black Sea over a period of 31 years are investigated in this study. The state-of-the art spectral wave model SWAN is implemented to perform a 31- year wave hindcast in the area of interest. The simulation results are used to assess the inter-annual variability and long-term changes in wind and wave climate in the Black Sea for the period 1979–2009. The SWAN model is forced with the Climate Forecast System Reanalysis (CFSR) winds. The model is calibrated and validated against available wave measurements at six offshore and near-shore locations spread over a large region in the Black Sea. The calibration was performed by tuning parameters in the white-capping and wind input formulations against available measurements for 1996 at three offshore locations (Gelendzhik, Hopa, and Sinop). The validation was carried out using measured data, at Gelendzhik, Hopa, and Sinop offshore locations, Gloria drilling platform and Karaburun and Filyos near-shore locations. From the 31-year simulation results, the long-term spatial distributions and changes of the mean wind and mean and maximum wave characteristics and their inter-annual variabilities were determined. The calibration improved SWAN model performance by 11.6% for Hm0 and 3.3% for Tm02 on average at three locations. The mean annual significant wave height (Hm0) and mean wind speed (WS) indicate the occurrence of higher wave heights and wind speeds in the western Black Sea compared to the south eastern coasts of the Black Sea. The coefficient of variation over the Black Sea for Hm0 and WS shows that the variability for Hm0 is higher than that of WS. It is also observed that the variability for Hm0 is higher in areas (such as offshore Gelendzhik, Russia) where the variability of WS is high. Besides, the storms mentioned in the previous studies (such as Galabov and Kortcheva, 2013; Tarakcioglu et al., 2015) are observed in four interesting characteristic areas with maximum Hm0 determined in this study over the Black Sea.

Wave atlas for Swiss lakes: modeling design waves in mountainous lakes

The present study aims to provide a comprehensive wave atlas for Swiss lakes. The atlas is an adequate tool that contains wave characteristics of extreme events derived from numerical calculations. Swiss lakes are surrounded by mountains, and the wind field over the lakes is highly influenced by the region’s topography. Therefore, it is important to apply the winds with appropriate direction distribution. The numerical weather prediction model COSMO-2 is used for defining the wind directional distribution, whereas the spectral wave model SWAN is used for wave simulations. The evaluation results are ultimately published on a geographical information internet platform, where a wave rose correlates the wave height to the event frequency for each point on a lake. An accurate knowledge of wave parameters is essential for the design of new shore protection measures, beach erosion control and harbor silting studies.

Assessments of wave energy in the Bohai Sea, China

Wave fields in the Bohai Sea are continuously simulated by the third-generation wave model SWAN in order to determine the wave energy resources from 1985 to 2010. The wind parameters used to simulate waves are obtained by the Regional Atmospheric Modeling System (RAMS). Comparisons of significant wave heights between simulations and observations show good agreement. The spatial distributions of mean monthly and annual averaged significant wave height and wave power flux are presented. Wave energy roses and temporal variations of average wave power density at five typical points in the Bohai Sea are calculated. Furthermore, the correlations between significant wave height and wave energy period are studied in scatter and energy diagrams.

Wave dynamics of a Pacific Atoll with high frictional effects

AbstractWe report field measurements of waves and currents made from September 2011 to July 2014 on Palmyra Atoll in the central Pacific that were used in conjunction with the SWAN wave model to characterize the wave dynamics operant on the atoll. Our results indicate that wave energy is primarily from the north during the northern hemisphere winter and from the south in the northern hemisphere summer. Refraction of waves along the reef terraces due to variations in bathymetry leads to focusing of waves in specific locations. Bottom friction, modeled with a modified bottom roughness formulation, is the significant source of wave energy dissipation on the atoll, a result that is consistent with available observations of wave damping on Palmyra. Indeed modeled wave dissipation rates from bottom friction are on average larger than dissipation rates due to breaking and are an order of magnitude larger than what has been observed on other, less geometrically complex reefs, a result which should be corroborated with future in situ measurements. The SWAN wave model with a modified bottom friction formulation better predicts bulk wave energy properties than the existing formulation at our measurement stations. The near bed squared velocity, a proxy for bottom stress, shows strong spatial variability across the atoll and exerts control over geomorphic structure and benthic community composition.

Wave model predictions in the Black Sea: Sensitivity to wind fields

This paper evaluates the impact of using different wind field products on the performance of the third generation wave model SWAN in the Black Sea and its capability for predicting both normal and extreme wave conditions during 1996. Wind data were obtained from NCEP CFSR, NASA MERRA, JRA-25, ECMWF Operational, ECMWF ERA40, and ECMWF ERA-Interim. Wave data were obtained in 1996 at three locations in the Black Sea within the NATO TU-WAVES project. The quality of wind fields was assessed by comparing them with satellite data. These wind data were used as forcing fields for the generation of wind waves. Time series of predicted significant wave height (Hmo), mean wave period (Tm02), and mean wave direction (DIR) were compared with observations at three offshore buoys in the Black Sea and its performance was quantified in terms of statistical parameters. In addition, wave model performance in terms of significant wave height was also assessed by comparing them against satellite data.The main scope of this work is the impact of the different available wind field products on the wave hindcast performance. In addition, the sensitivity of wave model forecasts due to variations in spatial and temporal resolutions of the wind field products was investigated. Finally, the impact of using various wind field products on predicting extreme wave events was analyzed by focussing on storm peaks and on an individual storm event in October 1996. The numerical results revealed that the CFSR winds are more suitable in comparison with the others for modelling both normal and extreme events in the Black Sea. The results also show that wave model output is critically sensitive to the choice of the wind field product, such that the quality of the wind fields is reflected in the quality of the wave predictions. A finer wind spatial resolution leads to an improvement of the wave model predictions, while a finer temporal resolution in the wind fields generally does not significantly improve agreement between observed and simulated wave data.

Wave forces limit the establishment of submerged macrophytes in large shallow lakes

AbstractWe studied the effect of waves on submerged macrophytes and hypothesized that exposure to large wave forces can hamper seedling establishment. In an indoor experiment in cylindrical mesocosms we tested whether large wave forces indeed inhibited the establishment of Chara globularis and Potamogeton pusillus from the propagule bank. We mimicked the effect of wave forces by generating a circular flow that caused resuspension of the sediment. Four treatments were applied, consisting of different repetition frequencies of resuspension events. Emergence and early growth of both species were monitored over 8 weeks. The resuspension treatments significantly reduced the emergence of both species, by 91% and 45% on average for Chara sp. and P. pusillus, respectively. We analysed field observations on the two species in the lakes of the IJsselmeer area in the Netherlands to evaluate whether wave forces may also inhibit establishment of macrophytes in the field. The field data seemed to support the hypothesis as both species hardly occurred in areas where a large bottom shear stress had occurred in spring, according to simulations with the SWAN wave model. The calculated maximum bottom shear stress correlated well with the occurrence of both Chara sp. and P. pusillus in the field. Regressions showed that this effect of wave forces was additional to the effect of light availability. Our study indicates that large wave forces may inhibit the establishment of macrophytes in large lakes. Reducing large wave forces can therefore potentially promote macrophyte development in these large lakes.

Numerical simulation of Typhoon Muifa (2011) using a Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system

The newly developed Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System is applied to investigate typhoon-ocean interactions in this study. The COAWST modeling system represents the state-of-the-art numerical simulation technique comprising several coupled models to study coastal and environmental processes. The modeling system is applied to simulate Typhoon Muifa (2011), which strengthened from a tropical storm to a super typhoon in the Northwestern Pacific, to explore the heat fluxes exchanged among the processes simulated using the atmosphere model WRF, ocean model ROMS and wave model SWAN. These three models adopted the same horizontal grid. Three numerical experiments with different coupling configurations are performed in order to investigate the impact of typhoon-ocean interaction on the intensity and ocean response to typhoon. The simulated typhoon tracks and intensities agree with observations. Comparisons of the simulated variables with available atmospheric and oceanic observations show the good performance of using the coupled modeling system for simulating the ocean and atmosphere processes during a typhoon event. The fully coupled simulation that includes a ocean model identifies a decreased SST as a result of the typhoon-forced entrainment. Typhoon intensity and wind speed are reduced due to the decrease of the sea surface temperature when using a coupled ocean model. The experiments with ocean coupled to atmosphere also results in decreased sea surface heat flux and air temperature. The heat flux decreases by about 29% compared to the WRF only case. The reduction of the energy induced by SST decreases, resulting in weakening of the typhoon. Coupling of the waves to the atmosphere and ocean model induces a slight increase of SST in the typhoon center area with the ocean-atmosphere interaction increased as a result of wave feedback to atmosphere.

IMPACT OF HARBOR NAVIGATION CHANNELS ON WAVES: A NUMERICAL MODELLING GUIDELINE

This study presents an intercomparison of a SWAN and SWASH wave model and 3D laboratory experiments for an existing navigation channel towards a harbor. Results show that the spectral refraction model SWAN underestimates the wave conditions in the channel and at the lee side of the channel especially - for longer waves travelling under a small angle with the channel - due to the neglect of the effects of wave tunneling by diffraction and evanescent modes. The phase-resolving non-hydrostatic 3D model SWASH does include these effects and performs reasonably well, though computational CPU demands are much higher. A modelling guideline is presented for application to other situations, showing that these effects need to be taken into account in optimizing the (layout) of the harbor; otherwise it may lead to under- or overestimation of the wave conditions and wave penetration into a harbor basin.

PREDICTION OF LONG-PERIOD BIG WAVES IN EAST COAST OF KOREA

In this study, we develop an algorithm to predict long-period big waves in the east coast of Korea using the directional wave gauge which is installed near Sokcho. Using the numerical wave model SWAN (Booij et al., 1999), we estimate wave data in open sea from the wave data observed through the directional wave gauge. Then, using the wave ray method (Munk and Arthur, 1952; Wilson, 1966) with the open-sea wave data as offshore boundary conditions, we predict the travel time of long-period long waves from the measurement site to several target points in the east coast of Korea. And, we predict significant wave height and peak wave direction at the target point using SWAN which simulates wave propagation in a domain including the point. We verify the prediction methods with the SWAN and wave ray method by comparing predicted wave data against 4 measured data of long-period big waves. The wave data were observed from October 2012 to April 2013 at the observation site of Whangdolcho which is located 200km southward from Sokcho. The prediction method can be improved using more measurement sites which will be installed in the future.

Wind waves in the Black Sea: results of a hindcast study

Abstract. In this study we describe the wind wave fields in the Black Sea. The general aims of the work were the estimation of statistical wave parameters and the assessment of interannual and seasonal wave parameter variability. The domain of this study was the entire Black Sea. Wave parameters were calculated by means of the SWAN wave model on a 5 × 5 km rectangular grid. Initial conditions (wind speed and direction) for the period between 1949 and 2010 were derived from the NCEP/NCAR reanalysis. According to our calculations the average significant wave height on the Black Sea does not exceed 0.7 m. Areas of most significant heavy sea are the southwestern and the northeastern parts of the sea as expressed in the spatial distribution of significant wave heights, wave lengths and periods. Besides, long-term annual variations of wave parameters were estimated. Thus, linear trends of the annual total duration of storms and of their quantity are nearly stable over the hindcast period. However, an intensification of storm activity is observed in the 1960s–1970s.

Impact of Sea Level Rise on Storm Surges around the Changjiang Estuary

ABSTRACT Zhao, C.; Ge, J., and Ding, P., 2014. Impact of sea level rise on storm surges around the Changjiang Estuary. The potential impacts of sea level rise (SLR) on storm surge around the Changjiang Estuary and the Hangzhou Bay are investigated using a shallow-water circulation model ADCIRC coupled with a spectral wave model SWAN. The validated model is applied to two typical typhoons under three scenarios: 1.0 m SLR, 0.483 m SLR and present sea-level condition. In consideration of interactions of tide, waves and surge, the impact exerted by SLR on tide and waves are also discussed. The migration of the amphidromes generated by SLR causes the co-phase lines to defect relatively counterclockwise near the Changjiang Estuary. The amplitude of tide increases slightly at the inner mouth of the Changjiang River, and more notable increases are presented at the northern part of the Changjiang River Mouth. The amplitude of tide decreases in adjacent areas of the Hangzhou Bay. The wave heights respond to the sea...

NUMERICAL SIMULATION OF CYCLONIC STORM SURGES OVER THE BAY OF BENGAL USING A METEOROLOGY-WAVE-SURGE-TIDE COUPLED MODEL

The purpose of this study was to evaluate the performance of an integrated meteorology and storm surge model for both the hind casting and prediction of future climate change intensified cyclones over the Bay of Bengal. This meteorology based storm surge model was developed and applied by integrating a mesoscale WRF model with the wave model SWAN and coastal ocean model FVCOM. The coupled model could capture the actual phenomena of historical cyclone Nargis, which made landfall in Myanmar in 2008, accurately and reproduce the total water level rise due to this event. The model was then used to also simulate future cyclones over the Bay of Bengal taking into account climate change by the year 2100.

SEDIMENT TRANSPORT AND DISPERSAL IN THE NEARSHORE OF “FLASH-FLOOD” RIVERS.

River sediment dispersal on the near-shore of "flash-flood† rivers is investigated using a coupled wave-current-sediment transport model. Besòs and Llobregat rivers (short and mountainous rivers in NW Mediterranean Sea, near to Barcelona City) are used as examples to study the sediment transport under "flash-flood† regime. The modeling system COWAST which includes the coupling between the water circulation model ROMS and the wave model SWAN, is applied to assess the sediment dispersal mechanisms and deposition in the coastal area off the two river mouths. Preferential depositional areas such as mud-belts were identified from the simulations. The sediment dispersal pattern obtained by the model agrees with observational measurments. Complementary numerical simulations revealed sorting of sediment grain size in the cross-shelf direction.

Realistic wave conditions and their influence on quantifying the tidal stream energy resource

When selecting suitable sites for tidal stream energy arrays a wide range of factors must be considered, from the magnitude of the tidal stream resource, to realistic oceanographic conditions. Previous computational and laboratory-scale investigations into the impact of waves upon tidal turbines (such as turbine blade loadings) and turbine arrays (such as array configuration) typically assume that waves propagate “inline” to the tidal current (waves following or waves opposing the tidal current with a 20° tolerance limit). We investigated the wave climate at typical tidal stream energy sites across the British Isles. The wave climate was simulated at 18 sites using a 7-year (2005–2011) SWAN wave model simulation of the northwest European shelf seas. The principal semi-diurnal lunar constituent (M2) was also estimated at these sites using the three-dimensional ROMS tidal model. A significant proportion of the wave climate (between 49% and 93% of the time), including extreme wave events (>10m wave heights), was found to be propagating in a direction which was “oblique” to the major axis of tidal flow (i.e. waves which propagate at an angle to the tidal current with a 20° tolerance limit) at all 18 selected sites. Furthermore, the average “inline” wave climate was 2.25m less in height and 2s less in wave period in comparison to the oblique wave climate. To understand the direct effect of waves upon the tidal stream resource, the dynamically wave-tide coupled COAWST modelling system was applied to an idealized headland case study, which represented the typical tide and wave conditions expected at first generation tidal stream energy sites. Waves were found to alter the simulated tidal velocity profile, which, because tidal stream power is proportional to velocity cubed, reduced the theoretical resource by 10% for every metre increase in wave height (R2 94% with 22 degrees of freedom) – depending upon wave period and direction. Our research indicates that wave angle should be considered when quantifying the impact of waves upon tidal turbines, such as computational fluid dynamic (CFD) studies, or laboratory-scale experiments of wake characteristics and turbine fatigue loading. Further, dynamically coupled tide-wave models may be necessary for a thorough resource assessment, since the complex wave-tide interaction affected the tidal resource; however, in situ observations of tidal velocity profiles during a range of wave events will be essential in validating such modelling approaches in the future.

A coupled tide-wave model for the NW European shelf seas

Understanding the interaction of tides and waves is essential in many studies, including marine renewable energy, sediment transport, long-term seabed morphodynamics, storm surges and the impacts of climate change. In the present research, a COAWST model of the NW European shelf seas has been developed and applied to a number of physical processes. Although many aspects of wave–current interaction can be investigated by this model, our focus is on the interaction of barotropic tides and waves at shelf scale. While the COWAST model was about five times more computationally expensive than running decoupled ROMS (ocean model) and SWAN (wave model), it provided an integrated modelling system which could incorporate many wave–tide interaction processes, and produce the tide and wave parameters in a unified file system with a convenient post-processing capacity. Some applications of the model such as the effect of tides on quantifying the wave energy resource, which exceeded 10% in parts of the region, and the effect of waves on the calculation of the bottom stress, which was dominant in parts of the North Sea and Scotland, during an energetic wave period are presented, and some challenges are discussed. It was also shown that the model performance in the prediction of the wave parameters can improve by 25% in some places where the wave-tide interaction is significant.

Inter-annual and inter-seasonal variability of the Orkney wave power resource

The waters surrounding the Orkney archipelago in the north of Scotland are one of the key regions in the world suitable for exploitation of both wave and tidal energy resources. Accordingly, Orkney waters are currently host to 1.08GW of UK Crown Estate leased wave and tidal energy projects, with a further 0.5GW leased in the southern part of the adjacent Pentland Firth. Although several wave resource models exist of the region, most of these models are commercial, and hence the results not publicly available, or have insufficient spatial/temporal resolution to accurately quantify the wave power resource of the region. In particular, no study has satisfactorily resolved the inter-annual and inter-seasonal variability of the wave resource around Orkney. Here, the SWAN wave model was run at high resolution on a high performance computing system, quantifying the Orkney wave power resource over a ten year period (2003–2012), a decade which witnessed considerable inter-annual variability in the wave climate. The results of the validated wave model demonstrate that there is considerable variability of the wave resource surrounding Orkney, with an extended winter (December–January–February–March, DJFM) mean wave power ranging from 10 to 25kW/m over the decade of our study. Further, the results demonstrate that there is considerably less uncertainty (30%) in the high energy region to the west of Orkney during winter months, in contrast to much greater uncertainty (60%) in the lower energy region to the east of Orkney. The DJFM wave resource to the west of Orkney correlated well with the DJFM North Atlantic Oscillation (NAO). Although a longer simulated time period would be required to fully resolve inter-decadal variability, these preliminary results demonstrate that due to considerable inter-annual variability in the NAO, it is important to carefully consider the time period used to quantify the wave power resource of Orkney, or regions with similar exposure to the North Atlantic. Finally, our study reveals that there is significantly less variability in the practical wave power resource, since much of the variability in the theoretical resource is contained within relatively few extreme events, when a wave device enters survival mode.

NearCoM-TVD — A quasi-3D nearshore circulation and sediment transport model

The newly developed nearshore circulation model, SHORECIRC, using a hybrid finite-difference finite-volume TVD-type scheme, is coupled with the wave model SWAN in the Nearshore Community Model (NearCoM) system. The new modeling system is named NearCoM-TVD and the purpose of this study is to report the capability and limitation of NearCoM-TVD for several coastal applications. For tidal inlet applications, the model is verified with the semi-analytical solution of Keulegan (1967) for an idealized inlet-bay system. To further evaluate the model performance in predicting nearshore circulation under intense wave–current interaction over complex bathymetry, modeled circulation patterns are validated with measured data during RCEX field experiment (MacMahan et al., 2010). For sediment transport applications, two sediment transport models are applied to predict three sandbar migration events at Duck, NC, during August to October 1994 (Gallagher et al., 1998). The model of Kobayashi et al. (2008) incorporates wave-induced onshore sediment transport rate as a function of the standard deviation of wave-induced horizontal velocities. The modeled beach profile evolution for two offshore events and one onshore event agrees well with the measured data. The second model investigated here combines two published sediment transport models, namely, the total load model driven by currents under the effect of wave stirring (Soulsby, 1997) and the wave-driven sediment transport model due to wave asymmetry/skewness (van Rijn et al., 2011). The model study with limited field data suggests that the parameterization of wave stirring is appropriate during energetic wave conditions. However, during low energy wave conditions, the effect of wave stirring needs to be re-calibrated.

Salt-marsh erosion and restoration in relation to flood protection on the Wadden Sea barrier island Terschelling

This paper explores the impact of erosion and restoration measures on habitat development and on wave damping by a small salt marsh nestled alongside a dike on the Wadden island of Terschelling. The aim is to advance knowledge about the benefits and possible side-effects of salt-marsh restoration. Analysis of a time series of aerial photographs from 1944 to 2010 indicates that the salt marsh decreased steadily in size after maintenance of accretion works was terminated. In the western part of the marsh, which is accessible to sheep, vegetation is low (5–15 cm) and dominated by Salicornia europaea and by Spartina anglica. In the most intensively grazed parts, vegetation is very scarce. The eastern, inaccessible part of the salt marsh is covered by dense patches of the shrubby perennial Atriplex portulacoides and Spartina anglica (15–25 cm in height). SWAN wave models show that wave height at this location is significantly affected by the areal extent of the salt marsh as well as by the vegetation. High or dense vegetation are in the models nearly as effective in damping waves (with an initial height of 0.15 and 0.5 m) as widening the salt-marsh area by 350 m. A low density of low plants, as observed in the grazed part of the marsh, has almost no wave-damping effect. Even under conditions of sea level rise, a broader salt marsh vegetated with high plants significantly affects modelled wave height. Therefore, salt-marsh restoration is an adaptation measure worth exploring, though an array of effect types must be considered.

Predicting the large-scale consequences of offshore wind turbine array development on a North Sea ecosystem

Three models were applied to obtain a first assessment of some of the potential impacts of large-scale operational wind turbine arrays on the marine ecosystem in a well-mixed area in a shelf sea: a biogeochemical model, a wave propagation model and an acoustic energy flux model. The results of the models are discussed separately and together to elucidate the combined effects. Overall, all three models suggested relatively weak environmental changes for the mechanisms included in this study, however these are only a subset of all the potential impacts, and a number of assumptions had to be made. Further work is required to address these assumptions and additional mechanisms. All three models suggested most of the changes within the wind turbine array, and small changes up to several tens of km outside the array. Within the array, the acoustic model indicated the most concentrated, spatially repetitive changes to the environment, followed by the SWAN wave model, and the biogeochemical model being the most diffuse. Because of the different spatial scales of the response of the three models, the combined results suggested a spectrum of combinations of environmental changes within the wind turbine array that marine organisms might respond to. The SWAN wave model and the acoustic model suggested a reduction in changes with increasing distance between turbines. The SWAN wave model suggested that the biogeochemical model, because of the inability of its simple wave model to simulate wave propagation, over-estimated the biogeochemical changes by a factor of 2 or more. The biogeochemical model suggested that the benthic system was more sensitive to the environmental changes than the pelagic system.