Phase-averaged Wave Model Research Articles

Breaching of the Sillon de Talbert gravel spit (North Brittany, France) and coastal flooding risk assessment

Coastal barriers are a natural defence against the marine submersion of low-lying back-barrier areas. In March 2018, a breach opened up in the proximal section of the Sillon de Talbert spit barrier (North Brittany, France), causing great concern among local residents and elected representatives. A study to assess the risk of submersion of the low-lying areas of the Laneros Peninsula, located behind the barrier, was carried out in 2020. Hydrodynamic conditions (waves, currents, and water levels) were modeled using Telemac-2D coupled with the phase-averaged wave model TOMAWAC. Simulations were compiled for the storm of February 1, 2014, corresponding to the most morphogenic event to occur in the last two decades. Five topomorphological configurations were considered for the numerical simulations: configurations #1 and #2 correspond to the ante-breach and post-breach morphological settings, while configurations #3, #4, and #5 correspond to hypothetical scenarios, from the enlargement of the breach (from 300 to 500 m width), to complete crest lowering of the barrier after a complete removing of the crest due to catastrophic overwash events. The results show that the breach accelerates the draining of the back-barrier sandflat, thereby reducing the height of extreme water levels at the coastline. This is mainly due to different tidal impacts on either side of the barrier, with a higher water level and the tidal peak occurring 15 min later and the on the back-barrier than on the ocean-side zone. As a result, ebb currents begin to flow before the high tide level is reached, inducing a significant draining with a lowering of the extreme water level at the back-barrier zone. However, these simulations do not take into account the effects of climate change over the coming years, in particular the future rise in sea level and/or the acceleration of erosion processes already underway.

Modeling wave attenuation by submerged flexible vegetation with XBeach phase-averaged model

Understanding and modeling wave attenuation by flexible vegetation have emerged as essential issues in coastal and ocean engineering. Given that the existing XBeach phase-averaged wave model can only simulate wave attenuation by rigid vegetation, and previous studies of wave energy dissipation by vegetation are mainly restricted to linear wave theory. This study aims to extend the XBeach phase-averaged wave model to simulate wave attenuation by both homogeneous and heterogeneous submerged flexible vegetation, and determine wave attenuation simulation deviation based on different wave theories. Hence, the existing flexible vegetation dynamic model is incorporated into the XBeach phase-averaged wave model. The reliability of the coupled model in simulating wave attenuation by both homogeneous and heterogeneous submerged flexible vegetation is revealed. Simulation results also demonstrate that the computed wave attenuation rate based on Stokes second-order wave theory is larger than that based on linear wave theory within the parameters used in this investigation. Meanwhile, wave attenuation simulation deviation between these two wave theories enlarges as the Ursell number increases within the parameters used in this investigation. This study can broaden the application of the XBeach model and lay the technical foundation for future engineering practice of nature-based coastal defence.

The impact of surface currents on the wave climate in narrow fjords

This study investigates the effect of surface currents on wind-generated waves in a complex coastal system with narrow fjords. The simulations are based on a phase-averaged wave model forced with surface currents from a high-resolution coastal ocean and fjord circulation model, and high-resolution winds from a nested atmospheric model. Wave simulations with and without ocean forcing are evaluated by comparing integrated wave parameters and modelled spectra with observations from five wave buoys. The comparison covers three winter seasons (2017–2020) and a case study. The wind sea part of the spectrum is better simulated at all locations when using the current forcing. At the most sheltered location, where wind sea dominated the wave climate, the wave height estimates improved by 12 percentage points when including current forcing. Spectral moments and the shape of the average spectra are also improved at most of the locations when current forcing is applied. The effect of wave–current interactions was found to be more pronounced at inner locations where the relative difference of spectral bandwidth is up 5%, the difference in directional spreading is greater than 5 degrees during strong surface currents, and the relative difference in peak frequency is exceeding 10%. Our results are consequential for narrow, deep and sheltered water bodies, but are not expected to carry over to shallow water areas.

Modeling nearshore dynamics of extreme storms in complex environments of Connecticut

Flood hazard planning requires the accurate estimation of total water elevation due to predicted tide, surge, and wave runup to design flood protection structures and improve coastal risk planning for severe storms. The beach geomorphology and nearshore hydrodynamic conditions impact the conclusive flood inundation mapping in complex environments. The conventional approaches of flood modeling are limited due to either i) simple static estimates, ii) the application of a coupled circulation and phase-averaged wave models in coarse resolution, iii) failing to calibrate and validate with in-situ data, or iv) not considering sea-level rise projections in mapping the flood extent. We used a fully nonlinear Boussinesq wave model (FUNWAVE-TVD) on the nearshore area with a high-resolution grid to determine total water elevation on the shores. The model adopted the boundary conditions from coupled circulation and wave model of the Long Island Sound (Liu et al., 2020).We applied the model to Branford, Norwalk, and New Haven, CT to represent the complex topo-bathymetric features and the structural interference in shallow water wave dynamics. We compared the model results with the FEMA base flood elevation, the North Atlantic Coast Comprehensive Study (NACCS), and (Liu et al., 2020)’s Long Island Sound FVCOM-SWAVE model. The FUNWAVE-TVD model is found to model wave processes more accurately in shallow water regions compared to the empirical equation application of FEMA and coupled circulation-phase averaged model application of NACCS and FVCOM-SWAVE. We also examined local sea-level rise predictions of storms with 1% and 10% annual exceedance probability by the year 2050 in Connecticut and found that the flood extent of these two storms showed little to no difference. We suggest the planning approaches should consider the increase in the frequency of the storms in the predicted inundation zones due to sea-level rise.

NEW BREAKER INDEX FORMULA IN PARAMETRIC WAVE MODEL

Phase-averaged parametric wave models have been widely used to predict nearshore wave height transformation. The performance of parametric models depends significantly on the wave breaker index (), which controls the amount of breaking energy dissipation. Previous parameterizations improved the model predictability by considering the breaker index as a tunable coefficient, while made less effort to the physical interpretation for the proposed formulas. Indeed, inconsistency from the physical perspectives might exist. Therefore, the parameterization of still requires further investigation by considering the comprehensive influences of the offshore wave parameters and the local water depth, as well as the possible relationships with the breaker type and the surf zone state.

Ensemble-Based Variational Method for Nonlinear Inversion of Surface Gravity Waves

ABSTRACTFreak/rogue waves are considered to be the causes of marine accidents and their generation mechanism is closely related to the formation of wave groups. However, observations that capture the spatiotemporal evolution of coherent wave groups in directional windsea are rather limited. The paper presents a new technique known as the surface wave reconstruction by ensemble adjoint-free data assimilation (SWEAD) method that enables reconstruction of a spatiotemporal wave field covering a large area from wave records limited in observational density and spatial extent. We reconstructed spatiotemporal profiles of nonlinear surface gravity waves from virtual observational data using the adjoint-free four-dimensional variational data assimilation (a4DVar) scheme. The higher-order spectral method (HOSM) is used as a forward deep-water nonlinear wave model in a realistic sea state. The a4DVar scheme uses perturbed ensemble simulations to calculate the cost function gradient and Hessian; thus, construction of an adjoint model is not needed. A few extensions of the a4DVar scheme are proposed in this study. For efficient wave reconstruction, perturbed ensemble simulation results are reused by increasing the searching direction dimension at each iteration while assuring conformity to the perturbed model’s linearity. For regularization, Fourier coefficient magnitudes are constrained by a known power spectrum from the phase-averaged wave model. Twin experiments were conducted for a unidirectional wave with virtual wave gauge data and a multidirectional wave with virtual stereo camera imaging data. For both unidirectional and multidirectional cases, nonlinear freak wave–related wave groups were well reproduced, which is impossible using a linear model.

Seasonal and interannual variability of the wave climate at a wave energy hotspot off the southwestern coast of Australia

Despite Australia having one of the most abundant offshore wave energy resources globally, there remains a lack of understanding of how this offshore resource extends into the coastal zone where most wave energy converters would be deployed. We used the phase-averaged wave model SWAN to simulate 38 years (1980–2017) of wave conditions near Albany, Western Australia, which has been proposed as a future commercial wave energy development site. The nearshore (30 m depth) wave resource varied seasonally and interannually with the wave energy flux and mean wave direction negatively correlated to the phase of the Southern Annular Mode (SAM) and positively correlated to the latitudinal position of the subtropical high-pressure ridge. As a result, the observed positive trend in SAM over recent decades may cause a decrease in nearshore wave energy (including fewer storm events) and an anti-clockwise (more southerly) rotation in wave direction. These changes may facilitate wave energy development and extraction by reducing the number and magnitude of extreme events during which wave energy cannot be extracted and equipment can be damaged. The interannual fluctuations in the wave resource can be significant and should be considered during the site selection for wave energy projects.

Wind Wave Prediction by using Autoregressive Integrated Moving Average model : Case Study in Jakarta Bay

Prediction of wind wave is highly needed to support safe navigation, especially for ship. Besides that, loading and unloading activities in a harbour, as well as for design purpose of coastal and offshore structures, data of prediction of wave height are needed. Based on its nature, the wind wave has random behaviour that is highly depending on behaviour of wind as the main driving force. In this paper, we propose a prediction method for wind wave by using Autoregressive Integrated Moving Average or ARIMA. To obtain historical data of wind wave, we perform wave simulation by using a phase-averaged wave model SWAN (Simulating Wave Near Shore). From the simulation, time series of wind wave is obtained. The prediction of wind wave is performed to calculate forecast of 24 hours ahead. Here, we perform wind wave prediction in a location in Jakarta Bay, Indonesia. We perform several combination of ARIMA model to obtain best fit model for wind wave prediction in the location in Jakarta Bay. Results of prediction show that ARIMA model give an accurate prediction especially for short term prediction.

VALIDATION OF INTEGRATED COASTAL PROCESS MODEL FOR SIMULATING WAVE-CURRENT-VEGETATION INTERACTION

Vegetation plays an important role in reducing wetland and coastal erosion. It is well-known that vegetation in coastal regions acts as a buffer to reduce wave impact and wave setup, as waves are deformed and energy of waves is attenuated through vegetation zones. Modeling wave deformation and transformation through vegetation zones can quantify wave attenuation effects as a function of vegetation properties, improve our understanding of physical processes, and eventually provide an effective assessment tool for management, control, and design of vegetation zones and wetlands. Wave attenuation effects have been studied for several decades. In addition to physical modeling and field observation, two types of mathematical approaches for modeling effects of vegetation on waves can be identified, i.e., phase resolving wave model and phase-averaged wave model. The former directly simulates dynamic wave shape deformations through a vegetation field by solving the time-dependent fluid flow governing equations. The latter directly computes wave spectral variations in space and time by using a time-averaged wave energy equation. Although a phase-resolving model can give detailed wave phase information as waves propagate through vegetation zones, it needs extensive computing efforts to solve dynamic fluid flow problems.

WAVE MODELLING IN A COMPLEX ESTUARY: STUDY IN PREPARATION OF FIELD MEASUREMENT CAMPAIGN EEMS-DOLLARD ESTUARY

This paper considers the Eems-Dollard estuary in the north of the Netherlands, which is part of the shallow shelf sea the Wadden Sea. This estuary is a highly complex area with tidal flats and deep channels and is characterised by an offshore directed wind, posing a challenge to wave prediction models. As little measurements are available, a measurement campaign is set up to provide field data for verifying and improving these wave models. The paper presents the locations that are most suited for wave measurements in the estuary, insight in the performance of the phase-averaged numerical wave model SWAN, and insight in the processes that play a role in the area around the corner of the Eemshaven. Furthermore, it presents insight into the reliability and applicability of SWAN in this highly complex area. An analysis of propagation effects is performed, as well as a comparison between the SWAN version as used for the Dutch dike safety assessment and the newest version, used for development, which includes the state of the art parameterisations of the physics. Furthermore, modelling is done for a schematised version of the area around the corner of the Eemshaven, considering several different model settings. Large differences occur in the results between the two SWAN versions. These differences are studied in more detail, and the causes of these differences are identified.

Numerical Modeling of the Wave Energy Propagation in the Iberian Nearshore

In the present work the wave energy propagation patterns in the western side of the Iberian nearshore were evaluated. This assessment takes into account the results provided by a wave modelling system based on spectral phase averaged wave models, which considers subsequent computational domains with increasing resolution towards the coast. The system was previously validated against both in situ measurements and remotely sensed data. Moreover, several data assimilation techniques were implemented as well. In this way, the reliability of the wave predictions was significantly increased. Although extended wave hindcasts have already been carried out close to the Iberian coast of the Atlantic Ocean, including wave energy assessments, they might not be completely accurate because of recent changes in the dynamics of the ocean and coastal wave climate. Thus, the present work considers wave nowcasts that correspond to the most recent and relevant wave energy propagation patterns in the targeted coastal environment. In order to perform this analysis, four different computational levels were considered. The first level corresponds to the sub oceanic domain and it is linked directly to the oceanic wave model, which is implemented over the entire North Atlantic Ocean. The second is related to the coarser computational domains of the coastal areas, while the third relates to the high-resolution domains. These three levels are defined as spherical coordinates (longitude, latitude). Finally, the last computational level includes some coastal areas which have the highest spatial resolution, defined considering the Cartesian coordinates. Moreover, for each level several computational domains have been considered. This work illustrates the most recent and significant wave transformation and energy propagation patterns corresponding to 18 computational domains with various resolutions in the western Iberian coastal environment.

Evaluation of the shoreline effect of the marine energy farms in different coastal environments

The objective of the proposed work is to assess the possible effects on the shoreline dynamics of the marine energy farms. Three different coastal environments have been considered as case studies. The first area is located in the Portuguese continental nearshore at the European western coast of the Atlantic Ocean. The second in the Mediterranean Sea in the coastal environment of Sardinia Island and the third on the western side of the Black Sea in the Romanian nearshore. Besides the fact that, from a geographical point of view, the three areas targeted are located in very different zones, they represent also coastal environments with very different characteristics. A computational framework joining a spectral phase averaged wave model with a 1D parametric circulation model has been used. The numerical models are SWAN (acronym from Simulating Waves Nearshore) for the waves and NSSM (the Navy Standard Surf Model) for estimating the longshore currents. For each case study, the main characteristics of the environmental matrix have been analyzed first. Then, various transmission scenarios have been, designed considering also in each case different distances from the marine energy farm to the shoreline. A general conclusion of the proposed work is that the longshore currents are very sensitive to the presence of the marine energy farms and that is why such farms can also play an active role in coastal protection. Moreover, since it presents in parallel similar analyses for three coastal environments that have very different features, the present study provides a more comprehensive picture concerning the medium to long term impact on the shoreline dynamics of the future marine energy farms operating in the nearshore.

Evaluation of the shoreline effect of the marine energy farms in different coastal environments

The objective of the proposed work is to assess the possible effects on the shoreline dynamics of the marine energy farms. Three different coastal environments have been considered as case studies. The first area is located in the Portuguese continental nearshore at the European western coast of the Atlantic Ocean. The second in the Mediterranean Sea in the coastal environment of Sardinia Island and the third on the western side of the Black Sea in the Romanian nearshore. Besides the fact that, from a geographical point of view, the three areas targeted are located in very different zones, they represent also coastal environments with very different characteristics. A computational framework joining a spectral phase averaged wave model with a 1D parametric circulation model has been used. The numerical models are SWAN (acronym from Simulating Waves Nearshore) for the waves and NSSM (the Navy Standard Surf Model) for estimating the longshore currents. For each case study, the main characteristics of the environmental matrix have been analyzed first. Then, various transmission scenarios have been, designed considering also in each case different distances from the marine energy farm to the shoreline. A general conclusion of the proposed work is that the longshore currents are very sensitive to the presence of the marine energy farms and that is why such farms can also play an active role in coastal protection. Moreover, since it presents in parallel similar analyses for three coastal environments that have very different features, the present study provides a more comprehensive picture concerning the medium to long term impact on the shoreline dynamics of the future marine energy farms operating in the nearshore.

The influence of waves on the tidal kinetic energy resource at a tidal stream energy site

Successful deployment of tidal energy converters relies on access to accurate and high resolution numerical assessments of available tidal stream power. However, since suitable tidal stream sites are located in relatively shallow waters of the continental shelf where tidal currents are enhanced, tidal energy converters may experience effects of wind-generated surface-gravity waves. Waves may thus influence tidal currents, and associated kinetic energy, through two non-linear processes: the interaction of wave and current bottom boundary layers, and the generation of wave-induced currents. Here, we develop a three-dimensional tidal circulation model coupled with a phase-averaged wave model to quantify the impact of the waves on the tidal kinetic energy resource of the Fromveur Strait (western Brittany) - a region that has been identified with strong potential for tidal array development. Numerical results are compared with in situ observations of wave parameters (significant wave height, peak period and mean wave direction) and current amplitude and direction 10m above the seabed (the assumed technology hub height for this region). The introduction of waves is found to improve predictions of tidal stream power at 10m above the seabed at the measurement site in the Strait, reducing kinetic energy by up to 9% during storm conditions. Synoptic effects of wave radiation stresses and enhanced bottom friction are more specifically identified at the scale of the Strait. Waves contribute to a slight increase in the spatial gradient of available mean tidal stream potential between the north-western area and the south-eastern part of the Strait. At the scale of the region within the Strait that has been identified for tidal stream array development, the available mean spring tidal stream potential is furthermore reduced by 12% during extreme waves conditions. Isolated effects of wave radiation stresses and enhanced bottom friction lead to a reduction in spring tidal potential of 7.8% and 5.3%, respectively. It is therefore suggested that models used for tidal resource assessment consider the effect of waves in appropriately wave-exposed regions.

Wave Propagation into Complex Coastal Systems and the Role of Nonlinear Interactions

The phase-averaged wave model Simulating WAves Nearshore (SWAN) is often used for the design of dikes and harbors. However, various hindcast studies have shown that SWAN underpredicts the wave energy when waves are penetrating into bathymetries with shallow areas traversed by channels, such as tidal inlets or harbor entrances. The underprediction of these waves could lead to dike failure or shipping downtime as a consequence of incorrect hydraulic loads. This paper presents an explanation for the underprediction of this wave penetration. By comparing a series of SWAN computations with laboratory measurements and computations with the Boussinesq-type wave model TRITON, it is demonstrated that the absence of various subharmonic and superharmonic interactions in SWAN causes an unrealistic amount of energy to be trapped on the channel slopes owing to wave refraction. The two-dimensional nonlinear interactions, which appear to be present in the measurements and TRITON results, broaden the directional range of the energy density spectrum when waves propagate over a sloping bottom. Owing to the directional broadening of the spectrum, more energy exists at angles smaller than the frequency-dependent critical angle for refraction, and therefore more wave energy is transmitted into and across channels, especially when waves approach the channel under an angle. It is recommended that this insight be used to find an alternative formulation for the present one-dimensional three-wave interaction formulation in SWAN.

Wave-energy dissipation by bottom friction in the English Channel

The energy dissipation by bottom friction of wind-generated surface-gravity waves is evaluated in relation to the seabed roughness magnitude in the English Channel (western Europe). The investigation is based on the phase-averaged wave model SWAN (Simulating WAves Nearshore) modified to account for a new parameterisation of the wind-drag coefficient at high wind speeds. Two formulations of the bottom-drag coefficient are evaluated: (1) the default constant empirical values derived from the JONSWAP experiment and (2) the eddy-viscosity model of Madsen et al. (1988) integrating the hydrodynamic conditions and the bottom roughness length scale considered successively constant and parametrised according to the grain size of bed sediments. Model performances are evaluated by comparing predictions with available measurements of the significant wave height and the peak period at (1) three offshore lightships and (2) two nearshore wave buoys off Le Havre and Cherbourg harbours. The heterogeneous bottom roughness length scale associated with the grain size of seabed sediments improves globally numerical estimates. Mappings of coastal regions influenced by bottom friction are produced exhibiting significant energy dissipation in areas of pebbles and gravels of the Normano-Breton Gulf and the surroundings of the Isle of Wight exposed to the incoming waves from the North-Atlantic ocean.

Waves in the Red Sea: Response to monsoonal and mountain gap winds

An unstructured grid, phase-averaged wave model forced with winds from a high resolution atmospheric model is used to evaluate wind wave conditions in the Red Sea over an approximately 2-year period. The Red Sea lies in a narrow rift valley, and the steep topography surrounding the basin steers the dominant wind patterns and consequently the wave climate. At large scales, the model results indicated that the primary seasonal variability in waves was due to the monsoonal wind reversal. During the winter, monsoon winds from the southeast generated waves with mean significant wave heights in excess of 2m and mean periods of 8s in the southern Red Sea, while in the northern part of the basin waves were smaller, shorter period, and from northwest. The zone of convergence of winds and waves typically occurred around 19–20°N, but the location varied between 15 and 21.5°N. During the summer, waves were generally smaller and from the northwest over most of the basin. While the seasonal winds oriented along the axis of the Red Sea drove much of the variability in the waves, the maximum wave heights in the simulations were not due to the monsoonal winds but instead were generated by localized mountain wind jets oriented across the basin (roughly east–west). During the summer, a mountain wind jet from the Tokar Gap enhanced the waves in the region of 18 and 20°N, with monthly mean wave heights exceeding 2m and maximum wave heights of 14m during a period when the rest of the Red Sea was relatively calm. Smaller mountain gap wind jets along the northeast coast created large waves during the fall and winter, with a series of jets providing a dominant source of wave energy during these periods. Evaluation of the wave model results against observations from a buoy and satellites found that the spatial resolution of the wind model significantly affected the quality of the wave model results. Wind forcing from a 10-km grid produced higher skills for waves than winds from a 30-km grid, largely due to under-prediction of the mean wind speed and wave height with the coarser grid. The 30-km grid did not resolve the mountain gap wind jets, and thus predicted lower wave heights in the central Red Sea during the summer and along the northeast coast in the winter.

The role of fringing coral reefs on beach morphodynamics

This paper examines the degree of energy dissipation provided by a fringing coral reef, and its role on the morphodynamics of adjacent beaches in terms of volumetric sediment transport. Morphological data were collected from the microtidal Mexican Caribbean beaches of Puerto Morelos, fringed by a reef, and Cancun, without a reef, from September 2007 to May 2011. Being exposed to the same offshore wave conditions, the morphodynamics of the coral reef-fronted beach were compared with those of the adjacent beach without a coral reef. Spatio-temporal changes in beach morphology were determined applying empirical orthogonal functions (EOF) to the shorelines extracted from the topographic data, and it was concluded that Puerto Morelos was considerably less dynamic than Cancun. The longshore energy fluxes were larger in Cancun, and the subaerial morphological differences in both beaches and under the same offshore conditions demonstrated that Puerto Morelos was particularly stable under shore-normal easterly waves. A calibrated phase-averaged wave model was implemented to determine the amount of wave energy dissipation across the coral reef. For energetic shore-normal waves the model determined that the semi-emerged coral reef was capable of reducing up to 85% of the incident wave height. The reef-crest height controlled the amount of wave energy dissipation, and the distance between the reef-crest and the shore determined the vulnerability of the beach to morphological changes. Reef-crest degradation by 1m resulted in a 10% increase in incoming wave energy, which resulted in 0.9m3/h/m of sand being mobilised along the beaches closer to the reef.

The Environmental Impact of a Wave Dragon Array Operating in the Black Sea

The present work describes a study related to the influence on the shoreline dynamics of a wave farm consisting of Wave Dragon devices operating in the western side of the Black Sea. Based on historical data analysis of the wave climate, the most relevant environmental conditions that could occur were defined, and for these cases, simulations with SWAN spectral phase averaged wave model were performed. Two situations were considered for the most representative patterns: model simulations without any wave energy converter and simulations considering a wave farm consisting of six Wave Dragon devices. Comparisons of the wave model outputs have been carried out in both geographical and spectral spaces. The results show that although a significant influence appears near the wave farm, this gradually decreases to the coast line level. In order to evaluate the influence of the wave farm on the longshore currents, a nearshore circulation modeling system was used. In relative terms, the longshore current velocities appear to be more sensitive to the presence of the wave farm than the significant wave height. Finally, the possible impact on the marine flora and fauna specific to the target area was also considered and discussed.

Evaluation of the wave transformation in an open bay with two spectral models

An evaluation of two state of the art phase averaged wave models for the transformation scale, SWAN and STWAVE, is carried out in the present work. The target area is the Obidos Bay located in the central part of the Portuguese continental nearshore. The wave input for the two models is provided by an offshore buoy. In order to compare the nearshore outputs of the wave models against in-situ measurements, a directional buoy and an ADCP, operating in intermediate water depth, are used. The wave parameters considered for comparisons are significant wave height, peak period and wave direction. Sensitivity analyses studies and evaluations in the spectral and geographical spaces concerning the results of the two models are also carried out in both intermediate and shallow water. The present study provides some information on the performances of the two wave models in different forcing conditions as well as on their sensitivity in relationship with various input parameters and some physical processes. STWAVE appears to be faster and more robust than SWAN, which on the other hand has more options and flexibility. In statistical terms the results are comparable.