Spectral Wave Model Research Articles

Characterization of Extreme Wave Conditions for Wave Energy Converter Design and Project Risk Assessment

Best practices and international standards for determining n-year return period extreme wave (sea states) conditions allow wave energy converter designers and project developers the option to apply simple univariate or more complex bivariate extreme value analysis methods. The present study compares extreme sea state estimates derived from univariate and bivariate methods and investigates the performance of spectral wave models for predicting extreme sea states at buoy locations within several regional wave climates along the US East and West Coasts. Two common third-generation spectral wave models are evaluated, a WAVEWATCH III® model with a grid resolution of 4 arc-minutes (6–7 km), and a Simulating WAves Nearshore model, with a coastal resolution of 200–300 m. Both models are used to generate multi-year hindcasts, from which extreme sea state statistics used for wave conditions characterization can be derived and compared to those based on in-situ observations at National Data Buoy Center stations. Comparison of results using different univariate and bivariate methods from the same data source indicates reasonable agreement on average. Discrepancies are predominantly random. Large discrepancies are common and increase with return period. There is a systematic underbias for extreme significant wave heights derived from model hindcasts compared to those derived from buoy measurements. This underbias is dependent on model spatial resolution. However, simple linear corrections can effectively compensate for this bias. A similar approach is not possible for correcting model-derived environmental contours, but other methods, e.g., machine learning, should be explored.

Stormtools Design Elevation (SDE) Maps: Including Impact of Sea Level Rise

Many coastal communities in the US use base flood elevation (BFE) maps for the 100-year return period, specified on Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs), to design structures and infrastructure. The FIRMs are increasingly known to have serious problems in accurately specifying the risk coastal communities face, as most recently evidenced during hurricanes Harvey and Irma in 2017 and Florence and Michael in 2018. The FIRM BFE maps also do not include the impact of sea level rise, which clearly needs to be considered in the design of coastal structures over the next several decades given recent National Oceanic and Atmospheric Administration (NOAA) sea level rise (SLR) projections. Here, we generate alternative BFE maps (STORMTOOLS Design Elevation (SDE) maps) for coastal waters of Rhode Island (RI) using surge predictions from tropical and extratropical storms of the coupled surge-wave models from the US Army Corp of Engineers, North Atlantic Comprehensive Coast Study (NACCS). Wave predictions are based on application of a steady state, spectral wave model (STWAVE), while impacts of coastal erosion/accretion and changes of geomorphology are modeled using XBeach. The high-resolution application of XBeach to the southern RI shoreline has dramatically increased the ability to represent the details of dune erosion and overtopping and the associated development of surge channels and over-wash fans and the resulting landward impact on inundation and waves. All methods used were consistent with FEMA guidelines for the study area and used FEMA-approved models. Maps were generated for 0, 2 ft (0.6 m), 5 ft (1.5 m), 7 ft (2.1 m), and 10 ft (3.1 m) of sea level rise, reflecting NOAA high estimates at various times for the study area through 2100. Results of the simulations are shown for both the southern RI shoreline (South Coast) and Narragansett Bay, to facilitate communication of projected BFEs to the general public. The maps are hosted on the STORMTOOLS ESRI Hub to facilitate access to the data. They are also now part of the RI Coastal Resources Management Council (CRMC) risk-based permitting system. The user interface allows access to all supporting data including grade elevation, inundation depth, and wave crest heights as well as corresponding FEMA FIRM BFEs and associated zones.

Wave Energy Assessment in the South Aquitaine Nearshore Zone from a 44-Year Hindcast

Wave resource assessment is the first step toward the installation of a wave energy converter (WEC). To support initiatives for wave energy development in the southwest of France, a coastal wave database is built from a 44-year hindcast simulation with the spectral wave model SWAN (Simulating WAve Nearshore) run on a high-resolution unstructured grid. The simulation includes shallow-water processes such as refraction, shoaling, and breaking. The model is validated against a five-year coastal wave buoy recording. The study shows that most of the resource is provided by sea states with wave heights ranging from 2 to 5 m, with wave periods from 10 and 15 s, and coming from a very narrow angular sector. The long hindcast duration and the refined unstructured grid used for the simulation allow assessment of the spatiotemporal distribution of wave energy across the coastal area. On the one hand, large longshore variations of the resource caused by steep bathymetric gradients such as the Capbreton submarine canyon are underlined. On the other hand, the study highlights that no specific long-term trend can be extracted regarding the coastal wave energy resource evolution. The provided downscaled local wave resource information may be used to optimize the location and design of a future WEC that could be deployed in the region.

A methodology for data gap filling in wave records using Artificial Neural Networks

Wave measuring equipment are subject to malfunction that can be caused by various reasons such as maintenance, navigation accidents, errors in communications and biofouling of sensors. This can eventually result in loss of valuable data before the equipment is fixed. A continuous data set is often critical for modelling and/or analysis of wave conditions. Data gaps in wave records can be filled using numerical modelling. This study presents an alternative method for filling missing data based on publicly available wind and wave information and artificial neural networks (ANN). The ANN developed in this study uses offshore hindcast wave and wind information (significant wave height, peak wave period and direction, wind speed and direction) from a data set publicly available for any region of the globe. The results of the application of this method to two one-week gaps in wave measurements are compared against a spectral wave model developed and implemented to estimate wave conditions at the measurement location. The ANN results for the validation period show slightly better statistical performance when compared to the wave model results in terms of the correlation coefficient, root mean square error, bias and scatter index. ANN are viable alternative methods to wave modelling for filling gaps in data and have several advantages since specific knowledge of wave models is not required, input data to feed the neural network is available for any region of the globe and the processing time is highly reduced when compared to numerical modelling.

Skill Assessment of an Atmosphere–Wave Regional Coupled Model over the East China Sea with a Focus on Typhoons

This study performed several sensitivity experiments to investigate the impact of atmosphere–wave coupling on the simulated wind and waves over the East China Sea (ECS) with a focus on typhoon events. These experiments include stand-alone regional atmosphere model (CCLM) simulations, stand-alone spectral wave model (WAM) simulations driven by the regional atmospheric model CCLM or ERA5 reanalysis, and two-way (CCLM-WAM) coupled simulations. We assessed the simulated wind speed and significant wave height against in situ observations and remote sensing data and focused on typhoon events in 2010. We analyzed the differences between the experiments in capturing the surface pressure, wind speed, and roughness length. Both ERA5 reanalysis data and our regional model simulations demonstrate high quality in capturing wind and wave conditions over the ECS. The results show that downscaled simulations tend to be closer to in situ observations than ERA5 reanalysis data in capturing wind variability and probability distribution, dominant wind and wave directions, strong typhoon intensity and related extreme significant wave height. In comparison with satellite observations, the CCLM-WAM simulation outperforms the CCLM in reducing wind bias. The coupled and uncoupled simulations are very similar in terms of other wind and wave statistics. Though there is much improvement in capturing typhoon intensity to ERA5, regional downscaled simulations still underestimate the wind intensity of tropical cyclones.

Numerical Modeling of Beach Morphological Change: A Case Study of Kaanapali Beach, Maui, Hawaii

This case study reports results from field observations and numerical simulations of waves and morphological changes along a portion of Kaanapali Beach on West Maui, Hawaii, which is protected by a hard coral reef and experiences shoreline changes from season to season. The SWAN spectral wave model shows reasonable agreement with ADCP observations of wave-heights for the winter months. Simulated beach profile change over one-month time frame was able to reasonably capture the trend of beach face migration (accretion or erosion); the modeled shoreline also shows satisfactory agreement with beach survey data. This case study suggests that Delft3D is able to capture key features of sediment transport along a narrow beach protected by a fringing reef.

Numerical Study of Longshore Variation in Beach Morphodynamics along Eastern Lake Erie Shoreline due to Seiche

Sogut, E. and Farhadzadeh, A., 2021. Numerical study of longshore variation in beach morphodynamics along Eastern Lake Erie shoreline due to seiche. Journal of Coastal Research, 37(1), 92–103. Coconut Creek (Florida), ISSN 0749-0208.Extreme coastal events and abrupt changes in atmospheric pressure in an enclosed or semi-enclosed basin can trigger low-frequency water surface oscillations known as seiches. This exploratory study numerically quantifies the effects of seiche on morphological changes along the eastern Lake Erie shoreline. The quantification is made by simulating the hydrodynamics of the lake using a coupled circulation and spectral wave model (ADCIRC+SWAN), providing the flow boundary conditions to the two-dimensional nearshore morphodynamic model, XBeach. The process-based XBeach model is used to simulate the nearshore morphological variations under two water-level conditions: the lake's actual water level and the synthetically generated seiche-free water level. The XBeach model is cross-calibrated using the process-based nearshore morphodynamic model, CSHORE, which was extensively validated using lab and field data. It is found that the seiching motions have a contribution of ∼1.5% to the erosion of beaches along the 2-km stretch of the shoreline in eastern Lake Erie shoreline.

Validating Ocean Wind Wave Global Hindcast with Visual Observations from VOS

Joint analysis of wind wave characteristics derived from Voluntary Observing Ships data (VOS) and the third-generation spectral wave model WAVEWATCH III v.5.16 hindcast is presented. Global distributions of significant wave heights and mean periods in both datasets demonstrate good qualitative and quantitative agreement, especially in regions with a high spatiotemporal density of observations. Simulation results and visually observed wind sea and swell show perfectly consistent patterns of directional steadiness. However, wind sea heights in WW3 are overestimated predominantly in stormy regions, while swell heights are globally underestimated. The reasons for these discrepancies in assessment of wave system components are investigated in the framework of various wave regime analyses.

Implementation and evaluation of coupled discontinuous Galerkin methods for simulating overtopping of flood defenses by storm waves

In coastal regions, accurate prediction of the inundation risks posed by tropical cyclones and other severe weather is key to mitigating disaster impacts. Predictions of these risks are often provided by numerical models; however, these models also frequently do not consider inundation due to wind-sea wave run-up. Neglecting such effects may be misleading for evaluating the risks posed to areas protected by flood defense systems such as levees and dykes. To effectively evaluate the flooding risks posed by storm waves, accurate wind-sea modeling should be undertaken in parallel with storm surge modeling, and accurate wave-overtopping flow rates at flood barriers must be prescribed based on the modeled sea conditions and structure geometries. To this end, we present development of a loosely coupled finite element based shallow-water equations and parametric spectral wave model. Wave-overtopping flow rates are computed by formulas provided in the guidance of the EurOtop manual. We demonstrate that the parametric spectral wave modeling approach is sufficiently accurate for modeling of wave-overtopping events, despite its simplifying assumptions, such as those based on deep water (e.g., discounting foreshore effects such as wave shoaling and breaking), and disregarding internal forces due to wave breaking and turbulence, as opposed to more commonly used discrete (third generation) spectral wave models, which provide more detailed descriptions of wave fields but are much more computationally expensive. Comparisons of numerically modeled wave-overtopping flow rates with measurements from the CLASH database show good agreement by linear regression (slope: 0.9911, R2: 0.9584) for more than 90% of the test cases considered, where for the relatively few outlier cases the model performs poorly due to either uncertainties in the EurOtop guidance or the lack of foreshore effects in the wave model.

Analysis of wave climate and trends in a semi-enclosed basin (Persian Gulf) using a validated SWAN model

Results from a spectral wave model of the Persian Gulf for a long-term period (32-year) are used to describe and quantify the seasonal wave climate and trends. The model is forced by a wind reanalysis dataset and validated against a set of field measurements and altimeter data from various satellite missions. The results revealed the existence of a more energetic wave (higher Hm0 and larger TP) zone located at the southern regions of the Gulf that is linked to the predominant and stronger north-westerly winds. The wave heights and periods are intensified in winter (Hm0>2.7m and TP > 8s for the 99th percentile) followed by spring. The waves conditions are milder in autumn. The analysis of wave climate trends throughout the region clearly show an overall slight positive trend (0–0.2%/year) in mean Hm0, which is associated with long-term positive trends in wind speed. There is generally a positive trend in the mean significant wave height during winter (0–0.8%/year), spring (0–0.6%/year) and autumn (0–1.0%/year), and negative during summer (0–0.8%/year). The results indicate that the more statistically significant results are at the areas with larger magnitude of trends.

Numerical Simulation of Ocean Wave Using High-Order Spectral Modeling Techniques: Its Influence on Transport Sediment in Benoa Bay, Bali, Indonesia

Benoa Bay is threatened by sedimentation issue within the bay. It impairs the water mass circulation, influencing scour, mixing, and turbulence as well as sediment transport processes. This study focuses on the wave characteristic and its relation to sediment transport within Benoa Bay. Spectral wave modeling techniques were employed. The equation was discretized based on the condition of winds, tidal, and currents. Total sediment transported was calculated according to the wave model result. Total suspended sediment (TSS) model was simulated which the simulation considers bed load and suspended load transport. Significant wave height (Hs) ranged 0-0.48 m and 0-0.44 m during high tidal condition and low tidal condition respectively. Wave undulation propagates toward the West and Northwest within Bay. The wave period (Ts) ranged 2-6.5 second. Total sediment transport ranged 2828.16 - 86235.66 m3/year. TSS concentration ranged 1-100 mg/L and 1-155 mg/L during high tidal condition and low tidal condition respectively. Those conditions indicate that the sedimentation has been extremely occurred within the bay. The areas around Benoa peninsula, Benoa harbor, and Serangan Island are heavily polluted by suspended sediment. Bottom sediment is stirred by hydraulic jump off wave propagation. The first wave crest induces scour, which its train carries the stirred sediment entering the bay. If ongoing, this condition will exacerbate the existing ecosystem. Benoa Bay development has a big role evoking the level of TSS and turbidity. The more the sedimentation occurs, the more the ecological problem takes place.

Parameterization of Wave Boundary Layer

It is known that drag coefficient varies in broad limits depending on wind velocity and wave age as well as on wave spectrum and some other parameters. All those effects produce large scatter of the drag coefficient, so, the data is plotted as a function of wind velocity forming a cloud of points with no distinct regularities. Such uncertainty can be overcome by the implementation of the WBL model instead of the calculations of drag with different formulas. The paper is devoted to the formulation of the Wave Boundary Layer (WBL) model for the parameterization of the ocean-atmosphere interactions in coupled ocean-atmosphere models and wave prediction models. The equations explicitly take into account the vertical flux of momentum generated by the wave-produced fluctuations of pressure, velocity and stresses (WPMF). Their surface values are calculated with the use of the spectral beta-functions whose expression was obtained by means of the 2-D simulation of the WBL. Hence, the model directly connects the properties of the WBL with an arbitrary wave spectrum. The spectral and direct wave modeling should also take into account the momentum flux to a subgrid part of the spectrum. The parameterization of this effect in the present paper is formulated in terms of wind and cut-off frequency of the spectrum.

Generation of 100-year-return value maps of maximum significant wave heights with automated threshold value estimation

Spectral numerical wave models are unable to predict extreme sea conditions accurately, and the probability of occurrence of a given wave parameter for fixed return periods (RPs) remains unexplored by such an approach. In this study, the significant wave height (SWH) parameter that plays a major role in ocean engineering and marine and naval activities was estimated spatially for the entire Bay of Bengal (BOB) region for the given RPs. Spatially gridded 44 years of SWH data at a spatial resolution of 1° × 1° were considered, and return value maps were generated for the BOB for 10-, 15-, 25-, 50-, and 100-year RPs. The peak over threshold (POT) model was chosen over a generalized Pareto distribution for data fitting and calculating the return values. As there is no existing method to determine the threshold value for use in the POT model, in this study, for the first time, an algorithm was designed to calculate the threshold value, which determined the accuracy of the estimation. Along with the conventional time-series analysis of the SWH data using POT, the novelty of the study lies in the spatial return period maps generated for the entire BOB region leading to the basin scale estimation.

Comparing Nearshore Wave Parameters in Amurang Bay location using MIKE-21 Spectral Wave Model

Wave parameters as an accurate prediction in ocean environment are important thing for good coastal development. Spectral wind wave model as a tools in MIKE 21 SW based on unstructured mesh is used in this study which the model simulates the growth, decay and transformation of wind generated waves and swell in offshore and coastal areas. The Amurang Bay as the province of North Sulawesi Indonesia was selected as the study area which the geography position around 1012’16.16” N-124027’04.33” E to 1015’43.80” N-124032’01.06”E. The bathymetry and tide data used in this research from Indonesian Coastline Environmental map of year 1995 with scale 1:50.000 from BIG (Badan Informasi Geospasial) with a satellite data from Google earth of year 2018 and LANTAMAL Manado, the wind and current data was obtained from BMKG Manado. Time simulations are taken from 25 November to 23 December 2016 as a wet season and 25 Mei to 23 June 2016 as a dry season.The model computed the wave parameters using the forecast wind input. The synoptic map of significant wave height (Hs), wave period, wave direction are obtained from the result of simulation. During the dry and wet season conditions the predicted wave parameters as the result of the simulation with tide and wind show to be higher than with tide and no wind simulation. The average condition of significant wave height is higher in outside of bay than inside of bay.

The wave-current interaction in the coastal area

In our investigation of the effect of wind-waves on barotropic mean flow in coastal areas, we compare two methods for calculating wave-induced force. The wave field is simulated by the nearshore spectral wave model SWAN. The wave-induced force (calculated using the radiation stress gradient and dissipation methods) and the Stokes drift are integrated in the COHERENS circulation model in the depth-averaged mode. The coupled set is validated using well-known academic test cases of planar beach and single-barred beach. Finally, in a two-dimensional test case based on Belgian coastal waters we compare simulations of mean flow using the two methods of calculating waveinduced force against field data.<br/> We show clearly that the two methods for calculation of wave-induced force yield very different results even in depth-averaged mode, depending on the angle of incident wave. Simulation of waveinduced circulation using the wave dissipation approach gives better results than using the radiation stress gradient approach. This is clearly visible for strong wave conditions in which the wind is blowing almost parallel to the shore. Under these conditions, the white-capping type of wave breaking is the dominant dissipation mechanism; in the radiation stress gradient, the dissipation signal is not visible, because the energy loss in the spectrum is compensated by wind input.

Ocean Wind Wave Climate Responses to Wintertime North Atlantic Atmospheric Transient Eddies and Low-Frequency Flow

AbstractAtmospheric transient eddies and low-frequency flow contributions to the ocean surface wave climate in the North Atlantic during boreal winter are investigated (1980–2016). We conduct a set of numerical simulations with a spectral wave model (WAVEWATCH III) forced by decomposed wind fields derived from the ERA-Interim reanalysis (0.7° horizontal resolution). Synoptic-scale processes (2–10-day bandpassed winds) are found to have the largest impact on the formation of wind waves in the western midlatitudes of the North Atlantic along the North American and western Greenland coasts. The eastern North Atlantic is found to be influenced by the combination of low-frequency forcing (>10-day bandpassed winds) and synoptic processes, contributing up to 60% and 30% of the mean wave heights, respectively. Midlatitude storm track variability is found to have a direct relationship with wave height variability along the eastern and western margins of the North Atlantic, implying an association between cyclogenesis over the North American eastern seaboard and wave height anomalies in the eastern North Atlantic. A change in wave height regimes defined using canonical correlation analysis is reflected in changes to their wave height distribution shapes. The results highlight the important role of transient eddies for the ocean surface wave climatology in the midlatitudes of the eastern North Atlantic both locally and through association with cyclone formation in the western part of the basin. These conclusions are presented and discussed particularly within the context of long-term storm track shifts projected as a possible response to climate warming over the coming century.

Extreme wind waves in the Black Sea

Summary Results of the analysis of a long-term data set, including fields of significant wave heights of the surface wave components, and mixed (total) wave field in the Black Sea are presented. The data set was collected on the basis of retrospective calculations using the MIKE 21 SW spectral wave model with the atmospheric forcing based on the ERA-Interim data in the period from 1979 to 2017. A criterion is used to isolate the swell waves from the initial wave data set that takes into account the wave age. We used the experimental data to develop a regression relationship showing that the maximum possible wave height can exceed the significant wave height approximately one and a half times. Analysis of the spatial distribution of wave heights in the Black Sea suggests that a possibility exists that significant wave height of storm waves can be as high as ∼12 m. This result indicates that the actual heights of maximum waves in the Black Sea can reach 18–19 m. Three regions are distinguished on the basis of the wave potential. The times of manifestation of extreme situations in these regions are different: in the southwestern part of the sea, extreme storm situations occur, as a rule, in December–January; in the region south of the Crimea Peninsula this happens in February; in the northeastern part of the sea they occur in November. It was also found that the south-southeastern and eastern parts of the sea are most affected by swell.

Predicting the hydrodynamic response of a coastal reef-lagoon system to a tropical cyclone using phase-averaged and surfbeat-resolving wave models

Tropical Cyclone (TC) Olwyn travelled along the coast of Ningaloo Reef in northwestern Australia as a Category 3 TC during 12–13 March 2015 with sustained wind speeds of over 130 km h−1 and significant wave heights on the forereef reaching 6 m. Observations from TC Olwyn showed that, contrary to typical wave transformation patterns across a coastal reef-lagoon, under the TC conditions there was a substantial shoreward increase of wave height. This unusual pattern was primarily due to strong local wind wave generation under the extreme wind conditions, despite the lagoon itself being relatively small (order 1 km wide) and shallow (order 1 m depth). Two common types of coastal numerical wave models that include either wave growth (i.e. the phase-averaged model SWAN) or infragravity waves (i.e. the surfbeat resolving model XBeach), but not both processes, were used to assess the capabilities of numerical models to predict the cyclone-induced hydrodynamics. Model results showed that wave conditions in the lagoon and at the shoreline were dominated by locally-generated wind waves, with the reef efficiently dissipating the large incident waves offshore. The infragravity waves in the lagoon were only of secondary importance during the TC compared to the locally-generated sea-swell waves. Although both the phase-averaged (SWAN) and surfbeat-resolving (XBeach) models were able to predict some of the local hydrodynamic responses, for this specific site and storm, the use of a phase-averaged model yielded the most accurate predictions of the hydrodynamics given that it included wind wave growth. However, because the influence of infragravity waves cannot be neglected in reef environments in general, these results emphasize the need for a more general modeling approach that requires the addition of wind wave growth formulations into surfbeat- or fully-phase resolving classes of numerical wave models and/or the addition of an infragravity wave formulation in spectral wave models.

Nearshore Wave Dynamics at Mangalia Beach Simulated by Spectral Models

The objective of this paper is to present an integrated picture of the relationship between the waves and the modifications induced by them in the Romanian shoreline. Thus, the hydrodynamic processes at the Mangalia beaches, located in the southern side of the Romanian nearshore, are simulated using the modeling system Mike 21 SW (MIKE 21 Spectral Waves), developed by the Danish Hydration Institute (DHI). This is one of the newest spectral wave models, which can be used for regional- and local-scale simulations. The model has been calibrated and validated using buoy measurements. The analysis of the statistical parameters shows a good match between the model and the observed data. Furthermore, a model to compare the differences that occur on the beach profiles between the cold and warm seasons was developed. The results obtained indicate a reinforcement of the coastal erosion in the winter, when the waves are stronger (especially in January and February).

Numerical study on the stochastic response of a long-span sea-crossing bridge subjected to extreme nonlinear wave loads

In current practice, linear wave load theory is widely employed in the wave force evaluation of sea-crossing bridges. However, the observation of nonlinear wave loads in shallow waters suggests that second-order wave loads might be essential to the dynamic behavior of marine structures. In this paper, a framework to investigate the stochastic response of a long-span sea-crossing bridge under extreme nonlinear wave loads was proposed based on the combined use of the spectral wave model MIKE21 SW, the 3D diffraction theory hydrodynamic solver AQWA and the structural analysis tool ANSYS. An extreme wave condition of a 300-year return period at the exact bridge site is estimated from offshore statistical wave data by utilizing the state-of-the-art spectral wave model MIKE21 SW by considering of the complex nearshore wave transformation. Linear as well as nonlinear stochastic wave loads are modeled in AQWA and subsequently fed into the finite-element model to assess the responses. The results indicate that the nonlinear wave loads could remarkably increase the dynamic responses of the internal base forces of bridge foundations as well as displacements of superstructures. The sum-frequency second-order components in nonlinear loads, though not large in magnitude, might overlap with the first few eigenfrequencies of the bridge and thus excite these modes and increase the responses.