Offshore Wave Characteristics Research Articles

Combined storm surge and wave overtopping inundation based on fully coupled storm surge-wave-tide model

Inundations in low-lying coastal areas are commonly caused by compound flooding due to the combination of surge overflow and wave overtopping. The increasing frequency of strong tropical cyclones accompanied by high waves is exacerbating flooding risks. In recent studies, wave-induced flooding (i.e., wave run-up and overtopping) has been reported as a critical component of coastal inundations. However, no clear standard has been established for estimating compound flooding caused by the nonlinear interactions of each component. The present study developed a fully coupled model of surge, wave, tide, and wave overtopping to propose a framework for assessing compound coastal inundations. A series of numerical experiments were conducted in an idealized coastal area to analyze flooding risk comprehensively. The experimental results revealed that the conventional decoupled calculation processes for wave overtopping/run-up tend to overestimate the inundation. The advanced approaches provided stable estimations of offshore wave characteristics that change dynamically under extreme wave conditions, resulting in a reliable wave overtopping calculation. Furthermore, a considerable amount of inundation can occur even when typhoons make landfall during low tidal conditions due to the prolonged occurrence of wave overtopping. The proposed framework can be considered an optimal method for practical flooding risk assessments in coastal areas by estimating compound inundations and including transition processes of surge overflow and wave overtopping/run-up.

On the General Longshore Transport model for resilient beaches

In the present paper, a set of 319 field and laboratory data on longshore transport rate has been gathered, checked, and finally adopted in order to overcome a noticed limitation in the previous version of General Longshore Transport (GLT) procedure for the estimation of Longshore Transport (LT). In particular, the extensive number of additional data, characterized by a wide mobility range, has allowed to obtain a single equation able to determine the longshore transport rate at coastal slopes consisting of loose not-cohesive materials, from reshaping dynamically stable berm breakwaters to sandy beaches. The reliability of the GLT procedure has been assessed by an independent verification with 39 field and laboratory data, for both offshore and breaking wave characteristics as an input. In addition, the GLT procedure has been compared with the van Rijn (2014) formula.

Sediment Transport Equivalent Waves for Estimating Annually Averaged Sedimentation and Erosion Trends in Sandy Coastal Areas

In this paper, a simple approach to determine representative offshore wave characteristics for estimating the annually averaged sedimentation and erosion trends in sandy coastal areas is presented. Given the offshore wave climate, the proposed approach breaks down the climate into fixed 22.5-degree bins and based on the sediment transport potential it determines the equivalent wave characteristics for each bin, i.e., a significant wave height, a peak period, a mean wave direction, and a corresponding frequency of occurrence. The approach is validated in idealized cases of uniformly sloping beaches with the presence of a breakwater, for various sediment diameters, sea bottom slopes, and different offshore wave characteristics. The performance of the proposed approach is evaluated against the full climate, returning good results. Furthermore, the proposed approach is applied in a real-life challenge, in the coastal area of Therma in the Island of Samothraki in Greece, where the presence of a fishing shelter has led to sedimentation and erosion problems. The performance of the proposed approach is very satisfactory, given the complexity of the problem. The generic nature of the proposed methodological approach allows it to be applied in numerous sandy coastal regions to estimate the sedimentation and erosion trends, reducing the amount of input parameters and thus requiring significantly less computational efforts.

Semi-Analytical Model for the Evaluation of Shoreline Recession Due to Waves and Sea Level Rise

The climate change process is leading to an increase in the sea level and the storm intensity. The associated shoreline recession can damage coastal facilities and also beaches protected by submerged/emerged breakwaters whose defense action can become ineffective. The application of cross-shore numerical models does not allow the performance of long-term analyses. In this paper, a semi-analytical model for the evaluation of shoreline recession due to waves and sea-level rise for free and protected beaches is proposed. The model is an extension of the Dean’s approach in which some limitations on the beach profile are overcome and the effects of breakwaters on the wave height (wave transmission) and on the water level (piling-up) are considered. The model takes into account a wide range of parameters for wave, sea level, beach profile, and breakwater characteristics. Among the breakwater parameters, the freeboard and the berm width are found to mainly affect the shoreline recession. For submerged breakwaters, an optimal value of the freeboard can be computed depending on the sea level and the offshore wave characteristics. The results of the model are then used to find prediction relations of the shoreline recession, with r2 > 0.99, for both free and protected beaches, depending on the main hydrodynamic/geometrical characteristics.

A Coastal Flood Early-Warning System Based on Offshore Sea State Forecasts and Artificial Neural Networks

An integrated methodological approach to the development of a coastal flood early-warning system is presented in this paper to improve societal preparedness for coastal flood events. The approach consists of two frameworks, namely the Hindcast Framework and the Forecast Framework. The aim of the former is to implement a suite of high-credibility numerical models and validate them according to past flooding events, while the latter takes advantage of these validated models and runs a plethora of scenarios representing distinct sea-state events to train an Artificial Neural Network (ANN) that is capable of predicting the impending coastal flood risks. The proposed approach was applied in the flood-prone coastal area of Rethymno in the Island of Crete in Greece. The performance of the developed ANN is good, given the complexity of the problem, accurately predicting the targeted coastal flood risks. It is capable of predicting such risks without requiring time-consuming numerical simulations; the ANN only requires the offshore wave characteristics (height, period and direction) and sea-water-level elevation, which can be obtained from open databases. The generic nature of the proposed methodological approach allows its application in numerous coastal regions.

Extreme waves induced by cyclone Nargis at Myanmar coast: numerical modeling versus satellite observations

The deltaic coast of Myanmar was severely hit by tropical cyclone Nargis in May 2008. In the present study, a top-down numerical simulation approach using the Weather Research and Forecasting (WRF) and Simulating WAves Nearshore (SWAN) models was conducted to study the meteorological and offshore wave characteristics of cyclone Nargis near the coast of Myanmar. The WRF simulation results agree well with the observed data from the India Meteorological Department. SWAN simulation results were compared with the WaveWatch 3 model by National Oceanic and Atmospheric Administration and validated against available measurement data from satellites. The model results show relatively good agreement, and hindcast with satellites data (significant wave height only) shows a correlation coefficient value of 0.89. The SWAN and satellite comparisons also show better fit for high wave conditions. The resulted maximum significant wave height of 7.3 m by SWAN is considerably higher in energy than the seasonal waves normally prevalent at Myanmar’s deltaic coast. The possibility of high energy waves due to cyclones should be considered during the design and operation of coastal and offshore projects in the area, particularly given the risks that climate change can intensify cyclones in the future. Since Myanmar lacks a dense network of in-situ observational stations, the methodology used in the current study presents the potential application of various numerical techniques and satellite data to estimate extreme wave conditions near the Myanmar coast.

COASTAL FLOODING RISK ASSESSMENT BY A NEURAL NETWORK APPROACH

An effective system of coastal flooding forecasting in the case of storm is essential to mitigate coastal risks for the population living in low-land coastal zones (less than 10 m above MSL). Nowadays, predictions of coastal flooding are usually carried out by adopting nested numerical models. However, the models adopted to obtain the data in the nearshore area require high computational costs, which are often too demanding and not viable for large scale forecasting. Data-driven models, such as Artificial Neural Networks (ANNs) can help to solve the problem as they can map complex nonlinear relationships between input and output variables once a suitable dataset of process realizations is available. In the present study a forecasting model for coastal flooding based on ANNs, in which the input data are the offshore wave characteristics from large scale model and the output results are the flooded areas, is proposed. These outputs provided a straightforward prediction of the area interested by coastal flooding during storms. Here an application of the model to assess the flooding risk in the village of Granelli, in the Southeast of Sicily (Italy) is presented.

Variations in extreme wave events near a South Pacific Island under global warming: case study of Tropical Cyclone Tomas

The intensification of tropical cyclones (TCs) and wind-induced ocean waves is expected to be amplified under global warming conditions. In 2010, strong TC Tomas approached the Fiji Islands and caused severe damage. Here, an ensemble simulation technique is combined with a pseudo-global warming (PGW) method to investigate future variations in TCs and wind-induced ocean waves. Ensemble PGW simulations were implemented using the weather research and forecasting (WRF) model with five different future projections. Hindcast and PGW simulations showed similar tracks of Tomas. In four PGW simulations, the central pressures of the simulated TCs decreased. Enhanced near-surface wind was recognized in three PGW simulations around the Fiji main island (Viti Levu). In the other two future simulations, the surface wind speed was weaker than the one in the present climate because of the slight eastward shift in the track and delayed development of the TC. WaveWatchIII (WW3) was applied for offshore wave simulations forced by the wind field obtained by WRF simulation results. In three future simulations, a clear increase in the maximum significant wave height (Hs) was found on the southeastern coast of Viti Levu. One future simulation yielded almost the same offshore wave characteristics as those under the present climate. In another future simulation, the ensemble mean Hs was as high as that in the present climate, but extremely large Hs values were found in several ensemble members. Future simulations using multiple global climate model (GCM) projections showed possible variations in TCs and wind-induced ocean waves which is useful for the risk assessment of various hazards.

Wave runup parameterization for sandy, gravel and platform beaches in a fetch-limited, large estuarine system

Empirical models of wave runup are commonly derived from open beaches, but their applicability in fetch-limited, sheltered environments are yet to be properly assessed. Based on original video-derived runup observations on 5 beaches with distinct morphodynamic states in the Estuary and Gulf of St. Lawrence (EGSL), this paper presents a runup analysis at the regional scale. A wide range of environmental parameters (varying wave climate, beach slopes, tidal range) and beach types (platform-beach, with and without nearshore bars, sandy/gravel) are considered. A total of 430 15 min timestacks have been analyzed over 35 different days across a 3-year survey. Runup, setup, incident (f > 0.05 Hz) and infragravity (f < 0.05 Hz) swash are regressed against offshore wave characteristics and beach slope. Results indicate that the influence of the hydrodynamic parameters on wave runup, setup and swash is a function of offshore wave height and wavelength. While hydrodynamic forcing explain most of the coastal water level components variability, setup and swash are affected differently by the beach slope. The role of the beach morphology on wave setup can be parameterized through the inverse function of the beach slope, while swash height is written as the square root of the beach slope. The parameterizations of R2% for the EGSL beaches have been validated against a new set of observations. Overall, the hydrodynamic forcing is the dominant driver of wave runup, but including the beach slope improves the understanding of the runup components. The original formulations offer a strong potential for runup assessment on various morphological fetch-limited coastal environments.

Nonstationary joint probability analysis of extreme marine variables to assess design water levels at the shoreline in a changing climate

In the present study, a recently developed novel approach (Bender et al. in J Hydrol 514:123–130, 2014) has been further extended to investigate the changes in the joint probabilities of extreme offshore and nearshore marine variables with time and to assess design the total water level (TWL) at the shoreline under the effects of climate change. The nonstationary generalised extreme value (GEV) distribution has been utilised to model the marginal distribution functions of marine variables (wave characteristics and sea levels), within a 40-year moving window. All parameters of the GEV were tested for statistically significant linear and polynomial trends over time, and best-fitted trends have been detected. Different copula functions were fitted at the 40-year moving windows, to model the dependence structure of extreme offshore significant wave heights and peak spectral periods, and of wave-induced sea levels on the shoreline and nearshore sea levels due to storm surges. The most appropriate bivariate models were then selected. Statistically significant polynomial trends were detected in the dependence parameters of the selected copulas, and time-dependent most likely bivariate events were extracted to be used in the estimation of the TWL at the shoreline. The methods of the present work were implemented in three selected Greek coastal areas in the Aegean Sea. The analysis revealed different variations in the most likely estimates of the offshore wave characteristics and nearshore storm surges in the three study areas, as well as in the time-dependent estimates of TWL at the shoreline. The approach combines nonstationarity and bivariate analysis, blends coastal and offshore marine features and finally provides non-trivial alterations in the response of coastal sea level dynamics to climate change signals, compared to former work on the subject. The methodology produces reasonable estimates of design quantities for coastal structures and boundary conditions for the assessment of flood hazard and risk in coastal areas.

A Multi-Year Data Set of Beach-Foredune Topography and Environmental Forcing Conditions at Egmond aan Zee, The Netherlands

Coastal dunes offer numerous functions to society, such as sea defense and recreation, and host unique habitats with high biodiversity. Research on coastal dune dynamics has traditionally focused on the erosional impact of short-duration (hours to days), high-wave storm events on the most seaward dune, called the foredune. In contrast, research data on its subsequent slow (months to years), wind-driven recovery are rather rare, yet essential to aid studying wind-driven processes, identifying the most relevant wind-forcing conditions, and testing and improving dune-growth models. The present data set contains 39 digital elevation models and 11 orthophotos of the beach-foredune system near Egmond aan Zee, The Netherlands. The novelty of the data set lies in the combination of long-term observations (6 years; January 2013 to January 2019), with high temporal (intervals of 2–4 months) and spatial resolution (1 × 1 m) covering an extensive spatial domain (1.4 km alongshore). The 25-m high foredune eroded substantially in October 2014, with a maximum recession of 75 m3/m, and subsequently recovered with a rate of approximately 15 m3/m/yr, although with substantial alongshore variability. The data set is supplemented with high-frequency time series of offshore wave, water level, and wind characteristics, as well as various annual subtidal cross-shore profiles, to facilitate its future application in coastal dune research.

Evaluation and modification of JONSWAP spectral parameters in the Persian Gulf considering offshore wave characteristics under storm conditions

Some efforts have been done by previous researchers and scientists to represent the spectral behavior of wind waves in the Persian Gulf by analyzing real-time data, but in most of them, only wave data collected from nearshore stations with water depths of 25 m or less were considered. Therefore, in order to consider offshore wave characteristics of the region, a 5-month wave data through a mooring buoy around Farzad A natural gas field, at a water depth of 45 m, was collected and analyzed. Spectral analysis of offshore measured data along with the previous spectral analyses of coastal wave stations revealed that the wave spectra in the Persian Gulf are mainly single-peaked during all stages of the storms considering the possible uncertainty existing in the shape of wave spectrums depending on storm stage and can be represented by JONSWAP spectrum model using the newly proposed modified key parameters (i.e., Phillip’s constant and peak-enhancement parameters). In addition, the spectral behavior of waves during various stages of extreme events/storms showed that the mean values of the key parameters for growing and developed portions of the storms are slightly different from the traditionally recommended values for the JONSWAP parameters, while the estimated average values of decaying portions are considerably different from generally utilized values.

FIELD MEASUREMENTS AND SWASH PARAMETERIZATION ON BEACHES

The wave runup, defined as the vertical oscillation of the water edge at the coast, is commonly used as criterion for coastal design projects and flooding/erosion risk analysis. Due to the complexity of nearshore wave processes, most runup studies are based on empirical approaches which directly relate these oscillations to the beach and offshore wave characteristics. However, there is still considerable debate about just how runup is related to these environmental parameters, as well as about the range of application of empirical models due to site specific conditions. Recent works emphasized the importance of including site specific conditions to reduce the scatter in available parameterizations. Parameters related to beach characteristics like the sediment size, the amount of reflection, wave spectral shape and morphodynamic beach state may improve runup predictions (Poate et al., 2016, Guza and Feddersen, 2012).

Machine learning based prediction of wave breaking over a fringing reef

The characteristics of wave breaking in shallow waters that are of interest include whether a wave will break, the type of breaking that will occur, the breaking wave height, breaking depth, the position of breaking, the wave setup, and the transformation of the broken wave for given offshore wave characteristics and given bottom profile. Various methods have been proposed in the literature to estimate these wave-breaking characteristics. Deo et al. (2003) used a neural network approach to predict the breaking wave height and breaking depth for waves transforming over a range of simply sloped bottoms. The Deo et al. approach is extended here to predict other characteristics of wave breaking, including the type of wave breaking, the position of breaking, the wave setup, and the rate of dissipation of wave energy, in the case of waves impinging on a fringing reef. Observations from a series of specially conducted laboratory experiments involving monochromatic waves impinging on an idealized reef are used to develop and train respective models. The input parameters to the neural network models are the ratio of offshore wave height to the shallow-water depth of the flat section of the reef, H1/hs and the wave frequency parameter fH1/g. The breaker type classification model developed predicts the type of breaker with a success rate of 96%, outperforming previously used criteria for classifying breaker types. The numeric prediction models for the dimensionless position of wave breaking for plunging and spilling breakers, for wave setup, and for the reduction in energy flux across the reef have performance ratings characterized by respective correlation coefficients of 0.99, 0.82, 0.89, and 0.94. The modest value for the correlation between prediction and the actual result for the position of breaking of spilling breakers is believed to be associated with inaccuracies in determination of the exact position of breaking and to difficulty in visually capturing spilling breakers in observations. High correlation between predicted and actual values of the reduction in energy flux across the reef is achieved in spite of the fact the model was trained using data from a wave tank that included partial reflection (characterized by 7% mean deviation among non-breaking waves) from the downstream end of the tank. The method can be extended to provide predictive models for consideration of a range of natural coastal conditions, random waves, and various bottom profiles and complex geometry, based on training and testing of the models using representative laboratory, field, and/or flow simulation, in support of accurate prediction of near-shore wave phenomena.

Physical modelling of tsunami onshore propagation, peak pressures, and shielding effects in an urban building array

Wave experiments were conducted on a 1:20 length scale to measure water surface elevations and extreme pressures on and around idealized structural elements and arrays of structures. Experiments varied offshore wave characteristics and onshore structural configurations. Conditions in which waves broke on or just before the specimen caused maximum impulsive pressures. Pressures measured under nonbreaking wave conditions agreed with predicted values using design equations suggested by the Japanese Cabinet Office; however bare-earth water surface elevation inputs produced nonconservative estimates in breaking wave trials. Shielded structures experienced pressure reductions of 40–70% under breaking wave conditions. Results indicate that shielding elements constructed nearshore may reduce wave-induced damage. This dataset may be used to validate numerical models of tsunami propagation through urban environments.

Estimation and Validation of Offshore Wave Characteristics of Bay of Bengal Cyclones (2008-2009)

Offshore wave characteristics viz; significant wave height (Hs) and peak wave period (Tp) during cyclonic storms are important in planning and design of offshore structures, ship operations and other offshore activities. During June, 2008 to May, 2009 four cyclonic storm (CS) and one depression (D) originated over Bay of Bengal (BoB). In this paper, with the help of empirical formulae (Young's model, 1984; SMB method, 1984 and Sanil et al. (2003) we have estimated the offshore wave characteristics along the tracks of the four CS and one depression using the cyclone wind climate of Indian Meteorological Department (IMD). The spectral wave model of MIKE 21 is used to simulate wave characteristics of BOB while the observed wave characteristics off Gopalpur measured at 23 m water depth is used for validation. Various statistical parameters viz. root mean square error (RMSE), scatter index (SI), and correlation coefficient (R) are derived to inter-compare the observed, simulated and estimated results. The observed and simulated values of Hs show very good agreement (correlation coefficient of 0.86) while the agreement in case of wave period is relatively lower (R=0.55). Comparison between simulated and estimated Hs shows good agreement for Young's model and SMB method, while Sanil et al. equation shows over estimation as compared to simulation. Similar comparison for Tp shows good agreement with Young's model and Sanil et al. equation and over prediction for SMB method.

HIGH FREQUENCY MONITORING OF SWASH HYDRO-MORPHODYNAMICS ON A REFLECTIVE BEACH (GRAND POPO, BENIN): A NEW METHOD

A new remote sensing video-based method for measuring rapid variations of the bed elevation and free-surface in the swash zone is tested on a steep reflective beach at Grand Popo. The key assumption is that the free-surface has a distinguishable optical signature with respect to the bed level. This new set up enables a high frequency description of the wave transformation, dissipation and reflection in the swash zone as well as the variations of the bed evolution at the wave-by-wave scale. At a more integrated scale, a good correlation is found between the offshore reflected energy measured by an ADCP, the incident offshore wave characteristics and the swash local morphology, suggesting the importance of the swash dynamics on the energy reflection mechanism.

Nearshore and foreshore influence on overwash of a barrier island

Matias, A., Carrasco, A.R., Loureiro, C., Almeida, S., Ferreira, Ó., 2014. Nearshore and foreshore influence on overwash of a barrier island. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 675–680, ISSN 0749-0208.Accurate prediction of the occurrence and morphological consequences of overwash are important for coastal flood risk assessment and management. A number of morphological and oceanographic factors controlling overwash have been identified by several authors, including nearshore bathymetry. This work intends to identify alongshore variations in storm impact and evaluate the role of sub-aerial and submerged morphological variations in overwash occurrence. For this study, 24 cross-shore topo-bathymetric profiles were set on Barreta Island (Ria Formosa barrier island system, Portugal), extending from the nearshore to the lagoon level on the island backbarrier. Pre- and post-overwash surveys were made between August 2012 and April 2013. Offshore wave characteristics were obtained during overwash events including offshore significant wave height (Hs), peak period and wave direction. Wave propagation and wave parameters at breaking were obtained from SWAN model simulations. During the study period, overwash occurred both under storm conditions (e.g., Hs = 3.8 m) and non-storm conditions (e.g., Hs = 0.8 m), the latter coincident with spring high-tide. Overwash water intrusion distance across the barrier varied alongshore between 0 m (no overwash) and 40 m (reaching lagoon waters). Beach morphology was spatially variable, and changeable from one overwash episode to the next. Important morphological variations of the barrier include modification of barrier crest elevation, beach slope, and presence of swash bars in the vicinity of Ancão Inlet. Predictions of overwash occurrence were made by means of the computation of the Overwash Potential (OP), defined as the difference between the wave runup and the barrier elevation. Several runup equations were tested, and the results of obtained OP were compared to the observations of actual overwash occurrence to determine the most reliable approach. The selected predictor provided an accuracy of 88% for the identification of the locations where overwash processes occurred along the barrier. The predictions were variable alongshore due to variations in breaking waves and beach slope, which in turn are related to the existence of nearshore bars, associated with the presence of the updrift Ancão Inlet. This study proves that nearshore and foreshore morphologies have a major impact on the longshore variation of overwash distribution.

Beach response to a sequence of extreme storms

A sequence of daily beach surveys acquired over one month covering an area larger than 100,000m2, was analyzed to study morphological changes resulting from a cluster of storms. The beach response was highly variable in both the cross- and alongshore. A cumulative storm effect was not observed, despite one storm being characterized by a 10-year return period that had significant wave height (Hs) of 8.1m and a peak wave period (Tp) of 17s. Instead, storms that can potentially cause significant erosion in terms of Hs had a limited effect on the morphology because the large wave height was coupled to either neap tides, normally-incident short-waves (f>0.04Hz), or low levels of infragravity (0.004<f<0.04Hz) energy. Multiple linear regression analysis shows that volumetric changes in the upper part of the beachface are explained by offshore wave characteristics (period, height and direction), tidal range or by infragravity energy in the inner surf zone (assessed using pressure and velocity measurements). The results indicate that it is not possible to scale-up single-storm erosion studies into predictions of cluster-storm erosion.

A meta-modelling strategy to identify the critical offshore conditions for coastal flooding

Abstract. High water level at the coast may be the result of different combinations of offshore hydrodynamic conditions (e.g. wave characteristics, offshore water level, etc.). Providing the contour of the "critical" set of offshore conditions leading to high water level is of primary importance either to constrain early warning networks based on hydro-meteorological forecast or observations, or for the assessment of the coastal flood hazard return period. The challenge arises from the use of computationally intensive simulators, which prevent the application of a grid approach consisting in extracting the contour through the systematic evaluation of the simulator on a fine design grid defined in the offshore conditions domain. To overcome such a computational difficulty, we propose a strategy based on the kriging meta-modelling technique combined with an adaptive sampling procedure aiming at improving the local accuracy in the regions of the offshore conditions that contribute the most to the estimate of the targeted contour. This methodology is applied to two cases using an idealized site on the Mediterranean coast (southern France): (1) a two-dimensional case to ease the visual analysis and aiming at identifying the combination of offshore water level and of significant wave height; (2) a more complex case aiming at identifying four offshore conditions (offshore water level and offshore wave characteristics: height, direction and period). By using a simulator of moderate computation time cost (a few tens of minutes), the targeted contour can be estimated using a cluster composed of a moderate number of computer units. This reference contour is then compared with the results of the meta-model-based strategy. In both cases, we show that the critical offshore conditions can be estimated with a good level of accuracy using a very limited number (of a few tens) of computationally intensive hydrodynamic simulations.