Wave Height Reduction Research Articles

Experimental study of flow structures of a solitary wave propagating over a submerged thin plate in different angles using PIV technique

This research presents an experimental investigation into the interaction of a solitary wave and a submerged thin plate under different angles. Experiments are conducted to measure both velocity and vorticity by using the Particle Image Velocimetry (PIV) technique. Effects of changing the thin plate angle on the wave height and wave speed are analyzed through the use of wave height gages. Vortices are generated when the solitary wave is transmitted over the obstacle. Understanding the formation and location of the vortices will help analyze the obstacle effect on the flow. In the initial stage, a thin plate is located vertically, and flow structures are visualized. The plate is then deviated from the vertical direction towards positive angles (the direction of wave propagation) and towards negative angles in the opposite direction. In order to study the effects of the plate angle on the flow structures, experiments are carried out for three positive and three negative angles. Comparison of the formed vortices at different angles shows the formation of an additional vortex near the bottom of the channel for positive angles, as opposed to negative angles. The larger the angle is, the less the formation time of the vortex at the bottom of the channel will be. The study of the clockwise vortices formed behind the obstacle shows that increasing the plate angle in both directions decreases their strength. The clockwise vortices of negative angles are stronger than those of positive angles. In addition, changing the plate angle to the negative direction causes more wave height and wave speed reduction than changing it to the positive direction.

Wave attenuation in presence of mangroves: A sensitivity study for varying bottom slopes

Mangroves thrive in the intertidal areas (interface between land and sea) of tropical and sub-tropical belt and play an important role in overall attenuation of nearshore waves. Multiple interactions of waves with mangrove trunks and roots and bottom friction are the two primary mechanisms responsible for wave attenuation in mangrove forests. Earlier studies, comprising both analytical and experimental, reported an exponential decay in wave height for waves propagating over vegetation with idealized bottom topography and a few on sloping bottom. But hardly studies have attempted to characterize the wave attenuation by vegetation over varying seabed slopes since mangroves generally grow luxuriantly on gradual topography having large tidal amplitudes. Nowadays, several studies are being carried out on development of artificial mangroves to reduce the coastal hazard risks; thenceforth, there is an imperative requirement to study the wave damping characteristics of mangroves on varying seabed slopes. Consequently, this study performs sensitivity experiments to analyze the wave attenuation over mangroves with different sea-bottom slopes using a third-generation wave model. The study exposes sensitivity of wave attenuation characteristics to different beach slopes in the presence of mangroves and aims at understanding how the wave attenuation characteristics by mangroves differ with varying bottom slopes. The total percentage energy reduction for waves reaching the shoreline after propagating through mangroves on mild slope (1:80, 1:40) is observed to be 93%–98%, nearly 84% for 1:20 slope, and 67% for steep slope (1:10). The study reveals that the wave height decays exponentially for the mild slope and found to be consistent with the earlier studies, but as the degree of bottom steepness increases, the wave height reduction becomes gradual, and this can be attributed to the water depth variation, shoaling, breaking, and reflection characteristics associated with different slopes, in the presence of mangroves.

Projected changes in significant wave height toward the end of the 21st century: Northeast Atlantic

AbstractWind field ensembles from six CMIP5 models force wave model time slices of the northeast Atlantic over the last three decades of the 20th and the 21st centuries. The future wave climate is investigated by considering the RCP4.5 and RCP8.5 emission scenarios. The CMIP5 model selection is based on their ability to reconstruct the present (1971–2000) extratropical cyclone activity, but increased spatial resolution has also been emphasized. In total, the study comprises 35 wave model integrations, each about 30 years long, in total more than 1000 years. Here annual statistics of significant wave height are analyzed, including mean parameters and upper percentiles. There is general agreement among all models considered that the mean significant wave height is expected to decrease by the end of the 21st century. This signal is statistically significant also for higher percentiles, but less evident for annual maxima. The RCP8.5 scenario yields the strongest reduction in wave height. The exception to this is the north western part of the Norwegian Sea and the Barents Sea, where receding ice cover gives longer fetch and higher waves. The upper percentiles are reduced less than the mean wave height, suggesting that the future wave climate has higher variance than the historical period.

On wave damping occurring during source-based generation of steep waves in deep and near-shallow water

Generation of steep waves (H/λ>0.03) in a NSE-based NWT is a challenging task and is seldom attempted in near-shallow water (kh<1.0). Failure towards attainment of the target steepness is characterized by an under-prediction of the wave height (H) along the length of the NWT. The issue of height damping has received limited attention in literature. In this context, a NSE based NWT has been developed using a PLIC-VOF formulation and a mass source-based wave generator. Wave damping at the NWT boundaries has been achieved using sponge layers. A total of nine wave designs have been simulated in three categories each of steepness (low, moderate and large) and relative depth (near-shallow, intermediate and deep). Criteria for selecting spatio-temporal resolution within the tank have been determined through parametric analysis. For H/λ>0.03, it is seen that wave height reduction in excess of 5% occurs in near-shallow and deep water. It is found that height reduction in deep water is largely attributable to free-surface damping as predicted by Airy theory. However, numerical height damping in near-shallow water is observed to be much stronger, especially near the source. A deeper analysis reveals that mass source-based wave generation induces strong and persistent vortical activity in near-shallow water which in turn leads to dissipation of source-injected momentum. Appropriate modifications in source design, that are aimed at reducing wave-vorticity interactions in the near-field, are proposed for steep wave generation in kh<1 and kh>2.5. Substantial improvement in the prediction of far-field wave height is hence reported.

Comparative Study on Hydrodynamic Performance of Porous and Non-Porous Submerged Breakwater

The paper offers a novel comparative case study on the performance of solid versus porous breakwater physically modeled under similar laboratory and wave conditions. Unidirectional waves were generated in a two dimensional laboratory flume having dimensions of 21.3 m length, 0.76 m width and 0.74 m height. Keeping still water depth of 50 cm, waves with T=1.6 sec, 1.7 sec, 1.8 sec and 2 sec were generated from a piston type wave generator. At 800 cm from the wave generator, horizontally slotted submerged breakwater of 100cm width,75cm length and 40 cm height were constructed with varying porosities of n=0.4, 0.5 and 0.6. Water level data were collected at six different locations for 12 unique run conditions. Using the measured data different hydrodynamic coefficients were calculated which include transmission co-efficient Kt, reflection co-efficient, Kr and wave energy loss co-efficient, KL. These co-efficient values were then, analyzed with respect to relative breakwater width (k.B), [where, k = wave number (2π/L), B = breakwater width] and porosity of breakwater. For the comparison, hydraulic performance of solid rectangular submerged breakwater of 40 cm height and 100 cm width was also reviewed which was modeled in the same laboratory under same wave condition of T=1.6,1.7 1.8 and 2.0 seconds and a still water depth of 50 cm. Experimental results from the porous breakwater and subsequent comparison of the results with solid breakwater indicate that, for transmitting less wave energy through the breakwater, maximum transmission co-efficient, Kt = 0.526 was obtained for porous breakwater with the lowest porosity (n=0.4) for T = 1.6 sec whereas for the solid breakwater maximum Kt= 0.5 was obtained for T=1.6 second. In terms of wave height, percentage wave height reduction was maximum 47% for porous breakwater whereas it was 68% for solid breakwater both at T=1.6 second. The hydrodynamic performance shows a significantly higher wave height reduction percentage for solid breakwater as anticipated by the physics of the interaction of the breakwater and the wave too. Besides, the position of wave breaking for different case scenario for both types of breakwater were recorded and analyzed which demonstrates that for porous breakwater waves tend to transmit through the slots and break at a certain distance beyond the breakwater whereas in case of solid breakwater, waves tend to break before or on the breakwater.

Rough weather avoidance effect on the wave climate experienced by oceangoing vessels

The quantitative assessment of the influence of rough weather avoidance on the wave climate ships experience in their oceanic routes is presented. The only meteorological source that naturally includes this effect is the database derived from the Voluntary Observing Ships’ programme, which is here compared with the state-of-the-art ERA-interim numerical re-analysis. The results of this work allow quantifying the reduction of the mean significant wave height encountered by ships in their operations for a more aware planning of the missions and better assessment of the environmental conditions.

Co-located wind and wave energy farms: Uniformly distributed arrays

Co-located wave and wind energy farms can serve to tackle one of the downsides of offshore wind energy relative to its onshore counterpart: the longer non-operational periods. These are partly caused by delays to maintenance tasks due to energetic sea states preventing access. By co-locating Wave Energy Converters (WECs) in an appropriate configuration it may be possible to reduce the wave heights within the wind farm area (shielding effect) and thereby increase the weather windows for maintenance. Previous works analysed the improvements in accessibility obtained by configuring the co-located WECs as a peripheral barrier or interspersed within the farm. However, the former led to an insufficient wave height reduction as the distance to the barrier increased and the latter presented other handicaps, notably in respect of the submarine cable installation and the navigation of workboats. The objectives of this work are: (i) to analyse whether a uniformly distributed array may be more convenient in these respects and (ii) to carry out a comparative economic assessment. This investigation is carried out through a case study at the Horns Rev 1 wind farm by means of a high-resolution spectral wave model. Annual cost savings of up to 900,000 € are found.

Estimation of the transmission coefficients of wave height and period after smooth submerged breakwater using a non-hydrostatic wave model

In the present study, new empirical formulas of wave transmission coefficients, including Kp for the mean wave period and Kt for the significant wave height, defined as the ratios between the transmitted and incident values of a smooth submerged breakwater, are proposed based on results simulated using the SWASH non-hydrostatic wave model. According to the analysis of the numerical results, both Kt and Kp are less than 1.0, and the transmitted peak period is close to the incident peak period in most cases. The reduction of wave height is mainly caused by energy loss due to wave breaking, and the reduction of the mean period is mainly caused by the generation of high-frequency harmonics after the submerged breakwater. Moreover, both Kt and Kp have good linear correlations with the relative submergences, and Kp has a good linear relationship with wave steepness. The relative crest width and slope have no significant effect on the transmission coefficient of the submerged breakwater compared with the effects of the relative submergences and wave steepness. Based on the factors mentioned above, new empirical formulas are formed. The coefficients of determination (RCor2) of Kt and Kp between measurements and values calculated using the new empirical formulas are 0.90 and 0.95, respectively, representing an improvement over the models of Van der Meer et al. (2003) and Carevic et al. (2013). The sensitivities of Kt and Kp to the variables in the empirical formulas are also investigated. According to the sensitivity analysis, a simplified formula of Kt is suggested based on the relative submergence and breaker parameter, and a simplified formula of Kp is proposed based on the relative submergence and wave steepness.

Nature-based flood protection: The efficiency of vegetated foreshores for reducing wave loads on coastal dikes

This paper analyses the effect of vegetation on wave damping under severe storm conditions, based on a combination of field measurements and numerical modelling. The field measurements of wave attenuation by vegetation were performed on two salt marshes with two representative but contrasting coastal wetland vegetation types: cordgrass (Spartina anglica) and grassweed (Scirpus maritimus). The former is found in salty environments, whereas the latter is found in brackish environments. The measurements have added to the range with the highest water depths and wave heights presented in the literature so far. A numerical wave model (SWAN) has been calibrated and validated using the new field data. It appeared that the model was well capable of reproducing the observed decay in wave height over the salt marsh. The model has been applied to compute the reduction of the incident wave height on a dike for various realistic foreshore configurations and hydraulic loading conditions. Additionally, the efficiency of vegetated foreshores in reducing wave loads on the dike has been investigated, where wave loads were quantified using a computed wave run-up height and wave overtopping discharge. The outcomes show that vegetated foreshores reduce wave loads on coastal dikes significantly, also for the large inundation depths that occur during storms and with the vegetation being in winter state. The effect of the foreshore on the wave loads varies with wave height to water depth ratio on the foreshore. The presence of vegetation on the foreshore extends the range of water depths for which a foreshore can be applied for effective reduction of wave loads, and prevents intense wave breaking on the foreshore to occur. This research demonstrates that vegetated foreshores can be considered as a promising supplement to conventional engineering methods for dike reinforcement.

Two-dimensional numerical study on the effect of water depth on resonance behaviour of the fluid trapped between two side-by-side bodies

The effect of water depth on gap resonance of two side-by-side rectangular boxes is examined in a 2D numerical wave flume at various gap widths and body drafts. The numerical model used is based on the volume of fluid (VOF) method for the solution of Navier–Stokes equations combined with free surfaces. The water depth covered ranges from shallow to deep water regimes. It is found that the resonant frequencies and wave heights depend apparently on water depth. For a given body draft and gap width, the resonant wave frequency shifts to higher values whereas the resonant wave height inside the gap decreases as water depth increases. The amount of reduction of wave height is found to be dependent on the depth to draft and gap width to breadth ratios. It is observed that the effect of water depth on resonance frequency is very small when the ratio of water depth to body draft is equal to or greater than six, while the effect of water depth on resonant water level diminishes when this ratio increases beyond 10. Furthermore, it is observed that the variation trends of the resonant frequency and the reduction of the resonant wave height with water depth are similar at different drafts and gap widths. The present results suggest that the water depth, body draft and the ratio of water depth to body draft are the key factors that influence the resonant responses. It is also found that the potential flow model not only over-predicts the resonant wave heights but also fails to predict the variation trends of the resonant wave height with water depth.

The Effectiveness, Costs and Coastal Protection Benefits of Natural and Nature-Based Defences.

There is great interest in the restoration and conservation of coastal habitats for protection from flooding and erosion. This is evidenced by the growing number of analyses and reviews of the effectiveness of habitats as natural defences and increasing funding world-wide for nature-based defences–i.e. restoration projects aimed at coastal protection; yet, there is no synthetic information on what kinds of projects are effective and cost effective for this purpose. This paper addresses two issues critical for designing restoration projects for coastal protection: (i) a synthesis of the costs and benefits of projects designed for coastal protection (nature-based defences) and (ii) analyses of the effectiveness of coastal habitats (natural defences) in reducing wave heights and the biophysical parameters that influence this effectiveness. We (i) analyse data from sixty-nine field measurements in coastal habitats globally and examine measures of effectiveness of mangroves, salt-marshes, coral reefs and seagrass/kelp beds for wave height reduction; (ii) synthesise the costs and coastal protection benefits of fifty-two nature-based defence projects and; (iii) estimate the benefits of each restoration project by combining information on restoration costs with data from nearby field measurements. The analyses of field measurements show that coastal habitats have significant potential for reducing wave heights that varies by habitat and site. In general, coral reefs and salt-marshes have the highest overall potential. Habitat effectiveness is influenced by: a) the ratios of wave height-to-water depth and habitat width-to-wavelength in coral reefs; and b) the ratio of vegetation height-to-water depth in salt-marshes. The comparison of costs of nature-based defence projects and engineering structures show that salt-marshes and mangroves can be two to five times cheaper than a submerged breakwater for wave heights up to half a metre and, within their limits, become more cost effective at greater depths. Nature-based defence projects also report benefits ranging from reductions in storm damage to reductions in coastal structure costs.

Hybrid wave and offshore wind farms: A comparative case study of co-located layouts

Marine energy is one of the most promising alternatives to fossil fuels due to the enormous energy resource available. However, it is often considered uneconomical and difficult. Co-located offshore wind turbines and wave energy converters have emerged as a solution to increase the competitiveness of marine energy. Among the benefits of co-located farms, this work focuses on the shadow effect, i.e. the reduction in wave height in the inner part of the farm, which can lead to significant savings in operation and maintenance (O&M) costs thanks to the augmented weather windows for accessing the wind turbines. The aim of this study is to quantify the wave height reduction achieved within a co-located wave-wind farm. Different locations and a large number of layouts are analysed in order to define the optimum disposition.

1314 美観を備えた新型津波防波堤の波高軽減効果(GS1-3 熱工学・流体工学)

1314 美観を備えた新型津波防波堤の波高軽減効果(GS1-3 熱工学・流体工学)

Modeling wave processes over fringing reefs with an excavation pit

The excavation of reef-flat sand and aggregate for engineering projects is a common practice on atoll islands, with attendant coastal management and hazard mitigation issues. To assess the impact of reef-flat excavation pits on wave transformation, a numerical study is carried out based on one-dimensional weakly dispersive, highly nonlinear Boussinesq equations. Model simulations are compared to field observations made at Majuro Atoll, Republic of the Marshall Islands (Ford et al., 2013a), at a 75-m-wide fringing reef flat with a 17-m-wide, 4-m-deep excavation pit and at an adjacent unmodified reef flat. With a calibrated empirical breaking model and uniform friction coefficient, the numerical model is shown to satisfactorily reproduce the observed sea and swell (SS) and infragravity (IG) frequency band wave heights for a moderate amplitude wave event. By removing the lateral morphological differences of the field experiment, the model shows that an excavation pit reduces/increases shoreline IG/SS wave heights compared to numerical tests without the pit, in qualitative agreement with Ford et al. (2013a). An EOF analysis demonstrates that the reduction in IG wave heights is associated with modifications of the ¼ and ¾ wavelength IG standing modes across the reef caused by the pit. Model tests are used to evaluate the effects of cross-reef pit location and width on wave transformation. Shoreline wave heights are least affected by a pit located near the reef crest, and both SS and IG wave heights increase as the pit location moves shoreward. Shoreline wave heights are an increasing function of pit width as the increase in SS wave height exceeds the decrease in IG wave height produced by the pit. The effects of incident wave height and spectral bandwidth are also examined for the reef with and without pit. It is found that both shoreline SS and IG wave heights increase with the increasing incident wave height for both profiles. The numerical model results also demonstrate that the shoreline response is sensitive to the bandwidth of the incident wave spectrum, and larger IG shoreline wave heights are excited for narrower spectral bandwidths until wave reflection limits the forcing.

Experimental Study of Reduction of Solitary Wave Run-Up by Emergent Rigid Vegetation on a Beach

Extensive studies have been carried out to study the performance of mangrove forests in wave height reduction. In this study, the reduction of the inundation and run-up of leading tsunami waves by mangrove forests was investigated through a series of laboratory experiments conducted in a long wave tank. The inundation and run-up were measured using a high speed CCD camera. Solitary waves were used to model the leading tsunami waves. Five vegetation models representing three forest densities and two tree distributions were examined on an impermeable sloping beach, and they were compared with the non-vegetated slope in view of wave reflection, transmission, and run-up. Results show that both incident wave height and run-up could be reduced by up to 50% when the vegetation was present on the slope. Dense vegetation reduced the wave transmission because of the increased wave reflection and energy dissipation into turbulence in vegetation. Normalized wave run-up on the beach decreased with the increase of both normalized incident wave height and forest density. Effect of forest density on the wave run-up on the sloping beach was further examined, and an empirical formula with the density incorporated was proposed. The study also highlighted the importance of tree distribution to wave interaction with vegetation on the slope when the forest density was unaltered, and run-up reduction difference between tandem and staggered arrangements of the trees could reach up to 20%.

Accessibility for operation and maintenance tasks in co-located wind and wave energy farms with non-uniformly distributed arrays

Operation and maintenance is a particularly challenging aspect of offshore wind energy. First, the harsh marine environment requires more frequent tasks relative to onshore wind turbines; and second, these labours can be delayed by difficult sea conditions – in particular, large wave heights – leading to down time and, consequently, increased costs. This can be offset in part by combining offshore wind and wave energy systems: wave energy converters adequately deployed extract part of the incoming wave energy, resulting in a milder wave climate within the wind park, and hence better accessibility for maintenance tasks and reduced down time. The actual results depend on the layout of the co-located wave-wind energy farm. Previous works showed that peripherally distributed arrays, i.e., co-located wave energy converters deployed along the periphery of the wind farm to act as a barrier, bring about good results in terms of wave height reduction and average accessibility. However, important differences between the accessibility to the wind turbines located directly behind the barrier and the other turbines in the farm were found. On this basis, the objective of this paper is to analyse whether non-uniformly distributed arrays can lead to a more uniform wave height reduction throughout the farm. The analysis is based on 4 offshore wind farms currently in operation: Alpha Ventus, Bard 1, Horns Rev 1 and Lincs. It was found that non-uniformly distributed arrays lead to a more significant average wave height reduction (by over 4.7%) and more uniformly distributed throughout the farm, as well as a greater power output from the co-located devices (by up to 16.5%).

Modeling of Seadome as artificial reefs for coastal wave attenuation

A newly designed marine structure, referred to as Seadome, is introduced and evaluated for its capability for surface wave attenuation in the nearshore zone for coastal protection purposes. More than 2600 laboratory tests showed that a range of 20–80% of wave height reduction could be achieved by use of a standard array consisting of 5 shore-parallel rows of the Seadome. The structure performance was found to increase almost linearly following the structure height to water depth ratio and the incident wave steepness, and decreases exponentially as the relative wave length to structure width increases. Semi-empirical formulas were developed and tested for prediction of the performance. A process-based predictive model with wider applicability and provision for qualitative insights is also introduced for the prediction purpose. A comparison between the modeled results and laboratory data was found to associate with root-mean-square error and standard deviation both less than 10%. A nomogram is developed as a ready-to-use tool for a practical design of the Seadome. According to the chart, a structure height to water depth ratio greater than 0.7 and a relative wave length to structure width smaller than 2.5 would be minimally required to achieve a wave amplitude reduction of 50%.

The wave energy scattering by interaction with the refracted breakwater and varying trench depth

The purpose of this study is to estimate the wave height located at the front face of the offshore refracted breakwater when the trench is dredged in the exterior region of the breakwater at various depths. The problems that arise with regard to the different obliquely incident wave field involve having two-dimensional planes and having the configuration of the trench region designated by a single horizontal rectangular system. The numerical approach involves applying the Green function based on the boundary integral approach. The results of the present numerical works are illustrated. The ratio of the wave height reduction at the front face of the refracted breakwater is shown to be more than 25% due to the trench’s effect on the seabed. The corner of the refracted breakwater is where the wave energy is concentrated, and through utilizing the trench, the wave energy can be subdued to reduce the damage on the breakwater. The trench brings about a reduction in wave amplitude and wave energy in, around, and directly downstream of the trench.

Co-located wind-wave farm synergies (Operation & Maintenance): A case study

Operation and maintenance can jeopardise the financial viability of an offshore wind energy project due to the cost of downtime, repairs and, above all, the inevitable uncertainties. The variability of wave climate can impede or hinder emergency repairs when a failure occurs, and the resulting delays imply additional costs which ultimately reduce the competitiveness of offshore wind energy as an alternative to fossil fuels. Co-located wind turbines and Wave Energy Converters (WECs) are proposed in this paper as a novel solution: the reduction of the significant wave height brought about by the WECs along the periphery of the wind farm results in a milder wave climate within the farm. This reduction, also called shadow effect, enlarges weather windows for Operation & Maintenance (O&M). The objective of this paper is to investigate the increase in the accessibility time to the turbines and to optimise the layout for the co-located wind-wave farm in order to maximise this time. The investigation is carried out through a case study: Alpha Ventus, an operating offshore wind farm. To maximise the reduction of wave height in the turbine area no fewer than 15 layouts are tested using high-resolution numerical modelling, and a sensitivity analysis is conducted. The results show that, thanks to the wave energy extraction by the WECs, weather windows (access time) can increase very significantly – over 80%. This substantial effect, together with other benefits from the combination of wave and offshore wind power in a co-located farm (common electrical infrastructures, shared O&M equipment and crews, etc.) will enhance the economic viability of these marine renewables, and hence their potential to reduce our carbon footprint on the planet.

LIMITS OF WETLAND WAVE DISSIPATION

Wetlands provide a natural protective buffer against coastal storms by dissipating wave energy, but there is limited quantitative information to evaluate the effectiveness of such natural features over a range of hydrodynamic conditions. Laboratory experiments were conducted in a large-scale flume with artificial vegetation. The measurements show good agreement to the wave dissipation formulation of Mendez and Losada (2004). Drag coefficients correlated well with Reynolds number for the submerged vegetation, but emergent vegetation gave higher coefficients than the submerged vegetation. Applying the drag coefficients derived from the lab, the efficiency of vegetation dissipation was explored analytically. For dense vegetation (400 stems/m2), it was found that significant reductions in wave height (50%) are possible over tens and hundreds of meters for relative depths (depth times wavenumber) less than 1.5. The distance required to dissipate wave energy increases exponentially as water depth increases. A hypothetical field example using the spectral wave model STWAVE with the Mendez and Losada (2004) expression for wave dissipation is also presented. The field example shows reduction in wave height of 70-80 percent along the shoreline for extensive vegetation coverage during severe wind and surge conditions. These hypothetical simulations demonstrate the potential for vegetation to reduce the storm intensity at the coast by reducing wave height under severe wind and water level conditions. However, the simulations cannot show whether the vegetation’s capacity to reduce storm damages can be sustained during storms or over longer periods of time as wetlands evolve.