Wave Height Reduction Research Articles

An approximate solution for the wave energy shadow in the lee of an array of overtopping type wave energy converters

In this study we investigate how the wave energy deficit in the lee of an array of overtopping type wave energy converting devices (WECs), redistributes with distance from the array due to the natural variability of the wave climate and wave structure interactions. Wave directional spreading has previously been identified as the dominant mechanism that disperses the wave energy deficit, reducing the maximum wave height reduction with increasing distance from the array. In addition to this when waves pass by objects such as an overtopping type WEC device, diffracted waves re-distribute the incident wave energy and create a complex interference pattern. The effect of wave energy redistribution from diffraction on the wave energy shadow in the near and far field is less obvious. In this study, we present an approximate analytical solution that describes the diffracted and transmitted wave field about a single row array of overtopping type WECs, under random wave conditions. This is achieved with multiple superpositions of the analytical solutions for monochromatic unidirectional waves about a semi-infinite breakwater, extended to account for partial reflection and transmission. The solution is used to investigate the sensitivity of the far field wave energy shadow to the array configuration, level of energy extraction, incident wave climate, and diffraction. Our results suggest that diffraction spreads part of the wave energy passing through the array, away from the direct shadow region of the array. This, in part, counteracts the dispersion of the wave energy deficit from directional spreading.

Supercritical flow profile improvement by means of a convex corner at a bend inlet

A novel method for the reduction of wave height along the outer wall of a bend is introduced by adding a convex corner to the inner bend wall. The negative waves emitted from the convex corner are then superimposed on the positive waves caused by the outer bend wall. The interaction of positive and negative waves transforms the original single-humped wave along the outer wall to a lower double-humped wave. The optimum expansion angle of the convex corner is a function of the offset width to the bend width, the radius of curvature to the bend width, and the approach flow Froude number. Numerical studies indicate that the convex corner may be used in a wide range of safe conditions free from cavitation damage. By using an optimized convex corner, the wave height may be reduced between 10 and 45%.

Scattering of gravity waves by multiple surface-piercing floating membrane

Interaction of surface gravity waves with multiple vertically moored surface-piercing membrane breakwaters in finite water depth is analyzed based on the linearized theory of water waves. The study is carried out using least square approximation method to understand the effect of the vertical membrane as effective breakwater. Initially the problem is studied for a single membrane wave barrier but for the case of multiple membrane breakwaters the study is carried out using the method of wide-spacing approximation. In the present study, it is observed that the deflection of the membrane is reduced with the increase in the stiffness parameter of the mooring lines attached to the membrane. In the case of single surface-piercing membrane with moored and fixed edge conditions, the reflection and transmission coefficients are compared and analyzed in detail. The resonating pattern in the reflection coefficients are also observed for multiple floating membrane which can also be referred as Bragg's resonance. In the presence of the porosity constant the wave reflection is also observed to be decreasing and the change in the distance between the vertical floating breakwaters also helps in the attenuation of wave height. It is observed that the presence of multiple floating breakwater helps in the reduction of wave height in the transmitted region.

FIELD INVESTIGATION OF WAVE DISSIPATION OVER SALT MARSH VEGETATION DURING TROPICAL CYCLONE

Wave data were measured along a 28 m transect using 3 pressure transducers over a 2-day period during a tropical storm. The tropical storm force winds produced waves up to 0.4 m high (zero-moment) that propagated over vegetation of Spartina alterniflora submerged under a surge of over 1 m above the marsh floor. Measured wave heights, energy losses between gages and spectral energy dissipation models of rigid vegetation were utilized to estimate wave height decay rates, integral and frequency-dependent bulk drag coefficients, and frequency distribution of energy dissipation induced by the vegetation. Measurements showed that incident waves attenuated exponentially over the vegetation. The exponential wave height decay rate decreased as Reynolds number increased. The swell was observed to decay at a slower rate than the wind sea regardless of the wave height. The linear spatial wave height reduction rate increased from 1.5% to 4% /m as incident wave height decreased. The bulk drag coefficient estimated from the field measurement decreased with increasing Reynolds and Keulegan-Carpenter numbers. The energy dissipation varied across the frequency scales with the largest magnitude observed near the spectral peaks, above which the dissipation gradually decreased. The wave energy dissipation did not linearly follow the incident energy, and the degree of non-linearity varied with the frequency. For a given spectrum, the frequency-distributed drag coefficient increased gradually up to the peak frequency and remained approximately at a stable value at the higher frequencies. This spectral variation was parameterized by introducing a frequency-dependent velocity attenuation parameter inside the canopy. The spectral drag coefficient is shown to predict the distribution of energy dissipation with more accuracy than the integral coefficients, which results in a more accurate prediction of the mean wave period and spectral width of a wave field with vegetation.

WAVE ATTENUATION IN MANGROVE FORESTS; FIELD DATA OBTAINED IN TRANG, THAILAND

Mangroves thrive in sheltered intertidal areas in the tropics and sub-tropics. Due to this position at the interface between land and sea, mangroves play an important role in the attenuation of waves. Dissipation of wave energy in mangrove forests is an interesting feature from the viewpoint of coastal protection. Nevertheless, field data are sparse and modeling attempts reveal the need for additional data. This paper presents the results of an extensive field campaign, lasting 6 months, in mangroves along the Andaman coast of Trang Province in southern Thailand. Wave attenuation has been studied along two contrasting transects with different elevation and vegetation characteristics and different orientations towards the Andaman Sea. Along the Kantang transect, which is mostly exposed to swell waves, vegetation densities increased from 4.5 to 9.3 volume-†° along the transect and on average 63% of the incident wave energy was attenuated over a distance of 246 m. Along the Palian transect, mostly exposed to sea waves instead, vegetation increased from 4.3 to 19 volume-†° and 72% of the incident wave energy was attenuated over this 98 m transect. It was found that standardized wave attenuation correlates well with incident wave energy, when attenuation is analyzed per vegetation zone. Energy reduction rates of these zones, defined by the gradient of the correlations between the standardized wave attenuation and incident wave energy, are found to increase significantly with vegetation density. Consistently, wave reduction rates, expressing the gradient of the correlation between wave height reduction and incident wave heights, are found to be 0.001-0.014 for the study sites and also show a significant and increasing trend with vegetation densities.

Turbulent structure of air flow over wind-induced gravity waves

This is the second paper in a group of three that reports the systematic measurements of wind-generated water waves in a wind tunnel experiment. Here, the structure of the boundary layer on the air side of the water–air interface was analysed and compared with the boundary layer over a smooth plane rigid wall. The contribution of the wave-induced Reynolds stress was detected through filtering the spectrum of velocity fluctuations. Wave-induced Reynolds stresses became negligible for z > 5 H rms. The intermittency factor in the boundary layer over water waves was similar to that in a boundary layer over a rigid plane wall, with several differences near the interface. Here, the presence/absence of water damps out the turbulence. The quadrant analyses revealed that ejection and sweep events were dominant and more concentrated. At small fetches, the large-amplitude negative streamwise perturbations were preferentially lifted. Turbulence energy production peaked at z/δ = 0.2 and had a distribution similar to that observed for a self-preserving boundary layer with a strong adverse gradient pressure. The quadrant analysis contribution to the energy production revealed that ejections still dominated the balance and that the production was spatially modulated in the wind direction with a couple of cells and with a minimum in the area of the free surface wave height reduction.

Applicability of CADMAS-SURF to evaluate detached breakwater effects on solitary tsunami wave reduction

Detached breakwaters, made with wave-dissipating concrete blocks such as Tetrapods, have been widely applied in Japan, but the effectiveness of such kinds of detached breakwaters on tsunami disaster prevention has never been discussed in detail. A numerical wave flume called CADMAS-SURF has been developed for advanced maritime structure design. CADMAS-SURF has been applied mainly to ordinary wave conditions such as wind waves, and little attempt has been made for expanding its application to tsunami waves. In this study, the applicability of CADMAS-SURF for evaluating the effectiveness of detached breakwaters on a solitary tsunami wave reduction is investigated by comparing the calculated results with those from hydraulic experiments. First, the effectiveness of a detached breakwater on the reduction of wave height and wave pressure was confirmed both by hydraulic experiments and numerical simulations. Finally, CADMAS-SURF has been found to be a useful tool for evaluating the effects of detached breakwaters on tsunami wave height and pressure reduction, as a first step in a challenging study.

Simulation of Wave Damping Near Coast due to Offshore Wind Farms

Abstract ALARI, V. and RAUDSEPP, U., 2012. Simulation of wave damping near coast due to offshore wind farms. Two hundred wind turbines with an annual productivity of 2.3 TWh, which could produce up to 30% of the energy Estonia needs, are scheduled to be constructed on separate shallows in the NW Estonian coastal waters (the Baltic Sea), 5–20 km off shore. With numerical modeling, we have established a potential impact of the wind farms on wave heights. We concluded that the impact exists in terms of the reduction of significant wave height, but it is very marginal, not more than 1% below 10-m isobaths. This is due to a very small ratio between the turbine diameter and dominant wavelength and the favorable setup of turbines with respect to each other and the coast.

The Protective Role of Coastal Marshes: A Systematic Review and Meta-analysis

BackgroundSalt marshes lie between many human communities and the coast and have been presumed to protect these communities from coastal hazards by providing important ecosystem services. However, previous characterizations of these ecosystem services have typically been based on a small number of historical studies, and the consistency and extent to which marshes provide these services has not been investigated. Here, we review the current evidence for the specific processes of wave attenuation, shoreline stabilization and floodwater attenuation to determine if and under what conditions salt marshes offer these coastal protection services.Methodology/Principal FindingsWe conducted a thorough search and synthesis of the literature with reference to these processes. Seventy-five publications met our selection criteria, and we conducted meta-analyses for publications with sufficient data available for quantitative analysis. We found that combined across all studies (n = 7), salt marsh vegetation had a significant positive effect on wave attenuation as measured by reductions in wave height per unit distance across marsh vegetation. Salt marsh vegetation also had a significant positive effect on shoreline stabilization as measured by accretion, lateral erosion reduction, and marsh surface elevation change (n = 30). Salt marsh characteristics that were positively correlated to both wave attenuation and shoreline stabilization were vegetation density, biomass production, and marsh size. Although we could not find studies quantitatively evaluating floodwater attenuation within salt marshes, there are several studies noting the negative effects of wetland alteration on water quantity regulation within coastal areas.Conclusions/SignificanceOur results show that salt marshes have value for coastal hazard mitigation and climate change adaptation. Because we do not yet fully understand the magnitude of this value, we propose that decision makers employ natural systems to maximize the benefits and ecosystem services provided by salt marshes and exercise caution when making decisions that erode these services.

A new wave absorbing quay-wall for wave height reduction in a harbor basin

In this study, a new type of wave absorbing quay-wall with a partial wave chamber is introduced. In addition to the basic design containing a wave chamber with an impermeable back wall, there are several alternative absorbing systems that use rock within the core of the wave chamber. These systems have been experimentally investigated to determine whether they further reduce wave heights in front of the wall. Then, they have been compared to a conventional quay-wall with a solid front wall. Their improved performance indicates that this is an effective new scheme for a wave absorbing quay-wall. This new scheme, which consists of a partial wave chamber containing a rock-armored slope, was experimentally compared with a conventional quay-wall with a solid front wall in the 3D physical model of the port of Piraeus, Greece. Results show that wave heights in front of the new absorbing quay-wall can be reduced by 20–30%. The experimental results were verified in the field, where the extension of the quay-wall of the port of Piraeus was finally constructed with the new and effective wave absorbing system.

Effect of mangrove forest structures on wave attenuation in coastal Vietnam

This paper analyses wave attenuation in coastal mangrove forests in Vietnam. Data from 32mangrove plots of six species located in 2 coastal regions are used for this study. In each plot, mangrove forest structures and wave height at different cross-shore distances are measured. Wave height closely relates to crossshore distances. 92 exponential regression equations are highly significant with R2 >0.95 and P val. <0.001. Wave height reduction depends on initial wave height, cross-shore distances, and mangrove forest structures. This relationship is used to define minimum mangrove band width for coastal protection from waves in Vietnam.

Medium timescale beach rotation; gale climate and offshore island influences

Beach profile surveys, gale climate and atmospheric variations were utilized to assess medium timescale morphological change at South Sands, Tenby, West Wales. Due to beach aspect in relation to offshore islands, gale wave height decreased as wave direction rotated eastwards (r = 0.83) and westwards (r = 0.88). Similarly, wave heights were in attuned to variations in positive (r = 0.68) and negative (r = − 0.72) NAO Index, showing a wave height reduction occurred during weakly negative/positive or transitory phases; morphological change was attuned to atmospheric variation at a 2-year timelag. Shelter from offshore islands is given to waves from the predominant southwesterly direction and was confirmed by negligible correlation with South Sands morphology. However, outside the shelter of these offshore islands, correlation was found between south-eastward rotating wave directions (135°–180°) and morphological change, which resulted in southern and central beach erosion and accretion to the north. With a southwesterly rotation (243°–256°) the opposite was true. Beach rotation expressed by volume change within the sub-aerial zone had a negative phased relationship between beach extremities (r = − 0.94) and a timelagged association within the intertidal zone (r = 0.55). Analyses resulted in the development of two medium timescale rotation models based on incident wave direction and climatic variability. Results have global implications for headland bays in the lee of offshore islands, as well as macro-tidal beach areas; and consequently similar models could inform local, regional and national beach management strategies

Field Measurements of Wave Characteristics on a Near‐Horizontal Shore Platform, Mahia Peninsula, North Island, New Zealand

AbstractWaves are an important process responsible for the initiation and subsequent development of intertidal shore platforms. However, few field studies to date have described wave processes on shore platforms. A field experiment was conducted using an across‐shore array of wave gauges, including a directional sensor, on a near‐horizontal platform at Mahia Peninsula, North Island, New Zealand. Results show that the platform is very efficient in filtering wave energy at gravity wave frequencies (&gt;0.05 Hz), with 40–80% reduction in wave height measured across the 140 m wide platform. By contrast, infragravity waves are present on the platform and increase in magnitude towards the cliff toe. Directional wave analysis indicates that wave reflection is mainly restricted to infragravity wave frequencies, demonstrating the differences in gravity and infragravity wave behaviour on the platform studied. Results indicate that, under fair weather conditions, the role of waves as an agent of cliff toe erosion is likely to be limited at present, although they are probably important for removing sediments accumulated at the cliff toe. The observed increase in infragravity wave energy towards the cliff toe implies that these long‐period waves may be important geomorphic agents on shore platforms.

Short-term beach rotation, wave climate and the North Atlantic Oscillation (NAO)

Beach profile surveys, offshore wave climate and variations in atmospheric conditions have been utilized to assess a short-term beach rotation phenomenon in a headland embayment Tenby, West Wales. Beach rotation, expressed by subaerial volumetric change, was shown by a negative phase relationship between beach extremities (r = —0.67), while cross-correlation at a one-month timelag increased statistical significance (r = 0.84). Due to beach aspect, gale wave heights decreased as wave direction rotated to the south (R2 = 0.4) and west (R 2 = 0.65), while offshore wave direction influenced change at the southern and northern extremities (R2 = 0.52 and 0.34, respectively). Shelter from offshore islands and Giltar Headland contributes via wave diffraction to accretive, erosive and rotational patterns, and these are sensitive to variations around the predominant wave direction (229°). A southerly shift induces north/south sediment movement, as waves diffract around the offshore islands, while a westerly change results in south/north sediment movement (i.e. beach rotation), as diffracted wave domination transfers to the headland. A general gale wave height reduction occurred when the North Atlantic Oscillation (NAO) was weak or in a transitional phase between positive or negative phases (R2 = 0.69 and R2 = 0.72, respectively). Morphological change was also attuned to atmospheric variation where a reversal in beach rotation was influenced by variations in positive and negative NAO/volume correlations and longshore profile location (R2 = 0.54 and 0.69, respectively). The results of this study have wider implications for coastal management; it is suggested that models developed in similar systems elsewhere will form the basis of human intervention or no active intervention strategies.

Relationship between Teleconnection Pattern and Winter Wave Climate around Japan

The long-term change of waves is known to have some kind of relation with the climate change. In order to project long-term wave change due to climate change requires the understanding of the relationship between wave climate and teleconnection pattern since climate variability has strong relations with teleconnection behavior. This study examined the influence of teleconnection on winter wave climate around Japan. The Western Pacific (WP) pattern has the most important effect on waves over the region. Under future global warming climate, the trend of WP would shift to positive, which indicates wave height reduction around Japan.

Numerical implementation and sensitivity analysis of a wave energy converter in a time-dependent mild-slope equation model

Several Wave Energy Converters (abbreviated as WECs) have intensively been studied and developed during the last decade and currently small farms of WECs are getting installed. WECs in a farm are partly absorbing, partly redistributing the incident wave power. Consequently, the power absorption of each individual WEC in a farm is affected by its neighbouring WECs. The knowledge of the wave climate around the WEC is needed to predict its performance in the farm. In this paper a technique is developed to implement a single and multiple WECs based on the overtopping principle in a time-dependent mild-slope equation model. So far, the mild-slope equations have been widely used to study wave transformations around coastal and offshore structures, such as breakwaters, piles of windmills and offshore platforms. First the limitations of the WEC implementation are discussed through a sensitivity analysis. Next the developed approach is applied to study the wave height reduction behind a single WEC and a farm. The wake behind an isolated WEC is investigated for uni- and multidirectional waves; it is observed that an increase of the directional spread leads to a faster wave redistribution behind the WEC. Further the wake in the lee of multiple WECs is calculated for two different farm lay-outs, i.e. an aligned grid and a staggered grid, by adapting the performance of each WEC to its incident wave power. The evolved technique is a fast tool to find the optimal lay-out of WECs in a farm and to study the possible influence on surrounding activities in the sea.

Impact of surface waves in a Regional Climate Model

A coupled regional atmosphere-wave model system is developed with the purpose of investigating the impact of climate changes on the wave field, as well as feed-back effects of the wave field on the atmospheric parameters. This study focuses on the effects of introducing a two-way atmosphere-wave coupling on the atmosphere as well as on wave parameters. The model components are the regional climate model RCA, and the third generation wave model WAM. Two different methods are used for the coupling, using the roughness length and only including the effect of growing sea, and using the wave age and introducing the reduction of roughness due to decaying sea (swell). Introducing a two-way coupling results in an altered frequency distribution of wind speed and wave heights. When only including growing sea the impact of waves on the long term mean atmospheric parameters is limited, inducing a reduction of wind speed and significant wave height. When also the impact of swell is introduced, there is a shift towards higher wind speeds as well as higher significant wave heights in the four investigated areas. There is a reduction of surface heat fluxes and a decrease in near surface temperature as well as a significant increase in near surface humidity. The major conclusion is that when introducing a more realistic surface description over sea, the air-sea interaction represented by waves has a significant impact also on long term averages of parameters in the atmosphere. Waves should thus be introduced in climate models for a realistic description of processes over sea.

Boundary element modelling of wave diffraction by interaction with wave-offshore structure and dredged region

Boundary element modelling of wave diffraction by interaction with wave-offshore structure and dredged region The purpose of this study is to estimate the wave height at the front face of breakwater (Refracted breakwater and Straight breakwater), when dredging like the submarine pit is performed in the distant offshore from outer breakwater. The wave field of the problem is considered to be two dimensional planes and the configuration of the pit region is designated by a single horizontal long-rectangular system. The numerical approach uses the Green function based on the boundary integral approach. The results of the present numerical works are illustrated by applying the normal and inclined incidence. It is shown that in the case of normal incidence, the ratio of wave height reduction at the front face of both types of breakwaters is approximately more than 20% due to the effect of the submarine pit on the sea bed. Furthermore, regardless of the type of breakwater and the difference in incident wave angles, the ratio of wave height was shown to be reduced.

Impacts of jetty construction on the wave heights off the Kiashahr lagoon

Coastal managers and ecologists are confronted with tasks to estimate wave heights in the Kiashahr sea area, since hydrodynamic factors can profoundly impact this environment. In this study, wind data and an empirical wind wave predicting model were used to estimate deepwater wave characteristics in the Kiashahr sea area; while a numerical model was used to determine wave characteristics in shallow waters. Wave heights were also determined and compared at different stations before and after construction of 2 jetties at the lagoon mouth to evaluate their efficiency in wave height reduction. Results revealed that wave height is in the range of 0.8 to 1.2 m at Sefid Rud river mouth. Significant wave height reduction in the range of 0.15 to 1.2 m occurred at the entrance of the lagoon due to construction of the jetties. This attenuation of wave energy may result in some impacts on the ecosystem health of Kiashahr lagoon such as increased sedimentation, reduction in flushing rates, dumping of garbage and consequently, more pollution.

Throwing in the towel: When do adverse conditions dictate a weather day during a bottom trawl survey?

Abstract Fishermen and field biologists routinely work under difficult conditions. It is natural in this setting to continue fishing during bad weather as long as safety is not compromised. However, as conditions deteriorate, the effectiveness of trawl survey gear may be reduced well before safety considerations force the cessation of sampling activity. Through comparison of 1754 hauls conducted on chartered vessels between 2003 and 2005 on the shelf and slope of the U.S. West Coast, we show a significant reduction in catch rates of bottom dwelling fish and invertebrates as a function of visually estimated wave height. Comparing randomly located trawl sets that occurred fortuitously close to one another, we conclude that to reduce the estimated effect of wave height on benthic species catch rates (and potentially relative biomass estimates) survey trawl activities on these vessels should be halted when wave heights reach six feet (1.83 m). Only 41 hauls from the 2003–2005 data were conducted when wave heights exceeded five feet (1.52 m), however their exclusion reduces the maximum expected bias in any single tow due to wave height by ∼31%. Further reductions in maximum wave height for routine sampling would begin to exclude geographic areas of the survey introducing a new bias to the randomized design. We further encourage the routine electronic measurement of vertical motion due to wave action during sampling so that future analyses can improve our understanding of its effect on trawl survey catch rates.