Moderate Wave Climate Research Articles

Numerical Simulations of Effects of the Layout of Permeable Pile Groin Systems on Longshore Currents

Coastal permeable groins have been used to protect beaches from erosion for centuries. However, the hydraulic functioning of permeable groins has not been fully understood and their design heavily depends on engineering experiences. In this study, numerical experiments were executed to investigate the effects of layout configurations of a permeable groin system on longshore currents. The non-hydrostatic SWASH (Simulating WAve till SHore) model was employed to carry out the numerical simulations. Two data sets obtained from physical laboratory experiments with different permeable groin layouts on different slopes are used to validate the accuracy of the model. Then, the longshore current reduction by the permeable groin system with varying configuration parameters (e.g., groin spacing, groin length) was numerically investigated under different environmental conditions (e.g., a slight or a moderate wave climate). From the calculation results of numerical experiments, it is indicated that permeable groins function efficiently to reduce the maximal longshore current velocity under the condition that the groin length ranges from 84% and 109% of the wave breaker zone width. The longshore current reduction rate monotonously decreases with the increase in groin spacing; permeable pile groin functions best to reduce longshore current with the minimal groin spacing-groin length ratio 1:1 among the range between 1:1 and 2:1. When the groin spacing–groin length ratios are 1:1 and 1.5:1, the longshore current reduction is not sensitive to the investigated wave conditions in this study. When the spatial ratio is 2:1, the permeable pile groin system functions worse under a moderate wave climate than under a slight wave climate, from the view of longshore current reduction.

EXTREME WAVE CLIMATE OF THE NEW SOUTH WALES COAST

The NSW coast is subject to a relatively moderate wave climate that is periodically affected by large coastal storm events arising from a range of synoptic weather systems. Extreme wave events may cause coastal inundation, beach erosion, damage to property and marine structures, and risks to public safety. This paper describes the statistical analysis of the extreme directional wave climate along the NSW coast based on historical wave buoy data extending back to 1974. From the analysis, long-term trends in the extreme wave climate (storm peaks, storm duration and peak directions) are also identified. The work is largely an extension of the analysis completed by Shand et al (2011), that incorporates an additional 13 years of wave data up to 2022.

Exergy assessment and sustainability of a simple off-shore oscillating water column device

This paper present a research on the performance efficiency and sustainability of an Oscillating Water Column (OWC) simple off-shore device, accounting for the influence of governing thermodynamic variables (moisture, temperature, pressure) in the compression/expansion polytropic process. The work proposes a simple off-shore OWC experimental set up as the basis of the study. The analysis takes into consideration both gas subsystems inside and outside the OWC, to achieve a better understanding of the conservative nature of entropy system variable, the net exchange balance, the effects on efficiency and exergy destruction, and the interpretation of the OWC as a thermodynamic engine. Results show that, within the context of the set up, moderate wave climate conditions contribute to a better efficiency of the device in terms of output power, providing with a low impact on exergy destruction and high sustainability in terms of renewability index.

Application of integrated wave-to-wire modelling for the preliminary design of oscillating water column systems for installations in moderate wave climates

Oscillating water column (OWC) devices are the technical solution more favourably considered for a significant future exploitation of the energy potential of sea waves. While theoretically quite simple, these systems are indeed rather complex in terms of correct tuning between the caisson and turbine. Recent studies showed that only a holistic approach, also including turbine control, is the way of effectively performing their precise design.To this end, the research presents the application of an analytical wave-to-wire model of an OWC device for the preliminary optimisation of these systems for a small installation in a moderate wave climate in the Mediterranean Sea. More specifically, scatter matrices were determined for imposing the wave states of a selected location positioned in Tuscany (Italy). Regarding previous similar studies, a novel joint optimisation of the caisson and controlled turbine is proposed for the specific site. Either monoplane isolated Wells turbines and axial impulse turbines were analysed. The operating curves, performance parameters, and attended annual energy harvesting were calculated, providing an interesting overview of the functioning of the system. The optimised devices with the Wells and impulse turbomachines convert 36.71 and 42.17 MWh/year operating with a global efficiency of 6.82% and 7.84%, respectively.

Variability of Shoreline Water Levels on an Atoll in the Maldives

AbstractMany atolls in tropical oceans are under the threat of sea‐level rise. Mitigation planning requires reliable estimates of increasing wave runup in response to sea‐level rise and changes in wave climate. However, shoreline dynamics on reef‐lined coasts, particularly the effect of low‐frequency waves, are not yet fully understood. Herein, we present a field study on the southernmost atoll of the Maldives to clarify the variability of shoreline water levels driven by waves and tides. The atoll is located just south of the equator and has a moderate wave climate; it is occasionally impacted by long‐distance swells from the South Indian Ocean. Wave evolution on a reef‐lined coast was observed in situ for a period of seven months. The shallow water depth tightly constrains reef‐flat waves under normal wave conditions; however, this constraint is broken under extreme conditions by low‐frequency waves generated from long‐period swells. Particularly, very low‐frequency waves (<0.005 Hz) are amplified via wave resonance over the reef platform and significantly contribute to shoreline motions. Our observation of shoreline motions confirmed that, under energetic conditions, extreme wave run‐ups are characterized by swell run‐ups riding on crests of low‐frequency waves. A combined approach of observations and analytical modeling revealed the long‐term variability of shoreline water levels, highlighting the stochastic nature of extreme water levels influenced by a joint probability of far‐field swells and high tides. Our results suggested that extreme shoreline water levels would increase at a significantly higher rate than sea‐level rise in the future.

Sediment Exchange Between the Created Saltmarshes of Living Shorelines and Adjacent Submersed Aquatic Vegetation in the Chesapeake Bay

Rising sea levels and the increased frequency of extreme events put coastal communities at serious risk. In response, shoreline armoring for stabilization has been widespread. However, this solution does not take the ecological aspects of the coasts into account. The “living shoreline” technique includes coastal ecology by incorporating natural habitat features, such as saltmarshes, into shoreline stabilization. However, the impacts of living shorelines on adjacent benthic communities, such as submersed aquatic vegetation (SAV), are not yet clear. In particular, while both marshes and SAV trap the sediment necessary for their resilience to environmental change, the synergies between the communities are not well-understood. To help quantify the ecological and protective (shoreline stabilization) aspects of living shorelines, we presented modeling results using the Delft3D-SWAN system on sediment transport between the created saltmarshes of the living shorelines and adjacent SAV in a subestuary of Chesapeake Bay. We used a double numerical approach to primarily validate deposition measurements made in the field and to further quantify the sediment balance between the two vegetation communities using an idealized model. This model used the same numerical domain with different wave heights, periods, and basin slopes and includes the presence of rip-rap, which is often used together with marsh plantings in living shorelines, to look at the influences of artificial structures on the sediment exchange between the plant communities. The results of this study indicated lower shear stress, lower erosion rates, and higher deposition rates within the SAV bed compared with the scenario with the marsh only, which helped stabilize bottom sediments by making the sediment balance positive in case of moderate wave climate (deposition within the two vegetations higher than the sediment loss). The presence of rip-rap resulted in a positive sediment balance, especially in the case of extreme events, where sediment balance was magnified. Overall, this study concluded that SAV helps stabilize bed level and shoreline, and rip-rap works better with extreme conditions, demonstrating how the right combination of natural and built solutions can work well in terms of ecology and coastal protection.

Hydrodynamic performance of tire-based floating breakwater

The hydrodynamic characteristics (wave transmission, reflection, and dissipation) of nine different scrap-tire floating breakwater configurations are studied in a wave flume for a wide range of input wave conditions. The breakwater has 3 different configurations: a) four and b) eight rows of tires as well as c) four rows in two levels. The effect of these configurations is assessed on the hydrodynamic performance as determined by wave transmission and reflection. Wave transmission coefficients > 0.75 were obtained for relatively longer wavelengths. For relatively shorter wavelengths (d/L 0.4, where d is the water depth and L the wavelength), the wave transmission coefficients decreased to ≤ 0.25. Increasing the number of rows, from four to eight, reduced the coefficients of transmission and prevented resonance for both the longer and shorter wavelengths. Furthermore, increasing the number of layers, from one to two, increased the coefficient of reflection from 0.4 to 0.5. To predict the wave transmission coefficients for all configurations, multiple regression analysis was implemented to produce empirical equations. Based on the performance of all tested configurations with regular waves, an optimal configuration was selected to delay beach erosion under the influence of moderate wave climates.

Thermodynamics of the OWC chamber: Experimental turbine performance under stationary flow

Among the different devices designed to extract energy from waves, the Oscillating Water Column (OWC) operating an air turbine has been one of the most studied in recent years. The aim of this paper is to study the polytropic exponent associated to the thermodynamic process that takes place through the turbine in a non-idealised environment. A real gas model is applied, considering the influence of the moisture in the air chamber. Experimental data from a simplified OWC chamber set up under stationary flow are interpreted within the frame of an analytical real gas model. For that purpose, thermodynamic variables involved in the compression/expansion process have been calculated with the implementation of a real gas model. Following the results, in which differences between the ideal gas adiabatic polytropic process and the real gas model are observed, a new value for the polytropic exponent is proposed, representing a non–adiabatic real gas behaviour for the air–water vapour mixture. This approach allows a fine adjustment prediction of OWC performance, that can result more realistic in the case of OWC performance prediction under moderate wave climate conditions. From that on, new management guidelines can be developed, in which eventually moderate/low production prospects can be counterbalanced with more efficient and low–cost design.

Optimization of the geometry and the turbine induced damping for fixed detached and asymmetric OWC devices: A numerical study

This paper presents the set-up, the validation and the application of a two-phase incompressible three-dimensional Computational Fluid Dynamic (CFD) model for an extensive parameter study on the performance of a fixed, detached Oscillating Water Column (OWC) wave energy converter. The numerical study aims to assess the combined effect of relevant design parameters (chamber length, front wall draught, damping applied by the turbine) and of the wave conditions (wave height, wave period and water depth) on the device performance, with reference to its hypothetical installation in moderate wave climates (e.g. in the Central Mediterranean Sea). The OWC device examined here is fixed, detached from the seabed, and with asymmetric front and back walls as a characteristic feature. The results show that, by appropriately tuning the design parameters, a maximum value of the capture width ratio of approximately 0.87 can be achieved.

A Power Take-Off and Control Strategy in a Test Wave Energy Converter for a Moderate Wave Climate

The energy in the waves of oceans and seas can be converted to electricity by different types of Wave Energy Converters (WECs). Wave energy conversion is currently widely studied to contribute to the world’s rising energy needs. This paper describes a point absorber test WEC that was built for electrical energy production in moderate wave climates as can be found in the Belgian part of the North Sea. A robust design was put forward to assess the feasibility of a full electric rotational Power Take-Off (PTO) system. A stable reactive control algorithm was implemented to optimise the absorbed energy from the waves by tuning the natural frequency of the WEC towards the frequency of the waves. From simulations it is shown that also for real irregular waves, this tuning shows a significant beneficial effect on the absorbed energy. The control parameters for different wave conditions are discussed as well as the effect of the chosen PTO system and its constraints on the absorbed power and optimum control parameters.

Wave climate scatter performance of a cycloidal wave energy converter

A lift based cycloidal wave energy converter (WEC) was investigated using potential flow numerical simulations in combination with viscous loss estimates based on published hydrofoil data. This type of wave energy converter consists of a shaft with one or more hydrofoils attached eccentrically at a radius. The main shaft is aligned parallel to the wave crests and submerged at a fixed depth. The operation of the WEC as a wave-to-shaft energy converter interacting with straight crested waves was estimated for an actual ocean wave climate. The climate chosen was the climate recorded by a buoy off the north-east shore of Oahu/Hawaii, which was a typical moderate wave climate featuring an average annual wave power PW=17kWh/m of wave crest. The impact of the design variables radius, chord, span and maximum generator power on the average annual shaft energy yield, capacity factor and power production time fraction were explored. In the selected wave climate, a radius R=5m, chord C=5m and span of S=60m along with a maximum generator power of PG=1.25MW were found to be optimal in terms of annual shaft energy yield. At the design point, the CycWEC achieved a wave-to-shaft power efficiency of 70%. In the annual average, 40% of the incoming wave energy was converted to shaft energy, and a capacity factor of 42% was achieved. These numbers exceeded the typical performance of competing renewables like wind power, and demonstrated that the WEC was able to convert wave energy to shaft energy efficiently for a range of wave periods and wave heights as encountered in a typical wave climate.

Limanların Yüzer Dalgakıranla Modellenmesi

Floating breakwaters which have usually been constructed for marinas in the milder wave climate regions of Turkey can be an efficient alternative for the harbors with high water depth, steep bottom slope and poor soil conditions. This study presents a model for Candarli Container Terminal with a floating breakwater where the water depth is reaching to 40 m and the soil is too weak. However, breakwater of Candarli Container Terminal was designed as a combination of rubble mound breakwater with 800m length and piled breakwater with 1000 m length. According to the design, two rows of 90m length piles with a diameter of 1.778m will be driven with 0.2m distance between two piles. Although the construction of the breakwater has already been started in Candarli, in this study, floating breakwater scenario is studied to investigate the performance of the floating breakwater and the possibility of its usage for the harbors with moderate wave climate. The results of the model study show that the required wave agitation could be gained when a port is constructed through floating breakwater scenario

GENERATING ELECTRICITY AT A BREAKWATER IN A MODERATE WAVE CLIMATE

A new concept for wave energy conversion is examined as a proof of concept for generating electricity in a moderate wave climate while being integrated in a caisson breakwater. Physical model testing is performed to analyse the preliminary efficiency of the device and to identify areas of improvement. The resulting device is calculated for two sample locations in order to gain an understanding of its feasibility. Recommendations are made for continued research and possible improvements of the design.

Experimental simulation of shoreface nourishments under storm events: A morphological, hydrodynamic, and sediment grain size analysis

This study focuses on barred beach shoreface nourishments physically simulated in a wave flume. The attack of a schematic storm on three different nourishments is analysed. The apex and waning storm phases lead respectively to offshore and onshore sediment transports. Nourishments in the trough and on the outer bar feed the bar and increase wave dissipation offshore. The bar acts as a wave filter and reduces shore erosion (lee effect). In contrast, nourishment on the beach face leads mostly to shore feeding and reconstruction (feeder effect). With successive nourishments, the beach face clearly becomes steeper and onshore sediment transport is reduced during moderate wave climates. The surface grain size analysis reveals marked variations. Coarser sediments are sorted on the bar and the upper beach face. These locations correspond to large wave dissipation zones during the storm apex.

Contrasting Morphologic Behaviour at Embayed Beaches in Southern Portugal

Embayed beaches protected by rocky headlands evidence particular morphologic responses to episodic, seasonal and cyclic variations in external forcing depending on the degree of protection and shoreline orientation. This paper is concerned with the morphological behaviour of three embayed beaches (Cabanas Velhas, Boca do Rio and Salema) oil the rocky South coast of Portugal that are subject to a moderate wave climate and have low sediment supply. The selected beaches show different geomorphological settings and have been regularly surveyed for measurement of morphological variability. Results show that when storms impact on an accreted profile with a well-developed berm all three beaches were highly responsive, with differences depending oil coastal orientation and direction of incident waves. Severe erosion with depletion of sand and exposure of underlying layers of gravel and cobble or lag deposits of boulders Occurred under single storms and storm groups. After severe erosion during storms, Cabanas Velhas and Boca do Rio did not show subsequent beach recovery. This may result from sand transport to outside the coastal cell and lack of an effective sediment source that enables recovery. The bi-directional storm wave climate, with storms occurring both from SE and SW, and the reduced headland confinement produced differential sediment displacement along opposing ends of Salema beach. Morphological adjustments, morphodynamic processes and the contrasting behaviour between these three embayed beaches appears to be strongly controlled by geomorphological inheritance.

Wave energy potential in the Baltic Sea and the Danish part of the North Sea, with reflections on the Skagerrak

Wave power, along with renewable energy-generating sources like tides and streams, is underestimated considering its advantageous physical properties and predictability. This paper examines possible examples of wave power installations in the Baltic Sea and the Danish part of the North Sea. Hindcasting data is used allowing estimations of wave energy generated and results show promising areas in the North Sea, but also several parts of the Baltic Sea are of interest. The study is based upon linear generator technique, placed on the seabed using point-absorbers arranged in arrays of up to several thousand units. The study aims at showing the physical possibilities of wave energy, including economical feasibility and environmental advantages of wave energy even in moderate wave climates. With discussion from two examples in the Baltic Sea, one in the Danish North Sea and a new pilot study site in the Swedish part of Skagerrak, this study show feasible illustrations of wave energy takeouts. Project examples vary in size due to distance to grid, grid voltage, and may thus be economically feasible. Examples also show considerations in societal and nature conservation matters, including aspects such as industrial and military interests, archaeological or marine reserves and local geology. The authors conclude that wave energy electric conversion is an option that needs more attention and which has several advantages compared to conventional renewable sources. Sound engineering, in combination with producer, consumer and broad societal perspective is advised for a sustainable development of wave energy conversion.