Wave Hydrodynamics

Wave flume facilities that are primarily designed for engineering studies are often complex and expensive to operate, and hence not ideal for long-term replicated experiments as commonly used in biology. This study describes a low-cost small wave flume that can be used for biological purposes using fresh-or seawater with or without sediment. The wave flume can be used as a mesocosm to study interactions between wave hydrodynamics and benthic organisms in aquatic ecosystems. The low-costs wave maker (< 2000 USD) allows for experimental setups which can be easily replicated and used for longer term studies; hence the term wave mesocosm. Waves were generated with a pneumatic piston and wave heights ranged between 3 and 6 cm. Maximum orbital flow velocities ranged between 10 and 50 cm s −1 representing shallow coastal areas with a short fetch. The system can generate both regular waves (i.e., the wave period and orbital velocity remains constant), using a wave absorber, and irregular waves (i.e., varying wave period and orbital velocity) using a fast push and slow pull motion of the wave paddle. This wave mesocosm system is particularly useful in biogeomorphology to quantify interactions between organisms, sediment, and hydrodynamics and for aquatic ecologist aiming to simulate realistic bed shear stress where short-and long-term experiments (weeks-months) can be replicated.

An integrated cartographic approach has been used to summarize different data (geomorphological, sedimentological, hydrodynamic, ecological and anthropic) from an urban microtidal, wave-dominated beach and adjacent inner shelf in a comprehensive and easily readable mapping output. The study area is located in S Sardinia (Italy,  Mediterranean Sea) and focuses on Poetto beach. All the data in this study were processed to produce a Main Map (1:6400 scale) showing the key characteristics of the entire area and three detailed secondary maps (1:56,000 and 1:59,000 scale) that include topographic and eco-graphic profiles, the distribution of sedimentary facies and the main anthropic impact. This map, providing detailed information on the beach dynamics, human impact and the marine ecological status of the Poetto urban beach, represents a useful new tool to facilitate environmental conservation and beach management.

Many low-elevation, coral reef-lined, tropical coasts are vulnerable to the effects of climate change, sea level rise, and wave-induced flooding. The considerable morphological diversity of these coasts and the variability of the hydrodynamic forcing that they are exposed to make predicting wave-induced flooding a challenge. A process-based wave-resolving hydrodynamic model (XBeach Non-Hydrostatic, ''XBNH'') was used to create a large synthetic database for use in a ''Bayesian Estimator for Wave Attack in Reef Environments'' (BEWARE), relating incident hydrodynamics and coral reef geomorphology to coastal flooding hazards on reef-lined coasts. Building on previous work, BEWARE improves system understanding of reef hydrodynamics by examining the intrinsic reef and extrinsic forcing factors controlling runup and flooding on reef-lined coasts. The Bayesian estimator has high predictive skill for the XBNH model outputs that are flooding indicators, and was validated for a number of available field cases. It was found that, in order to accurately predict flooding hazards, water depth over the reef flat, incident wave conditions, and reef flat width are the most essential factors, whereas other factors such as beach slope and bed friction due to the presence or absence of corals are less important. BEWARE is a potentially powerful tool for use in early warning systems or risk assessment studies, and can be used to make projections about how wave-induced flooding on coral reef-lined coasts may change due to climate change. Plain Language Summary Low-lying tropical coasts fronted by coral reefs are threatened by the effects of climate change, sea level rise, and flooding caused by waves. However, the reefs on these coasts differ widely in their shape, size, and physical characteristics; the wave and water level conditions affecting these coastlines also vary in space and time. These factors make it difficult to predict flooding caused by waves along coral reef-lined coasts. We created a system (''BEWARE'') that estimates how different wave, water level, and reef combinations can lead to flooding. This tool tells us what information is needed to make good predictions of flooding. We found that information on water levels and waves is most important, followed by the width of the reef. BEWARE can be used to make short-term predictions of flooding in early warning systems, or long-term predictions of how climate change will affect flooding caused by waves on coral reef-lined coasts.

Coastal vegetation is widely attributed to stabilize sediment. While most studies focused on how canopy causes flow reduction and thereby affects sediment dynamics, the role of roots and rhizomes on stabilizing the surface sediment has been less well studied. This study aims to quantify interactions between above-and below-ground biomass of eelgrass (i.e., living Zostera marina plants and mimics) with surface sediment erosion (i.e., bed load and suspended load), under different hydrodynamic forcing that was created using a wave flume. Belowground biomass played an important role preventing bed-load erosion, by roughly halving the amount of sediment transported after being exposed to maximal orbital velocities of 27 cm s −1 , with and without canopy. Surprisingly, for suspended sediment transport, we found opposite effects. In the presence of eelgrass, the critical erosion threshold started at lower velocities than on bare sediment, including sand and mud treatments. Moreover, in muddy systems, such resuspension reduced the light level below the minimum requirement of Z. marina. This surprising result for sediment resuspension was ascribed to a too small eelgrass patch for reducing waves but rather showing enhanced turbulence and scouring at meadow edges. Overall, we conclude that the conservation of the existent eelgrass meadows with developed roots and rhizomes is important for the sediment stabilization and the meadow scale should be taken into account to decrease sediment resuspension.

Existing hydrodynamic flow transport equations for coastal boulder transport are useful for estimating post-event the characteristics of extreme storm waves and tsunamis. However, the effect of suspended sediment concentration (SSC) on seawater density is normally ignored. This is unrealistic given that turbulent runup flows easily entrain available fine sediment. Proper consideration of SSC can be encouraged by including a mixed-fluid density coefficient (C ρ) as a multiplier for clear-seawater density, where elevated sediment content can be assumed and estimated. Minimum flow velocities required for boulder transport are shown to reduce as sediment concentrations increase.

The present work describes an operational strategy for the development of a multiscale modeling system, based on a multiple-nesting approach and open-source numerical models. The strategy was applied and validated for the Gulf of Taranto in southern Italy, scaling large-scale oceanographic model results to high-resolution coupled wave–3-D hydrodynamics simulations for the area of Mar Grande in the Taranto Sea. The spatial and temporal high-resolution simulations were performed using the open-source TELEMAC suite, forced by wind data from the COSMO-ME database, boundary wave spectra from the RON buoy at Crotone and results from the Southern Adriatic Northern Ionian coastal Forecasting System (SANIFS) regarding sea levels and current fields. Model validation was carried out using data collected in the Mar Grande basin from a fixed monitoring station and during an oceanographic campaign in October 2014. The overall agreement between measurements and model results in terms of waves, sea levels, surface currents, circulation patterns and vertical velocity profiles is deemed to be satisfactory, and the methodology followed in the process can constitute a useful tool for both research and operational applications in the same field and as support of decisions for management and design of infrastructures.

Seagrasses and bare sediment represent alternative stable states, with sediment resuspension being a key driver of system stability via the Seagrass-Sediment-Light (SSL) feedback. We explore the SSL feedback by quantifying the sediment stabilization by seagrass, and using these measurements to calculate under which conditions seagrass ends up in a turbid environment. We quantified in-situ sediment resuspension velocity thresholds (u cr) for Zostera marina growing in medium to fine sand, using a field flume inducing near-bed wave motion. u cr was determined for full length shoots, shoots clipped to 0.08 m, and removed shoots. We found that rhizomes did not influence u cr of the top sediment layer. Overall, u cr was linearly related to blade area, which became independent for sediment type when normalizing u cr for the resuspension threshold after shoot removal. Comparing measured u cr against natural wave conditions showed that the seagrass meadow at the study site is currently stable. Exploring the effects of changing hydrodynamic conditions revealed that effects of increasing storminess has limited influence on sediment resuspension and thus the SSL-feedback. Increasing mean wind velocity had a stronger influence on SSLfeedback dynamics by causing more frequent exceedance of u cr. The response of seagrasses to increasing wind pressure depends on bay topography. A fully exposed Z. marina meadow under low initial turbidity pressure trended toward bistability, as turbidity pressure increased mainly on bare sediments. The study site and a fully exposed Z. marina meadow under high initial turbidity pressure saw an increase in turbidity across all blade areas.

In the present work, an integrated coastal engineering numerical model is presented. The model simulates the linear wave propagation, wave-induced circulation, and sediment transport and bed morphology evolution. It consists of three main modules: WAVE_L, WICIR, and SEDTR. The nearshore wave transformation module WAVE_L (WAVE_Linear) is based on the hyperbolic-type mild slope equation and is valid for a compound linear wave field near coastal structures where the waves are subjected to the combined effects of shoaling, refraction, diffraction, reflection (total and partial), and breaking. Radiation stress components (calculated from WAVE_L) drive the depth averaged circulation module WICIR (Wave Induced CIRculation) for the description of the nearshore wave-induced currents. Sediment transport and bed morphology evolution in the nearshore, surf, and swash zone are simulated by the SEDTR (SEDiment TRansport) module. The model is tested against experimental data to study the effect of representative coastal protection structures and is applied to a real case study of a coastal engineering project in North Greece, producing accurate and consistent results for a versatile range of layouts.

It is shown that steady large-scale slowly eastward-moving twin-cyclone coherent structures, the equatorial modons, exist in both one-and two layer versions of the rotating shallow water model on the equatorial beta plane. They arise via the process of "ageostrophic adjustment" from the analytic asymptotic modon solutions of the vorticity equation obtained in the limit of small pressure perturbations. Evolution of these structures in adiabatic and moist-convective environments, and also in the presence of topography is analyzed, showing their robustness in the one-layer model. It is demonstrated that moist convection enhances and helps maintain the modons. In the two-layer model the barotropic and quasi-barotropic modons display similar to one-layer modon features, while increasing baroclinicity leads to eventual loss of coherence and arrest of the eastward propagation. Some features of equatorial modons resemble those observed in the Madden-Julian Oscillation events in tropical atmosphere, which hints at their possible relevance to the dynamics of this phenomenon.

Rapid coastal erosion threatens Arctic coastal infrastructure, including communities and industrial installations.  Erosion of permafrost depends on numerous processes, including thermal and mechanical behaviour of frozen and unfrozen soil, nearshore hydrodynamics, atmospheric forcing, and the presence of sea ice.  The quantification and numerical modelling of these processes is essential to predicting Arctic coastal erosion.  This paper presents a case study of Baydaratskaya Bay, Russia, using the COSMOS numerical model to predict thermal-mechanical erosion.  In particular, this study focuses on thermoabrasional rather than thermodenudational processes.  A field dataset of onshore thermal and mechanical soil characteristics was supplemented by sources from the literature to serve as input for the model.  A detailed sensitivity analysis has been conducted to determine the influence of key parameters on coastal erosion rates at the study site.  This case study highlights the need for expanded data collection on Arctic coastlines and provides direction for future investigations.

To improve the seakeeping capability, some devices, such as submerged plates, are often installed on floating structures. The attached plate can not only suppress the motion response but also provide an additional immersed body surface that receives fluid action, aggravating the wave loads. In this study, a theoretical model is developed within the context of linear potential theory to study the hydrodynamic characteristics of a floating column with a submerged plate attached at the bottom. The eigenfunction expansion matching method is applied to obtain the velocity potential, based on which the linear wave force and wave runup can be found immediately. A novel derivation of the mean drift force formulation is developed via the application of Green's second identity to the velocity potential and its derivative in finite fluid volume surrounding the body. Mean drift force formulation that involves control surfaces is then obtained. With the availability of the velocity potential, semi-analytical solution of the mean drift force on the compound column-plate structure is developed based on, respectively, the derived and the classic far-field formulations. After conducting convergence tests and validating the theoretical model, detailed numerical analysis is performed thereafter based on the theoretical model. The influence of the plate size, such as the radius and height, on the wave force and the associated wave runup is assessed.

The renewal energy technologies are increasingly  popular to ensure future energy sustenance and address environmental issues. The tides are enormous and consistent untapped resource of renewable energy. The growing interest in exploring tidal energy has compelling reasons such as security and diversity of supply, intermittent but predictable and limited social and environmental impacts. The tidal energy industry is undergoing an increasing shift towards diffuser augmented turbines. The reason is the higher power output of diffuser augmented turbines compared to conventional open turbines. The purpose of this study is to present a comprehensive review of diffuser augmented horizontal axis tidal current turbines. The components, relative advantages, limitations and design parameters of diffuser augmented horizontal axis tidal current turbines are presented in detail. CFD simulation of NACA 0016 airfoil is carried out to explore its potential for designing a diffuser. The core issues associated with diffuser augmented horizontal axis tidal current turbines are also discussed.

In this paper a 3D numerical model was developed to
study the complicated interaction between waves and a set of
tandem fixed cylinders. The fluid was considered to be inviscid and
irrotational. Therefore, the Helmholtz equation was used as a
governing equation. The boundary element method (BEM) was
adopted to discretize the relevant equations. Open boundaries were
used in far fields of the study domain. Linear waves were generated
and propagated towards tandem fixed cylinders to estimate the
forces applied on them. Special attention was paid to consideration
of the effect on varying non-dimensional cylinder radius and
distance between cylinders, ka and kd on forces and trapped modes.
The middle cylinder wave forces and trapped modes in a set of nine
tandem cylinders were validated utilizing analytical data. The
comparisons confirm the accuracy of the model. The results of the
inline wave force estimation on n tandem cylinders show that the
critical cylinder in the row is the middle one for odd numbers of
cylinders. Furthermore the results show that the critical trapped
mode effect occurs for normalized cylinder radiuses close to 0.5
and 1.0. Finally the force estimation for n tandem cylinders
confirms that force amplitude of the middle cylinder versus
normalized separation distance fluctuates about that of a single
cylinder.

A diffraction-radiation analysis is usually required when the hydrodynamic interactions between structural members occur in short waves. For bracings or small cylindrical members, which play important roles in the vicinity of the natural frequency of a floating platform, special care should be taken into account for the effect of viscous damping. Two hybrid panel-stick models are, therefore, developed, through the combination of the standard diffraction-radiation method and the Morison's formulae, considering the effect of small members differently. The fluid velocity is obtained directly by the panel model. The viscous fluid force is calculated for individual members by the stick model. A semi-submersible type platform with a number of fine cylindrical structures, which is designed as a floating foundation for multiple wind turbines, is analyzed as a numerical example. The results show that viscous force has significant influence on the hydrodynamic behavior of the floating body and can successfully be considered by the proposed hybrid models.

The world is heavily dependent on fossil fuels since most of its energy requirements are fulfilled by conventional methods of burning these fuels. The energy demand is increasing by day with growing population. The energy production by fossil fuels is devastating the environment and survival of life on globe is endangered. The renewal energy technologies are vital to ensure future energy sustenance and environmental issues. Ocean is a vast resource of renewable energy. The technology today makes it possible to extract energy from tides. The growing interest in exploring tidal current technologies has compelling reasons such as security and diversity of supply, intermittent but predictable and limited social and environmental impacts. The purpose of this study is to present a comprehensive review of tidal current technologies to harness ocean energy. The ocean energy resources are presented. The author discusses tidal energy technologies. The tidal current turbines are discussed in detail. The author reviews today’s popular tidal current technologies. The present status of ocean energy development is also reported.

This work presents the results of the numerical study implemented for the natural area of Lido di Spina, a touristic site along the Italian coast of the North Adriatic Sea, close to the mouth of River Reno. High-resolution simulations of nearshore dynamics are carried out under climate change conditions estimated for the site. The adopted modeling chain is based on the implementation of multiple-nested, open-source numerical models. More specifically, the coupled wave-2D hydrodynamics runs, using the open-source TELEMAC suite, are forced at the offshore boundary by waves resulting from the wave model (SWAN) simulations for the Adriatic Sea, and sea levels computed following a joint probability analysis approach. The system simulates present-day scenarios, as well as conditions reflecting the high IPCC greenhouse concentration trajectory named RCP8.5 under predicted climate changes. Selection of sea storms directed from SE (Sirocco events) and E-NE (Bora events) is performed together with Gumbel analysis, in order to define ordinary and extreme sea conditions. The numerical results are here presented in terms of local parameters such as wave breaking position, alongshore currents intensity and direction and flooded area, aiming to provide insights on how climate changes may impact hydrodynamics at a site scale. Although the wave energy intensity predicted for Sirocco events is expected to increase only slightly, modifications of the wave dynamics, current patterns, and inland flooding induced by climate changes are expected to be significant for extreme conditions, especially during Sirocco winds, with an increase in the maximum alongshore currents and in the inundated area compared to past conditions.

In order to understand the current weather pattern of the Sundarban, a comprehensive data base of meteorological parameters were collected from Satjelia Island of Sundarban and analyzed for a one year period and a long term set up has been commissioned to understand the climate change patterns of the region.

This study is focused on diffuser augmented tidal current turbines that capture the kinetic energy in a tidal stream. The energy that  can be extracted from tides is proportional to the cube of the current velocity. The role of the diffuser in diffuser augmented tidal turbines is to help accelerate the incoming current velocity. Consequently, the  efficiency of the turbine can be  significantly increased by using a diffuser. The research community is investing considerable time and financial resources in this growing domain. The diffuser augmented tidal turbines research data is rather scarce due to their emerging nature, large and costly research and development setup, start up cost and proprietary issues. The purpose of this study is to investigate the effect of length and angle on NACA 0018 airfoil for diffuser design. CFD simulation is carried out to investigate velocity and mass flow rate at the throat. The drag force due to diffuser installation is also calculated. Velocity inside the diffuser increases with diffuser length and angle of attack. Velocity increases up to stall angle and then drops due to flow separation. The drag force is also dominant compared to lift coefficient near stall angle region.

Tidal energy is one of the most predictable forms of renewable energy. Tides posses both potential and kinetic energy. Tidal energy can be utilized by capturing potential energy i.e. by means of tidal barrage and tidal fence or by capturing kinetic energy i.e. by means of tidal current technologies. This study is focused on diffuser augmented tidal turbines that capture the kinetic energy. The power generated by a tidal turbine is directly proportional to the cube of velocity of current flow. The role of the diffuser in diffuser augmented tidal turbines is to help accelerate the incoming current velocity. Consequently, the efficiency of the turbine is significantly increased by using a diffuser. The research community is investing considerable time and financial resources in this growing domain. The diffuser augmented tidal turbines research data is rather scarce due to their emerging nature, large and costly research & development setup, start up cost and proprietary issues. The purpose of this paper is to study the effect of length and angle on NACA 0010 airfoil for diffuser design. Numerical simulation is carried out to investigate velocity and mass flow rate at the throat. The drag force due to diffuser installation is also calculated.

Low-lying tropical islands are highly vulnerable to the effects of sea-level rise and climate change. Most pressing is the threat posed to their fresh water supplies by wave-induced flooding. This thesis attempts to generalize previous site-specific studies of flooding on coral atolls and apply it in a framework that can be used for early warning systems or long-term climate change impact studies. To do so, a large synthetic database of representative reef properties and hydrodynamics was developed using the numerical wave model XBeach, and then analyzed using a Bayesian probabilistic network. The resulting tool allows us to make real-time flood predictions based on offshore wave or sea level conditions, and the unique characteristics of a given island (e.g. topography). Narrow, smooth reefs with steep fore reef slopes are the most vulnerable to runup. Extreme flooding is associated with anomalously high, resonant low frequency waves, which are more likely to occur on narrow, smooth reefs subjected to extreme swell waves (large wave heights with long periods). These extreme swells are "blue sky" events which originate from distant storms. Thus, they may thus arrive without warning, since they are completely independent of local weather conditions. This bolsters the need for more effective prediction tools. Validation of the results presented in this study is limited by the small number of field observations against which the model can be compared. Thus, there is a need to develop a comprehensive database of reef morphology and hydrodynamics. Offshore wave conditions, water levels, and reef width are the most essential variables for predicting runup, so future research efforts should be directed towards collecting those data. This model can also be used to ask questions like, "which islands will be most severely impacted by climate change?", or "can we increase flood resilience for a given island by restoring its coral reefs?". Hence, there is also potential for its use in guiding decision-makers to allocate limited funding in the places where it will have the most impact.

Diffuser augmented tidal turbines are getting enormous attention due to their immense potential to increase the generated power output. Researchers around the globe are investing  considerable time and financial resources in this domain. Limited  research  results  are  available  for diffuser augmented tidal turbines due to their emerging nature, large and costly research and development setup, start up cost and proprietary issues. Turbine enclosed in a diffuser  is based on the principle that the generated power output by a tidal turbine is directly proportional to the cube of velocity of incoming fluid flow. Thus, even a minor increase in velocity considerably increases the generated power output. The diffuser helps accelerate the incoming fluid flow. Hence, the efficiency of the turbine is significantly increased by using  a diffuser. It is challenging to accelerate the  incoming  flow by using a diffuser due to its shape, geometry and fabrication limitations. The  diffuser design requires great deal of innovation and time investment. The purpose of this paper is to present the study of  2D model of diffuser for tidal current turbine. The study involves developing a 2D CFD model of diffuser, acquiring simulation results and comparison with experimental results. The mesh is generated in ICEM followed by simulation in CFX. The simulation results are compared  to experimental results and found in reasonable agreement. The research is essential to utilize CFD tools for diffuser design used for tidal current turbine.

Researchers and engineers around the globe are striving to improve green energy technologies. Among green energy technologies, diffuser augmented tidal turbines are attracting focus due to enormous potential for producing energy. The power output by a tidal turbine is directly proportional to the cube of velocity of incoming fluid flow. Thus, even a minor increase in velocity considerably increases the power output. The diffuser helps accelerate the velocity of incoming fluid flow. Hence, the efficiency of the turbine is significantly increased by using a diffuser. It is challenging to accelerate the incoming flow by a diffuser due to its shape, geometry and fabrication limitations. The diffuser design requires great deal of innovation and time investment. The research community is investing  considerable time and financial resources in this arena. However, limited  research results  are  available  for diffuser augmented tidal turbines due to their emerging nature, large and costly research & development setup, startup cost and proprietary issues. The purpose of this paper is to present the numerical simulation of 2D model of diffuser for tidal turbine. Numerical simulation results of velocity profile for fifteen models with different mesh sizes is presented in detail. The effect of mesh density on coefficient of velocity is discussed. Predicted results are then compared to experimental results and found in reasonable agreement. The research is essential for utilizing CFD tools for diffuser design for tidal turbine.

The Hanson Larson evolution model assumes that wave height grows or decays from an initial condition towards an equilibrium wave height through a simple exponential growth. The model uses wind input data, fetch length and depth measurements. Using this input parameters, it both determines the wave height and period values, thus predicts wave climate. The model is developed using deep water conditions where group velocity is assumed be constant.

The model is validated using measured wave data from the Baltic Sea. The wave and input wind data are retrieved from the Swedish meteorological and hydraulic institute’s (SMHI) data base.  The wind stations used in the analysis have a correlation coefficient of 0.72. The wind inputs were also checked for their reliability to represent the wind condition at the wave buoy.

This was done by comparing the wind speed and wind direction measured at the wind stations. Whenever the difference between speeds was less than 5 m/s and wind direction difference less than 45 degrees, the wind input was taken as reliable.

The wave heights calculated were in a fair agreement with observed values for most of the trial runs, except when wind direction was abruptly changing. The error observed was more for wave periods as compared to wave height, but the growth and decay pattern calculated by the model was similar as the observed pattern. 

Other two more models based on wave energy analysis are also tested as to predict wave height and wave period, from wind input data. In comparison to the Hanson Larson evolution model, these models resulted in more disagreement between calculated and observed values, rendering them less useful.