Action Of Sea Waves Research Articles

SAR Imaging of Swing Ships Based on Two-Dimensional Spatial Variance Correction

The ship on the sea has the phenomenon of swing under the combined action of sea waves, wind and its stability system in the case of synthetic aperture radar (SAR) imaging, which makes it difficult to focus. In order to correct the two-dimensional (2-D) spatial variance and accomplish the imaging of swing ship, this paper proposes a new imaging algorithm based on spatial variant phase gradient autofocus (SV-PGA). Firstly, the entropy difference is used to determine the specific position of the ship. Then the ship image is extracted and segmented by a variable-size segmentation strategy. PGA is used to focus each sub image, and a well-focused ship image is obtained. The results of simulation experiments and real data show that the algorithm has good focusing performance.

Mineralogy, Geochemistry and Provenance of Coastal Sands from Greece: New Insights on the REE Content of Black Coastal Sands from Aggelochori Area, N.-Greece

Beach sands from Aggelochori coast line are investigated for their geochemistry and REE content, mineralogy and their provenance. These fluvial sands bear heavy minerals enriched horizons (containing minerals such as magnetite, zircon, ilmenite, hematite, rutile and titanite) that can be distinguished due to their black color and are formed usually due to the action of sea waves that deposit the heavy minerals and remove the lighter ones. After a suitable processing (washing, sieving, drying and magnetic separation) of the samples, the mineral constituents and their presence (wt.%) were estimated by XRD. Among the samples, the one being simultaneously the more fine grained and the more zircon-enriched (as suggested by XRPD data and optical microscopy analysis) has been selected for further geochemical analyses. The major and trace elements contents were compared to previously studied REE enriched beach sands from Kavala and Sithonia. Beach sands from Aggelochori area appear to have relatively low REE contents. Considering the provenance of these sediments, we suggest that these sands, are a product of the erosion of multi-sources, including the near-by Monopigado granite, as well as metamorphic rocks, as indicated by the presence of rutile and both ilmenite and magnetite in some samples. Therefore, there are indications of a complex flow pattern that existed at the paleo-catchment area of the deposition.

Non-cohesive and cohesive sediment transport due to tidal currents and sea waves: A case study

A model is proposed to determine the sediment transport rate generated by the combined action of sea waves and tidal currents, the latter being modelled as a sequence of steady currents because of the large values of the Keulegan-Carpenter number of the tidal flows, which allow to neglect inertial effects in their modelling. The hydrodynamic module solves Reynolds-averaged momentum equations by introducing a two-equation turbulence model, which considers also the flow in the region closest to the sea bottom and enforces the no-slip condition and the vanishing of the turbulent kinetic energy at the bottom without the need to assume the existence of a logarithmic velocity profile and to fix the normal derivative of the turbulent kinetic energy close to the sea bed. Hence, the evaluation of the velocity profile in the near-bed region turns out to be accurate. The sediment transport module determines the sediment concentration and sediment transport for a mixture of non-cohesive and cohesive sediments. To validate the model, its predictions are compared with laboratory measurements and field data collected over rippled beds at two different tidal estuaries. Then, results are obtained for values of the parameters chosen to mimic a site close to Punta Umbría (Huelva, Spain) and considering different hydrodynamic conditions and different sediment mixtures. The influence of the waves on the suspended sediment concentration is lower for the cohesive fraction than for the non-cohesive fraction. For all the sediment mixtures, it turns out that the cohesive fraction is well mixed over the whole water depth, meanwhile the concentration of the non-cohesive fraction has significant values only near the bottom, thus showing that an accurate description of turbulence dynamics in the buffer layer and viscous sublayer (if it exists) is important to quantify the sediment transport rate.

Viscous Effects and Energy Recovery Optimization for Freely Floating and Bottom Fixed Wave Energy Converters

This work deals with the study of two types of wave energy converters (WEC’s) taken as representative models of the bottom fixed systems and the freely floating ones. The considered freely floating WEC (FFWEC), a multi-body articulated system, consists of two horizontal cylinders connected with a flat plate and with axis parallel to the waves. The bottom fixed WEC, a point absorber (PAWEC), consists of a unique horizontal cylinder oscillating under the action of sea waves and connected to the seabed through an extensible Power Take Off device. For each system, a mathematical model is developed and is based on a balance of forces involving, in addition to the gravity and the Archimedean thrust, the Morison force, representing the added mass and damping effects as well as additional viscous effects. An evolutionary algorithm based method is used for the identification of the appropriate values of added mass and drag coefficients. The importance of the optimization of the sizing parameters for each of the systems with respect to the energy recoverable by the WEC’s is shown, and an evolutionary algorithm method is also used for their optimization. For the PAWEC, the results shows that the recovered energy is a decreasing function of the drag coefficient and that the decay is more marked when the wave pulsation increase. This decrease remains however limited and is about 5% when varies from 0 to 6. However, for the FFWEC the results show a less predictable situation as the drag coefficient increase due to the fact that the FFWEC model is more sensitive to a parameter’s variation. For the FFWEC, it is found that the viscous effects can act in both directions, in favor or against a recovery of energy recovery according to the range of values of the other coefficients. It is also important to note that taking account of viscous effects can lead to corrections of more than 50% in the calculation of the energy recovered by the FFWEC.

Numerical investigation of the stability of armour units in low-crested breakwaters using combined SPH–Polyhedral DEM method

Low-crested breakwaters have become more attractive because they do not hinder the beautiful coastal landscape and are more environmentally friendly comparing to traditional breakwaters. The available formulae for design of these types of structures mainly focus on wave transmission, reflection and dissipation aspects. However, stability of the armour blocks are not directly taken into account in the design process. In this study the Lagrangian meshfree method of SPH along with a discrete element method i.e. Polyhedral DEM are used to simulate the interaction between the sea water waves with different characteristics and low-crested breakwaters constructed using cubic armour blocks. SPH is used for computing the wave forces and moments applied to each armour unit subjected to sea wave action under various conditions. The fluid velocity and pressure time series near the armour units are calculated and based on those, the applied forces and moments are determined. Stability analysis for armour units are conducted by employing DEM which can determine whether the armour units can be displaced by wave action, a phenomenon that causes failure of the whole breakwater over time. Based on the results of a large number of numerical simulations, a practical non-dimensional relationship is proposed for calculating the required dimension of the cubic armour units that can be used for preliminary design of low-crested breakwaters. The newly proposed relationship can fill the gap that currently exists in the literature regarding using cubic armour units for this type of marine structures.

A preliminary study on coastal debris in Nallathanni Island, Gulf of Mannar Biosphere Reserve, Southeast coast of India

Nine sampling stations were fixed to carry out the preliminary investigation on coastal debris from Nallathanni island, Gulf of Mannar Biosphere Reserve, Southeast coast of India. The coastal debris were separated and identified using hand picking, visual identification and microscopic studies. The coastal areas of the study region were less dominated by micro plastic contamination. The coastal debris was dominated by polyethylene bottles and fibrous fishnet materials. The distribution of the coastal debris along the coastal region is chiefly controlled by winnowing action of sea waves and Aeolian action along the berm region.

Monitoring the tidal response of a sea levee with ambient seismic noise

Internal erosion, a major cause of failure of earthen dams and levees, is often difficult to detect at early stages using traditional visual inspection. The passive seismic-interferometry technique could enable the early detection of internal changes taking place within these structures. We test this technique on a portion of the sea levee of Colijnsplaat, Netherlands, which presents signs of concentrated seepage in the form of sandboils. Applying seismic interferometry to ambient noise collected over a 12-hour period, we retrieve surface waves propagating along the levee. We identify the contribution of two dominant ambient seismic noise sources: the traffic on the Zeeland bridge and a nearby wind turbine. Here, the sea-wave action does not constitute a suitable noise source for seismic interferometry. Using the retrieved surface waves, we compute time-lapse variations of the surface-wave group velocities during the 12-hour tidal cycle for different frequency bands, i.e., for different depth ranges. The estimated group-velocity variations correlate with variations in on-site pore-water pressure measurements that respond to tidal loading. We present lateral profiles of these group-velocity variations along a 180-meter section of the levee, at four different depth ranges (0m–40m). On these profiles, we observe some spatially localized relative group-velocity variations of up to 5% that might be related to concentrated seepage.

Geochemistry of beach sands from Kavala, Northern Greece

Thirty-seven beach sand samples from the shoreline close to Kavala pluton were analyzed for REE and major element contents. Results are compared with the adjacent plutonic rock-types, in order to determine any enrichments or depletions. Among the samples, several are significantly enriched in REE, being seasonal deposits of heavy minerals and their concentrations are controlled by the action of sea-waves and local winds. The available geochemical characteristics were also used to confirm the parent rocks of the beach sands, which were the rock-types of the Kavala pluton. The heavy fractions (total, total magnetic and total non-magnetic) of the beach sands are moderately correlated with the REE concentrations. Among the minerals of the heavy non-magnetic fraction, allanite seems to control the REE content in the heavy mineral-enriched samples.

Characterizing the 2D shape complexity dynamics of the islands of Sundarbans, Bangladesh: a fractal dimension approach

Fractal dimension index (FDI) and shape index (SI) of the islands of Bangladesh Sundarbans situated at the estuarine part of the Ganges–Brahmaputra–Meghna (GBM) delta were estimated with the aid of remote sensing techniques between the years 1989 and 2010. The main objective was to correlate the temporal change in these two indices within two decades with the possible geomorphic processes regulating these patterns. The Sundarban region is composed of almost 200 islands having self-similar coastlines with FDI and SI ranged from 1.002 to 1.202 and 1.016 to 7.546, respectively. Both FDI and SI values along with their temporal change showed that the complexity and irregularity in the island shape increased in the northern and north-eastern parts of the Sundarbans, while in the southern end it exhibited an opposite trend. Apart from a few exceptions, the longer serrated coastlines of the north (FDI > 1.18) became more serrated and rough due to the predominant tidal effect (particularly ebb tidal flow) within the two decades. The comparatively smoother coastlines of the southern islands (FDI < 1.18) became more compact and circular due to strong sea wave action during the same course of time. The spatial variation in FDI was explained in terms of the relative elevation of the islands, which was found to be lower in the northern part, their differential response to rising sea level and the amplified tidal action in this funnel shaped estuary. Apart from changes in fractal dimension, a net land loss of ≈ 60 km2 was observed within this island system during the study period. These observations imply that with an increasing sea level rise, the northern and southern parts of Sundarbans Island system will respond differentially, resulting in inundation in northern area (water encroachment in the low-lying islands) and land loss in the southern part (erosion in the coastal boundary) causing a severe loss of ecosystem services provided by the mangrove forest.

A new time domain Rankine panel method for simulations involving multiple bodies with large relative displacements

Many important offshore operations involve multiple vessels whose relative positions change considerably in the course of the operational procedures. When the action of sea waves on the dynamics of these vessels is concerned, an accurate modeling of the hydrodynamic problem is not an easy task. The use of frequency-domain panel codes is compromised by the variations of the bodies’ relative positions and may lead to inconsistent results. A time-domain approach is clearly more appropriate, but even in this case the representation of the moving bodies requires a re-meshing procedure and considerable computational efforts. This paper addresses the problem of wave interaction with multiple bodies with large relative displacement by integrating a re-meshing procedure to a time-domain Rankine panel code. The mathematical algorithms adopted for generating the new free-surface grids and performing the interpolation of wave elevation and velocity potential are described in detail. The paper also presents a set of dedicated captive model tests performed in a wave basin as a means of verifying the performance of the numerical method. Tests involved models with simple geometries that performed slow prescribed drift motions in the presence of incoming regular waves. Hydrodynamic forces were measured on one of the models and different levels of transient interaction effects could be reached by varying the frequency ratios between waves and body motions. The good performance of the method is then attested by directly comparing the hydrodynamic forces predicted by the computational code with those measured during the model tests. Moreover, it will be shown that the numerical model was able to capture some interesting Doppler effects that will be discussed together with other validation results.

Boulder accumulations related to extreme wave events on the eastern coast of Malta

Abstract. The accumulation of large boulders related to waves generated by either tsunamis or extreme storm events have been observed in different areas of the Mediterranean Sea. Along the eastern low-lying rocky coasts of Malta, five sites with large boulder deposits have been investigated, measured and mapped. These boulders have been detached and moved from the nearshore and the lowest parts of the coast by sea wave action. In the Sicily–Malta channel, heavy storms are common and originate from the NE and NW winds. Conversely, few tsunamis have been recorded in historical documents to have reached the Maltese archipelago. We present a multi-disciplinary study, which aims to define the characteristics of these boulder accumulations, in order to assess the coastal geo-hazard implications triggered by the sheer ability of extreme waves to detach and move large rocky blocks inland. The wave heights required to transport 77 coastal boulders were calculated using various hydrodynamic equations. Particular attention was given to the quantification of the input parameters required in the workings of these equations, such as size, density and distance from the coast. In addition, accelerator mass spectrometry (AMS) 14C ages were determined from selected samples of marine organisms encrusted on some of the coastal boulders. The combination of the results obtained both by the hydrodynamic equations, which provided values comparable with those observed and measured during the storms, and radiocarbon dating suggests that the majority of the boulders have been detached and moved by intense storm waves. These boulders testify to the existence of a real hazard for the coasts of Malta, i.e. that of very high storm waves, which, during exceptional storms, are able to detach large blocks of volumes exceeding 10 m3 from the coastal edge and the nearshore bottom, and also to transport them inland. Nevertheless, the occurrence of one or more tsunami events cannot be ruled out, since radiocarbon dating of some marine organisms did reveal ages which may be related to historically known tsunamis in the Mediterranean region, such as the ones in AD 963, 1329, 1693 and 1743.

Numerical comparison between deep water and intermediate water depth expressions applied to a wave energy converter

The energy that can be captured from the sea waves and converted into electricity should be seen as a contribution to decrease the excessive dependency and growing demand of fossil fuels. Devices suitable to harness this kind of renewable energy source and convert it into electricity—wave energy converters (WECs)—are not yet commercially competitive. There are several types of WECs, with different designs and working principles. One possible classification is their distance to the shoreline and thus their depth. Near-shore devices are one of them since they are typically deployed at intermediate water depth (IWD). The selection of the WEC deployment site should be a balance between several parameters; water depth is one of them. Another way of classifying WECs is grouping them by their geometry, size and orientation. Considering a near-shore WEC belonging to the floating point category, this paper is focused on the numerical study about the differences arising in the power captured from the sea waves when the typical deep water (DW) assumption is compared with the more realistic IWD consideration. Actually, the production of electricity will depend, among other issues, on the depth of the deployment site. The development of a dynamic model including specific equations for the usual DW assumption as well as for IWD is also described. Derived equations were used to build a time domain simulator (TDS). Numerical results were obtained by means of simulations performed using the TDS. The objective is to simulate the dynamic behavior of the WEC due to the action of sea waves and to characterize the wave power variations according with the depth of the deployment site.

Dynamic Response of Pile-cap Structure Under Random Sea Wave Action

Based on Pierson-Moscowitz (P–M) sea wave spectrum and linear wave theory, this paper presents the techniques for simulating random wave action on slender piles of pile-cap structure in coastal engineering, and discusses the dynamic response of pile-cap structure under random wave action using Finite Element (FE) method. In this study, a full FE model of a realistic pile-cap structure of a sea platform is established. The dynamic time-history analysis of the structure under the random sea wave action is carried out in consideration of two different structural damping types. As contrast, the static and dynamic analyses of the structure under the characteristic wave are also conducted. By comparing the displacement and internal force induced in the structure based on the different approaches considered, it is found that the dynamic response under the random sea wave is largest, which could have serious implications for design of structures of this type.

Geochemistry of beach sands from Sithonia Peninsula (Chalkidiki, Northern Greece)

Thirty beach sand samples from the granitic shoreline of the Sithonia Plutonic Complex (SPC) were analyzed for their REE and major element contents. The obtained results are compared with the adjacent SPC rock-types, in order to determine any enrichments or depletions. Among the samples enriched in REE, three are seasonal deposits of heavy minerals and their concentrations are controlled by the action of sea-waves. The available geochemical characteristics were also used to confirm the parental rocks of the beach sands, which are the SPC rock-types. The heavy fractions (total, total magnetic and total non-magnetic) of the beach sands were correlated with the REE concentrations, revealing a strong correlation between the heavy non-magnetic fraction and REE content. Among the minerals of the heavy non-magnetic fraction, monazite seems to control the REE content in the heavy mineral-enriched samples, whereas in the rest of the samples allanite, belonging to the heavy magnetic fraction may be the most important REE mineral.

Numerical modelling of shoreline evolution in the Aveiro coast, Portugal – climate change scenarios

ABSTRACT Pereira, C., Coelho, C., Ribeiro, A., Fortunato, A., Lopes, C.L. and Dias, J.M., 2013. Numerical modelling of shoreline evolution in the Aveiro coast, Portugal – climate change scenarios In the Northwest coast of Portugal, the shoreline evolution is leading to several problems, such as loss of territory and infrastructural damages, mainly caused by the sea wave's action. This study aims to predict the impacts of climate change on the shoreline evolution in this coastal area. Coastal stretches studied were Esmoriz-Furadouro and Vagueira-Mira, due to their vulnerability related with the low lying sandy coast, the deficit of sediments, the presence of urban waterfronts and the narrow width of the sand spit that separates the ocean from the Aveiro lagoon. The paper describes the application of two shoreline evolution models (LTC and GENESIS) to the projection of shoreline evolution between 2010 and 2100. A reference scenario considering the present typical wave climate and the mean sea water level wa...

DOUBLE-HINGED ARTICULATED OFFSHORE TOWER UNDER SEISMIC EXCITATION

The seismic response and reliability analysis of a double-hinged articulated offshore tower under the action of sea waves, currents, and earthquakes have been investigated. Major nonlinearities associated with the system such as large deformation, variable submergence, drag force, and added mass have been incorporated. The responses are obtained by spectral analyses using El Centro earthquake ground motion records. The equations of motion are formulated by Lagrangian approach and solved by Newmark β integration scheme. A limit-state function for seismic demands at the universal joint has been derived. Using the derived limit-state function and the responses obtained after the dynamic analysis, reliability assessment of the critical joint has been carried out with a probabilistic method. The results of the study show that the earthquake response investigations are quite crucial for target-based probabilistic design of the articulated system, as seismic sea environment imposes significant demands for the survival of the tower.

Simulation of Sloshing in LNG-Tanks

The purpose of this paper was to demonstrate the application of a procedure to predict internal sloshing loads on partially filled tank walls of liquefied natural gas (LNG) tankers that are subject to the action of sea waves. The method is numerical. We used a moving grid approach and a finite-volume solution method designed to allow for arbitrary ship motions. An interface-capturing scheme that accounts for overturning and breaking waves computed the motion of liquid inside the tanks. The method suppressed numerical mixing. Mixing effects close to the interface were buried in the numerical treatment of the interface. This interface, which was at least one cell wide, amounted to about 20–50 cm at full scale. Droplets and bubbles smaller than mesh size were not resolved. Tank walls were considered rigid. The results are first presented for an LNG tank whose motion was prescribed in accordance with planned laboratory experiments. Both two-dimensional and three-dimensional simulations were performed. The aim was to demonstrate that (1) realistic loads can be predicted using grids of moderate fineness, (2) the numerical method accurately resolves the free surface even when severe fragmentation occurs, and (3) long-term simulations over many oscillation periods are possible without numerical mixing of liquid and gas. The coupled simulation of a sea-going full-sized LNG tanker with partially filled tanks demonstrated the plausibility of this approach. Comparative experimental data were unavailable for validation; however, results were plausible and encouraged further validation.

Forensic study of wave loads on a pier in Belgium

A small but significant number suspended-deck coastal structures – including highway bridges, piers, jetties and wharves – have been damaged by sea wave action in recent years, often in milder conditions than those assumed in the design. This paper discusses the forensic investigation of Blankenberge pier in Belgium, which was subjected to unexpected impulsive wave loads after its renovation in 2002. The investigation included field measurements and hydraulic physical model tests to assess wave-loads and optimise alternative mitigation strategies.

Frequency-based model validation and parameter identification of a sea-surface vehicle

This paper focuses on the modeling of the roll motion for a manned sea-surface vehicle. An extension of the Conolly model is proposed, where the uncertainties in the mechanical torques acting on the vehicle – torques coming out from wind and sea-wave action – are also included in a general state space representation. A frequency-based identification procedure is provided in order to identify both the ship and the disturbance parameters. The algorithm has been tested on real data and most significative results are proposed.

Oscillatory boundary layer close to a rough wall

The flow field, generated by an oscillating pressure gradient close to a rough wall, is investigated by means of direct numerical simulations of Navier–Stokes and continuity equations. The wall roughness consists of semi-spheres regularly placed on a plane wall. A comparison of the obtained results with the experimental measurements of Keiller and Sleath [D.C. Keiller, J.F.A. Sleath, Velocity measurements close to a rough plate oscillating in its own plane, J. Fluid Mech. 73 (1976) 673–691] supports the numerical findings. As in Keiller and Sleath [D.C. Keiller, J.F.A. Sleath, Velocity measurements close to a rough plate oscillating in its own plane, J. Fluid Mech. 73 (1976) 673–691], a secondary peak in the streamwise velocity component is observed close to flow reversal and the peak is shown to be generated by the coherent vortex structures which are shed by the roughness elements. The flow is found to be dominated by the shear layers which form at the top of the roughness elements during the accelerating phases of the cycle and by the horse-shoe vortices which form close to the base of the semi-spheres. The dynamics of the shear layers and of the horse-shoe vortices is found to have a relevant influence on the pressure distribution and on the force exerted by the fluid on the roughness elements. The obtained results shed light to the mechanism by which the sediment is picked-up from the bottom by the action of sea waves.