Ocean Swell Waves Research Articles

Evaluation of an experimental kelp farm’s structural behavior using regression modelling and response amplitude operators derived from in situ measurements

An experimental kelp farming system for exposed ocean conditions was designed, deployed, planted with kelp and instrumented for evaluation of its dynamic response to ocean waves, tides, and currents. The farm featured a novel “lattice” mooring design and anchor lines and cultivation lines (horizontal lines used as kelp growth substrate) made of fiberglass rods. The farm was deployed at a site in Saco Bay, Maine with 13 m (MLLW) water depth. There the farm was exposed to waves with heights up to 5.9 m. Anchor line tension, tide and wave height time series were gathered and processed into response amplitude operators (RAOs) and least squared error linear regression models enabling recognition of meaningful patterns between the forcing factors and the mooring response. Mean mooring line tensions were shown to increase nonlinearly with tide. Anchor line tension response amplitudes were shown to exhibit high sensitivity to both low and high frequency wave forcing. Numerical free-release test simulations suggested natural frequencies in heave of 0.91 Hz, indicating that tension response sensitivities at high frequency could be the result of resonance. Low frequency tension response disproportionate to the low frequency wave forcing could be explained by wave forcing on kelp cultivation arrays modulated by wave group envelopes. Instances of high magnitude, potentially damaging peak tensions, deemed shock loads, were prevalent in most load cases. Anchor line tension dynamics including RAO and shock load magnitudes were shown to be sensitive to mooring stiffness (ratio of tension to resulting elongation) and, in some cases, significant wave amplitude. Patterns of anchor line response indicated that additional mooring elasticity or geometric compliance and use of floatation with less sensitivity to high frequency waves could help avoid the cause of and costly consequences of amplified high frequency loading and high amplitude shock loading. RAOs and regression model results also indicated a subdued response in frequencies associated with ocean swell waves, suggesting desirable performance in waves most dominant in extreme storm events. With the proposed improvements, the farm system design suggests merit as a robust and durable macroalgae biomass production platform.

A simulator of Synthetic Aperture Radar (SAR) image spectra: the applications on oceanswell waves

ABSTRACT The Synthetic Aperture Radar (SAR) carried on-board satellites yields invaluable data of global wave spectra since the early 1990s, with several satellites in orbit at present and more launches scheduled in the near future. However, the retrieval of wave information from SAR images constitutes a complex set of procedures. In this context, we have presented here a methodology to simulate SAR image spectra of ocean swell waves. SAR simulators are important tools for the implementation and evaluation of wave spectra retrieval schemes. The one proposed here is based on the Hasselmann Transform whose Modulation Transfer Functions (MTF’s) account for the main physical processes involved in the imaging of ocean waves. A detailed description of its structure is provided. Through several test cases, we highlight some particularities of the relationship between SAR image spectra and wave spectra. We have evaluated the impact of the parameters settings and input information on the retrieval process, pinpointing possible shortcomings. The results indicated that useful information about the processes involved in the imaging of ocean swells can be derived from the SAR simulator.

Large-Eddy Simulation-Based Study of Effect of Swell-Induced Pitch Motion on Wake-Flow Statistics and Power Extraction of Offshore Wind Turbines

In this study, the effects of ocean swell waves and swell-induced pitch motion on the wake-flow statistics and power extraction of floating wind turbines are numerically investigated. A hybrid numerical model coupling wind large-eddy (LES) and high-order spectral-wave simulations is employed to capture the effects of ocean swell waves on offshore wind. In the simulation, 3 × 3 floating wind turbines with prescribed pitch motions were modeled using the actuator disk model. The turbulence statistics and wind-power extraction rate for the floating turbines are quantified and compared to a reference case with fixed turbines. Statistical analysis based on the phase-average approach shows significant swell-correlated wind-velocity variations in both cases, and the swell-induced pitch motion of floating turbines is found to cause oscillations of wind-turbulence intensity and Reynolds stress, as well as an increase of vertical velocity variance in the near-wake region. Swells also cause periodic oscillation in extracted power density in the fixed turbine case, and the turbine pitch motion in the floating turbine case could further modulate this oscillation by shifting the phase dependence by about 180 degrees with respect to the swell-wave phase.

Convective boundary‐layer structure in the presence of wind‐following swell

AbstractThe marine boundary layer is known to be influenced by fast long ocean swell waves travelling away from their generation area, where they were initiated by momentum transferred to the ocean wave field during storms. The atmospheric boundary layer during wind‐following swell and various stability states has been investigated using large‐eddy simulation (LES) data. The dominant energy‐containing motions in the near‐neutral atmospheric boundary layer over flat terrain are known to be dominated by near‐ground shear‐induced regions of high‐ and low‐speed flow. Wind fields and momentum fluxes from LES for swell‐dominated situations have been used to interpret field measurements suggesting that these motions are disrupted by effects related to the underlying wave field in the presence of swell waves. Statistical analysis and visualization are used to further describe the effects of stratification during swell for convective boundary‐layer winds and fluxes. A mechanism for transport of momentum to the upper levels of the boundary layer is suggested from interpretation of LES data. Coherent detached eddies from the directly wave‐induced motions near the surface are found to maintain an upward momentum transfer. This mechanism is found to strengthen during stronger swell conditions and also during slightly convective conditions. In this way, it is argued that processes related to both the wave field and surface convection can have a significant influence on the global structure of neutral and convective boundary layers during swell. This has implication for the turbulence length‐scales during wind‐following swell. Copyright © 2012 Royal Meteorological Society

Waves and tides responsible for the intermittent closure of the entrance of a small, sheltered tidal wetland at San Francisco, CA

Crissy Field Marsh (CFM; http://www.nps.gov/prsf/planyourvisit/crissy-field-marsh-and-beach.htm) is a small, restored tidal wetland located in the entrance to San Francisco Bay just east of the Golden Gate. The marsh is small but otherwise fairly typical of many such restored wetlands worldwide. The marsh is hydraulically connected to the bay and the adjacent Pacific Ocean by a narrow sandy channel. The channel often migrates and sometimes closes completely, which effectively blocks the tidal connection to the ocean and disrupts the hydraulics and ecology of the marsh. Field measurements of waves and tides have been examined in order to evaluate the conditions responsible for the intermittent closure of the marsh entrance. The most important factor found to bring about the entrance channel closure is the occurrence of large ocean waves. However, there were also a few closure events during times with relatively small offshore waves. Examination of the deep-water directional wave spectra during these times indicates the presence of a small secondary peak corresponding to long period swell from the southern hemisphere, indicating that CFM and San Francisco Bay in general may be more susceptible to long period ocean swell emanating from the south or southwest than the more common ocean waves coming from the northwest. The tidal records during closure events show no strong relationship between closures and tides, other than that closures tend to occur during multi-day periods with successively increasing high tides. It can be inferred from these findings that the most important process to the intermittent closure of the entrance to CFM is littoral sediment transport driven by the influence of ocean swell waves breaking along the CFM shoreline at oblique angles. During periods of large, oblique waves the littoral transport of sand likely overwhelms the scour potential of the tidal flow in the entrance channel.

INVESTIGATION OF WAVE INDUCED STORM SURGE WITHIN A LARGE COASTAL EMBAYMENT - MORETON BAY (AUSTRALIA)

Moreton Bay is a large coastal embayment on the south-east Queensland coast which is surrounded by the urbanised areas of greater Brisbane on its western and southern shorelines. It is protected from the open coast by a number of islands, including South Stradbroke, North Stradbroke and Moreton Islands. Tropical cyclones occasionally track far enough south to cause significant damage to south-east Queensland due to flooding, winds, waves and elevated ocean water levels. Distant tropical cyclones which may be several hundred kilometres north of Moreton Bay have been known to cause storm surge, high waves and erosion inside Moreton Bay. These events generally do not generate gale force winds within Moreton Bay, but can generate large ocean swell waves. It has been identified that the wave conditions generated from distant cyclones can cause a variation in water levels inside Moreton Bay. A detailed study was undertaken to investigate the regional wave set-up process which affects Moreton Bay. The simulation of the residual water levels within Moreton Bay using a coupled hydrodynamic and wave model system developed for this study is considerably more accurate than applying a hydrodynamic model alone and explains water level anomalies that have a tidal frequency. The paper discusses the physical process of regional wave set-up inside a large embayment, analysis of observed residual water level and also the modelling study undertaken to quantify the influence of waves on storm tide levels inside Moreton Bay. The storm tide hazard study for the Moreton Bay Councils included the effects of regional wave set-up in the specification of design water levels.

Origin and Formation of an Estuarine Barrier Island, Tapora Island, New Zealand

SMITH, Q.H.T.; HEAP, A.D., and NICHOL, S.L., 2010. Origin and formation of an estuarine barrier island, Tapora Island, New Zealand. Journal of Coastal Research, 26(2), 292–300. West Palm Beach (Florida), ISSN 0749-0208. Barrier islands in sheltered settings are rare coastal geomorphic features. Here we present a case study of controls on the evolution of Tapora Island, North Island, New Zealand. Tapora Island is an active barrier island located opposite the entrance to the Kaipara Harbour on a high-energy coast. Subsurface facies form an aggradational barrier island succession from subtidal to subaerial elevations. This facies succession, combined with surface samples and geomorphic and geologic relationships, indicates that Tapora Island is the most recent barrier island at this location in the estuary and forms part of a prograded coast opposite the entrance. Wave data indicate that ocean swell waves penetrate the inlet for approximately 2 hours either side of high tide and are capable of transporting sand onto the island. The combined effects of swell waves, abundant sediment supply, and exposed aspect are the critical factors that have formed the barrier island. Despite the ‘‘sheltered’ estuarine setting, Tapora Island has formed under conditions that are more akin to open ocean coasts. The origin and development of Tapora Island broadly conforms to the accumulating barrier island model.

Multichannel ATI-SAR with application to the adaptive Doppler filtering of ocean swell waves

Multichannel along track interferometric (MATI) SAR systems are discussed from the point of view of ocean remote sensing. It is shown that the signals scattered by range moving scatterers can be Doppler filtered using a MATI-SAR. Processing techniques are presented, including a phase screen method for the removal of phase errors (including systematic errors and also those resulting from platform motions). A particular radar system (the ESR, enhanced surveillance radar) is described; this has a 3 cm wavelength MATI with two, three or four beams. It is used for investigating and exploiting Doppler effects in ocean radar scattering. It is demonstrated that, by applying adaptive Doppler filtering techniques, the MATI-SAR can significantly increase the imaging sensitivity of ocean features with respect to a conventional SAR. Examples illustrate the use of the ESR MATI-SAR system for imaging ship generated swell waves. Using three beams, the visibility of the wave images is increased by 9.5 dB looking downwind and 5.0 dB looking upwind for horizontal polarisation at grazing angles of 39° and 32°, respectively. Associated Doppler filter responses are shown which show that the scatterers modulated by the swell wave are located in the neighbourhood of the wave crests.

Teaching ocean wave forecasting using computer-generated visualization and animation—Part 2: swell forecasting

This paper, the second of a two-part series, introduces undergraduate students to ocean wave forecasting using interactive computer-generated visualization and animation. Verbal descriptions and two-dimensional illustrations are often insufficient for student comprehension. Fortunately, the introduction of computers in the geosciences provides a tool for addressing this problem. Computer-generated visualization and animation, accompanied by oral explanation, have been shown to be a pedagogical improvement to more traditional methods of instruction. Cartographic science and other disciplines using geographical information systems have been especially aggressive in pioneering the use of visualization and animation, whereas oceanography has not. This paper will focus on the teaching of ocean swell wave forecasting, often considered a difficult oceanographic topic due to the mathematics and physics required, as well as its interdependence on time and space. Several MATLAB ® software programs are described and offered to visualize and animate group speed, frequency dispersion, angular dispersion, propagation, and wave height forecasting of deep water ocean swell waves. Teachers may use these interactive visualizations and animations without requiring an extensive background in computer programming.

Mineral Sands Occurrences in the Murray Basin, SoutheasternAustralia

The Murray Basin in southeastern Australia is proving to be a major mineral sand province that eventually will replace Australia’s east and west coasts in production of rutile, zircon, and ilmenite. Concentrations of relatively coarse-grained heavy mineral occur as beach placers in the Pliocene Loxton-Parilla sands in the upper part of the Murray Basin sequence. These formed as 400-km-long barrier complexes in the Murravian Gulf under the action of long-period ocean swell waves. We think the main source of barrier sand, at least initially, was from erosion of Miocene sands on the bed of the Murravian Gulf; progradation was a response to sea level fluctuations linked to Milankovitch climatic cycles in the Pliocene. In most areas, the resulting 400-km-wide barrier strand plain is now overlain by fluvial, aeolian, and lacustrine deposits. Typically, the heavy mineral deposits are ilmenite rich, with 30 to 40 percent rutile and zircon. They occur as single, or as multiple, stacked strandline deposits, are often more than 10 m thick, have mineral grades that exceed 20 percent in places, are several hundred meters wide and 10 to 25 km long; some contain up to several million tonnes of heavy mineral. The rutile and zircon are comparable in grain size and quality to minerals traditionally mined in Australia. Many of the deposits are associated with topographic ridges—the Neckarboo and Iona Ridges are the best known—that appear to be fault-bounded blocks. Deposits of major commercial significance found so far contain a total of over 12 million tonnes (Mt) of rutile, zircon, and ilmenite. The total, coarse-grained mineral sand resources in the Murray Basin are conservatively estimated to be over 50 Mt. The distribution of mineral sands in the Murray Basin seems to be associated with two aspects of the region’s geology and geomorphology: (1) a zone bordering the central part of the basin where the Pliocene barriers were derived from underlying Miocene sands that probably already contained some mineral concentrations and (2) growth faulting with deposits preferentially occurring on upfaulted blocks especially in the zone defined by (1) above. We speculate that localized uplift during the formation of the Loxton-Parilla barriers was sufficient to modify coastal processes on uplifted blocks so as to increase the rate of alongshore sediment bypass compared to nearby areas. This phenomenon has been simulated in computer modeling. Where the barrier sands were already enriched in heavy minerals, winnowing by storm waves formed beach placers on the uplifted fault-blocks. Based on criteria (1) and (2), the prospective areas of the Murray Basin account for 80 percent of the beach placers found to date.

Investigation of nonlinear interactions in ocean swell waves using the reduced bispectrum

In this paper, a two-dimensional (2-D) reduced bicoherence function which can be calculated via the reduced bispectrum is introduced. The statistical and computational aspects of the 2-D reduced bicoherence function are compared with those of the conventional bicoherence function. It is shown that the proposed technique is less noise-sensitive than the conventional bicoherence estimate. The use of the 2-D reduced bicoherence function to investigate nonlinear interactions in ocean swell waves is also demonstrated. As the 2-D reduced bicoherence can be efficiently calculated, it is proposed to use this novel technique in oceanographic field experiments that require real-time data processing and interpretation.

Longshore variations in nearshore wave processes at magilligan point, northern ireland

AbstractMagilligan Point is a recurved cuspate foreland at the mouth of Lough Foyle. Two wave regimes intersect in the estuary mouth and the manner of their interplay controls shoreline changes. Ocean swell waves from the N and NE are refracted around the recurve, losing both height and energy longshore. Width of the surf zone decreases and waves tend to steepen, although both these changes and wave refraction owe something to nearshore geometry. Angle of wave approach becomes more acute and a westerly flowing longshore current moves sand S and SW along the beach. Estuary waves from the S and SW are wind‐driven with high‐frequencies and steepnesses. They generate a northeasterly current which returns material N, but dies out as the waves become obliterated by nearshore attenuation and breaking of swell. It is possible to identify a time‐averaged null‐point where shoreline wave power is balanced, although this tends to shift over short periods causing rapid morphological changes. The existence of two independent, but counteractive cells ensures the long‐term maintenance of the foreland, without requiring major or continuous supplies of fresh sediment.