Changes In Wave Energy Research Articles

A global wave power resource and its seasonal, interannual and long-term variability

Abstract Assessments of global wave power have been receiving increasing attention currently; however, a characterization of the global resources that holistically consider the different temporal scales that influence wave climate (monthly and seasonal, interannual and long-term) is still lacking. Moreover, the debate around the global figure of available resource is still widely open. This study provides a new global wave power assessment using a dataset that covers the period from 1948 to 2008, which was corrected using altimetry data and validated with buoys in terms of wave power. This study characterizes the mean wave power globally as well as its monthly and seasonal variability. Furthermore, it provides a link with the most relevant climate indices globally. The effect of the interannual variability is especially noteworthy for the Northern Hemisphere, where the seasonality is strongest. Additionally, we detect decadal long-term changes and determine that natural variability could explain a few of the differences found between decades. Lastly, we provide an assessment of the global theoretical wave power that covers the last six decades, compare approaches and estimates, and discuss factors of discrepancy. The global offshore wave power is estimated at 32,000 TW h/yr, which is reduced to 16,000 TW h/yr when considering the direction of the energy. The historical average change is 580 TW h/decade. Our results indicate that the global natural variability could be a more relevant factor in the lifetime of wave farms than the historical long-term changes in wave energy.

ESTIMATING SHORELINE RESPONSE IN A CHANGING WAVE CLIMATE

Estimates of the impacts of sea level rise and changing wave climates on shoreline change in the future are an increasing area of interest for many coastal stakeholders, engineers, and scientists. The ability to predict such change requires accurate information on the temporal evolution of key forcing parameters, such as wave height and period. An added complexity in modelling shoreline change is that beaches evolve in time. Higher wave conditions produce more dissipative beaches, resilient to further change, while periods of low waves produce highly dynamic low to intermediate beach states. Here, a simple shoreline model based on an equilibrium principle and forced by wave height and period is used to explore potential shoreline change projections at two different beach types: a highly dynamic storm dominated beach and a more dissipative seasonally dominated coastline. In all cases, the model parameters (which are previously shown to be a function of mean beach state) are assumed to not change after the 7-year calibration period, despite changes in wave energy. At both sites, an increase in average wave energy is predicted to either slightly accrete or stabilize a beach, while increases to storm waves results in severe erosion at the dynamic storm-dominated beach and only mild erosion at the dissipative beach. The paper concludes with a discussion on the validity of constant model parameters and poses the question over what timescales does a beach evolve to changing waves and how can this information be incorporated into simple empirical-based equilibrium shoreline models.

A comparative study on hydrocarbon detection using three EMD-based time–frequency analysis methods

Abstract Due to strong heterogeneity of marine carbonate reservoir, seismic signals become more complex, thus, it is very difficult for hydrocarbon detection. In hydrocarbon reservoir, there usually exist some changes in seismic wave energy and frequency. In their instantaneous spectrums there often exist such phenomena that show the characteristics of attenuation of high frequency energy and enhancement of low-frequency energy. The three EMD-based time-frequency analysis methods' instantaneous spectra all have certain oil and gas detection capability. In this paper, we introduced the Normalized Hilbert Transform (NHT) and a new method named the HU method for hydrocarbon detection. The model results in the Jingbian Gas Field which is located in the eastern Ordos Basin, China, show that NHT and HU methods can be adopted. They also detect the gas-bearing reservoir efficiently as the HHT method does. The three EMD-based methods, that is, the Hilbert–Huang transformation (HHT) and NHT and HU methods, were respectively applied to analyze the seismic data from the Jingbian Gas Field. Firstly, the seismic signals were decomposed into a finite number of intrinsic mode functions (IMFs) by empirical mode decomposition (EMD) method. The second IMF signal (IMF2) of the original seismic section better indicates the distribution of the reservoir. Information on hydrocarbon-bearing reservoir is mainly in IMF2. Secondly, the HHT, NHT and HU methods were respectively used to obtain different frequency division sections from IMF2. Hydrocarbon detection was realized from the energy distribution of the different frequency division sections with these three EMD-based methods. The practical application results show that the three EMD-based methods can all be employed to hydrocarbon detection. Frequency division section of IMF2 using NHT method was better for the seismic data from the Jingbian Gas Field than when using the HHT method and HU method.

Evidence for a transgressive barrier within a regressive strandplain system: Implications for complex coastal response to environmental change

AbstractClastic, depositional strandplain systems have the potential to record changes in the primary drivers of coastal evolution: climate, sea‐level, and the frequency of major meteorological and oceanographic events. This study seeks to use one such record from a southern Brazilian strandplain to highlight the potentially‐complex nature of coastal sedimentological response to small changes in these drivers. Following a 2 to 4 m highstand at ca 5·8 ka in southern Brazil, falling sea‐level reworked shelf sediment onshore, forcing coastal progradation, smoothing the irregular coastline and forming the 5 km wide Pinheira Strandplain, composed of ca 500 successive beach and dune ridges. Sediment cores, grab samples and >11 km of ground‐penetrating radar profiles reveal that the strandplain sequence is composed of well‐sorted, fine to very‐fine quartz sand. Since the mid‐Holocene highstand, the shoreline prograded at a rate of ca 1 to 2 m yr−1 through the deposition of a 4 to 6 m thick shoreface unit; a 1 to 3 m thick foreshore unit containing ubiquitous ridge and runnel facies; and an uppermost beach and foredune unit. However, the discovery of a linear, 100 m wide barrier ridge with associated washover units, a 3 to 4 m deep lagoon and 250 m wide tidal inlet within the strandplain sequence reveals a period of shoreline transgression at 3·3 to 2·8 ka during the otherwise regressive developmental history of the plain. The protected nature of Pinheira largely buffered it from changes in precipitation patterns, wave energy and fluvial sediment supply during the time of its formation. However, multiple lines of evidence indicate that a change in the rate of relative sea‐level fall, probably due to either steric or ice‐volume effects, may have affected this coastline. Thus, whereas these other potential drivers cannot be fully discounted, this study provides insights into the complexity of decadal‐scale to millennial‐scale coastal response to likely variability in sea‐level change rates.

Optimization of Ship Hull Lines of Bulbous Bow by Using CFD Approach

The proposal of this investigation is the optimization design of bulbous bow-lines for a container vessel. The results of wave-making coefficients and relative data of wave-making field both from computation based on Green function method and Michell integral method are provided for 7 different designs of bulbous bow lines respectively in this paper. According to the minimum energy from the sample of average wave-making energy for each design of bulbous bow-lines, which is to be objective function in this computation, the geometric parameters for optimization lines of bulbous bow can be determined. Adopting CFD method, it is respectively calculated that the average wave energy change with the ahead extendable length of bulbous bow, the height above the baseline of bulbous bow and the transverse area of the bulbous bow at fore pole. By above three change curves, the optimization shape of the bulbous bow is gained. The resistance components and the wave-field for wave-making for ship hull by using CFD method have been validated based on corresponding model test results. Therefore, the optimized bulbous bow-lines can be used in the ship design practice.

Pressure-Wave Energy Relationship during IABP Counterpulsation in a Mock Circulation: Changes with Angle and Assisting Frequency

Despite decades of successful clinical use of the intra aortic balloon pump (IABP), certain aspects of its operation are not yet fully understood. This work aims to investigate in vitro the mechanism underlying balloon inflation and deflation with varying assisting frequency and operating angle with respect to the horizontal, by studying the corresponding pressure and wave energy changes. A mock circulatory system (MCS), with physiological distribution of peripheral resistance and compliance, presented a controllable test bed. We used Wave Intensity Analysis (WIA) to identify balloon-generated waves and quantify their energy. Conventional hemodynamic parameters were also calculated. Tests were repeated at varying operating angles (0°-45°), resembling the semi-recumbent position in the ICU, and at different assisting frequencies (1:1, 1:2, 1:3). Two balloons (25 cc and 40 cc in volume) were tested. The main waves associated with counterpulsation were identified as a backward compression wave associated with balloon inflation and a backward expansion wave associated with balloon deflation. Results showed that the IABP inflation and deflation benefits are reduced with increasing angle, in terms of the size of the inflation and deflation waves as well as in terms of diastolic pressure augmentation and end-diastolic pressure reduction. Both WIA findings and pressure parameters indicated 1:1 as the most effective mode of pumping. This study shows that, in vitro, a greater benefit of counterpulsation can be achieved in the horizontal position at 1:1 assisting frequency, with a good correlation between wave and pressure results.

Characteristics of sediment in transport in the swash zone of a steep estuarine foreshore

AbstractCross‐shore grading of sediment has been observed on the surface of estuarine beaches but the swash zone processes responsible for this grading have not been measured. This study was conducted to provide an explanation for the cross‐shore grading of sediment on a predominantly sandy estuarine foreshore. Data on wave and swash characteristics and sediment trapped in the uprush and backwash during 25 swash events were gathered from mid‐rising to mid‐falling tide on a small transgressive barrier in Delaware Bay, New Jersey, USA. Sediment is predominantly quartz and feldspar, medium to coarse sands with a gravel fraction of granules and pebbles. Wave energies increased with tidal rise. The percentage of gravel in transport in the uprush and backwash is similar (11% and 13%) during the rising tide when the swash zone is at mid‐foreshore, decreases in the uprush (9%) and increases in the backwash (18%) when the swash zone is on the upper foreshore. When the swash zone is at mid‐foreshore on the falling tide, the quantity of gravel in the backwash (30%) is greater than in the uprush (24%). The low proportion of gravel within the foreshore prior to trapping, and the increase in the percentage of gravel when the waves and swash are on the upper foreshore, suggests that the step is the primary source of gravel high on the foreshore. The size of the step increases as wave heights increase with tidal rise. The rate of delivery of gravel into the swash is enhanced by sediment entrained during wave breaking and interaction of the uprush with the previous backwash. The lag in the rate of step migration relative to breaker migration during the falling tide increases the likelihood of mining gravel from the step and subsequent transport in the uprush and backwash. These findings are important for low energy estuarine beaches sensitive to small changes in tidal range and wave energy that cause sedimentological change across the foreshore.

Thermal Erosion of a Permafrost Coastline: Improving Process-Based Models Using Time-Lapse Photography

Coastal erosion rates locally exceeding 30 m y−1 have been documented along Alaska’s Beaufort Sea coastline, and a number of studies suggest that these erosion rates have accelerated as a result of climate change. However, a lack of direct observational evidence has limited our progress in quantifying the specific processes that connect climate change to coastal erosion rates in the Arctic. In particular, while longer ice-free periods are likely to lead to both warmer surface waters and longer fetch, the relative roles of thermal and mechanical (wave) erosion in driving coastal retreat have not been comprehensively quantified. We focus on a permafrost coastline in the northern National Petroleum Reserve–Alaska (NPR-A), where coastal erosion rates have averaged 10–15 m y−1 over two years of direct monitoring. We take advantage of these extraordinary rates of coastal erosion to observe and quantify coastal erosion directly via time-lapse photography in combination with meteorological observations. Our observations indicate that the erosion of these bluffs is largely thermally driven, but that surface winds play a crucial role in exposing the frozen bluffs to the radiatively warmed seawater that drives melting of interstitial ice. To first order, erosion in this setting can be modeled using formulations developed to describe iceberg deterioration in the open ocean. These simple models provide a conceptual framework for evaluating how climate-induced changes in thermal and wave energy might influence future erosion rates in this setting.

A vortex force analysis of the interaction of rip currents and surface gravity waves

[1] We use the vortex force formalism to analyze the effect of rip currents on their own wave forcing. The vortex force formalism allows us to decompose the wave forcing into the nonconservative flux of momentum due to wave breaking and the conservative vortex force. Following Yu and Slinn (2003), we consider rip currents initially generated by alongshore variation of wave breaking due to a perturbation of a barred bottom topography. This variation is reduced in magnitude by two current effects on waves: wave ray bending and the flux of wave energy by currents. We compute the change in wave energy caused by these two effects on their own and use this to show that their relative magnitude scales with the square of the ratio of the length to width of the rip current. Both effects increase the wave height over the channels of the longshore bar, which leads to more wave breaking and counterbalances its longshore variation due to bottom refraction. In comparison to wave breaking, the change in the vortex force is negligible. Next, we show how the reduction in wave breaking is similar to an enhanced bottom friction. We then analyze the dependence of this relationship on the breaking parameterization, angle of incidence of the waves, and bottom drag law.

Development of Portable Measurement System of 3-D Beach Topography for Monitoring of Beach Changes

Instead of the ordinary surveying method, a portable measurement system of 3-D beach topography for monitoring of beach changes was developed. Seasonal beach changes between the foreshore and backshore in response to the seasonal change in wave energy can be measured. The system was applied for the monitoring after the beach nourishment at the Hamamatsu-shinohara coast facing the Pacific Ocean. It becomes a practical method for the measurement of beach changes.

Global trends in extremal microseism intensity

Globally ubiquitous seismic background noise peaks near 7 and 14 s period are generated via distinct mechanisms that transfer storm‐generated gravity wave energy to the seismic wave field. We utilize continuous digital ground motion data recorded by the Global Seismographic Network and precursor instrumentation to chronicle microseism power extreme events for 1972–2009. Because most land‐observed microseism surface‐wave energy is generated at or near coasts, microseism metrics are particularly relevant to assessing changes in coastal ocean wave energy. Extreme microseism winter storm season event counts reveal the widespread influence of the El Niño Southern Oscillation (ENSO). Individual station and ensemble slopes trend positive for this study period for Northern Hemisphere stations. The double‐frequency microseism is particularly volatile, suggesting that the weaker single‐frequency microseism directly generated by ocean swell at coasts is likely a more representative seismic proxy for broad‐scale ocean wave energy estimation.

Daily morphological changes determined by high‐energy events on an embayed beach: a qualitative model

AbstractThis paper examines the daily morphological responses of Sununga Beach, an embayed beach located on the south‐eastern Brazilian coast, to storms in the South Atlantic Ocean. The main mechanisms and timing of beach erosion and accretion, the relationship between wave height and direction, and beach volume changes are considered, to establish a qualitative model for short‐term embayed beach morphological changes.The methodology consisted of daily topographic surveys during the month of May in 2001, 2002, and 2003, using an RTK‐GPS (real‐time kinematics – global positioning system). Weather and wave model results were used to correlate hydrodynamics and beach morphology.The results indicate that the morphodynamics of Sununga Beach are characterized by a process of beach rotation, which occurred more or less clearly during all three surveys. Unlike what has been commonly described in the literature for longer time intervals and alternations of fair and stormy weather, the beach rotation processes on Sununga Beach occurred under conditions of moderate‐to‐high wave energy change (wave heights greater than 2 m).An integrated evaluation of the behaviour of the meteorological aspects, together with beach morphology, enabled us to recognize that extra‐tropical cyclones were the most important agent in remobilizing the beach planform, whether in beach rotation or in cross‐shore erosion. Copyright © 2010 John Wiley & Sons, Ltd.

Generation of polarized shear Alfvén waves by a rotating magnetic field source

Experiments are performed in the Large Plasma Device at the University of California, Los Angeles to study the propagation of field-aligned, polarized kinetic shear Alfvén waves radiated from a rotating magnetic field source created via a novel phased orthogonal loop antenna. Both right and left hand circular polarizations are generated at a wide range of frequencies from 0.21≤ω/Ωci<0.93. Propagation parallel to the background magnetic field near the Alfvén velocity is observed along with a small parallel wave magnetic field component implying a shear mode. The peak-to-peak magnitude of the wave magnetic field, 33 cm away from the antenna, is on the order of 0.8% of the background field and drops off in the far field. The full width at half maximum of the wave energy changes little over a distance of 2.5 parallel wavelengths while the exponential decrease in wave energy as a function of distance can be attributed to collisional damping. Evidence of electron heating and ionization is observed during the pulse.

Study of Relative Roles of Nonlinearity and Depth Refraction in Wave Spectrum Evolution in Shallow Water

For modelling a series of depth profiles covering the relative depth parameter interval 2>kph>0.3, evolution of two-dimensional gravity wave spectra is calculated in the frame of three-wave quasi-kinetic approximation derived by Zaslavskii and Polnikov (1988). The relative impact of refraction and nonlinearity on a change of two-dimensional spectra shape for gravity waves is estimated. It is shown that on the background of refraction impact on the spectrum shape, the three-wave nonlinearity results in a remarkable change of angular and frequency distribution for a wave energy spectrum. Herewith, in the spectral peak domain the nonlinearity reduces the value of the angular narrowness parameter by 20–30%, counteracting the refraction during the wave propagation into a shoal zone. In contrast to the high frequency domain of the spectrum, the angular narrowness parameter is increased due to the nonlinearity. For this reason, the nonlinearity can result in more than 10% change of wave energy in a shallow water zone with respect to the linear wave evolution case. These conclusions were checked by using the SWAN model under the same conditions. It was found that the SWAN model describes some of the main peculiarities of nonlinear waves in shallow water. Some recommendations were made to elaborate the three-wave nonlinear term in the source function of the SWAN model.

Collective bubble oscillations as a component of surf infrasound

Plunging surf is a known generator of infrasound, though the mechanisms have not been clearly identified. A model based on collective bubble oscillations created by demise of the initially entrained air pocket is examined. Computed spectra are compared to infrasound data from the island of Kauai during periods of medium, large, and extreme surf. Model results suggest that bubble oscillations generated by plunging waves are plausible generators of infrasound, and that dynamic bubble plume evolution on a temporal scale comparable to the breaking wave period may contribute to the broad spectral lobe of dominant infrasonic energy observed in measured data. Application of an inverse model has potential to characterize breaking wave size distributions, energy, and temporal changes in seafloor morphology based on remotely sensed infrasound.

The role of forcing in the local stability of stationary long waves. Part 1. Linear dynamics

The local linear stability of forced, stationary long waves produced by topography or potential vorticity (PV) sources is examined using a quasi-geostrophic barotropic model. A multiple scale analysis yields coupled equations for the background stationary wave and low-frequency (LF) disturbance field. Forcing structures for which the LF dynamics are Hamiltonian are shown to yield conservation laws that provide necessary conditions for instability and a constraint on the LF structures that can develop. Explicit knowledge of the forcings that produce the stationary waves is shown to be crucial to predicting a unique LF field. Various topographies or external PV sources can be chosen to produce stationary waves that differ by asymptotically small amounts, yet the LF instabilities that develop can have fundamentally different structures and growth rates. If the stationary wave field is forced solely by topography, LF oscillatory modes always emerge. In contrast, if the stationary wave field is forced solely by PV, two LF structures are possible: oscillatory modes or non-oscillatory envelope modes. The development of the envelope modes within the context of a linear LF theory is novel.An analysis of the complex WKB branch points, which yields an analytical expres-sion for the leading-order eigenfrequency, shows that the streamwise distribution of absolute instability and convective growth is central to understanding and predicting the types of LF structures that develop on the forced stationary wave. The location of the absolute instability region with respect to the stationary wave determines whether oscillatory modes or envelope modes develop. In the absence of absolute instability, eastward propagating wavetrains generated in the far field can amplify via local convective growth in the stationary wave region. If the stationary wave region is streamwise symmetric (asymmetric), the local convective growth results in a local change in wave energy that is transient (permanent).

Wave energy gradients across a Maldivian atoll: Implications for island geomorphology

Exposure to wave energy has been used to account for a range of ecological, geological and geomorphic processes in coral reef systems, but few attempts have been made to quantify spatial variations in energy at the atoll scale. This study presents results of measurements of wave energy on reef platforms across South Maalhosmadulu Atoll, Maldives and their implications for island geomorphology. The atoll has a perforated rim (37% effective aperture) and experiences predictable shifts in monsoon winds from the west (8 months) and northeast (4 months). Results show that wave energy affecting the atoll is considerably greater during the westerly monsoon. Atoll structure promotes significant changes in wave energy and wave characteristics across the atoll. Short period (3–8 s) monsoon-driven wave energy, which is significant on windward reefs, is dissipated on the peripheral reef network and the density of lagoonal patch reefs limits development of locally generated wind-wave energy across the lagoon. However, longer period swell (8–20 s) propagates through the lagoon to leeward reefs. A windward to leeward decay in wave energy is evident in the westerly monsoon, but not in the northeast monsoon, when long period swell (from the southwest) remains significant on western reefs. Net energy calculations that account for seasonal changes in wave energy across the atoll identify a steep west–east gradient that has geomorphic significance for island building. Western reefs are dominated by westerly flowing energy that is 4.5–7 times the total energy input elsewhere in the atoll. Wave energy on central reefs is balanced, whereas net energy on eastern reef platforms is dominated by eastward propagating waves. This steep energy gradient provides a physical explanation for the presence and distribution of islands on reef platforms across the atoll and provides quantitative support for the theory of Gardiner [Gardiner, J.S., 1903. The Fauna and Geography of the Maldives and Laccadives Archipelagoes. Cambridge University Press, London] to explain the prevalence of islands on the eastern atoll reef rim.

On the Interaction of Surface Waves and Upper Ocean Turbulence

Abstract The phase-averaged energy evolution for random surface waves interacting with oceanic turbulence is investigated. The change in wave energy balances the change in the production of turbulent kinetic energy (TKE). Outside the surface viscous layer and the bottom boundary layer the turbulent flux is not related to the wave-induced shear so that eddy viscosity parameterizations cannot be applied. Instead, it is assumed that the wave motion and the turbulent fluxes are not correlated on the scale of the wave period. Using a generalized Lagrangian average it is found that the mean wave-induced shears, despite zero vorticity, yield a production of TKE as if the Stokes drift shear were a mean flow shear. This result provides a new interpretation of a previous derivation from phase-averaged equations by McWilliams et al. It is found that the present source or sink of wave energy is smaller but is still on the order of the empirically adjusted functions used for the dissipation of swell energy in operational wave models, as well as observations of that phenomenon by Snodgrass et al.

AO/NAO response to climate change: 1. Respective influences of stratospheric and tropospheric climate changes

We utilize the GISS Global Climate Middle Atmosphere Model and eight different climate change experiments, many of them focused on stratospheric climate forcings, to assess the relative influence of tropospheric and stratospheric climate change on the extratropical circulation indices (Arctic Oscillation, AO; North Atlantic Oscillation, NAO). The experiments are run in two different ways: with variable sea surface temperatures (SSTs) to allow for a full tropospheric climate response, and with specified SSTs to minimize the tropospheric change. The results show that experiments with tropospheric warming or stratospheric cooling produce more positive AO/NAO indices. Experiments with tropospheric cooling or stratospheric warming produce a negative AO/NAO response. For the typical magnitudes of tropospheric and stratospheric climate changes, the tropospheric response dominates; results are strongest when the tropospheric and stratospheric influences are producing similar phase changes. Both regions produce their effect primarily by altering wave propagation and angular momentum transports, but planetary wave energy changes accompanying tropospheric climate change are also important. Stratospheric forcing has a larger impact on the NAO than on the AO, and the angular momentum transport changes associated with it peak in the upper troposphere, affecting all wavenumbers. Tropospheric climate changes influence both the AO and NAO with effects that extend throughout the troposphere. For both forcings there is often vertical consistency in the sign of the momentum transport changes, obscuring the difference between direct and indirect mechanisms for influencing the surface circulation.

Continuous reef growth during the last 23 cal kyr BP in a tectonically active zone (Vanuatu, SouthWest Pacific)

At Urélapa island, Vanuatu, sea-level rise and a tectonic uplift of ∼3 mm yr −1 have controlled water depth and the colonization and reef development on a substrate from 23 until 6 cal kyr BP. Analysis of coralgal assemblages offers insights on the mode of reef growth and on water depth over the reef for the studied period. Those assemblages, precisely dated by the 230Th/ 234U method, give valuable information on the rate and timing of sea-level rise since the Last Glacial Maximum. The Urélapa reef sequences are characterized by upwards replacement of reef frameworks which indicates changes in wave energy through time. The sudden acceleration of sea-level rise associated with meltwater pulse-1B at around 11.3 cal kyr BP is marked by a change in reef growth strategy, from a keep-up mode to a catch-up mode. We examine the delay in timing of the occurrence of suitable substrates, the role of paleoceanographic factors and the influence of possible jumps in sea level.