Wave Energy Attenuation Research Articles

Um sistema deposicional de planície de maré aberta, dominado por ondas: a Formação Lagarto, Ordoviciano (?), Domínio Estância, Sergipe

The Lagarto Formation was interpreted as a depositional system of tidal flat where storm waves and tides controlled the sedimentation. This type of depositional system, which is presently relatively common, but uncommon in the geological record, contains hummocky cross-stratifications (HCS) formed in shallow water. Detailed facies analysis was used to define the physical characteristics of the sediments, a depositional architectural organisation, a depositional model and the controlling factors that preserved HCS in shallow-water. Seven facies, formed by oscillatory, unidirectional and combined flows, were recognised. They compose three architectural elements that represent the intertidal zone of a wave-dominated, open-coast tidal flat. The most distal element is characterised by thick and spaced HCS and small amount of mudstone. On the contrary, the most proximal elements display small wave or combined ripples and a high amount of mudstone. This organisation is due to the progressive attenuation of wave and tide energy approaching to the coast. Physical, biological, and geological factors influenced the preservation of HCS in shallow-waters. Fair-weather waves and tidal currents were not able to rework the storm deposited sandstone in the intertidal area. Environmental conditions did not allow the development of a biological community that could disrupt the sedimentary structure of the HCS. High sedimentation rate has allowed the rapid burial and preservation of the HCS beds.

Ordnance gelatine as an in vitro tissue simulation scaffold for extracorporeal shock wave lithotripsy

In vitro shock wave lithotripsy (SWL) research is typically performed utilizing wet coupling lithotriptors with a mesh basket model. This model does not take into account shock wave energy attenuation through tissue. Models using dry coupling lithotriptors rely on immersion chambers and face similar limitations. Ordnance gelatin (OG) displays strength and viscous properties similar to human tissue and is therefore widely used for ballistic tissue injury research. We present our initial experience using an OG tissue simulating scaffold for dry coupling SWL research. Using 10% OG prepared in a disc-shaped mold (five stone wells/gel), we tested the model using a Modulith SLX-F2 lithotriptor and artificial stone phantoms. Following a test of concept run on an empty gel mold and a material integrity check for leakage, we shocked 60 stones (30 narrow focus [NF], 30 wide focus [WF]) in human pooled urine. Half were shocked using gels containing open-ended wells with the remainder closed-ended wells. Fragmentation coefficients (FC) were calculated across both foci and gel models. All gels successfully completed 5,000 shocks (1,000/well) without loss of gel integrity or fluid leakage. The mean FC using open-ended wells was 77.9 ± 7.6% NF and 74.4 ± 4.8% WF, and for closed wells 75.9 ± 8.0% NF and 67.1 ± 3.5% WF. The total model cost including the preparation of gels and begostones was assessed at approximately $1 per stone (Canadian). Ordnance gel serves as an excellent surrogate tissue shockwave scaffold providing an easily manufactured, reproducible and inexpensive model for dry coupling SWL research.

The wave energy resource along Australia’s Southern margin

The Southern Australian margin is one of the most energetic regions in the world suitable for the extraction of wave energy for electricity generation. We have produced a data set in which the deep-water wave energy resource for the region is described by three representative deep-water wave states, equivalent to the 10th, 50th, and 90th percentiles of the deep-water wave energy flux, derived from archives of the USA National Oceanic and Atmospheric Administration (NOAA) WaveWatch III (NWW3) operational wave model. The Simulating WAves Nearshore (SWAN) wave model is then applied along the full Southern Australian margin to propagate these representative wave states into the near-shore region to quantify the effects of shallow water processes such as refraction, shoaling, and bottom friction. The wave energy incident on the 25-m isobath (∼30–50 kW/m) is approximately 35%–50% less than the World Energy Council estimates of offshore wave energy but is approximately 20% greater than the energy observed from long-term buoy deployments on the midshelf. The latter discrepancy is attributed to an overestimation of significant wave height along the Southern Australian margin by the NWW3 model. The near-shore model applied in this study adequately simulates the attenuation of wave heights across the continental shelf when compared with estimates of wave height attenuation obtained from the Topex satellite altimeter. The attenuation of wave energy across the continental shelf reduces the estimates of offshore wave energy as given by the World Energy Council; however the wave energy resource incident on the Southern Australian margin remains considerable. We estimate that if 10% of the incident near-shore energy in this region, which is an ambitious target when conversion efficiency is considered, were converted to electricity, approximately 130 TW h/yr (one-half of Australia’s total present-day electricity consumption) would be produced.

Sound absorption characteristics of porous asphalt concrete pavements

This study investigates the sound absorption characteristics of porous asphalt concrete (PAC) pavements using an impedance tube to assess noise reduction performance. It is clearly shown that an improved attenuation of sound wave energy is achieved by adjusting the air void content, aggregate gradation, and PAC pavement thickness, based on measurements of sound absorption coefficients. In addition, traffic noise measurements taken from PAC pavements in the field are compared to those taken from dense-graded asphalt concrete (DGAC), which is widely used in pavement materials.

Impacts of jetty construction on the wave heights off the Kiashahr lagoon

Coastal managers and ecologists are confronted with tasks to estimate wave heights in the Kiashahr sea area, since hydrodynamic factors can profoundly impact this environment. In this study, wind data and an empirical wind wave predicting model were used to estimate deepwater wave characteristics in the Kiashahr sea area; while a numerical model was used to determine wave characteristics in shallow waters. Wave heights were also determined and compared at different stations before and after construction of 2 jetties at the lagoon mouth to evaluate their efficiency in wave height reduction. Results revealed that wave height is in the range of 0.8 to 1.2 m at Sefid Rud river mouth. Significant wave height reduction in the range of 0.15 to 1.2 m occurred at the entrance of the lagoon due to construction of the jetties. This attenuation of wave energy may result in some impacts on the ecosystem health of Kiashahr lagoon such as increased sedimentation, reduction in flushing rates, dumping of garbage and consequently, more pollution.

Comparison of long-, medium- and short-term variations of beach profiles with and without submerged geological control

Abstract Victoria Beach (Cadiz, Spain) comprises a rocky flat outcrop in its northern zone and a sand-rich southern zone. These natural features allowed for a 5-year monitoring period and subsequent analysis of two different profiles (one in each zone) based on differences in bottom contours. Topo-bathymetric data were analysed using empirical orthogonal functions (EOFs) to determine changes over the short-, medium- and long-term. Several morphologic phenomena were identified (generalised erosion, seasonal or summer–winter tilting of the profile around different hinge points, berm development and its posterior destruction, etc.) in terms of their importance in explaining the variability of the collected data for both profiles. It is worth mentioning that both profiles undergo parallel regression in the medium-term. Thus, the 1st eigenfunction enabled us to identify the true regression of the beach shoreline, independent of seasonal or summer–winter slope changes. Reconstruction of profiles using EOF components demonstrated that though accretion periods in the medium-term were similar for both types of profiles, the accretion speed was much faster in the sand-rich profile than in the reef-protected profile (1.01 m 3 /day versus 0.33 m 3 /day). Moreover, the seasonal erosion rate and the subsequent shoreline retreat for the sand-rich profile were much larger than for the reef-protected profile (121 m 3 /year versus 29 m 3 /year). Analysis in the short-term (changes induced by a single day's storm) showed an instantaneous tilting of the profile, with the mobilised sand volume being much greater for the sand-rich than for the reef-protected profile (68 m 3 /m versus 12 m 3 /m). In brief, we can assume that the wave energy attenuation provoked by the existence of a reef flat acted as a geological control for profile evolution, which is essentially different for both profiles. Among the most important differences, special attention should be given to the larger slope, smaller mobilised sand volume and slower accretion rate of the reef-flat profile versus the non-protected profile.

Relative velocity of seagrass blades: Implications for wave attenuation in low‐energy environments

While the ability of subaquatic vegetation to attenuate wave energy is well recognized in general, there is a paucity of data from the field to describe the rate and mechanisms of wave decay, particularly with respect to the relative motion of the vegetation. The purpose of this study was to quantify the attenuation of incident wave height through a seagrass meadow and characterize the blade movement under oscillatory flow under the low‐energy conditions characteristic of fetch‐limited and sheltered environments. The horizontal motion of the seagrass blades and the velocity just above the seagrass canopy were measured using a digital video camera and an acoustic Doppler velicometer (ADV) respectively in order to refine the estimates of the drag coefficient based on the relative velocity. Significant wave heights (Hs) were observed to increase by ∼0.02 m (∼20%) through the first 5 m of the seagrass bed but subsequently decrease exponentially over the remainder of the bed. The exponential decay coefficient varied in response to the Reynolds number calculated using blade width (as the length scale) and the oscillatory velocity measured immediately above the canopy. The ability of the seagrass to attenuate wave energy decreases as incident wave heights increase and conditions become more turbulent. Estimates of the time‐averaged canopy height and the calculated hydraulic roughness suggest that, as the oscillatory velocity increases, the seagrass becomes fully extended and leans in the direction of flow for a longer part of the wave cycle. The relationship between the drag coefficient and the Reynolds number further suggests that the vegetation is swaying (going with the flow) at low‐energy conditions but becomes increasingly rigid as oscillatory velocities increase over the limited range of the conditions observed (200 < Re < 800). In addition to the changing behavior of the seagrass motion, the attenuation was not uniform with wave frequency, and waves at a secondary frequency of 0.38 Hz (2.6 s) appeared to be unaffected by the seagrass. Cospectral analysis between the oscillatory and blade velocity suggests that the seagrass was moving in phase with the current at the (lower) secondary frequency and out of phase at the (higher) peak frequency. In this respect, seagrass is not only an attenuator of wave energy but also serves as a low‐pass filter; higher frequencies in the spectra tend to be more attenuated.

Ultrabroadband electromagnetic wave propagation in hydrocarbon reservoirs in the presence of an oil-water interface

Numerical experiments are applied to simulate propagation of broadband pulses which are transmitted by an electric dipole and travel in a uniform oil-bearing formation or in a layered reservoir with an oil-water interface. We calculate the frequency responses of the oil- and water-saturated layers as low-frequency filters for the propagating waves and find the respective cutoff frequencies. Attenuation rates are analyzed in the cases of incident nanosecond and picosecond pulses in uniform oil-bearing formations and those reflected from the oil-water interface. The low frequency filtering of pulses is investigated in terms of its effect on wave energy attenuation during propagation in oil-bearing rocks.

Progress in the study of physic-optics characteristics of atmospheric aerosols

Atmospheric aerosol particles, as one of the important members in the earth atmosphere, can influence the radiative transfer from violet and visible light to infrared in a wide wave band. On one hand, by scattering and absorption of electromagnetic waves, they can induce the energy attenuation of light waves; on the other hand, by turning the absorbed energy into the heat energy, they can heat up the atmosphere. This process directly or indirectly plays an important role in the simulation of radiate climate effect, the propagation of laser atmosphere, the high resolution spectrum, the ground and space remote sensing, the atmospheric correction, the environmental monitoring, the target detection and identification, and the overland traffics, aviation and navigation. We review the study of physics-optics characteristics of atmospheric aerosols, aiming at providing a reference for further study of atmospheric aerosols.

Sediment phosphorus fractions and profile distribution at different vegetation growth zones in a macrophyte dominated shallow Wuliangsuhai Lake, China

Sediment phosphorus (P) fractions and profile distribution at submerged macrophyte growth zone, emergent macrophyte growth zone and open-water zone were studied in Wuliangsuhai Lake, China, as well as the correlations among water content, grain size, and organic matter and P fractions. Among the three surveyed zones, the highest concentrations of most P forms occurred in the surface sediment and the lowest between a depth of 12 and 22 cm, except HCl-P and NaOH-P. Sediment phosphorus was mainly associated to inorganic forms (>50%) in three surveyed areas, and the highest value of inorganic phosphorus (IP) in the surface sediment was obtained from submerged macrophyte growth zone. Submerged and emergent macrophytes increased the IP content by 107 and 44 μg/g, respectively, in the surface sediment compared with open-water zone. Vertical profiles of IP in the three surveyed zones showed that the concentrations decreased from surface to 12 cm depth and then increased. There was a similar trend in the sediment profiles of organic phosphorus (OP) and total phosphorus (TP), but the highest concentration of OP and TP in surface sediment was obtained from the emergent macrophyte growth zone. Compared with open-water zone, the emergent macrophyte increased TP content in surface sediment by 1.73 times to 1,320 μg/g, while submerged macrophyte enhanced TP content in the surface sediment by 1.13 times to 865 μg/g. It was observed that in macrophyte growth zones, a strong linear correlation existed between organic matter and OP (r > 0.98), and the maximum concentrations of OP were present in the areas with maximum concentrations of organic matter. Results show that, although rooted macrophyte could uptake directly P from sediments, it is responsible for increasing the internal P loading especially OP by reducing current velocities, attenuating wave energy and generating organic residue in Wuliangsuhai Lake.

Seismic time-frequency spectral decomposition by matching pursuit

A seismic trace may be decomposed into a series of wavelets that match their time-frequency signature by using a matching pursuit algorithm, an iterative procedure of wavelet selection among a large and redundant dictionary. For reflection seismic signals, the Morlet wavelet may be employed, because it can represent quantitatively the energy attenuation and velocity dispersion of acoustic waves propagating through porous media. The efficiency of an adaptive wavelet selection is improved by making first a preliminary estimate and then a localized refining search, whereas complex-trace attributes and derived analytical expressions are also used in various stages. For a constituent wavelet, the scale is an important adaptive parameter that controls the width of wavelet in time and the bandwidth of the frequency spectrum. After matching pursuit decomposition, deleting wavelets with either very small or very large scale values can suppress spikes and sinusoid functions effectively from the time-frequency spectrum. This time-frequency spectrum may be used in turn for lithological analysis—for instance, detection of a gas reservoir. Investigation shows that the low-frequency shadow associated with a carbonate gas reservoir still exists, even high-frequency amplitudes are compensated by inverse-[Formula: see text] filtering.

Fine-sediment transport associated with cold-front passages on the shallow shelf, Gulf of Mexico

The eastern part of the chenier plain of the Louisiana coast has been prograding seaward over the last few decades while much of the rest of the Louisiana coast is experiencing high erosion rates. The source of sediment is the Atchafalaya River, which has been delivering sediment to the coastal ocean since the 1940s. Researchers have suggested that the repeated passage of cold fronts during winter and early spring plays an important role in delivering sediment to the coast. A sediment-transport study on the Atchafalaya coast was conducted between October 1997 and March 2001, which included several field experiments in early March, the period of high discharge from the Atchafalaya and frequent cold-front activity. A combination of shipboard profiling and time-series measurements from a bottom tripod and array of wave sensors on the inner shelf has resulted in a data set that illustrates the mechanism of onshore transport. For a cold-front passage sampled in 2001, during pre-front conditions, sediment is resuspended and mixed throughout the water column, with transport rates onshore and to the west of 53 and 184 g s −1 m −1, respectively. Post-front conditions also result in onshore transport due to onshore flow (upwelling) in the lower meter of the water column and formation of a high-concentration bottom layer. Post-front onshore transport rates are 32 g s −1 m −1 and most of the transport occurs in the bottom meter of the water column. The repeated cycling of cold-front passages leads to a positive feedback with transport onshore during both pre- and post-front conditions, and effective attenuation of wave energy over the muddy inner shelf inhibits erosion at the coast. Thus, the chenier-plain coast is experiencing high progradation rates (up to 29 m yr −1), while most of the Gulf coast is eroding.

Estimation of coda wave attenuation for NW Himalayan region using local earthquakes

The attenuation of seismic wave energy in NW Himalayas has been estimated using local earthquakes. Most of the analyzed events are from the vicinity of the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT), which are well-defined tectonic discontinuities in the Himalayas. The time-domain coda-decay method of a single back-scattering model is employed to calculate frequency dependent values of Coda Q (Qc). A total of 36 local earthquakes of magnitude range 2.1–4.8 have been used for Q c estimation at central frequencies 1.5, 3.0, 6.0, 9.0, 12.0 and 18.0 Hz through eight lapse time windows from 25 to 60 s starting at double the time of the primary S-wave from the origin time. The estimated average frequency dependence quality factor gives the relation, Q c = 158 f 1.05, while the average Q c values vary from 210 at 1.5 Hz to 2861 at 18 Hz central frequencies. The observed coda quality factor is strongly dependent on frequency, which indicates that the region is seismic and tectonically active with high heterogeneities. The variation of the quality factor Q c has been estimated at different lapse times to observe its effect with depth. The estimated average frequency dependent relations of Q c vary from 85 f 1.16 to 216 f 0.91 at 25 to 60 s lapse window length respectively. For 25 s lapse time window, the average Q c value of the region varies from 131 ± 36 at 1.5 Hz to 2298 ± 397 at 18 Hz, while for 60 s lapse time window its variation is from 285 ± 95 at 1.5 Hz to 2868 ± 336 at 18 Hz of central frequency. The variation of Q c with frequency and lapse time shows that the upper crustal layers are seismically more active compared to the lower lithosphere. The decreasing value of the frequency parameter with increasing lapse time shows that the lithosphere acquires homogeneity with depth.

Hydrodynamic Implications of Large Offshore Mussel Farms

Around 3000 Ha of New Zealand's coastal waters are presently allocated to mussel farming of the Greenshell mussel Perna canaliculus mainly in small inshore farms of 30 000 to 80 000 m/sup 2/ each. The farms are constructed from blocks of parallel moored long-lines, with loops of spat-impregnated line hanging vertically in the upper 10-15 m of the water column (provided sufficient water depth). When fully laden, the mussel loops can present a rough diameter of 0.20 m or more to the flow. Industry expansion is likely to be in the form of larger offshore farms. Some of the larger proposed farms will have more than 1000 km of mussel line. This presents a potentially significant drag to currents and waves. In this paper, observations are used from one of the first large offshore farms constructed in New Zealand, in conjunction with scaling estimates of energy loss, to investigate the effect of the farm on waves, currents, and stratification. The farm, consisting of 230 long-lines arranged into 20 blocks, measured 650 m /spl times/ 2450 m. The hanging mussel loops at this site reached an average depth of 8 m, and water depth at midtide varied across the site from 10-12 m. Transects were taken through the farm using both conductivity, temperature, depth (CTD), and Acoustic Doppler Profiler (ADP) instruments. A second ADP was moored inside the farm providing velocity time series. The average current was reduced within the farm by 36%-63%. The moored ADP detected an undercurrent beneath the farm with velocities nearly twice that within the farm. Sharp changes in stratification coincide with the start of fully stocked mussel long-lines. Furthermore, other than at the very upstream end of the farm, stratification appears to have prevented significant vertical mixing between the fluid within the farm and that flowing under the farm. Wave energy was low during the 7-d deployment, with significant wave heights peaking at 0.25 m. The attenuation of transitional waves through the farm is examined by comparing simultaneous pressure sensor measurements from offshore and inshore of the farm. The observations show that wave energy attenuation was frequency dependent. Observed wave energy attenuation across the 650 m wide farm was approximately 5%, 10%, and 17% at wave frequencies of 0.1, 0.2, and 0.25 Hz, respectively.

Identifying storm impacts on an embayed, high-energy coastline: examples from western Ireland

Large sections of the western Irish coast are characterised by a highly compartmentalised series of headland-embayment cells in which sand and gravel beaches are backed by large vegetated dune systems. Exposure to modally high-energy swell renders most of these beaches dissipative in character. A mesotidal range (c. 3.5–4.5 m) exists along much of the coast. Analysis of instrumental wind records from three locations permitted the identification of a variety of storm types and the construction of storm catalogues. Few individual storms were recorded at all three stations indicating a lack of regional consistency in storm record. Of the total storms recorded, only a small percentage are potentially damaging (onshore directed) and even fewer span a high tide and thus potentially induce a measurable morphological response at the coast. Through a combination of historical records, meteorological records, field observations and wave modelling we attempt to assess the impact of storms. Quantifiable records of coastal morphology (maps, air photos and beach profiles) are few in number and do not generally record responses that may be definitely attributed to specific storms. Numerical wave simulations and observations at a variety of sites on the west Irish coast, however, provide insights into instantaneous and medium term (decadal) storm responses in such systems. We argue that beaches and dunes that are attuned to modally high-energy regimes require extreme storms to cause significant morphological impact. The varying orientation of beaches, a spatially nonuniform storm catalogue and the need for a storm to occur at high water to produce measurable change, impart site-specific storm susceptibility to these embayments. Furthermore, we argue that long-period wave energy attenuation across dissipative shorefaces and beaches reduces coastal response to distant storms whereas short-period, locally generated wind waves are more likely to cause major dune and beach erosion as they arrive at the shoreline unrefracted. This apparently variable response of beach and dune systems to storm forcing at a decadal scale over a coastline length of 200 km urges caution in generalising regarding regional-scale coastal responses to climatic change.

The influence of rock resistance on coastal morphology around Lord Howe Island, southwest Pacific

AbstractAn Erratum has been published for this article in Earth Surface Processes and Landforms 27(7) 2004, 931.Lord Howe Island, in the northern Tasman Sea, is a remnant of a much larger Late Miocene basaltic shield volcano. Much of the island's coastline is exposed to waves that have unlimited fetch, but a marked contrast is provided by a fringing coral reef and lagoon that very effectively attenuate wave energy along a portion of the western coastline. The geology of the island is varied, with hard and resistant basalt lavas, breccias and tuffs of intermediate resistance, and highly erodible eolianites. This variability provides an excellent opportunity to examine the influence of rock resistance on the development of the spectacular rock coast landforms that occur around the island. The hardness of rocks and the extent of weathering around the coastline were assessed using a Schmidt hammer, and statistical analysis was undertaken to remove outlying values. On all but one occasion, higher mean rebound values were returned from fresh surfaces than weathered surfaces, but only half of these differences were statistically significant. Shore platforms with two distinct levels are juxtaposed along two stretches of coastline and Schmidt hammer results lend support to hypotheses that the raised surfaces may be inherited features. Relative rock resistance was assessed through a combination of Schmidt hammer data and measurements of joint density, and constrained on the basis of morphological data. This approach formed a basis for examining threshold conditions for sea‐cliff erosion at Lord Howe Island in the context of the distribution of resistant plunging cliffs and erosional shore platforms. Copyright © 2004 John Wiley & Sons, Ltd.

Low–frequency acoustic waves in nematic elastomers

We study the spectral and polarization properties of acoustic waves propagating in nematic liquid-crystalline rubber materials. We apply the viscoelastic theory of nematic elastomers in the low-frequency (hydrodynamic) limit. Dynamic soft elasticity, exhibited by ideal nematic elastomers under certain geometries of shear at low frequencies, leads to anomalous anisotropy of energy transfer and attenuation of transverse waves. The results suggest an application of this class of materials as an acoustic polarizer medium.

Analysis of Surface Wave Attenuation in Mangrove Forests

This paper presents an analytical study on surface wave attenuation in mangrove forest using analytical model developed by Massel et.al. (1999). The energy dissipation in the frequency domain is determined by treating the mangrove forest as a random media with certain characteristics using the geometry of mangrove trunks and their locations. Initial nonlinear governing equations are linearized using the concept of minimalization in the stochastic sense and interactions between mangrove trunks and roots have been introduced through the modification of the drag coefficients. To see the effectiveness of the mangrove forest in attenuating wave energy the analytical model was applied to two types of mangrove forest i.e. Rhizophora and Ceriops forests. The resulting rate of wave energy attenuation depends strongly on the density of the mangrove forest, and on diameter of mangrove roots and trunks. More effective wave energy attenuation is shown by Rhizophora.

Wave dissipation over macro-tidal saltmarshes: Effects of marsh edge typology and vegetation change

To achieve sustainable coastal management and planning, the interaction between fine-grained - in particular, vegetated - intertidal environments and incoming waves needs to be better understood. Previous studies have established that wave attenuation over saltmarshes can be significantly greater than over unvegetated intertidal surfaces. However, detailed, quantitative information on the effect of marsh elevation in the tidal frame, marsh width, seaward marsh edge configuration (e.g. cliffed versus ramped marshes), seasonal changes in marsh surface roughness (e.g. creek density, vegetation composition) and incident wave conditions, however, has been lacking. Based on a 10-month-long wave/tide dataset from two sites on the Dengie marshes, eastern England, this study addresses the effect of (i) marsh edge topography; (ii) marsh width; (iii) inundation depths; and (iv) seasonal changes in marsh surface vegetation cover on wave height and wave energy dissipation. Directional waves and water levels were recorded at 21 locations across both shallow-sloping and cliffed (cliff height of ca. 1.5m) intertidal profiles. In addition, changes in marsh surface vegetation cover and composition were recorded on a seasonal basis.Wave height attenuation over 310m of the shallow-sloping profile averaged 92 % over the monitoring period. Further analysis shows that the most rapid reduction in wave heights occurs over the most seaward 10 meters of permanent saltmarsh vegetation, where wave height attenuation averaged 2.1% and 1.1% per meter at the shallow-sloping and cliffed site respectively. Across the mudflat and the saltmarsh as a whole, wave height dissipation rates were significantly lower with an average of 0.1% and 0.5% per meter respectively. The presence of a saltmarsh cliff increased average wave heights by up to 0.5% per meter. Observed wave height attenuation showed a seasonal pattern at both sites (average wave energy attenuation near the marsh edge was highest in September – November and lowest in March – July) and appears to be linked to the cycle of seasonal vegetation growth.The study provides criteria for the assessment of the wave dissipation potential of marshes characterised by different widths, edge configurations and slopes, variability of water depths, and seasonal variations in vegetation cover/density.

Energy partition and attenuation of Lg waves by numerical simulations using screen propagators

Energy partition and attenuation of Lg waves in complex crustal waveguides with both large-scale structures and small-scale random heterogeneities are studied by numerical simulations. A newly developed screen propagator method (half-space generalized screen propagators) is tested and applied to this problem. The screen method is two to three orders of magnitude faster than finite difference (FD) method and uses much less internal memory. The method has no numerical dispersion and can easily incorporate various Q models into the codes. After analyzing different attenuation mechanisms, this paper is concentrated in simulating the leakage attenuation of Lg waves caused by forward large-angle scattering from random heterogeneities, which scatters the guided waves out of the trapped modes and leaking into the mantle. In addition to energy attenuation curves, variations of angular spectra of Lg waves along the path are also shown to give insight on the energy partition and scattering effects. The curve of equivalent Q for leakage attenuation as a function of normalized scale length ( ka) of the random heterogeneities agrees well with the scattering theory. The comparisons of the method with wavenumber integration and FD method, and the results of the numerical simulations demonstrate the validity and capacity of the screen propagator method in studying Lg attenuation.