Showing papers on "Wave height published in 2013"

cBathy: A robust algorithm for estimating nearshore bathymetry

Abstract: [1] A three-part algorithm is described and tested to provide robust bathymetry maps based solely on long time series observations of surface wave motions. The first phase consists of frequency-dependent characterization of the wave field in which dominant frequencies are estimated by Fourier transform while corresponding wave numbers are derived from spatial gradients in cross-spectral phase over analysis tiles that can be small, allowing high-spatial resolution. Coherent spatial structures at each frequency are extracted by frequency-dependent empirical orthogonal function (EOF). In phase two, depths are found that best fit weighted sets of frequency-wave number pairs. These are subsequently smoothed in time in phase 3 using a Kalman filter that fills gaps in coverage and objectively averages new estimates of variable quality with prior estimates. Objective confidence intervals are returned. Tests at Duck, NC, using 16 surveys collected over 2 years showed a bias and root-mean-square (RMS) error of 0.19 and 0.51 m, respectively but were largest near the offshore limits of analysis (roughly 500 m from the camera) and near the steep shoreline where analysis tiles mix information from waves, swash and static dry sand. Performance was excellent for small waves but degraded somewhat with increasing wave height. Sand bars and their small-scale alongshore variability were well resolved. A single ground truth survey from a dissipative, low-sloping beach (Agate Beach, OR) showed similar errors over a region that extended several kilometers from the camera and reached depths of 14 m. Vector wave number estimates can also be incorporated into data assimilation models of nearshore dynamics.

A significant increase in wave height in the North Atlantic Ocean over the 20th century

Abstract: article i nfo A new 109 year numerical wind-wave hindcast is developed for the North Atlantic Ocean based on the 20th century atmospheric reanalysis (20CR). Wave results are validated directly against data originating from voluntary observing ships and satellite altimetry in the North-East Atlantic Ocean. The normalized error for yearly-mean significant wave height (Hs) is shown to be of the order of 5% for the second part of the 20th century. An indirect validation is also performed through 10 m wind speed and suggests that the accuracy of yearly-mean Hs only slightly decreases for the beginning of the 20th century. The comparison between Hs and the index of the North Atlantic Oscillation revealed that this phenomenon partly controls Hs inter-annual variability, with a positive (negative) correlation in the northeastern (southwestern) part of the study area. The analysis of model results shows an increase in Hs over the whole North Atlantic Ocean superimposed to the inter-annual variability, reaching 0.01 m.yr −1 (20 to 40% over the 20th century) north of 50°N. This increase is explained by a rise in wind speed exceeding 20% north of 50°N. The roughening in the wave climate demonstrated in this study is expected to have strong implications for the development of coastal zones and could explain the increase in erosion along the North Atlantic shorelines.

Improved landslide-tsunami prediction: Effects of block model parameters and slide model

Abstract: [1] Subaerial landslide-tsunamis and impulse waves are caused by mass movements impacting into a water body, and the hazards they pose have to be reliably assessed. Empirical equations developed with physical Froude model studies can be an efficient method for such predictions. The present study improves this methodology and addresses two significant shortcomings in detail for the first time: these are the effect of three commonly ignored block model parameters and whether the slide is represented by a rigid block or a deformable granular material. A total of 144 block slide tests were conducted in a wave flume under systematic variation of three important block model parameters, the slide Froude number, the relative slide thickness, and the relative slide mass. Empirical equations for the maximum wave amplitude, height, and period as well as their evolution with propagation distance are derived. For most wave parameters, remarkably small data scatter is achieved. The combined influence of the three block model parameters affects the wave amplitude and wave height by up to a factor of two. The newly derived equations for block slides are then related to published equations for granular slides. This comparison reveals that block slides do not necessarily generate larger waves than granular slides, as often argued in the technical literature. In fact, it is shown that they may also generate significant smaller waves. The new findings can readily be integrated in existing hazard assessment methodologies, and they explain a large part of the discrepancy between previously published data.

Is the Intensifying Wave Climate of the U.S. Pacific Northwest Increasing Flooding and Erosion Risk Faster Than Sea-Level Rise?

Abstract: The relative contributions of sea-level rise (SLR) and increasing extratropical storminess to the frequency with which waves attack coastal features is assessed with a simple total water level (TWL) model. For the coast of the U.S. Pacific Northwest over the period of wave- buoy observations (approximately 30 years), wave height (and period) increases have had a more significant role in the increased frequency of coastal flooding and erosion than has the rise in sea level. Where tectonic-induced vertical land motions are significant and coastlines are pres- entlyemergent relativetothemeansealevel,increasingwave heightsresultinthesestretchesofcoastbeingpossiblysubmergentrelativetothe TWL. Although it is uncertain whether wave height increases will continue into the future, it is clear that this process could remain more im- portant than, or at least as important as, SLR for the coming decades, and needs to be taken into account in terms of the increasing exposure of coastal communities and ecosystems to flooding and erosion. DOI: 10.1061/(ASCE)WW.1943-5460.0000172. © 2013 American Society of Civil Engineers.

The decay rate of ocean swell observed by altimeter

Abstract: Altimeter data from transects across the Southern Ocean are analyzed to determine the decay of oceanic swell. The resulting decay rate is shown to be proportional to wavenumber squared and swell amplitude cubed. Such a decay relationship is consistent with turbulent interaction with the background, either in the air or water. The present data cannot distinguish between these two cases. The results are consistent with the limited previous studies and present a source term suitable for use in wave prediction models.

Tsunami damping by mangrove forest: A laboratory study using parameterized trees

Abstract: . Tsunami attenuation by coastal vegetation was examined under laboratory conditions for mature mangroves Rhizophora sp. The developed novel tree parameterization concept, accounting for both bio-mechanical and structural tree properties, allowed to substitute the complex tree structure by a simplified tree model of identical hydraulic resistance. The most representative parameterized mangrove model was selected among the tested models with different frontal area and root density, based on hydraulic test results. The selected parameterized tree models were arranged in a forest model of different width and further tested systematically under varying incident tsunami conditions (solitary waves and tsunami bores). The damping performance of the forest models under these two flow regimes was compared in terms of wave height and force envelopes, wave transmission coefficient as well as drag and inertia coefficients. Unlike the previous studies, the results indicate a significant contribution of the foreshore topography to solitary wave energy reduction through wave breaking in comparison to that attributed to the forest itself. A similar rate of tsunami transmission (ca. 20%) was achieved for both flow conditions (solitary waves and tsunami bores) and the widest forest (75 m in prototype) investigated. Drag coefficient CD attributed to the solitary waves tends to be constant (CD = 1.5) over the investigated range of the Reynolds number.

Ocean Waves and Teleconnection Patterns in the Northern Hemisphere

Abstract: Understanding long-term, ocean wave climate variability is important to assess climate change impacts on coastal and ocean physics and engineering. Teleconnection patterns can represent wave climate variability in the context of climate change. The objective of this study is to identify how large-scale spatial distributions of wave heights vary on a monthly basis and how they are influenced by various teleconnection patterns using reanalysis datasets. The wave height climate responses to teleconnection patterns in the eastern part of the North Pacific and North Atlantic are more sensible than in the corresponding western parts. The dominant spatial patterns of monthly averaged wave height variability in winter were obtained by empirical orthogonal function analysis. The three dominant patterns in the North Pacific and North Atlantic are similar. It is remarkable that one of the three dominant patterns, a band-shaped pattern, exhibits a strong relation to the teleconnection pattern in each ocean. T...

Residual liquefaction of seabed under standing waves

Abstract: This paper presents the results of an experimental study of the seabed liquefaction beneath standing waves. Silt (with d50=0.070 mm) was used in the experiments. Two kinds of measurements were carried out: pore water pressure measurements and water surface elevation measurements. These measurements were synchronized with video recording of the liquefaction process from the side. The ranges of the various quantities in the experiments were wave height H=5.9–12.0 cm, wave period T=1.09 s, and water depth h=30 cm. The experiments show that the seabed liquefaction under standing waves, although qualitatively similar, show features different from that caused by progressive waves. The pore water pressure builds up (or accumulated) in the areas around the node and subsequently spreads out toward the antinodes. The experimental results imply that this transport is caused by a diffusion mechanism with a diffusion coefficient equal to the coefficient of consolidation. The experiments further show that the n...

Growth of river mouth bars in sheltered bays in the presence of frontal waves

Abstract: [1] One of the key processes for the formation of deltas and their fluvial networks is the deposition of mouth bars in front of prograding distributaries. Waves influence mouth bar growth, but it is not clear how and to what extent. Toward this end, we conduct a modeling study on deltas forming in sheltered bays, where waves are locally generated and both longshore currents and surf zone are absent. We focus on the simplified case of a homopycnal river plume subject to frontal wave attack, and we begin by isolating the effects of waves on jet spreading. An analytical model for the hydrodynamic interaction between incoming waves and a turbulent expanding jet is developed and tested with the numerical model Delft3D coupled to the wave model SWAN. Both the analytical model and Delft3D predict that incoming surface gravity waves increase the spreading of the jet and the interaction between wave and current boundary layers causes an increase in bottom friction. To investigate how waves influence mouth bar morphodynamics, a set of numerical simulations is run with Delft3D-SWAN utilizing a geometry and wave characteristics typical of sheltered bays. Our numerical results show that in the presence of waves, mouth bars form up to 35% closer to the river mouth and 40% faster when compared to cases without waves. The distance from the river mouth to the stagnated mouth bar decreases with increasing wave height and wave period. The timescale of bar formation is inversely proportional to wave height and directly proportional to wave period. Our modeling study suggests that wave influence on mouth bar growth is complex. Small waves, like the ones modeled here, promote mouth bar formation via increased jet spreading and faster formation time, which in turn should create deltas with more distributary channels. On the other hand, large waves suppress mouth bar formation, as seen in other studies, leading to fewer distributary channels.

Waves in the Red Sea : response to monsoonal and mountain gap winds

Abstract: An unstructured grid, phase-averaged wave model forced with winds from a high resolution atmospheric model is used to evaluate wind wave conditions in the Red Sea over an approximately 2-year period. The Red Sea lies in a narrow rift valley, and the steep topography surrounding the basin steers the dominant wind patterns and consequently the wave climate. At large scales, the model results indicated that the primary seasonal variability in waves was due to the monsoonal wind reversal. During the winter, monsoon winds from the southeast generated waves with mean significant wave heights in excess of 2 m and mean periods of 8 s in the southern Red Sea, while in the northern part of the basin waves were smaller, shorter period, and from northwest. The zone of convergence of winds and waves typically occurred around 19–20°N, but the location varied between 15 and 21.5°N. During the summer, waves were generally smaller and from the northwest over most of the basin. While the seasonal winds oriented along the axis of the Red Sea drove much of the variability in the waves, the maximum wave heights in the simulations were not due to the monsoonal winds but instead were generated by localized mountain wind jets oriented across the basin (roughly east–west). During the summer, a mountain wind jet from the Tokar Gap enhanced the waves in the region of 18 and 20°N, with monthly mean wave heights exceeding 2 m and maximum wave heights of 14 m during a period when the rest of the Red Sea was relatively calm. Smaller mountain gap wind jets along the northeast coast created large waves during the fall and winter, with a series of jets providing a dominant source of wave energy during these periods. Evaluation of the wave model results against observations from a buoy and satellites found that the spatial resolution of the wind model significantly affected the quality of the wave model results. Wind forcing from a 10-km grid produced higher skills for waves than winds from a 30-km grid, largely due to under-prediction of the mean wind speed and wave height with the coarser grid. The 30-km grid did not resolve the mountain gap wind jets, and thus predicted lower wave heights in the central Red Sea during the summer and along the northeast coast in the winter.

Hydrodynamics of spur and groove formations on a coral reef

Abstract: [1] Spur and groove (SAG) formations are found on the fore reefs of many coral reefs worldwide. Although these formations are primarily present in wave-dominated environments, their effect on wave-driven hydrodynamics is not well understood. A two-dimensional, depth-averaged, phase-resolving nonlinear Boussinesq model (funwaveC) was used to model hydrodynamics on a simplified SAG system. The modeling results show that the SAG formations together with shoaling waves induce a nearshore Lagrangian circulation pattern of counter-rotating circulation cells. The mechanism driving the modeled flow is an alongshore imbalance between the pressure gradient (PG) and nonlinear wave (NLW) terms in the momentum balance. Variations in model parameters suggest the strongest factors affecting circulation include spur-normal waves, increased wave height, weak alongshore currents, increased spur height, and decreased bottom drag. The modeled circulation is consistent with a simple scaling analysis based on the dynamical balance of NLW, PG, and bottom stress terms. Model results indicate that the SAG formations efficiently drive circulation cells when the alongshore SAG wavelength allows for the effects of diffraction to create alongshore differences in wave height without changing the mean wave angle.

Sea‐cliff retreat and shore platform widening: steady‐state equilibrium?

Abstract: We challenge the notion of steady-state equilibrium in the context of progressive cliff retreat on micro-tidal coasts. Ocean waves break at or close to the abrupt seaward edge of near-horizontal shore platforms and then rapidly lose height due to turbulence and friction. Conceptual models assume that wave height decays exponentially with distance from the platform edge, and that the platform edge does not erode under stable sea-level. These assumptions combine to a steady-state view of Holocene cliff retreat. We argue that this model is not generally applicable. Recent data show that: (1) exponential decay in wave height is not the most appropriate conceptual model of wave decay; (2) by solely considering wave energy at gravity wave frequencies the steady-state model neglects a possible formative role for infragravity waves. Here we draw attention to possible mechanisms through which infragravity waves may drive cliff retreat over much greater distances (and longer timescales) than imaginable under the established conceptual model. Copyright © 2013 John Wiley & Sons, Ltd.

Surge and Wave Modeling for the Louisiana 2012 Coastal Master Plan

Abstract: Cobell, Z.; Zhao, H.; Roberts, H.J.; Clark, F.R., and Zou, S., 2013. Surge and wave modeling for the Louisiana 2012 Coastal Master Plan. The goal of the study was to evaluate various coastal restoration and protection projects and the associated benefits for reductions in storm surge and wave height. Efforts in numerical modeling have been made to create a database of storm surge and wave responses to a set of hypothetical storms under current and various future conditions. The ADvanced CIRCulation (ADCIRC) and the Unstructured Simulating WAves Nearshore (UnSWAN) models were selected for this study. A coarser version of the state-of-the-art, southern Louisiana, unstructured mesh was developed to reduce computational overhead while maintaining critical hydraulic features. Model outputs were reviewed and analyzed from coastwide and onshore-transect points of view. The potential benefits of restoration and protection projects proposed in the Master Plan were examined by comparing Future without Acti...

A new model for calculating the design wave height in typhoon-affected sea areas

Abstract: High precision design wave height is required in extreme marine environments in typhoon-affected sea areas. A new model is built under typhoon effect to calculate the design wave heights. The new model has multiple undetermined parameters, and it is able to fit observed data more flexibly and accurately. In addition, the distribution functions of this new model are based on the maximum entropy principle. Therefore, they can avoid the apriority, which means arbitrarily assigning Poisson distribution to describe the distribution of typhoon occurrence frequency and assigning Gumbel distribution or Pearson-III distribution to describe the distribution of extreme events in the process of applying the compound distribution to deduce the design elevations. The observed data of 18-year (1984–2001) extreme wave heights and the corresponding typhoon events in Maidao are used to test the model. Test results show that the new model is theoretically more stable and more precise when predicting the design wave heights under the typhoon-affected sea areas.

The complex singularity of a Stokes wave

Abstract: Two-dimensional potential flow of the ideal incompressible fluid with free surface and infinite depth can be described by a conformal map of the fluid domain into the complex lower half-plane. Stokes wave is the fully nonlinear gravity wave propagating with the constant velocity. The increase of the scaled wave height $H/\lambda$ from the linear limit $H/\lambda=0$ to the critical value $H_{max}/\lambda$ marks the transition from the limit of almost linear wave to a strongly nonlinear limiting Stokes wave. Here $H$ is the wave height and $\lambda$ is the wavelength. We simulated fully nonlinear Euler equations, reformulated in terms of conformal variables, to find Stokes waves for different wave heights. Analyzing spectra of these solutions we found in conformal variables, at each Stokes wave height, the distance $v_c$ from the lowest singularity in the upper half-plane to the real line which corresponds to the fluid free surface. We also identified that this singularity is the square-root branch point. The limiting Stokes wave emerges as the singularity reaches the fluid surface. From the analysis of data for $v_c\to 0$ we suggest a new power law scaling $v_c\propto (H_{max}-H)^{3/2}$ as well as new estimate $H_{max}/\lambda \simeq 0.1410633$.

Infragravity response to variable wave forcing in the nearshore

Abstract: [1] Analysis of field observations of sea-surface elevation reveals the role of time-varying incident forcing and bound wave release mechanisms on the generation of infragravity waves on a barred beach. Observations of infragravity response were obtained during sea breeze and swell periods with wave height varying from 0.2 to 1.2 m and wave period from 3 to 16 s. The observations show a stronger infragravity response to longer period incident swell than to short period wind-sea. During the peak of the sea breeze, short period wind-sea is conducive to long wave generation by breakpoint forcing. Under swell-dominated conditions, the frequency and wave number of incoming bound waves associated with wave groups may satisfy the linear dispersion relationship, leading to resonant forcing of a free wave that appears to be released from the wave group. The transition between long wave generation due to bound wave release and time-varying breakpoint forcing agrees with the surf beat similarity parameter.