Showing papers on "Wave height published in 2011"

Rogue wave observation in a water wave tank.

Abstract: The conventional definition of rogue waves in the ocean is that their heights, from crest to trough, are more than about twice the significant wave height, which is the average wave height of the largest one-third of nearby waves. When modeling deep water waves using the nonlinear Schr\"odinger equation, the most likely candidate satisfying this criterion is the so-called Peregrine solution. It is localized in both space and time, thus describing a unique wave event. Until now, experiments specifically designed for observation of breather states in the evolution of deep water waves have never been made in this double limit. In the present work, we present the first experimental results with observations of the Peregrine soliton in a water wave tank.

Global Trends in Wind Speed and Wave Height

Abstract: Studies of climate change typically consider measurements or predictions of temperature over extended periods of time. Climate, however, is much more than temperature. Over the oceans, changes in wind speed and the surface gravity waves generated by such winds play an important role. We used a 23-year database of calibrated and validated satellite altimeter measurements to investigate global changes in oceanic wind speed and wave height over this period. We find a general global trend of increasing values of wind speed and, to a lesser degree, wave height, over this period. The rate of increase is greater for extreme events as compared to the mean condition.

Fifth order gravity wave theory

Abstract: In dealing with problems connected with gravity waves, scientists and engineers frequently find it necessary to make lengthy theoretical calculations involving such wave characteristics as wave height, wave length, period, and water depth. Several approximate theoretical expressions have been derived relating the above parameters. Airy, for instance, contributed a very valuable and complete theory for waves traveling over a horizontal bottom in any depth of water. Due to the simplicity of the Airy theory, it is frequently used by engineers. This theory, however, was developed for waves of very small heights and is inaccurate for waves of finite height. Stokes presented a similar solution for waves of finite height by use of trigonometric series. Using five terms in the series, this solution will extend the range covered by the Airy theory to waves of greater steepness. No attempt has been made in this paper to specify the range where the theory is applicable. The coefficients in these series are very complicated and for a numerical problem, the calculations become very tedious. Because of this difficulty, this theory would be very little used by engineers unless the value of the coefficient is presented in tabular form. The purpose of this paper is to present the results of the fifth order theory and values of the various coefficients as a function of the parameter d/L.

Wave runup during extreme storm conditions

Abstract: [1] Video measurements of wave runup were collected during extreme storm conditions characterized by energetic long swells (peak period of 16.4 s and offshore height up to 6.4 m) impinging on steep foreshore beach slopes (0.05–0.08). These conditions induced highly dissipative and saturated conditions over the low-sloping surf zone while the swash zone was associated with moderately reflective conditions (Iribarren parameters up to 0.87). Our data support previous observations on highly dissipative beaches showing that runup elevation (estimated from the variance of the energy spectrum) can be scaled using offshore wave height alone. The data is consistent with the hypothesis of runup saturation at low frequencies (down to 0.035 Hz) and a hyperbolic-tangent fit provides the best statistical predictor of runup elevations.

Global estimates of extreme wind speed and wave height

Abstract: A long-term dataset of satellite altimeter measurements of significant wave height and wind speed, spanning 23 years, is analyzed to determine extreme values corresponding to a 100-yr return period. The analysis considers the suitability of both the initial distribution method (IDM) and peaks-over-threshold (POT) approaches and concludes that for wave height both IDM and POT methods can yield reliable results. For the first time, the global POT results for wave height show spatial consistency, a feature afforded by the larger dataset. The analyses also show that the POT approach is sensitive to spatial resolution. Since wind speed has greater spatial and temporal variability than wave height, the POT approach yields unreliable results for wind speed as a result of undersampling of peak events. The IDM approach does, however, generate extreme wind speed values in reasonable agreement with buoy estimates. The results show that the altimeter database can estimate 100-yr return period significant wav...

Large-scale experiments on wave propagation over Posidonia oceanica

Abstract: Posidonia oceanica, the most abundant seagrass species in the Mediterranean, supports a highly bio-diverse habitat and is crucial in protecting against coastal erosion. In this work, experiments in a large-scale facility have been performed, for the measurement of wave attenuation, transmission and energy dissipation over artificial Posidonia oceanica. The effects of submergence ratio corresponding to the seagrass height divided by water depth, and seagrass density as the number of stems per square metre on the above characteristics are investigated. Measurements of wave height at different locations along the vegetation meadow indicate the wave attenuation along the Posidonia oceanica for three different submergence ratios and two seagrass densities. Results are also analysed with regard to the wave-induced flow within the meadow, and the effects of the submergence ratio and the seagrass density on the mean flow characteristics, based on data of mean velocities taken at three locations within the seagrass.

Ocean Wave Integral Parameter Measurements Using Envisat ASAR Wave Mode Data

Abstract: An empirical algorithm to retrieve integral ocean wave parameters such as significant wave height (SWH), mean wave period, and wave height of waves with period larger than 12 s (H12) from synthetic aperture radar (SAR) images over sea surface is presented. The algorithm is an extension to the Envisat Advanced SAR (ASAR) wave mode data based on the CWAVE approach developed for ERS-2 SAR wave mode data and is thus called CWAVE_ENV (CWAVE for Envisat). Calibrated ASAR images are used as the only source of input without needing prior information from an ocean wave model (WAM) as the standard algorithms used in weather centers. This algorithm makes SAR an independent instrument measuring integrated wave parameters like SWH and mean wave period to altimeter quality. A global data set of 25 000 pairs of ASAR wave mode images and collocated reanalysis WAM results from the European Centre for Medium-Range Weather Forecasts (ECMWF) is used to tune CWAVE_ENV model coefficients. Validation conducted by comparing the retrieved SWH to in situ buoy measurements shows a scatter index of 0.24 and 0.16 when compared to the ECMWF reanalysis WAM. Two case studies are presented to evaluate the performance of the CWAVE_ENV algorithm for high sea state. A North Atlantic storm during which SWH is above 18 m as observed by SAR and Radar Altimeter simultaneously is analyzed. For an extreme swell case that occurred in the Indian Ocean, the potential of using SWH measurements from ASAR wave mode data derived by the CWAVE_ENV algorithm is demonstrated.

The dissipation of wind wave energy across a fringing reef at Ipan, Guam

Abstract: Field observations over a fringing reef at Ipan, Guam, during trade wind and tropical storm conditions are used to assess the transformation of sea and swell energy from the fore reef to the shoreline. Parameterizations of wave breaking and bottom friction developed for sandy beaches are found to represent the observed decay in wave energy with an increased friction coefficient. These parameterizations are incorporated into the one-dimensional energy flux balance, which is integrated across the reef to assess the effects of varying tidal range, incident wave height and reef bathymetry on the sea and swell band wave height and wave setup near the shoreline. Wave energy on the reef is strongly depth-limited and controlled by the reef submergence level. Shoreline wave energy increases with incident wave height largely due to the increase in water level from breaking wave setup. Increased tidal levels result in increased shoreline energy, since wave setup is only weakly reduced. The wave height at the shore is shown to be inversely proportional to the width of the reef flat due to dissipation.

Nearshore circulation in a tropical fringing reef system

Abstract: [1] The role of waves, tide, and wind on the circulation of a fringing reef system was investigated using data collected during a 6 week field experiment in a section of Ningaloo Reef off Western Australia The high correlation observed between current velocities and wave height throughout the system revealed the dominant role wave breaking plays in driving the overall reef-lagoon circulation, whereas the modulation of the currents at tidal frequencies suggested that the wave-driven currents responded to tidal variations in the mean water level over the reef The influence of the various forcing mechanisms on the current field was investigated for both high- and low-frequency bands Wave breaking was found to be the dominant forcing mechanism for the low-frequency (subtidal) currents, with the subtidal flow pattern consisting of a cross-reef flow over the reef, alongshore flow in the lagoon, and water exiting back to the ocean through the main channel The tides controlled the high-frequency current variability via two mechanisms: one associated with the ebb-flood cycle of the tides and the second associated with tidal modulations of the wave-driven currents Wind-forcing and buoyancy effects were both found to be negligible in driving the circulation and flushing of the system during the observation period Flushing time scale estimates varied from as low as 2 h to more than a day for the wide range of observed incident wave heights The results suggest that the circulation of Ningaloo Reef will be strongly influenced by even a small mean sea level rise

Experimental Setup for a Large-Scale Bridge Superstructure Model Subjected to Waves

Abstract: To gain a better understanding of the wave forces that led to the failure of numerous causeway-type coastal highway bridges along the U.S. Gulf coast, a series of experiments was conducted on a realistic, 1:5 scale reinforced concrete bridge superstructure. The experimental setup is unique compared to other wave-in-deck studies in that the stiffness of the horizontal support system can be varied to represent different dynamic properties of the bridge system. The bridge specimen is subjected to a wide range of regular and random wave conditions at multiple water levels. In addition to measuring pressures and forces, the experiments measure the dynamic response of the bridge specimen using strain gauges, displacement sensors, and accelerometers. This paper presents the innovative experimental setup, and a preliminary analysis of the data showing the effect of wave height, wave period, and water level, on the forces experienced by the bridge superstructure.

Modeling wind waves and tidal flows in shallow micro-tidal basins

Abstract: Observational evidence and mathematical modeling have demonstrated the crucial role of wind waves on sediment resuspension in shallow micro-tidal basins, where tidal fluxes alone are unable to mobilize tidal flat sediments Carniello et al (2005) presented a numerical model which combines wind waves with tidal fluxes in a shallow micro-tidal basin The highly irregular bathymetry typical of these environments characterized by the presence of deep channels, emergent salt marshes and extensive tidal flats suggested the introduction of specific hypotheses while solving the wave action conservation equation to describe wind-wave generation and propagation In particular, as suggested by field measurements, the wave spectrum in this type of environment is quite often very narrow Thus Carniello et al (2005) originally followed a monochromatic approach, further assuming that the direction of wave propagation instantaneously adjusts to the wind direction and a constant wave period both in space and time In the present contribution, we relax the latter assumption by introducing a variable wave period through a suitable empirical power law which relates wave period to wind speed and flow depth, based on wind-wave data collected in the Venice Lagoon The same relationship came out to fit quite well also data collected in Lake George (AU) and, more recently, data collected in a system of lagoons at the Virginia Coast Reserve, USA (Mariotti et al, 2010) The noteworthy improvement in the estimation of wave height obtained by considering the local wave period is shown on the basis of the results of a number of simulations carried out for different storm events The improvement obtained in the wind-wave field description reflects also on the estimation of the bottom shear stress and, therefore, on the description of the processes responsible for the morphological evolution of shallow tidal environments

Flow depths and velocities at crest and landward slope of a dike, in theory and with the wave overtopping simulator

Abstract: Wave overtopping discharges at coastal structures are well described in the EurOtop Manual (2007), including the distribution of overtopping wave volumes. Each volume that overtops a dike or levee will have a certain flow velocity and depth record in time, often given by the maximum velocity and flow depth. This paper describes some further development of the theory on flow depth and velocities on the crest, but will also show an inconsistency with respect to the mass balance. The second part of the paper gives an analysis of measured values on real dikes, simulated by the Wave Overtopping Simulator. It gives also the method of "cumulative hydraulic load" to compare overtopping discharges for different wave conditions. A large wave height with less overtopping waves, but larger overtopping wave volumes, is more damaging than a small wave height with more, but smaller overtopping volumes, even if the overtopping discharge is similar. The reasons to develop the cumulative hydraulic load have been compared with the recently in the US developed method of erosional equivalence.

Wave Dissipation by Vegetation

Abstract: Flooding resulting from hurricanes and other extreme storm events is a prominent risk along the coasts. These coastal areas are typically of low elevation and relief,making land and infrastructure highly susceptible to inundation by storm surge and waves. These verity of this threat is exacerbated by sea level rise and a possible increase in storm frequency and strength due to climate change. Although hard protection structures such as levees and flood walls reduce flood risk, these structures may fail when storm conditions exceed the design threshold. There is a general consensus that wetlands, which often serve as transition zones between open water and dry land, could act as buffers and reduce storm surge and propagatingwaves substantially before they encounter coastal development. Unfortunately, the capability of wetlands to serve as protection during extreme storms is not understood fully or well documented; furthermore, water level and wave height reductions by vegetation are studied only in low-energy environments. Nonetheless, these studies present methods to quantify vegetation induced wave attenuation for both modeling and design. This technical note focuses on the damping of propagating water waves by vegetation, but also discusses surge reduction briefly.Although waves may be encountered in freshwater environments(e.g., boat wakes, lake fetch,flood waves, etc.), this review focuses on coastal vegetation and resultant effects on flood and storm damage reduction.

Predicting the response of hard and soft rock coasts to changes in sea level and wave height

Abstract: A mathematical model was used to predict the effect of climate change on soft and hard rock coasts in a 2 m tidal environment. Erosional equations represented the effect of wave impact and bottom generated shear stresses in the intertidal and subtidal zones. Model runs were made for: 2900 years with constant sea level; a further 100 years, representing the last century, with either constant or slow sea level rise (0.2 m per century); and another 100 years, representing the present century, with either slow or fast (1 m per century) sea level rise, and with either no change in storm frequency or with a 10% increase in the frequency of the highest waves. The results suggest that rising sea level will trigger faster rates of cliff recession, whereas increased storm wave frequency may have only a fairly minor effect on erosional efficacy. Model runs were used to derive a series of predictive equations relating cliff recession during the present and last centuries.

Modelling of multi-bodies in close proximity under water waves: Fluid resonance in narrow gaps

Abstract: Viscous fluid model and potential flow model with and without artificial damping force (f = −µV, µ the damping coefficient and V the local averaging flow velocity) are employed in this work to investigate the phenomenon of fluid resonance in narrow gaps between multi-bodies in close proximity under water waves. The numerical results are compared with experimental data available in the literature. The comparison demonstrates that both the viscous fluid model and the potential flow model are able to predict the resonant frequency reasonably well. However the conventional potential flow model (without artificial damping term) significantly over-predicts the wave height in narrow gaps around the resonant frequency. In order to calibrate the appropriate damping coefficient used for the potential model and make it work as well as the viscous fluid model in predicting the resonant wave height in narrow gaps but with little computational efforts, the dependence of damping coefficient µ on the body geometric dimensions is examined considering the parameters of gap width B g, body draft D, body breadth ratio B r and body number n (n = 2, 3), where B r = B B/B A for the case of two bodies (Body A and Body B) with different breadths of B A and B B, respectively. It was confirmed that the damping coefficient used for the potential flow model is not sensitive to the geometric dimensions and spatial arrangement. It was found that µ ∈ [0.4, 0.5] may guarantee the variation of H g/H 0 with kh to be generally in good agreement with the experimental data and the results of viscous fluid model, where H g is the excited wave height in narrow gaps under various dimensionless incident wave frequencies kh, H 0 is the incident wave height, k = 2π/L is the wave number and h is the water depth.

Nonlinear random wave field in shallow water: variable Korteweg-de Vries framework

Abstract: . The transformation of a random wave field in shallow water of variable depth is analyzed within the framework of the variable-coefficient Korteweg-de Vries equation. The characteristic wave height varies with depth according to Green's law, and this follows rigorously from the theoretical model. The skewness and kurtosis are computed, and it is shown that they increase when the depth decreases, and simultaneously the wave state deviates from the Gaussian. The probability of large-amplitude (rogue) waves increases within the transition zone. The characteristics of this process depend on the wave steepness, which is characterized in terms of the Ursell parameter. The results obtained show that the number of rogue waves may deviate significantly from the value expected for a flat bottom of a given depth. If the random wave field is represented as a soliton gas, the probabilities of soliton amplitudes increase to a high-amplitude range and the number of large-amplitude (rogue) solitons increases when the water shallows.

Assessment of future stability of breakwaters under climate change

Abstract: Climate change is expected to lead to increases in both sea level and typhoon intensity, which could threaten the stability of breakwaters in the future. In this study, calculations using the SWAN model showed that a 10% potential increase in the future wind speed of typhoons resulting from the warming of surface sea temperatures can lead to a 21% increase in the significant wave heights generated by these winds. To understand the effect that this would have on breakwater stability, the expected sliding distances for the breakwaters at Shibushi Ports in Japan were estimated using a probabilistic design method. The results show that in the future the expected sliding distances may become five times greater than at present, due to a combination of increases in sea level and wave height.

Turbulent Mixing due to Surface Waves Indicated by Remote Sensing of Suspended Particulate Matter and Its Implementation into Coupled Modeling of Waves, Turbulence, and Circulation

Abstract: This paper studies the impact of the surface waves on the turbulent mixing. The satellite observations of suspended particulate matter (SPM) at the ocean surface as an indicator of turbulent quantities of the flow are used. In a water column, SPM builds a vertical profile depending on settling velocities of the particles and on vertical mixing processes; thus, SPM is a perfect marker to study the turbulent quantities of the flow. Satellite observations in the North Sea show that surface SPM concentrations, in locations of its deposition, grow rapidly and build plume-shaped, long (many kilometers) uninterrupted and consistent structures during a storm. Also, satellites reveal that SPM rapidly sinks to the seabed after the storm peak has passed and wave height decreases (i.e., in the absence of strong turbulence). The nonbreaking wave-induced turbulence has been discussed, parameterized, and implemented into an equation of evolution of turbulent kinetic energy (TKE) in the frame of mean-flow concept, which can be used in existing circulation models. The ratio between dissipated and total wave energy is used to describe the influence of wave damping on the mean flow. The numerical tests reproduce experiments in a wave tank very well and are supported by observations of SPM in the North Sea. Their results show that the motion of an individual nonbreaking wave includes turbulent fluctuations if the critical Reynolds number for wave motion is exceeded, independent of the presence of currents due to wind or tides. These fluctuations can produce high diffusivity and strongly influence mixing in the upper water layer of the ocean.

Nearshore Tsunami Inundation Model Validation: Toward Sediment Transport Applications

Abstract: Model predictions from a numerical model, Delft3D, based on the nonlinear shallow water equations are compared with analytical results and laboratory observations from seven tsunami-like benchmark experiments, and with field observations from the 26 December 2004 Indian Ocean tsunami. The model accurately predicts the magnitude and timing of the measured water levels and flow velocities, as well as the magnitude of the maximum inundation distance and run-up, for both breaking and non-breaking waves. The shock-capturing numerical scheme employed describes well the total decrease in wave height due to breaking, but does not reproduce the observed shoaling near the break point. The maximum water levels observed onshore near Kuala Meurisi, Sumatra, following the 26 December 2004 tsunami are well predicted given the uncertainty in the model setup. The good agreement between the model predictions and the analytical results and observations demonstrates that the numerical solution and wetting and drying methods employed are appropriate for modeling tsunami inundation for breaking and non-breaking long waves. Extension of the model to include sediment transport may be appropriate for long, non-breaking tsunami waves. Using available sediment transport formulations, the sediment deposit thickness at Kuala Meurisi is predicted generally within a factor of 2.

On the Mach reflection of a solitary wave: revisited

Abstract: Reflection of an obliquely incident solitary wave at a vertical wall is studied experimentally in the laboratory wave tank Precision measurements of water-surface variations are achieved with the aid of laser-induced fluorescent (LIF) technique and detailed features of the Mach reflection are captured During the development stage of the reflection process, the stem wave is not in the form of a Korteweg- de Vries (KdV) soliton but a forced wave, trailing by a continuously broadening depression Evolution of stem-wave amplification is in good agreement with the Kadomtsev―Petviashvili (KP) theory The asymptotic characteristics and behaviours are also in agreement with the theory of Miles (J Fluid Mech, vol 79, 1977b, p 171) except those in the neighbourhood of the transition between the Mach reflection and the regular reflection The predicted maximum fourfold amplification of the stem wave is not realized in the laboratory environment On the other hand, the laboratory observations are in excellent agreement with the previous numerical results of the higher-order model of Tanaka (J Fluid Mech, vol 248, 1993, p 637) The present laboratory study is the first to sensibly analyse validation of the theory; note that substantial discrepancies exist from previous (both numerical and laboratory) experimental studies Agreement between experiments and theory can be partially attributed to the large-distance measurements that the precision laboratory apparatus is capable of More important, to compare the laboratory results with theory, the corrected interaction parameter is derived from proper interpretation of the theory in consideration of the finite incident wave angle Our laboratory data indicate that the maximum stem wave can reach higher than the maximum solitary wave height The wave breaking near the wall results in the substantial increase in wave height and slope away from the wall

Effects of climate change and wave direction on longshore sediment transport patterns in Southern California

Abstract: Changes in deep-water wave climate drive coastal morphologic change according to unique shoaling transformation patterns of waves over local shelf bathymetry. The Southern California Bight has a particularly complex shelf configuration, of tectonic origin, which poses a challenge to predictions of wave driven, morphologic coastal change. Northward shifts in cyclonic activity in the central Pacific Ocean, which may arise due to global climate change, will significantly alter the heights, periods, and directions of waves approaching the California coasts. In this paper, we present the results of a series of numerical experiments that explore the sensitivity of longshore sediment transport patterns to changes in deep water wave direction, for several wave height and period scenarios. We outline a numerical modeling procedure, which links a spectral wave transformation model (SWAN) with a calculation of gradients in potential longshore sediment transport rate (CGEM), to project magnitudes of potential coastal erosion and accretion, under proscribed deep water wave conditions. The sediment transport model employs two significant assumptions: (1) quantity of sediment movement is calculated for the transport-limited case, as opposed to supply-limited case, and (2) nearshore wave conditions used to evaluate transport are calculated at the 5-meter isobath, as opposed to the wave break point. To illustrate the sensitivity of the sedimentary system to changes in deep-water wave direction, we apply this modeling procedure to two sites that represent two different coastal exposures and bathymetric configurations. The Santa Barbara site, oriented with a roughly west-to-east trending coastline, provides an example where the behavior of the coastal erosional/accretional character is exacerbated by deep-water wave climate intensification. Where sheltered, an increase in wave height enhances accretion, and where exposed, increases in wave height and period enhance erosion. In contrast, all simulations run for the Torrey Pines site, oriented with a north-to-south trending coastline, resulted in erosion, the magnitude of which was strongly influenced by wave height and less so by wave period. At both sites, the absolute value of coastal accretion or erosion strongly increases with a shift from northwesterly to westerly waves. These results provide some examples of the potential outcomes, which may result from increases in cyclonic activity, El Nino frequency, or other changes in ocean storminess that may accompany global climate change.