Alvise Benetazzo

Linking reduced breaking crest speeds to unsteady nonlinear water wave group behavior

Observed crest speeds of maximally steep, breaking water waves are much slower than expected. Our fully nonlinear computations of unsteadily propagating deep water wave groups show that each wave crest approaching its maximum height slows down significantly and either breaks at this reduced speed, or accelerates forward unbroken. This previously noted crest slowdown behavior was validated as generic in our extensive laboratory and field observations. It is likely to occur in unsteady dispersive nonlinear wave groups in other natural systems.

A Variational Stereo Method for the Three-Dimensional Reconstruction of Ocean Waves

We develop a novel remote sensing technique for the observation of waves on the ocean surface. Our method infers the 3-D waveform and radiance of oceanic sea states via a variational stereo imagery formulation. In this setting, the shape and radiance of the wave surface are given by minimizers of a composite energy functional that combines a photometric matching term along with regularization terms involving the smoothness of the unknowns. The desired ocean surface shape and radiance are the solution of a system of coupled partial differential equations derived from the optimality conditions of the energy functional. The proposed method is naturally extended to study the spatiotemporal dynamics of ocean waves and applied to three sets of stereo video data. Statistical and spectral analysis are carried out. Our results provide evidence that the observed omnidirectional wavenumber spectrum S(k) decays as k-2.5 is in agreement with Zakharovs theory (1999). Furthermore, the 3-D spectrum of the reconstructed wave surface is exploited to estimate wave dispersion and currents.

Observation of Extreme Sea Waves in a Space–Time Ensemble

AbstractIn this paper, an observational space–time ensemble of sea surface elevations is investigated in search of the highest waves of the sea state. Wave data were gathered by means of a stereo camera system, which was installed on top of a fixed oceanographic platform located in the Adriatic Sea (Italy). Waves were measured during a mature sea state with an average wind speed of 11 m s−1. By examining the space–time ensemble, the 3D wave groups have been isolated while evolving in the 2D space and grabbed “when and where” they have been close to the apex of their development, thus exhibiting large surface displacements. The authors have selected the groups displaying maximal crest height exceeding the threshold adopted to define rogue waves in a time record, that is, 1.25 times the significant wave height (Hs). The records at the spatial positions where such large crests occurred have been analyzed to derive the empirical distributions of crest and wave heights, which have been compared against standar...

Analysis and Interpretation of Frequency–Wavenumber Spectra of Young Wind Waves

The energy level and its directional distribution are key observations for understanding the energy balance in the wind-wave spectrum between wind-wave generation, nonlinear interactions, and dissipation. Here, properties of gravity waves are investigated from a fixed platform in the Black Sea, equipped with a stereo video system that resolves waves with frequency f up to 1.4 Hz and wavelengths from 0.6 to 11 m. One representative record is analyzed, corresponding to young wind waves with a peak frequency fp = 0.33 Hz and a wind speed of 13 m s−1. These measurements allow for a separation of the linear waves from the bound second-order harmonics. These harmonics are negligible for frequencies f up to 3 times fp but account for most of the energy at higher frequencies. The full spectrum is well described by a combination of linear components and the second-order spectrum. In the range 2fp to 4fp, the full frequency spectrum decays like f−5, which means a steeper decay of the linear spectrum. The directional spectrum exhibits a very pronounced bimodal distribution, with two peaks on either side of the wind direction, separated by 150° at 4fp. This large separation is associated with a significant amount of energy traveling in opposite directions and thus sources of underwater acoustic and seismic noise. The magnitude of these sources can be quantified by the overlap integral I(f), which is found to increase sharply from less than 0.01 at f = 2fp to 0.11 at f = 4fp and possibly up to 0.2 at f = 5fp, close to the 0.5π value proposed in previous studies.

Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basin

ABSTRACT Sclavo, M., Benetazzo, A., Carniel, S., Bergamasco, A., Falcieri, F.M., and Bonaldo, D., 2013. Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basin The shallow northern Adriatic Sea (namely Gulf of Venice) serves as a model for exploring the interaction of surface gravity waves and oceanic currents and how they influence bottom sediment dispersal and bathymetry evolution. This wave-current interaction effect is investigated using the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system. COAWST relies on the Regional Ocean Modeling System (ROMS), the Simulating WAves Nearshore (SWAN) model, and the CSTMS (Community Sediment Transport Modeling System) models and routines. The 2-way data transfer between wave and ocean circulation models is synchronous via MCT (Model Coupling Toolkit), with ROMS providing to SWAN the 2-D current field, free surface elevation, and bathymetry. COAWST modeling system is implemented on two computational grids at different horizontal resolution: a parent grid (with resolution of 2.0 km) covering the whole Adriatic Sea and a child grid resolving the Gulf of Venice at a resolution of 0.5 km. Simulated waves and currents are validated against in-situ observations at the CNR-ISMAR Acqua Alta oceanographic tower, located 15 km off the Venice lagoon. The analysis of wave-current interaction effect on sediment dispersal and sea bottom evolution are performed over the 2011 winter season (January-March) with particular focus on the waves generated by dominant and prevailing winds blowing on the Adriatic Sea: Bora and Sirocco. Results show that while the effects on bottom stress may vary depending on wave propagation and current direction, the effects on advective dynamics may become dominant particularly in presence of severe storms with parallel wave propagation and global circulation, which is the case of Bora storms in northern Adriatic Sea.

Euler characteristics of oceanic sea states

We present an application of a novel Variational Wave Acquisition Stereo System (VWASS) for the estimation of the wave surface height of oceanic sea states. Specifically, we show that VWASS video technology combined with statistical techniques based on Euler Characteristics of random fields provides a new paradigm for the prediction of wave extremes expected over a given area of the ocean.

WAVE STATISTICS AND SPECTRA VIA A VARIATIONAL WAVE ACQUISITION STEREO SYSTEM

We propose a novel variational Wave Acquisition Stereo System (WASS) that exploits new stereo reconstruction techniques for accurate estimates of the spatio-temporal dynamics of ocean waves. WASS has a significant advantage as a low-cost system in both installation and maintenance. A stereo camera view provides three-dimensional data (both in space and time) whose statistical content is richer than that of a time series retrieved from wave gauges, ultrasonic instruments or buoys, the latter being expensive to install and maintain. Indeed, wave spectra can be easily estimated from the multi-dimensional images obtained with WASS. The estimated spectra present an inertial range that decays as k -2.5 , k being the wave number, in agreement with wave turbulence theory (Zakharov 1999, Socquet-Juglard et al. 2005). Further, the empirical probability density functions derived from the reconstructed surface data compare very well with theoretical models (Tayfun & Fedele 2007, Fedele 2008). The variational WASS is a promising technology with broader impacts in offshore engineering since it will enrich the understanding of the statistics of waves for an improved design of offshore structures.

Space-Time Waves and Spectra in the Northern Adriatic Sea via a Wave Acquisition Stereo System

A novel Wave Acquisition Stereo System (WASS ) is proposed for the stereo reconstruction of oceanic waves both in space and time. To test the performance of such video observational technology, we have deployed WASS at the oceanographic tower Acqua Alta in the Northern Adriatic Sea, off the Venice coast in Italy. The analysis of WASS video measurements yielded accurate estimates of the oceanic sea state dynamics, the associated directional spectra and wave surface statistics that agree well with theoretical models. Further, the expected largest wave surface height over an area is estimated via both Piterbarg’s theory (Piterbarg 1995) and Adler’s Euler characteristics (Adler 1981, Adler and Taylor 2007). It is found that the largest surface height over the imaged area is considerably larger than the maximum crest observed in time at a single point, in agreement with theoretical predictions.Copyright

Modelling wave-driven sediment transport in a changing climate: a case study for northern Adriatic Sea (Italy)

In this paper, we investigate the impact of climate change on coastal sediment transport in a deltaic system in the northern Adriatic Sea, with reference to the period 2070–2099 in the IPCC A1B emission scenario. Wind fields obtained by means of the high-resolution regional climate model Consortium for Small-scale Modelling–Climate Limited-area Modelling were employed for computing wave climate at basin scale by means of the spectral wave model Simulating Waves Nearshore. This was used as a constraint for a nearshore hydromorphodynamic model (MIKE LITPACK), which was applied to a test site on the Po River Delta, located in northern Italy. Relevant sediment transport processes have been studied at storm and decadal time scales in order to capture climate change effects on single events and as an overall trend. The transport rates in the A1B climate change scenario were then compared with the corresponding results of a control analysis (period 1965–1994) representing the actual climate. Although predicted wave climate in the investigated scenario displays an overall decrease in sea severity offshore in the northern Adriatic Sea, the effects of these modifications are modulated during the onshore propagation, with different impacts on sediment transport depending on the considered process and time scale. The strategy presented in this work can find fruitful applications in the long-term modelling of coastal and transitional environments, in which morphology is strongly influenced by sediment transport in the nearshore zone, suggesting a methodological approach for coastal planning and management.

On the two-dimensional structure of short gravity waves in a wind wave tank

Experiments on wind-waves in a laboratory tank are carried out for a variety of operational conditions. In addition to using a conventional wave gauge to determine the temporal variation of the surface elevation at the sensor location, independent measuring techniques such as 3D reconstruction of stereo video images and 2D laser slope gauge are applied in order to gain information on the three-dimensional structure of the wind-wave field. The consistency of results obtained by different methods under identical forcing conditions is examined to establish the limits of applicability and accuracy of each method. The accumulated results on the spatial and temporal coherence of the surface elevation variation, on the directional wave spectra, and on the probability distribution of the instantaneous surface slope directions demonstrate that the wind-wave field is essentially three-dimensional and short-crested.Experiments on wind-waves in a laboratory tank are carried out for a variety of operational conditions. In addition to using a conventional wave gauge to determine the temporal variation of the surface elevation at the sensor location, independent measuring techniques such as 3D reconstruction of stereo video images and 2D laser slope gauge are applied in order to gain information on the three-dimensional structure of the wind-wave field. The consistency of results obtained by different methods under identical forcing conditions is examined to establish the limits of applicability and accuracy of each method. The accumulated results on the spatial and temporal coherence of the surface elevation variation, on the directional wave spectra, and on the probability distribution of the instantaneous surface slope directions demonstrate that the wind-wave field is essentially three-dimensional and short-crested.