scholarly journals WAVE GROUPINESS AS A SOURCE OF NEARSHORE LONG WAVES

1986 ◽  
Vol 1 (20) ◽  
pp. 38 ◽  
Author(s):  
Jeffrey H. List
Keyword(s):  
Wave Amplitude ◽  
Cross Correlation ◽  
Surf Zone ◽  
Low Frequency ◽  
Long Waves ◽  
Progressive Wave ◽  
Wave Groups ◽  
Long Wave ◽  
Cross Correlation Analysis ◽  
Limited Usefulness

Data from a low energy swell-dominated surf zone are examined for indications that observed low frequency motions are simply group-forced bounded long waves. Time series of wave amplitude are compared to filtered long wave records through cross-spectral and cross-correlation analysis. These methods are found to have limited usefulness until long waves are separated into seaward and shoreward components. Then a clear picture of a rapidly shoaling bounded long wave emerges, with a minimum of nearly one fourth of the long wave amplitude being explainable by this type of motion close to shore. Through the zone in which waves were breaking, and incident wave amplitude variability decreased by 50%, the contribution from the bounded long wave continued to increase at a rate much greater than a simple shoaling effect. Also present are clear signs that this amplified bounded long wave is reflected from a position close to the shoreline, and is thus released from wave groups as a free, offshore-progressive wave.

Long wave generation by the shoaling and breaking of transient wave groups on a beach

2006 ◽  
Vol 462 (2070) ◽  
pp. 1853-1876 ◽  
Author(s):  
T.E Baldock
Keyword(s):  
Shallow Water ◽  
Water Waves ◽  
Surf Zone ◽  
Laboratory Data ◽  
Short Wave ◽  
Long Waves ◽  
Transient Wave ◽  
Wave Groups ◽  
Spatial Gradients ◽  
Long Wave

This paper presents new laboratory data on the generation of long waves by the shoaling and breaking of transient-focused short-wave groups. Direct offshore radiation of long waves from the breakpoint is shown experimentally for the first time. High spatial resolution enables identification of the relationship between the spatial gradients of the short-wave envelope and the long-wave surface. This relationship is consistent with radiation stress theory even well inside the surf zone and appears as a result of the strong nonlinear forcing associated with the transient group. In shallow water, the change in depth across the group leads to asymmetry in the forcing which generates significant dynamic setup in front of the group during shoaling. Strong amplification of the incident dynamic setup occurs after short-wave breaking. The data show the radiation of a transient long wave dominated by a pulse of positive elevation, preceded and followed by weaker trailing waves with negative elevation. The instantaneous cross-shore structure of the long wave shows the mechanics of the reflection process and the formation of a transient node in the inner surf zone. The wave run-up and relative amplitude of the radiated and incident long waves suggests significant modification of the incident bound wave in the inner surf zone and the dominance of long waves generated by the breaking process. It is proposed that these conditions occur when the primary short waves and bound wave are not shallow water waves at the breakpoint. A simple criterion is given to determine these conditions, which generally occur for the important case of storm waves.

scholarly journals SURF BEAT GENERATION ON A MILD-SLOPE BEACH

1988 ◽  
Vol 1 (21) ◽  
pp. 79 ◽  
Author(s):  
Hemming A. Schaffer ◽  
Ib A. Svendsen
Keyword(s):  
Surf Zone ◽  
Beat Generation ◽  
Radiation Stress ◽  
Inhomogeneous Wave ◽  
Wave Groups ◽  
Forcing Function ◽  
Long Wave ◽  
Inhomogeneous Wave Equation ◽  
Slope Beach ◽  
Resonant Generation

Two dimensional generation of surf beats by incident wave groups is examined theoretically. An inhomogeneous wave equation describes the amplitude of the surf beat wave. The forcing function is the modulation of the radiation stress. The short waves are amplitude modulated both outside and inside the surf zone causing the long wave generation to continue right to the shore line. Resonant generation as shallow water is approached is included. The analytical solution is evaluated numerically and shows a highly complicated amplitude variation of the surf beat depending on the parameters of the problem.

Low Frequency Surf Zone Response to Wave Groups

1999 ◽  
Author(s):  
Merrick C. Haller ◽  
Uday Putrevu ◽  
Joan Oltman-Shay ◽  
Robert A. Dalrymple
Keyword(s):  
Surf Zone ◽  
Low Frequency ◽  
Wave Groups

scholarly journals Upstream Propagating Long-Wave Modes at a Microtidal River Mouth

2020 ◽  
Vol 2 (1) ◽  
pp. 15
Author(s):  
Matteo Postacchini ◽  
Lorenzo Melito ◽  
Alex Sheremet ◽  
Joseph Calantoni ◽  
Giovanna Darvini ◽  
...  
Keyword(s):  
Wave Propagation ◽  
Field Data ◽  
Tide Gauge ◽  
Low Frequency ◽  
River Mouth ◽  
Long Waves ◽  
Tidal Forcing ◽  
Long Wave ◽  
Low Frequency Waves ◽  
Wave Modes

We illustrate recent findings on the upriver propagation of long waves entering the mouth of the Misa River (Senigallia, Italy). Such a microtidal environment has been recently studied to understand river–sea interactions: it has been found that the river forcing dominates over the marine actions in winter, especially during storms. However, upriver wave propagation is not negligible with low-frequency waves propagating upriver for distances of the order of kilometers. With the aim to better understand the behavior of low-frequency waves propagating upriver, the analysis of the present work builds on field data collected by instruments installed close to the mouth and along the final reach of the Misa River: a tide gauge, two hydrometers and an acoustic Doppler sensor. It has been here observed that the tidal forcing (periods of the order of hours/days) is significantly strong at a distance of more than one kilometer from the river mouth, while shorter waves, like seiches (periods of some hours), are less important and are supposed to largely dissipate at the estuary, although their role could be of importance during relatively short events (e.g., floods).

Infragravity waves induced by short-wave groups

1993 ◽  
Vol 247 ◽  
pp. 551-588 ◽  
Author(s):  
Hemming A. Schäffer
Keyword(s):  
Surf Zone ◽  
Break Point ◽  
Short Wave ◽  
Radiation Stress ◽  
Wave Groups ◽  
Infragravity Waves ◽  
Short Waves ◽  
Long Wave ◽  
Integrated Conservation ◽  
Infragravity Wave

A theoretical model for infragravity waves generated by incident short-wave groups is developed. Both normal and oblique short-wave incidence is considered. The depth-integrated conservation equations for mass and momentum averaged over a short-wave period are equivalent to the nonlinear shallow-water equations with a forcing term. In linearized form these equations combine to a second-order long-wave equation including forcing, and this is the equation we solve. The forcing term is expressed in terms of the short-wave radiation stress, and the modelling of these short waves in regard to their breaking and dynamic surf zone behaviour is essential. The model takes into account the time-varying position of the initial break point as well as a (partial) transmission of grouping into the surf zone. The former produces a dynamic set-up, while the latter is equivalent to the short-wave forcing that takes place outside the surf zone. These two effects have a mutual dependence which is modelled by a parameter K, and their relative strength is estimated. Before the waves break, the standard assumption of energy conservation leads to a variation of the radiation stress, which causes a bound, long wave, and the shoaling bottom results in a modification of the solution known for constant depth. The respective effects of this incident bound, long wave and of oscillations of the break-point position are shown to be of the same order of magnitude, and they oppose each other to some extent. The transfer of energy from the short waves to waves at infragravity frequencies is analysed using the depth-integrated conservation equation of energy. For the case of normally incident groups a semi-analytical steady-state solution for the infragravity wave motion is given for a plane beach and small primary-wave modulations. Examples of the resulting surface elevation as well as the corresponding particle velocity and mean infragravity-wave energy flux are presented. Also the sensitivity to the variation of input parameters is analysed. The model results are compared with laboratory experiments from the literature. The qualitative agreement is good, but quantitatively the model overestimates the infragravity wave activity. This can, in part, be attributed to the neglect of frictional effects.

Harbour excitations by incident wave groups

1990 ◽  
Vol 217 ◽  
pp. 595-613 ◽  
Author(s):  
Jiang-Kang Wu ◽  
Philip L.-F. Liu
Keyword(s):  
Incident Wave ◽  
Multiple Scales ◽  
Low Frequency ◽  
Second Order ◽  
Long Waves ◽  
Wave Groups ◽  
Order Of Magnitude ◽  
Harbour Resonance ◽  
Analytical Expressions ◽  
Multiple Scales Perturbation Method

By using the multiple-scales perturbation method, analytical solutions are obtained for the second-order low-frequency oscillations inside a rectangular harbour excited by incident wave groups. The water depth is a constant. The width of the harbour entrance is of the same order of magnitude as the wavelength of incident carrier (short) waves, but small in comparison with the wavelength of the wave envelope. Because of the modulations in the wave envelope, a second-order long wave is locked in with the wave envelope and propagates with the speed of the group velocity. Outside the harbour, locked long waves also exist in the reflected wave groups, but not in the radiated wave groups. Inside the harbour, the analytical expressions for the locked long waves are obtained. Owing to the discontinuity of the locked long waves across the harbour mouth, second-order free long waves are generated. The free long waves propagate with a speed of (gh)½ inside and outside the harbour. The free long waves inside the harbour may be resonated in a low-frequency range which is relevant to the harbour resonance.

scholarly journals Amplitude–phase calibration of tri-axial accelerometers in the low-frequency range by a LDV

2019 ◽  
Vol 8 (1) ◽  
pp. 223-231 ◽  
Author(s):  
Giulio D'Emilia ◽  
Antonella Gaspari ◽  
Emanuela Natale
Keyword(s):  
Cross Correlation ◽  
Laser Doppler Vibrometer ◽  
Low Frequency ◽  
Frequency Range ◽  
Critical Aspect ◽  
Calibration Uncertainty ◽  
Cross Correlation Analysis ◽  
Phase Calibration ◽  
Linear Slide ◽  
Phase Evaluation

Abstract. A method for amplitude–phase calibration of tri-axial accelerometers in the low-frequency range (0 to 20 Hz) is proposed, based on a linear slide, used to excite all the axes of the accelerometer at the same time, and a laser Doppler vibrometer (LDV) as a reference. For the phase evaluation different methods, like cross-correlation analysis and cross-spectrum analysis, have also been used for validation purposes. The procedure includes many further validation actions in order to verify the correctness of modelling of the systems being tested, of the data processing and to reduce the calibration uncertainty. Results show that the phase is a critical aspect to consider in calibration, more than the amplitude, and the comparison with the theoretical model is useful to verify the hypotheses. Different behaviours result depending on the elements of the measurement chain and not only on the type of accelerometer.

scholarly journals Generation, Transformation, and Scattering of Long Waves Induced by a Short-Wave Group over Finite Topography

2011 ◽  
Vol 41 (10) ◽  
pp. 1842-1859 ◽  
Author(s):  
Qingping Zou
Keyword(s):  
Analytical Solutions ◽  
Wave Train ◽  
Short Wave ◽  
Long Waves ◽  
Wave Group ◽  
Wave Groups ◽  
Long Wave ◽  
Wave Orbital Velocity ◽  
Variable Water ◽  
Horizontal Bottom

Abstract Second-order analytical solutions are constructed for various long waves generated by a gravity wave train propagating over finite variable depth h(x) using a multiphase Wentzel–Kramers–Brillouin (WKB) method. It is found that, along with the well-known long wave, locked to the envelope of the wave train and traveling at the group velocity Cg, a forced long wave and free long waves are induced by the depth variation in this region. The forced long wave depends on the depth derivatives hx and hxx and travels at Cg, whereas the free long waves depend on h, hx, and hxx and travel in the opposite directions at . They interfere with each other and generate free long waves radiating away from this region. The author found that this topography-induced forced long wave is in quadrature with the short-wave group and that a secondary long-wave orbital velocity is generated by variable water depth, which is in quadrature with its horizontal bottom counterpart. Both these processes play an important role in the energy transfer between the short-wave groups and long waves. These behaviors were not revealed by previous studies on long waves induced by a wave group over finite topography, which calculated the total amplitude of long-wave components numerically without consideration of the phase of the long waves. The analytical solutions here also indicate that the discontinuity of hx and hxx at the topography junctions has a significant effect on the scattered long waves. The controlling factors for the amplitudes of these long waves are identified and the underlying physical processes systematically investigated in this presentation.

scholarly journals SEPARATION OF LOW-FREQUENCY WAVES BY AN ANALYTICAL METHOD

2011 ◽  
Vol 1 (32) ◽  
pp. 64
Author(s):  
Yuxiang Ma ◽  
Guohai Dong ◽  
Xiaozhou Ma
Keyword(s):  
Fluid Mechanics ◽  
Gravity Waves ◽  
Present Method ◽  
Low Frequency ◽  
Wave Generation ◽  
Coastal Engineering ◽  
Wave Groups ◽  
International Conference ◽  
Long Wave ◽  
Low Frequency Waves

A new method for separating low-frequency waves in time domain is proposed by constructing the analytical signals of the measured waves. Using three simultaneous wave records, the time series of incident bound, free and reflected low-frequency waves can be obtained by the present method. This method is only suitable for separating monochromatic low-frequency waves. The applicability of the method is examined by numerical tests. The results show that the present method can give accurate results over sloping beaches when water depth (kh) is larger than 0.2. Then, the present method is used to study an experiment of low-frequency waves over a mild slope beach. References Bakkenes, H.J. 2002. Observation and separation of bound and free low-frequency waves in the nearshore zone, in Faculty of Civil Engineering and Geosciences. Delft University of Technology: Delft. Baldock, T.E., D.A., Huntley, P.A.D., Bird, T.O., Hare, and G.N., Bullock. 2000. Breakpoint generated surf beat induced by bichromatic wave groups. Coastal Engineering. 30 (2-4): 213-242. http://dx.doi.org/10.1016/S0378-3839(99)00061-7 Battjes, J.A., Bakkenes, H.J., Janssen, T.T., van Dongeren, A.R. 2004. Shoaling of subharmonic gravity waves. J. Geophys. Res., 109(C2): C02009. http://dx.doi.org/10.1029/2003JC001863 Bowers, E.C. 1977. Harbour resonance due to set-down beneath wave groups. Journal of Fluid Mechanics. 79: 71-92. http://dx.doi.org/10.1017/S0022112077000044 Cohen, L. 1995. Time Frequency Analysis: Theory and Applications. Prentice Hall Englewood Cliffs, New Jersey. Dong, G.H., X.Z., Ma, M., Perlin, Y.X., Ma, B., Yu, and G., Wang. 2009. Experimental Study of long wave generation on sloping bottoms. Coastal Engineering, 56(1), 82-89. http://dx.doi.org/10.1016/j.coastaleng.2008.10.002 Kamphuis, J.W. 2000. Designing for low frequency waves. Proceedings of 27th International Conference on Coastal Engineering. Sydney, Australian. 1434-1447. Kostense, J.K. 1984. Measurements of surf beat and set-down beneath wave groups. Proceedings of 19th International Conference on Coastal Engineering. Houston, USA. 724-740. Longuet-Higgins, M.S. and R.W., Stewart. 1962. Radiation stress and mass transport in gravity waves with application to 'surfbeat'. Journal of Fluid Mechanics. 13: 481-504 http://dx.doi.org/10.1017/S0022112062000877 Mallat, S. 1999. A Wavelet Tour of Signal Processing. Academic Press. PMCid:407895 Nagai, T., N., Hashimoto, T., Asai, et al. 1994. Relationship of a moored vessel in a harbor and a long wave caused by wave groups. Proceedings of 17th International Conference on Coastal Engineering. Kobe, Japan. 847-861. Schäffer, H.A. 1993. Second-orderwavemaker theory for irregularwaves.Ocean Engineering. 23 (1), 47–88. http://dx.doi.org/10.1016/0029-8018(95)00013-B Symonds, G.D.A., D.A., Huntley, and A.J., Bowen. 1982. Two-dimensional surf beat-long-wave generation by a time-varying breakpoint. Journal of Geophysical Research. 87(C1): 492-498. http://dx.doi.org/10.1029/JC087iC01p00492 Yu, J. and C.C., Mei. 2000. Formation of sand bars under surface waves. Journal of Fluid Mechanics. 416: 315-348. http://dx.doi.org/10.1017/S0022112000001063

scholarly journals LABORATORY EXPERIMENTS OF BICHROMATIC WAVE GROUPS PROPAGATION ON A GENTLE SLOPE BEACH PROFILE AND ENERGY TRANSFER TO LOW AND HIGH FREQUENCY COMPONENTS

2017 ◽  
pp. 6 ◽  
Author(s):  
Enrique M Padilla ◽  
Jose M Alsina
Keyword(s):  
High Frequency ◽  
Wave Breaking ◽  
Low Frequency ◽  
Primary Wave ◽  
Wave Group ◽  
Wave Groups ◽  
Long Wave ◽  
Wave Conditions ◽  
Hf Wave ◽  
Nonlinear Energy

This work presents a first analysis of experimental data studying the influence of the frequency bandwidth on the propagation of bichromatic wave groups over a constant 1:100 beach slope. The use of a large spatial cross-shore resolution and Bi-Spectral analysis techniques allows the identification of nonlinear energy transfers along the propagation of wave groups. During wave-group shoaling, nonlinear coupling between the primary wave frequencies results in a larger growth of superharmonics for narrow-banded wave conditions, increasing the skewness of the wave and leading to eventual instabilities and earlier high frequency (hf) wave breaking compared to the broad-banded wave condition. Regarding the growth of low frequency (lf) component, the data analysis has shown a larger growth of the incident bound long wave (IBLW) for broad-banded wave conditions. It is generally assumed that the transferred energy from the primary wave components to subharmonics does not affect the short wave energy budget. Here, the opposite is hypothesised, and a larger growth of the IBLW for broad-banded wave conditions is accompanied of a larger reduction of the primary wave components, a reduced growth of hf components and, consequently, a reduction in the growth of hf wave asymmetry during wave group shoaling. Conversely for narrow-banded wave conditions, a reduced IBLW growth is associated with a larger growth of hf wave asymmetry. After hf wave breaking, within the low frequency domain (lf), the IBLW decays slightly for narrow-banded conditions, consistent with a reduction in radiation stress forcing. This involves a nonlinear energy transfer from the wave group frequency back to hf components. The remaining lf energy, Outgoing Free Long Wave (OFLW), reflects back at the shoreline. However, for broad-banded wave conditions, strong dissipation and minimal reflection of lf components occurs close to the shoreline, which might be caused by lf wave breaking.

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