Surface Wave Processes on the Continental Shelf and Beach

LONG-TERM GOALS: Wind waves and swell dominate the hydrodynamic and sediment transport processes on many continental shelves and beaches, affect underwater acoustics, and play an important role in remote sensing applications. Wave prediction in coastal environments is a challenging task because waves are affected by many processes, including scattering by seafloor topography, strong nonlinear interactions, wave breaking, and friction in the bottom boundary layer. Several of these processes are poorly understood and existing wave prediction models rely on parameterizations and empirical calibration to represent them. The long term goals of this research are to obtain a better understanding of the physical processes that affect ocean surface waves in the coastal environment and develop improved wave prediction capability.Award Numbers: N0001408WR20154, N0001408WR20003, N00014-07-1-0365, N00014-07-1-040

Runup kinematics on a natural beach

Runup kinematics on a gently sloping natural beach are examined with detailed measurements from video images, resistance wires deployed at five elevations (between 5 and 25 cm) above and parallel to the beach face, and pressure sensors located in the inner surf zone. As suggested in a previous study comparing a single-level resistance wire and manually digitized films, runup measurements are sensitive to the sensor elevation above the bed, owing to the elongated shape of the runup tongue. The measured mean runup elevation (setup) and vertical excursion increase as the sensor elevation decreases, with the video-based runup estimates having the maximum means and variances. For the six data runs the average ratios of the video-based setup and significant runup excursion to estimates based on wires elevated 15 cm above the bed are 2.7 and 1.5, respectively. These trends, combined with the high coherence and small phase difference between the video and the lowest wire, demonstrate that the video-based estimates correspond to a very near-bed (less than a few centimeters elevation) wire measurement. The measured increase in runup excursion with decreasing sensor elevation and the cross-shore variation in the amplitudes of pressure fluctuations at infragravity frequencies, are consistent with the theory for linear, inviscid, normally incident standing waves. For example, valleys in the pressure spectra occur at approximately the predicted standing wave nodal frequencies. Also in accord with small-amplitude wave theory , observed swash excursions are nearly identical to pressure fluctuations at the location of the measured runup mean (for pressure sensors located seaward of the most offshore bed-level rundown). However, at very low frequencies, where reflection is typically assumed complete and dissipation negligible, the observed, near-bed swash magnitudes are overamplified relative to best fit of the linear standing wave model based on the amplitude and phase of the seaward observations

Data-Enhanced Modeling of Sea and Swell on the Continental Shelf

LONG-TERM GOAL: Our long-term goal is to contribute to the accurate prediction of surface gravity wave generation, propagation, and dissipation in coastal regions through the combined use of measurements and models.Award #s: N00014-98-1-0019; N0001499WX30036; N0001499WR3000

A new parameterisation for runup on gravel beaches

publisher: Elsevier articletitle: A new parameterisation for runup on gravel beaches journaltitle: Coastal Engineering articlelink: http://dx.doi.org/10.1016/j.coastaleng.2016.08.003 content_type: article copyright: © 2016 The Authors. Published by Elsevier B.V. Open Access funded by Engineering and Physical Sciences Research Counci

Excitation of Edge Waves and Their Role in the Formation of Beach Cusps

It is shown theoretically that surface waves incident on a beach from deep water can excite edge waves. In particular, a monochromatic wave train normally incident and reflected on a beach of constant gentle slope is found to transfer energy to edge waves through a weak resonant interaction resulting from an instability of the incident wave with respect to perturbation by edge waves. The analysis is based on the shallow water approximation and ignores the earths rotation and consequently applies only to relatively low mode, high frequency waves. Coupling coefficients. frequencies and longshore wave numbers of the excited waves are given. In accord with Hasselmann's (1967) rule, only edge waves with frequencies lower than the incident wave are excited by this mechanism. Viscous effects suggest that an edge wave with mode number zero and frequency one-half that of the incident wave (a subharmonic edge wave) is preferentially excited. The minimum incident wave amplitudes for which this resonance can occur are predicted by consideration of viscous effects. Higher order terms, not studied in detail, suggest that an edge wave with frequency equal that of the incident wave (a synchronous edge wave) may also be resonantly excited when the incident wave is strongly reflected.Experiments show that a very strong subharmonic edge wave resonance occurs on nonerodable plane laboratory beaches when single frequency incident waves are surging and strongly reflected at the beach. Smaller synchronous edge waves occur when the basin geometry or viscous effects exclude the subharmonic. Neither type edge wave is visible on the beach face when the incident wave is so large that it plunges rather than surges, incident wave energy being dissipated in the surf zone rather than reflected at the beach face as assumed in the resonance theory. This observation is quantified, and leads to a classification of beach face dynamics into reflective and dissipative systems.Large subharmonic edge waves on plane laboratory beaches are shown to rearrange sand tracers into accumulations which resemble natural beach cusps. These edge wave induced morphologies, however, interfere with the edge wave excitation process so the edge wave amplitudes decrease as the cusps grow.Small edge waves can form longshore periodic morphologies by providing destabilizing perturbations on a berm properly located in the swash zone. In this case, the retreating incident wave surge is channelized into breeches in the berm caused by the edge waves.Various theories for edge wave generation on dissipative beaches with complex incident wave fields are briefly reviewed, and differences and similarities with the reflective beach case are stressed

Excitation of Edge Waves and Their Role in the Formation of Beach Cusps

It is shown theoretically that surface waves incident on a beach from deep water can excite edge waves. In particular, a monochromatic wave train normally incident and reflected on a beach of constant gentle slope is found to transfer energy to edge waves through a weak resonant interaction resulting from an instability of the incident wave with respect to perturbation by edge waves. The analysis is based on the shallow water approximation and ignores the earths rotation and consequently applies only to relatively low mode, high frequency waves. Coupling coefficients. frequencies and longshore wave numbers of the excited waves are given. In accord with Hasselmann's (1967) rule, only edge waves with frequencies lower than the incident wave are excited by this mechanism. Viscous effects suggest that an edge wave with mode number zero and frequency one-half that of the incident wave (a subharmonic edge wave) is preferentially excited. The minimum incident wave amplitudes for which this resonance can occur are predicted by consideration of viscous effects. Higher order terms, not studied in detail, suggest that an edge wave with frequency equal that of the incident wave (a synchronous edge wave) may also be resonantly excited when the incident wave is strongly reflected.Experiments show that a very strong subharmonic edge wave resonance occurs on nonerodable plane laboratory beaches when single frequency incident waves are surging and strongly reflected at the beach. Smaller synchronous edge waves occur when the basin geometry or viscous effects exclude the subharmonic. Neither type edge wave is visible on the beach face when the incident wave is so large that it plunges rather than surges, incident wave energy being dissipated in the surf zone rather than reflected at the beach face as assumed in the resonance theory. This observation is quantified, and leads to a classification of beach face dynamics into reflective and dissipative systems.Large subharmonic edge waves on plane laboratory beaches are shown to rearrange sand tracers into accumulations which resemble natural beach cusps. These edge wave induced morphologies, however, interfere with the edge wave excitation process so the edge wave amplitudes decrease as the cusps grow.Small edge waves can form longshore periodic morphologies by providing destabilizing perturbations on a berm properly located in the swash zone. In this case, the retreating incident wave surge is channelized into breeches in the berm caused by the edge waves.Various theories for edge wave generation on dissipative beaches with complex incident wave fields are briefly reviewed, and differences and similarities with the reflective beach case are stressed