# Search Results

## You are looking at 11 - 18 of 18 items for :

- Author or Editor: R. T. Guza x

- Journal of Physical Oceanography x

- Refine by Access: Content accessible to me x

## Abstract

The energy of seaward and shoreward propagating ocean surface gravity waves on a natural beach was estimated with data from an army of 24 bottom-mounted pressure sensors in 13-m water depth, 2 km from the North Carolina coast. Consistent with a parameterization of surface wave reflection from a plane sloping beach by Miche, the ratio of seaward to shoreward propagating energy in the swell-sea frequency band (0.044â€“0.20 Hz) decreased with increasing wave frequency and increasing wave height, and increased with increasing beach-face slope. Although most incident swell-sea energy dissipated in the surf zone, reflection was sometimes significant (up to 18% of the incident swell-sea energy) when the beach face was steep (at high tide) and the wave field was dominated by low-energy, low-frequency swell. Frequency-directional spectra show that reflection of swell and sea was approximately specular. The ratio of seaward to shoreward propagating energy in the infragravity frequency band (0.010â€“0.044 Hz) varied between about 0.5 and 3 and increased with increasing swell energy. This trend suggests that infragravity waves generated in very shallow water, and refractively trapped on the sloping seabed, are significantly dissipated over a 50-km wide shelf during storms.

## Abstract

The energy of seaward and shoreward propagating ocean surface gravity waves on a natural beach was estimated with data from an army of 24 bottom-mounted pressure sensors in 13-m water depth, 2 km from the North Carolina coast. Consistent with a parameterization of surface wave reflection from a plane sloping beach by Miche, the ratio of seaward to shoreward propagating energy in the swell-sea frequency band (0.044â€“0.20 Hz) decreased with increasing wave frequency and increasing wave height, and increased with increasing beach-face slope. Although most incident swell-sea energy dissipated in the surf zone, reflection was sometimes significant (up to 18% of the incident swell-sea energy) when the beach face was steep (at high tide) and the wave field was dominated by low-energy, low-frequency swell. Frequency-directional spectra show that reflection of swell and sea was approximately specular. The ratio of seaward to shoreward propagating energy in the infragravity frequency band (0.010â€“0.044 Hz) varied between about 0.5 and 3 and increased with increasing swell energy. This trend suggests that infragravity waves generated in very shallow water, and refractively trapped on the sloping seabed, are significantly dissipated over a 50-km wide shelf during storms.

## Abstract

Steady surf-zone longshore currents and directional properties of the incident wave field were measured on a beach with nearly straight and parallel depth contours. Selected data were processed into 64 segments, each of 68.2 min. duration, irregularly spaced throughout an 18-day period. A wide variety of incident wave and longshore current conditions were observed. The radiation stress spectrum [*S _{xy}
*(

*f*)] was estimated from a slope array and two current meters located seaward of the surf zone. In many cases the total radiation stress [

*S*= Î£

_{xy}^{T}*S*(

_{xy}*f*)Î”

*f*] contains important contributions from a wide range of frequencies. In a few instances, sea and swell approach the beach from different directions quadrants resulting in a new zero

*S*. The strong shears and direction reversals of the longshore current that could conceivably occur in this circumstance were not observed. An EOF decomposition of the mean longshore current pattern shows that most (<90%) of the current spatial variation in the 64 runs is contained in a classical parabolic shape. The temporal expansion coefficients of the first EOF are equally highly correlated with both

_{xy}^{T}*S*, and a scale velocity suggested by radiation stress-based longshore current theories.

_{xy}^{T}## Abstract

Steady surf-zone longshore currents and directional properties of the incident wave field were measured on a beach with nearly straight and parallel depth contours. Selected data were processed into 64 segments, each of 68.2 min. duration, irregularly spaced throughout an 18-day period. A wide variety of incident wave and longshore current conditions were observed. The radiation stress spectrum [*S _{xy}
*(

*f*)] was estimated from a slope array and two current meters located seaward of the surf zone. In many cases the total radiation stress [

*S*= Î£

_{xy}^{T}*S*(

_{xy}*f*)Î”

*f*] contains important contributions from a wide range of frequencies. In a few instances, sea and swell approach the beach from different directions quadrants resulting in a new zero

*S*. The strong shears and direction reversals of the longshore current that could conceivably occur in this circumstance were not observed. An EOF decomposition of the mean longshore current pattern shows that most (<90%) of the current spatial variation in the 64 runs is contained in a classical parabolic shape. The temporal expansion coefficients of the first EOF are equally highly correlated with both

_{xy}^{T}*S*, and a scale velocity suggested by radiation stress-based longshore current theories.

_{xy}^{T}## Abstract

The radiation stresses *S _{ij}
* associated with the propagation of wind-generated waves are principal driving forces for several important surf-zone processes. The accurate estimation of the onshore flux of longshore-directed mean momentum

*S*, using a linear array of pressure sensors, is considered here. Three analysis methods are examined: integration of two high-resolution directional-spectrum estimators [maximum likelihood (MLM) and a modified version (IMLM)], and a direct estimator of the

_{yx}*S*directional moment (DMM

_{yx}_{v}) which is developed here.

The *S _{yx}
* estimation methods are compared using numerical simulations and field data from two experiments at Torrey Pines Beach, California. In the first field experiment, IMLM and DMM, estimates of

*S*(from a 3-element, 99 m long linear array) showed excellent agreement with a slope array (Higgins

_{yx}*et al*., 1981) in the frequency range 0.05â€“0.15 Hz. In the second experiment, IMLM and DMM, estimates of

*S*(from a 5-element, 360 m long array) agreed with values of

_{yx}*S*obtained from a nearby orthogonal-axis current meter for the frequency range 0.06â€“0.11 Hz. The integration of the MLM directional spectrum estimates yields biased (low) values of

_{yx}*S*. Although the DMM method is used here for the estimation of

_{yx}*S*, it can easily be adapted for the calculation of any arbitrary directional moment. While conventional methods are shown to be deficient in

_{yx}*S*estimation, they provide accurate estimates of

_{yx}*S*, the onshore flux of onshore-directed momentum.

_{xx}## Abstract

The radiation stresses *S _{ij}
* associated with the propagation of wind-generated waves are principal driving forces for several important surf-zone processes. The accurate estimation of the onshore flux of longshore-directed mean momentum

*S*, using a linear array of pressure sensors, is considered here. Three analysis methods are examined: integration of two high-resolution directional-spectrum estimators [maximum likelihood (MLM) and a modified version (IMLM)], and a direct estimator of the

_{yx}*S*directional moment (DMM

_{yx}_{v}) which is developed here.

The *S _{yx}
* estimation methods are compared using numerical simulations and field data from two experiments at Torrey Pines Beach, California. In the first field experiment, IMLM and DMM, estimates of

*S*(from a 3-element, 99 m long linear array) showed excellent agreement with a slope array (Higgins

_{yx}*et al*., 1981) in the frequency range 0.05â€“0.15 Hz. In the second experiment, IMLM and DMM, estimates of

*S*(from a 5-element, 360 m long array) agreed with values of

_{yx}*S*obtained from a nearby orthogonal-axis current meter for the frequency range 0.06â€“0.11 Hz. The integration of the MLM directional spectrum estimates yields biased (low) values of

_{yx}*S*. Although the DMM method is used here for the estimation of

_{yx}*S*, it can easily be adapted for the calculation of any arbitrary directional moment. While conventional methods are shown to be deficient in

_{yx}*S*estimation, they provide accurate estimates of

_{yx}*S*, the onshore flux of onshore-directed momentum.

_{xx}## Abstract

Low-salinity water from Chesapeake Bay forms an intermittent buoyant gravity current that propagates more than 100 km southward along the coast. During five events when wind and surface gravity-wave forcing were weak, the buoyant coastal current 90 km south of Chesapeake Bay was less than 5 km wide, was 5â€“10 m thick, and propagated alongshore at âˆ¼50 cm s^{âˆ’1}. The density decreased 2â€“3 kg m^{âˆ’3} over a few hundred meters at the nose of the buoyant coastal current, which was located about 1 km offshore in âˆ¼8 m of water. Water up to 4 km ahead of the advancing nose was displaced southward and offshore (maximum velocities near the nose of 20 and 10 cm s^{âˆ’1}, respectively). The southward alongshore current increased abruptly to âˆ¼50 cm s^{âˆ’1} at the nose and continued to increase to a supercritical maximum of âˆ¼70 cm s^{âˆ’1} about 1 km behind the nose. An onshore flow of between 5 and 15 cm s^{âˆ’1}, which extended at least 5 km behind the nose, supplied buoyant water to the onshore region of weak, subcritical alongshore flow. The observed flow structure is qualitatively similar to theoretical predictions and laboratory measurements of buoyant gravity currents propagating along a sloping bottom.

## Abstract

Low-salinity water from Chesapeake Bay forms an intermittent buoyant gravity current that propagates more than 100 km southward along the coast. During five events when wind and surface gravity-wave forcing were weak, the buoyant coastal current 90 km south of Chesapeake Bay was less than 5 km wide, was 5â€“10 m thick, and propagated alongshore at âˆ¼50 cm s^{âˆ’1}. The density decreased 2â€“3 kg m^{âˆ’3} over a few hundred meters at the nose of the buoyant coastal current, which was located about 1 km offshore in âˆ¼8 m of water. Water up to 4 km ahead of the advancing nose was displaced southward and offshore (maximum velocities near the nose of 20 and 10 cm s^{âˆ’1}, respectively). The southward alongshore current increased abruptly to âˆ¼50 cm s^{âˆ’1} at the nose and continued to increase to a supercritical maximum of âˆ¼70 cm s^{âˆ’1} about 1 km behind the nose. An onshore flow of between 5 and 15 cm s^{âˆ’1}, which extended at least 5 km behind the nose, supplied buoyant water to the onshore region of weak, subcritical alongshore flow. The observed flow structure is qualitatively similar to theoretical predictions and laboratory measurements of buoyant gravity currents propagating along a sloping bottom.

## Abstract

Surf-zone dispersion is studied using drifter observations collected within about 200 m of the shoreline (at depths of less than about 5 m) on a beach with approximately alongshore uniform bathymetry and waves. There were about 70 individual drifter releases, each 10â€“20 min in duration, on two consecutive days. On the first day, the sea-swell significant wave height *H _{s}
* was equal to 0.5 m and mean alongshore currents |

*Ï…*

^{âˆ’1}). On the second day, the obliquely incident waves were larger, with

*H*equal to 1.4 m, and at some surf-zone locations |

_{s}*Ï…*

^{âˆ’1}. The one-particle diffusivity was larger, with larger waves and stronger currents. On both days, the one-particle diffusivity tensor is nonisotropic and time-dependent. The major axis is initially parallel to the cross-shore direction, but after a few wave periods it is aligned with the alongshore direction. In both the along- and cross-shore directions, the asymptotic diffusivity is reached sooner within, rather than seaward of, the surf zone. Two-particle statistics indicate that relative dispersion grows like

*D*

^{2}(

*t*) âˆ¼

*t*

^{3/2}and that the relative diffusivity is scale-dependent as

*Î¼*âˆ¼

*l*

^{2/3}, with

*l*being the particle separation. The observed scalings differ from 2D inertial-subrange scalings [

*D*

^{2}(

*t*) âˆ¼

*t*

^{3}and

*Î¼*âˆ¼

*l*

^{4/3}]. Separations have a non-Gaussian self-similar distribution that is independent of time. The two-particle statistics are consistent with a nonconstant-coefficient diffusion equation for the separation probability density functions. The dispersion is explained by neither irrotational surface gravity waves nor shear dispersion. The observations imply the existence of a 2D eddy field with 5â€“50-m length scales, the source of which is speculated to be alongshore gradients in breaking-wave height associated with finite crest lengths.

## Abstract

Surf-zone dispersion is studied using drifter observations collected within about 200 m of the shoreline (at depths of less than about 5 m) on a beach with approximately alongshore uniform bathymetry and waves. There were about 70 individual drifter releases, each 10â€“20 min in duration, on two consecutive days. On the first day, the sea-swell significant wave height *H _{s}
* was equal to 0.5 m and mean alongshore currents |

*Ï…*

^{âˆ’1}). On the second day, the obliquely incident waves were larger, with

*H*equal to 1.4 m, and at some surf-zone locations |

_{s}*Ï…*

^{âˆ’1}. The one-particle diffusivity was larger, with larger waves and stronger currents. On both days, the one-particle diffusivity tensor is nonisotropic and time-dependent. The major axis is initially parallel to the cross-shore direction, but after a few wave periods it is aligned with the alongshore direction. In both the along- and cross-shore directions, the asymptotic diffusivity is reached sooner within, rather than seaward of, the surf zone. Two-particle statistics indicate that relative dispersion grows like

*D*

^{2}(

*t*) âˆ¼

*t*

^{3/2}and that the relative diffusivity is scale-dependent as

*Î¼*âˆ¼

*l*

^{2/3}, with

*l*being the particle separation. The observed scalings differ from 2D inertial-subrange scalings [

*D*

^{2}(

*t*) âˆ¼

*t*

^{3}and

*Î¼*âˆ¼

*l*

^{4/3}]. Separations have a non-Gaussian self-similar distribution that is independent of time. The two-particle statistics are consistent with a nonconstant-coefficient diffusion equation for the separation probability density functions. The dispersion is explained by neither irrotational surface gravity waves nor shear dispersion. The observations imply the existence of a 2D eddy field with 5â€“50-m length scales, the source of which is speculated to be alongshore gradients in breaking-wave height associated with finite crest lengths.

## Abstract

The nonlinear dispersion of random, directionally spread surface gravity waves in shallow water is examined with Boussinesq theory and field observations. A theoretical dispersion relationship giving a directionally averaged wavenumber magnitude as a function of frequency, the local water depth, and the local wave spectrum and bispectrum is derived for waves propagating over a gently sloping beach with straight and parallel depth contours. The linear, nondispersive shallow water relation is recovered as the first-order solution, with weak frequency and amplitude dispersion appearing as second-order corrections. Wavenumbers were estimated using four arrays of pressure sensors deployed in 2â€“6-m depth on a gently sloping sandy beach. When wave energy is low, the observed wavenumbers agree with the linear, finite-depth dispersion relation over a wide frequency range. In high energy conditions, the observed wavenumbers deviate from the linear dispersion relation by as much as 20%â€“30% in the frequency range from two to three times the frequency of the primary spectral peak, but agree well with the nonlinear Boussinesq dispersion relation, confirming that the deviations from linear theory are finite amplitude effects. In high energy conditions, the predicted frequency and amplitude dispersion tend to cancel, yielding a nearly nondispersive wave field in which waves of all frequencies travel with approximately the linear shallow water wave speed, consistent with the observations. The nonlinear Boussinesq theory wavenumber predictions (based on the assumption of irrotational wave motion) are accurate even within the surf zone, suggesting that wave breaking on gently sloping beaches has little effect on the dispersion relation.

## Abstract

The nonlinear dispersion of random, directionally spread surface gravity waves in shallow water is examined with Boussinesq theory and field observations. A theoretical dispersion relationship giving a directionally averaged wavenumber magnitude as a function of frequency, the local water depth, and the local wave spectrum and bispectrum is derived for waves propagating over a gently sloping beach with straight and parallel depth contours. The linear, nondispersive shallow water relation is recovered as the first-order solution, with weak frequency and amplitude dispersion appearing as second-order corrections. Wavenumbers were estimated using four arrays of pressure sensors deployed in 2â€“6-m depth on a gently sloping sandy beach. When wave energy is low, the observed wavenumbers agree with the linear, finite-depth dispersion relation over a wide frequency range. In high energy conditions, the observed wavenumbers deviate from the linear dispersion relation by as much as 20%â€“30% in the frequency range from two to three times the frequency of the primary spectral peak, but agree well with the nonlinear Boussinesq dispersion relation, confirming that the deviations from linear theory are finite amplitude effects. In high energy conditions, the predicted frequency and amplitude dispersion tend to cancel, yielding a nearly nondispersive wave field in which waves of all frequencies travel with approximately the linear shallow water wave speed, consistent with the observations. The nonlinear Boussinesq theory wavenumber predictions (based on the assumption of irrotational wave motion) are accurate even within the surf zone, suggesting that wave breaking on gently sloping beaches has little effect on the dispersion relation.

## Abstract

In Part I, the energy levels of ocean surface waves at infragravity frequencies (nominally 0.005â€“0.05 Hz) locally forced by swell in 13-m water depth were shown to be predicted accurately by second-order nonlinear wave theory. However, forced infragravity waves were consistently much less energetic than free infragravity waves. Here, in Part II, observations in depths between 8 and 204 m, on Atlantic and Pacific shelves, are used to investigate the sources and variability of free infragravity wave energy. Both free and forced infragravity energy levels generally increase with increasing swell energy and decreasing water depth, but their dependencies are markedly different. Although free waves usually dominate the infragravity frequency band, forced waves contribute a significant fraction of the total infragravity energy with high energy swell and/or in very shallow water. The observed *h*
^{âˆ’1} variation of free infragravity energy with increasing water depth *h* is stronger than the *h*
^{âˆ’1/2} dependence predicted for leaky surface gravity waves propagating approximately perpendicular to local depth contours, but is consistent with a heuristic, geometrical optics-based (WKB) model of the refractive trapping of a directionally broad wave field generated close to shore. Preliminary analysis shows that free infragravity waves are indeed directionally broad and that the propagation directions of infragravity waves and incident swell are related. Free infragravity energy levels also depend on the general geographic surroundings. Comparisons of observations from the same depth and with similar swell conditions, but on different shelves, suggest that more free infragravity wave energy is radiated from wide, sandy beaches than from rocky, cliffed coasts and that less energy is trapped on a narrow shelf than on a wide shelf.

## Abstract

In Part I, the energy levels of ocean surface waves at infragravity frequencies (nominally 0.005â€“0.05 Hz) locally forced by swell in 13-m water depth were shown to be predicted accurately by second-order nonlinear wave theory. However, forced infragravity waves were consistently much less energetic than free infragravity waves. Here, in Part II, observations in depths between 8 and 204 m, on Atlantic and Pacific shelves, are used to investigate the sources and variability of free infragravity wave energy. Both free and forced infragravity energy levels generally increase with increasing swell energy and decreasing water depth, but their dependencies are markedly different. Although free waves usually dominate the infragravity frequency band, forced waves contribute a significant fraction of the total infragravity energy with high energy swell and/or in very shallow water. The observed *h*
^{âˆ’1} variation of free infragravity energy with increasing water depth *h* is stronger than the *h*
^{âˆ’1/2} dependence predicted for leaky surface gravity waves propagating approximately perpendicular to local depth contours, but is consistent with a heuristic, geometrical optics-based (WKB) model of the refractive trapping of a directionally broad wave field generated close to shore. Preliminary analysis shows that free infragravity waves are indeed directionally broad and that the propagation directions of infragravity waves and incident swell are related. Free infragravity energy levels also depend on the general geographic surroundings. Comparisons of observations from the same depth and with similar swell conditions, but on different shelves, suggest that more free infragravity wave energy is radiated from wide, sandy beaches than from rocky, cliffed coasts and that less energy is trapped on a narrow shelf than on a wide shelf.

## Abstract

Previous field observations indicate that the directional spread of swell-frequency (nominally 0.1 Hz) surface gravity waves increases during shoreward propagation across the surf zone. This directional broadening contrasts with the narrowing observed seaward of the surf zone and predicted by Snellâ€™s law for bathymetric refraction. Field-observed broadening was predicted by a new model for refraction of swell by lower-frequency (nominally 0.01 Hz) current and elevation fluctuations. The observations and the model suggest that refraction by the cross-shore currents of energetic shear waves contributed substantially to the observed broadening.

## Abstract

Previous field observations indicate that the directional spread of swell-frequency (nominally 0.1 Hz) surface gravity waves increases during shoreward propagation across the surf zone. This directional broadening contrasts with the narrowing observed seaward of the surf zone and predicted by Snellâ€™s law for bathymetric refraction. Field-observed broadening was predicted by a new model for refraction of swell by lower-frequency (nominally 0.01 Hz) current and elevation fluctuations. The observations and the model suggest that refraction by the cross-shore currents of energetic shear waves contributed substantially to the observed broadening.