Alex Hay | Dalhousie University (original) (raw)
Papers by Alex Hay
2015 IEEE/OES Eleveth Current, Waves and Turbulence Measurement (CWTM), 2015
2011 IEEE/OES 10th Current, Waves and Turbulence Measurements (CWTM), 2011
Results are reported from an experiment carried out with the newly developed Nortek VectrinoII an... more Results are reported from an experiment carried out with the newly developed Nortek VectrinoII and the standard Nortek Vectrino in a turbulent axisymmetric jet at a Reynolds number of 5000. The mechanical and acoustic characteristics of these instruments are identical. However, the electronics and signal processing scheme in the VectrinoII represent advancements over those in the original Vectrino (referred to as VectrinoI in this paper). In addition, the VectrinoII provides for profiling over a ca. 3 cm range interval thereby allowing direct measurement of the spatial structure of the flow. The two instruments deliver comparable performance as measured by mean velocity profiles, turbulent kinetic energy spectra, and the derived values of Reynolds stress and dissipation. The Vectrino measurements are compared to the mean and turbulent properties observed by [1] using hot-film and Laser Doppler anemometry. Here, there is good agreement in mean velocity and Reynolds stress measurements. Significant differences are seen in dissipations and velocity variance.
Journal of Geophysical Research: Oceans, 2013
1] The shoaling of horizontally propagating internal waves may represent an important source of m... more 1] The shoaling of horizontally propagating internal waves may represent an important source of mixing and transport in estuaries and coastal seas. Including such effects in numerical models demands improvements in the understanding of several aspects of the energetics, especially those relating to turbulence generation, and observations are needed to build this understanding. To address some of these issues in the estuarine context, we undertook an intensive field program for 10 days in the summer of 2008 in the St. Lawrence Estuary. The sampling involved shore-based photogrammetry, ship-based surveys, and an array of moorings in the shoaling region that held both conventional and turbulence-resolving sensors. The measurements shed light on many aspects of the wave shoaling process. Wave arrivals were generally phase-locked with the M 2 tide, providing hints about far-field forcing. In the deeper part of the study domain, the waves propagated according to the predictions of linear theory. In intermediate-depth waters, the waves traversed the field site perpendicularly to isobaths, a pattern that continued as the waves transformed nonlinearly. Acoustic Doppler velocimeters permitted inference of the turbulent energetics, and two main features were studied. First, during a period of shoaling internal waves, turbulence dissipation rates exceeded values associated with tidal shear by an order of magnitude. Second, the evolving spectral signatures associated with a particular wave-shoaling event suggest that the turbulence is at least partly locally generated. Overall, the results of this study suggest that parameterizations of wave-induced mixing could employ relatively simple dynamics in deep water, but may have to handle a wide suite of turbulence generation and transport mechanisms in inshore regions.
Journal of Geophysical Research, 2003
Field observations of linear transition ripples during an autumn storm exhibit high correlation b... more Field observations of linear transition ripples during an autumn storm exhibit high correlation between cross-shore ripple migration rate and the skewness of the near-bed wave orbital velocity. For a bimodal spectrum of mixed sea and swell, negatively skewed near-bed orbital velocities were accompanied by offshore ripple migration, while for unimodal swell, ripples migrated onshore under positively skewed velocities. Bispectral analysis
Journal of Geophysical Research, 2001
Field observations were made in 3-4 m water depth of linear transition ripple geometry and migrat... more Field observations were made in 3-4 m water depth of linear transition ripple geometry and migration using a high-resolution laser-video bed profiling system and acoustic scanning sensors during both the growth and decay phases of an autumn storm event. Linear transition ripples are long-crested, low-steepness bedforms of the anorbital ripple type and were observed to occur here at relatively high
The Journal of the Acoustical Society of America, 1993
Phase shift of the nth wave wavelength of sound in fluid frequency density of fluid density of cy... more Phase shift of the nth wave wavelength of sound in fluid frequency density of fluid density of cylinder duration of transmitted pulse Eq. (13) Eq. ( 11 ), also Eq. (20)
The Journal of the Acoustical Society of America, 2005
Results are presented from the acoustic remote sensing component of Dalhousie University's i... more Results are presented from the acoustic remote sensing component of Dalhousie University's interdisciplinary Coastal Ocean Observatory in Lunenburg Bay, Nova Scotia, operational since June 2002 (www.cmep.ca/bay). Remote acoustic measurements of water column velocity and backscatter are made from bottom-mounted instruments hard-wired to surface buoys. The data are telemetered in near real-time to a shore station via wireless Ethernet, and to Dalhousie over the Internet. The fixed-point measurements from the bottom pods are augmented by a vessel-mounted profiling system, and by opportunistic deployments of internally-recording instruments. Highlights from the first three years of the observing system operation will be presented, including sediment suspension from the seabed and bubble penetration from the surface during Hurricane Juan.
The Journal of the Acoustical Society of America, 1988
The available data for scattered acoustic intensity and attenuation in dilute aqueous suspensions... more The available data for scattered acoustic intensity and attenuation in dilute aqueous suspensions of sand are compared with theory. In theoretical calculations, the scatterer is assumed to be spherical and elastic, or rigid and movable, or rigid and immovable. The rigid movable model provides the best fit to the data. The failure of the elastic model in comparison to the rigid sphere models indicates that resonance excitation does not occur in natural sand grains, probably because of irregularities in shape. The fact that better agreement with experiment is obtained with the rigid movable model than with the rigid immovable model indicates that the inertia of the particles is important. Additional approximate expressions for the form factor and attenuation coefficient have been constructed based on a modified form of the so-called high-pass model introduced by Johnson [J. Acoust. See. Am. 61, 275-277 (1977) ]. The modified high-pass model provides a fit to the data that is as good as, or better than, the rigid movable case. PACS numbers: 43.30.Ft, 43.30.Gv
The Journal of the Acoustical Society of America, 2000
Journal of Physical Oceanography, 2007
An extreme weather event (Hurricane Juan) made landfall in Nova Scotia, Canada, in September 2003... more An extreme weather event (Hurricane Juan) made landfall in Nova Scotia, Canada, in September 2003. The storm produced an ϳ70-cm storm surge and ϳ40 cm s Ϫ1 coastal currents in Lunenburg Bay, registered by a coastal observing system. A fine-resolution (60 m) coastal circulation model is used to examine the response of Lunenburg Bay to Hurricane Juan. The model is forced by local wind stress at the sea surface, and tides and remotely generated waves specified at model open boundaries. The model performance is assessed in terms of ␥ 2 , the variance of the model errors normalized by the observed variance. The model reproduces very well the observed surface elevations with ␥ 2 values of less than 0.05, and reasonably well the observed currents with ␥ 2 values between 0.2 and 1.1 in the bay during Hurricane Juan. The model-data comparisons demonstrate that the coastal circulation in the bay is significantly affected by local wind associated with the storm. The model results are also used to demonstrate the importance of nonlinear dynamics in the barotropic response of the bay to the storm.
Journal of Geophysical Research, 2008
1] The coupling between a quarter-wave resonance in a coastal bay and a Helmholtz mode in an adja... more 1] The coupling between a quarter-wave resonance in a coastal bay and a Helmholtz mode in an adjacent cove (connected to the bay through a narrow channel) is investigated by comparing field measurements to analytical and numerical model predictions. Pressure and velocity spectra from locations throughout the bay reveal an oscillation with a period of approximately 1 hour, consistent with a quarter-wave seiche mode. The associated sea surface displacements throughout the bay are small (less than 5 cm RMS, i.e., only 10% of the tidal elevation). Velocities within the channel are significantly modulated in the 1-hour band, with amplitudes up to 40% of the peak tidal current. The analytical model shows that the modulation of the channel flow results from the interaction between the quarter-wave mode in the main basin and a Helmholtz resonance in the cove, also with a period near 1 hour. The amplitude and phase of the 1-hour oscillation varies through the tidal cycle because of the change in Helmholtz frequency with tidal elevation. Good quantitative agreement between the data and the model predictions is obtained if a drag coefficient approximately 3-4 times larger than the classical value of 3 Â 10 À3 is used in the channel and cove.
Journal of Geophysical Research, 2007
1] Variations with wave energy of near-bed turbulence and the wave friction factor are investigat... more 1] Variations with wave energy of near-bed turbulence and the wave friction factor are investigated in the near-shore zone for bed states spanning low-steepness sand ripples and flat bed, and for wave energies extending well into the sheet flow regime. The measurements were made using a 1.7-MHz pulse-coherent Doppler profiler in ca. 3-m mean water depth. Near-bed turbulence intensities, phase-averaged over the highest-1/3 waves, peak at phases between 10°and 55°after the wave crest, this phase decreasing with increasing wave Reynolds number. Wave friction factors computed from near-bed vertical turbulence intensity fall within the range predicted by existing semiempirical formulae, and exhibit broadly similar trends. At the higher end of the observed wave energy range (i.e., in the sheet flow regime), however, the measured friction factors increase with sea-and-swell energy faster than the predictions. This anomalous increase is correlated with infragravity wave energy and with mean cross-shore current speed, but not with other forcing parameters including mean long-shore current speed, wave skewness, wave asymmetry and wave breaking frequency. It is argued that the anomaly is partly due to additional near-bed turbulence associated with infragravity waves, and therefore that these data are not inconsistent with parameterization for bottom roughness in oscillatory sheet flow. Peak near-bed turbulence intensities are independent of wave Reynolds number for Re ] 1.2 Â 10 6 , but proportional to Re for Re^1.2 Â 10 6 , this abrupt change possibly indicating a critical dependence on Re of turbulence production in the WBL over flat or nearly flat mobile beds. Citation: Newgard, J. P., and A. E. Hay (2007), Turbulence intensity in the wave boundary layer and bottom friction under (mainly) flat bed conditions,
Journal of Geophysical Research, 1992
Vertical profiles of suspended sand concentration and size are obtained from multifrequency acous... more Vertical profiles of suspended sand concentration and size are obtained from multifrequency acoustic profiling data collected in 1989 during a nearshore experiment at Stanhope Lane Beach, Prince Edward Island. The data were acquired with acoustic sounders operating at 1, 2.25, and 5 MHz. Independent estimates of concentration were made using optical backscatter sensors (OBSs). The algorithm for inversion of the three-frequency backscatter data to particle size and concentration, based on the ratios of the different signals, was tested in laboratory experiments with an unconfined high Reynolds number suspended sediment jet. Results from the Stanhope Beach experiment are presented for three different surface wave energy regimes, with significant wave orbital velocities ranging from 0.3 to 0.8 m/s and peak wave periods of 4-6 s. The acoustic estimates of mean concentration are shown to be within 10% on average of those determined with the OBS nearest the bottom at 5-to 10-cm height, over time scales ranging from 6.5 min to 4-6 s (one wave period). The acoustic estimates of suspended sediment size near the bottom are within 5-20% of the bottom sediment mean size. The statistical variability of the size estimates is high, with standard deviations in the estimates ranging between 30 and 50% of the mean. The time-averaged concentration profiles exhibit an exponential decrease with height above an O(10)-cm-thick near-bottom region of nonexponential decrease. In contrast, the time-averaged mean size profiles decrease approximately linearly with height, and rather slowly, about 25% in 0.5 m.
Journal of Geophysical Research, 2010
1] Field observations and model simulations are presented of flow generated by waves breaking ove... more 1] Field observations and model simulations are presented of flow generated by waves breaking over a shoal at the entrance to a shallow bay. The shoal is composed of a series of steep and narrow bedrock ridges with depths of 2-8 m at the ridge crests. Observations from instruments on the shoal indicated peak significant wave heights during a storm event were 2.4-4.0 m across the observation sites; this spatial variability is due to wave breaking over the ridges. The 2-D depth-averaged hydrodynamic model Delft3D, coupled to the wave model SWAN, was used to simulate the waves and wave-driven flow over the shoal and throughout the entire bay with a nested fine grid (5 m resolution) to resolve the shoal bathymetry. The model predicts a well-defined jet behind the shoal, with mean axial speeds of 0.4-0.7 m/s. The observations indicate maximum speeds behind the shoal of 0.3-0.4 m/s, exceeding the maximum tidal current speed by more than a factor of 4, that are consistent in timing and direction with the model predictions. The model overpredicts wave breaking over the steep slopes and as a result the current speeds are overestimated.
Journal of Geophysical Research, 2008
1] A coupled wave/flow model was used to simulate the currents in a coastal bay during the landfa... more 1] A coupled wave/flow model was used to simulate the currents in a coastal bay during the landfall of a hurricane with large waves. Extensive wave breaking along the shoreline and over a midbay shoal induced the development of a strong mean circulation in the bay, in combination with currents forced by wind, tide, and storm surge. The general circulation pattern consisted of inflows along the shoreline and over the shoal region that were driven by radiation stress gradients, and outflows due to mass balance of the wave-driven inflow that were observed in deeper channels. The predicted currents agreed with observations only when wave forcing was included in the circulation model. Wave-driven flows accounted for over 50% of the high flushing rates during the storm and induced strong horizontal velocity gradients over short ($200 m) length scales.
Journal of Geophysical Research, 1999
Observations of the temporal evolution of the geometric properties and migration of wave-formed r... more Observations of the temporal evolution of the geometric properties and migration of wave-formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements. Six weeks of bedform observations were taken at the sandy (medium to coarse sized sand) LEO-15 site located on Beach Haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar. During this period, six tropical storms, several of hurricane strength, passed to the east of the study site. Ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed. The predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3/4 of the wave orbital diameter. Although orbital diameters become larger than 130 cm during the maximum wave event, it is unclear if a transition to nonorbital scaling is occurring. Ripple migration is found to be directed primarily onshore at rates of up to 80 cm/day. Suspended transport due to wave motions, calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements, are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration. Thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near-bottom suspended transport. Bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates. Sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction, as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave-to-current velocities.
Journal of Geophysical Research, 2002
1] This paper reports on remote acoustic observations of vertical turbulence intensity and vertic... more 1] This paper reports on remote acoustic observations of vertical turbulence intensity and vertical suspended sediment flux profiles on a planar beach in 3-4 m water depth. The measurements of suspended sediment concentration and velocity are colocated and simultaneous and extend through the wave bottom boundary layer to the bed with 0.7 cm vertical resolution. Normalized cospectra of the suspended sediment flux and the vertical velocity for different bed states (irregular ripples, cross ripples, linear transition ripples, and flat bed) indicate a small but significant peak at incident wave frequencies but are otherwise rather flat, with weak redness. Estimates of the vertical flux components indicate a general balance between upward fluxes due to waves and turbulence and downward settling. Two exceptions to this balance are found immediately above the bed and for nonmigrating irregular ripples. The contribution from the high-frequency turbulent component is small. Wave phase averages for low-energy bed states exhibit near-bed peaks in the suspended sediment flux following wave phase reversal. Wave phase averages for the high-energy cases do not exhibit a diffusive signature. Observed vertical profiles of turbulence intensity for different bed states reveal that the near-bed turbulence levels are relatively independent of bed state. Friction velocity predictions from presently available models, including a bed stress model and a sediment eddy diffusion model, are compared to measured values of near-bed turbulence intensity. Reasonable agreement is found between measured and predicted bottom friction velocities when wave friction factors from Tolman [1994] are used.
Journal of Atmospheric and Oceanic Technology, 1999
A pulse-to-pulse coherent acoustic Doppler profiler has been developed for high-resolution partic... more A pulse-to-pulse coherent acoustic Doppler profiler has been developed for high-resolution particle velocimetry in the ocean, in particular for remote measurements of suspended sediment flux and turbulence in the nearshore and continental shelf bottom boundary layer. Acoustic backscatter estimates of suspended particle concentration and velocity are determined simultaneously from the phase and amplitude of the backscattered signal over an O(1
2015 IEEE/OES Eleveth Current, Waves and Turbulence Measurement (CWTM), 2015
2011 IEEE/OES 10th Current, Waves and Turbulence Measurements (CWTM), 2011
Results are reported from an experiment carried out with the newly developed Nortek VectrinoII an... more Results are reported from an experiment carried out with the newly developed Nortek VectrinoII and the standard Nortek Vectrino in a turbulent axisymmetric jet at a Reynolds number of 5000. The mechanical and acoustic characteristics of these instruments are identical. However, the electronics and signal processing scheme in the VectrinoII represent advancements over those in the original Vectrino (referred to as VectrinoI in this paper). In addition, the VectrinoII provides for profiling over a ca. 3 cm range interval thereby allowing direct measurement of the spatial structure of the flow. The two instruments deliver comparable performance as measured by mean velocity profiles, turbulent kinetic energy spectra, and the derived values of Reynolds stress and dissipation. The Vectrino measurements are compared to the mean and turbulent properties observed by [1] using hot-film and Laser Doppler anemometry. Here, there is good agreement in mean velocity and Reynolds stress measurements. Significant differences are seen in dissipations and velocity variance.
Journal of Geophysical Research: Oceans, 2013
1] The shoaling of horizontally propagating internal waves may represent an important source of m... more 1] The shoaling of horizontally propagating internal waves may represent an important source of mixing and transport in estuaries and coastal seas. Including such effects in numerical models demands improvements in the understanding of several aspects of the energetics, especially those relating to turbulence generation, and observations are needed to build this understanding. To address some of these issues in the estuarine context, we undertook an intensive field program for 10 days in the summer of 2008 in the St. Lawrence Estuary. The sampling involved shore-based photogrammetry, ship-based surveys, and an array of moorings in the shoaling region that held both conventional and turbulence-resolving sensors. The measurements shed light on many aspects of the wave shoaling process. Wave arrivals were generally phase-locked with the M 2 tide, providing hints about far-field forcing. In the deeper part of the study domain, the waves propagated according to the predictions of linear theory. In intermediate-depth waters, the waves traversed the field site perpendicularly to isobaths, a pattern that continued as the waves transformed nonlinearly. Acoustic Doppler velocimeters permitted inference of the turbulent energetics, and two main features were studied. First, during a period of shoaling internal waves, turbulence dissipation rates exceeded values associated with tidal shear by an order of magnitude. Second, the evolving spectral signatures associated with a particular wave-shoaling event suggest that the turbulence is at least partly locally generated. Overall, the results of this study suggest that parameterizations of wave-induced mixing could employ relatively simple dynamics in deep water, but may have to handle a wide suite of turbulence generation and transport mechanisms in inshore regions.
Journal of Geophysical Research, 2003
Field observations of linear transition ripples during an autumn storm exhibit high correlation b... more Field observations of linear transition ripples during an autumn storm exhibit high correlation between cross-shore ripple migration rate and the skewness of the near-bed wave orbital velocity. For a bimodal spectrum of mixed sea and swell, negatively skewed near-bed orbital velocities were accompanied by offshore ripple migration, while for unimodal swell, ripples migrated onshore under positively skewed velocities. Bispectral analysis
Journal of Geophysical Research, 2001
Field observations were made in 3-4 m water depth of linear transition ripple geometry and migrat... more Field observations were made in 3-4 m water depth of linear transition ripple geometry and migration using a high-resolution laser-video bed profiling system and acoustic scanning sensors during both the growth and decay phases of an autumn storm event. Linear transition ripples are long-crested, low-steepness bedforms of the anorbital ripple type and were observed to occur here at relatively high
The Journal of the Acoustical Society of America, 1993
Phase shift of the nth wave wavelength of sound in fluid frequency density of fluid density of cy... more Phase shift of the nth wave wavelength of sound in fluid frequency density of fluid density of cylinder duration of transmitted pulse Eq. (13) Eq. ( 11 ), also Eq. (20)
The Journal of the Acoustical Society of America, 2005
Results are presented from the acoustic remote sensing component of Dalhousie University's i... more Results are presented from the acoustic remote sensing component of Dalhousie University's interdisciplinary Coastal Ocean Observatory in Lunenburg Bay, Nova Scotia, operational since June 2002 (www.cmep.ca/bay). Remote acoustic measurements of water column velocity and backscatter are made from bottom-mounted instruments hard-wired to surface buoys. The data are telemetered in near real-time to a shore station via wireless Ethernet, and to Dalhousie over the Internet. The fixed-point measurements from the bottom pods are augmented by a vessel-mounted profiling system, and by opportunistic deployments of internally-recording instruments. Highlights from the first three years of the observing system operation will be presented, including sediment suspension from the seabed and bubble penetration from the surface during Hurricane Juan.
The Journal of the Acoustical Society of America, 1988
The available data for scattered acoustic intensity and attenuation in dilute aqueous suspensions... more The available data for scattered acoustic intensity and attenuation in dilute aqueous suspensions of sand are compared with theory. In theoretical calculations, the scatterer is assumed to be spherical and elastic, or rigid and movable, or rigid and immovable. The rigid movable model provides the best fit to the data. The failure of the elastic model in comparison to the rigid sphere models indicates that resonance excitation does not occur in natural sand grains, probably because of irregularities in shape. The fact that better agreement with experiment is obtained with the rigid movable model than with the rigid immovable model indicates that the inertia of the particles is important. Additional approximate expressions for the form factor and attenuation coefficient have been constructed based on a modified form of the so-called high-pass model introduced by Johnson [J. Acoust. See. Am. 61, 275-277 (1977) ]. The modified high-pass model provides a fit to the data that is as good as, or better than, the rigid movable case. PACS numbers: 43.30.Ft, 43.30.Gv
The Journal of the Acoustical Society of America, 2000
Journal of Physical Oceanography, 2007
An extreme weather event (Hurricane Juan) made landfall in Nova Scotia, Canada, in September 2003... more An extreme weather event (Hurricane Juan) made landfall in Nova Scotia, Canada, in September 2003. The storm produced an ϳ70-cm storm surge and ϳ40 cm s Ϫ1 coastal currents in Lunenburg Bay, registered by a coastal observing system. A fine-resolution (60 m) coastal circulation model is used to examine the response of Lunenburg Bay to Hurricane Juan. The model is forced by local wind stress at the sea surface, and tides and remotely generated waves specified at model open boundaries. The model performance is assessed in terms of ␥ 2 , the variance of the model errors normalized by the observed variance. The model reproduces very well the observed surface elevations with ␥ 2 values of less than 0.05, and reasonably well the observed currents with ␥ 2 values between 0.2 and 1.1 in the bay during Hurricane Juan. The model-data comparisons demonstrate that the coastal circulation in the bay is significantly affected by local wind associated with the storm. The model results are also used to demonstrate the importance of nonlinear dynamics in the barotropic response of the bay to the storm.
Journal of Geophysical Research, 2008
1] The coupling between a quarter-wave resonance in a coastal bay and a Helmholtz mode in an adja... more 1] The coupling between a quarter-wave resonance in a coastal bay and a Helmholtz mode in an adjacent cove (connected to the bay through a narrow channel) is investigated by comparing field measurements to analytical and numerical model predictions. Pressure and velocity spectra from locations throughout the bay reveal an oscillation with a period of approximately 1 hour, consistent with a quarter-wave seiche mode. The associated sea surface displacements throughout the bay are small (less than 5 cm RMS, i.e., only 10% of the tidal elevation). Velocities within the channel are significantly modulated in the 1-hour band, with amplitudes up to 40% of the peak tidal current. The analytical model shows that the modulation of the channel flow results from the interaction between the quarter-wave mode in the main basin and a Helmholtz resonance in the cove, also with a period near 1 hour. The amplitude and phase of the 1-hour oscillation varies through the tidal cycle because of the change in Helmholtz frequency with tidal elevation. Good quantitative agreement between the data and the model predictions is obtained if a drag coefficient approximately 3-4 times larger than the classical value of 3 Â 10 À3 is used in the channel and cove.
Journal of Geophysical Research, 2007
1] Variations with wave energy of near-bed turbulence and the wave friction factor are investigat... more 1] Variations with wave energy of near-bed turbulence and the wave friction factor are investigated in the near-shore zone for bed states spanning low-steepness sand ripples and flat bed, and for wave energies extending well into the sheet flow regime. The measurements were made using a 1.7-MHz pulse-coherent Doppler profiler in ca. 3-m mean water depth. Near-bed turbulence intensities, phase-averaged over the highest-1/3 waves, peak at phases between 10°and 55°after the wave crest, this phase decreasing with increasing wave Reynolds number. Wave friction factors computed from near-bed vertical turbulence intensity fall within the range predicted by existing semiempirical formulae, and exhibit broadly similar trends. At the higher end of the observed wave energy range (i.e., in the sheet flow regime), however, the measured friction factors increase with sea-and-swell energy faster than the predictions. This anomalous increase is correlated with infragravity wave energy and with mean cross-shore current speed, but not with other forcing parameters including mean long-shore current speed, wave skewness, wave asymmetry and wave breaking frequency. It is argued that the anomaly is partly due to additional near-bed turbulence associated with infragravity waves, and therefore that these data are not inconsistent with parameterization for bottom roughness in oscillatory sheet flow. Peak near-bed turbulence intensities are independent of wave Reynolds number for Re ] 1.2 Â 10 6 , but proportional to Re for Re^1.2 Â 10 6 , this abrupt change possibly indicating a critical dependence on Re of turbulence production in the WBL over flat or nearly flat mobile beds. Citation: Newgard, J. P., and A. E. Hay (2007), Turbulence intensity in the wave boundary layer and bottom friction under (mainly) flat bed conditions,
Journal of Geophysical Research, 1992
Vertical profiles of suspended sand concentration and size are obtained from multifrequency acous... more Vertical profiles of suspended sand concentration and size are obtained from multifrequency acoustic profiling data collected in 1989 during a nearshore experiment at Stanhope Lane Beach, Prince Edward Island. The data were acquired with acoustic sounders operating at 1, 2.25, and 5 MHz. Independent estimates of concentration were made using optical backscatter sensors (OBSs). The algorithm for inversion of the three-frequency backscatter data to particle size and concentration, based on the ratios of the different signals, was tested in laboratory experiments with an unconfined high Reynolds number suspended sediment jet. Results from the Stanhope Beach experiment are presented for three different surface wave energy regimes, with significant wave orbital velocities ranging from 0.3 to 0.8 m/s and peak wave periods of 4-6 s. The acoustic estimates of mean concentration are shown to be within 10% on average of those determined with the OBS nearest the bottom at 5-to 10-cm height, over time scales ranging from 6.5 min to 4-6 s (one wave period). The acoustic estimates of suspended sediment size near the bottom are within 5-20% of the bottom sediment mean size. The statistical variability of the size estimates is high, with standard deviations in the estimates ranging between 30 and 50% of the mean. The time-averaged concentration profiles exhibit an exponential decrease with height above an O(10)-cm-thick near-bottom region of nonexponential decrease. In contrast, the time-averaged mean size profiles decrease approximately linearly with height, and rather slowly, about 25% in 0.5 m.
Journal of Geophysical Research, 2010
1] Field observations and model simulations are presented of flow generated by waves breaking ove... more 1] Field observations and model simulations are presented of flow generated by waves breaking over a shoal at the entrance to a shallow bay. The shoal is composed of a series of steep and narrow bedrock ridges with depths of 2-8 m at the ridge crests. Observations from instruments on the shoal indicated peak significant wave heights during a storm event were 2.4-4.0 m across the observation sites; this spatial variability is due to wave breaking over the ridges. The 2-D depth-averaged hydrodynamic model Delft3D, coupled to the wave model SWAN, was used to simulate the waves and wave-driven flow over the shoal and throughout the entire bay with a nested fine grid (5 m resolution) to resolve the shoal bathymetry. The model predicts a well-defined jet behind the shoal, with mean axial speeds of 0.4-0.7 m/s. The observations indicate maximum speeds behind the shoal of 0.3-0.4 m/s, exceeding the maximum tidal current speed by more than a factor of 4, that are consistent in timing and direction with the model predictions. The model overpredicts wave breaking over the steep slopes and as a result the current speeds are overestimated.
Journal of Geophysical Research, 2008
1] A coupled wave/flow model was used to simulate the currents in a coastal bay during the landfa... more 1] A coupled wave/flow model was used to simulate the currents in a coastal bay during the landfall of a hurricane with large waves. Extensive wave breaking along the shoreline and over a midbay shoal induced the development of a strong mean circulation in the bay, in combination with currents forced by wind, tide, and storm surge. The general circulation pattern consisted of inflows along the shoreline and over the shoal region that were driven by radiation stress gradients, and outflows due to mass balance of the wave-driven inflow that were observed in deeper channels. The predicted currents agreed with observations only when wave forcing was included in the circulation model. Wave-driven flows accounted for over 50% of the high flushing rates during the storm and induced strong horizontal velocity gradients over short ($200 m) length scales.
Journal of Geophysical Research, 1999
Observations of the temporal evolution of the geometric properties and migration of wave-formed r... more Observations of the temporal evolution of the geometric properties and migration of wave-formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements. Six weeks of bedform observations were taken at the sandy (medium to coarse sized sand) LEO-15 site located on Beach Haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar. During this period, six tropical storms, several of hurricane strength, passed to the east of the study site. Ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed. The predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3/4 of the wave orbital diameter. Although orbital diameters become larger than 130 cm during the maximum wave event, it is unclear if a transition to nonorbital scaling is occurring. Ripple migration is found to be directed primarily onshore at rates of up to 80 cm/day. Suspended transport due to wave motions, calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements, are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration. Thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near-bottom suspended transport. Bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates. Sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction, as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave-to-current velocities.
Journal of Geophysical Research, 2002
1] This paper reports on remote acoustic observations of vertical turbulence intensity and vertic... more 1] This paper reports on remote acoustic observations of vertical turbulence intensity and vertical suspended sediment flux profiles on a planar beach in 3-4 m water depth. The measurements of suspended sediment concentration and velocity are colocated and simultaneous and extend through the wave bottom boundary layer to the bed with 0.7 cm vertical resolution. Normalized cospectra of the suspended sediment flux and the vertical velocity for different bed states (irregular ripples, cross ripples, linear transition ripples, and flat bed) indicate a small but significant peak at incident wave frequencies but are otherwise rather flat, with weak redness. Estimates of the vertical flux components indicate a general balance between upward fluxes due to waves and turbulence and downward settling. Two exceptions to this balance are found immediately above the bed and for nonmigrating irregular ripples. The contribution from the high-frequency turbulent component is small. Wave phase averages for low-energy bed states exhibit near-bed peaks in the suspended sediment flux following wave phase reversal. Wave phase averages for the high-energy cases do not exhibit a diffusive signature. Observed vertical profiles of turbulence intensity for different bed states reveal that the near-bed turbulence levels are relatively independent of bed state. Friction velocity predictions from presently available models, including a bed stress model and a sediment eddy diffusion model, are compared to measured values of near-bed turbulence intensity. Reasonable agreement is found between measured and predicted bottom friction velocities when wave friction factors from Tolman [1994] are used.
Journal of Atmospheric and Oceanic Technology, 1999
A pulse-to-pulse coherent acoustic Doppler profiler has been developed for high-resolution partic... more A pulse-to-pulse coherent acoustic Doppler profiler has been developed for high-resolution particle velocimetry in the ocean, in particular for remote measurements of suspended sediment flux and turbulence in the nearshore and continental shelf bottom boundary layer. Acoustic backscatter estimates of suspended particle concentration and velocity are determined simultaneously from the phase and amplitude of the backscattered signal over an O(1