OS11A-0476 0800h
Wave-Current Interactions at SandyDuck
Near shore, waves can drive significant flows. Conversely, where the water is shallow and the wave speeds slow, refraction of the waves by currents can also become noticeable. In SandyDuck (a major field experiment conducted off the shore of Duck, NC, in 1997) a pair of "Phased-Array Doppler Sonars" (PADS) were deployed, providing dense time and space coverage of the 2-component horizontal flows. These measurements resolve both the waves and underlying currents over an area some 300 to 400 m on a side. Using 3-D spectral analysis, the waves can be broken into components and examined to assess even fairly subtle refraction by currents. The two-way interaction between the waves and currents can be examined. Spatial variations of the wave-related momentum fluxes (or "Radiation Stress") can be estimated for the expected effects on the underlying flow, while the observed refraction can be compared to that expected from theory (is linear theory adequate?).
OS11A-0477 0800h
Refraction of Surface Gravity Waves by Shear Waves
Previous observations indicate that the directional spread of swell-frequency (0.05--0.15~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 Snell's Law for bathymetric refraction. We derived a model for the fluctuating refraction of swell by low frequency (0.001-0.05~Hz) infragravity and shear waves. Comparison of this model with field observations suggests that refraction by the low frequency cross-shore currents of shear waves caused much of the observed directional broadening, whereas infragravity waves contributed little to broadening.
http://www-ccs.ucsd.edu/~shenders/res.html
OS11A-0478 0800h
A Moored Airborne Video System with Nearshore Applications
Over the past two decades researchers have developed video-based remote sensing techniques to measure relevant nearshore variables. Measurements made include spatial patterns in sand bar morphology, run-up oscillations, wave breaking distributions, phase speed and wave angle, and most recently, surface currents within the surf zone and swash. In general, vertical (i.e., downward oriented) photography or videography is preferred to high-oblique land-based systems. However, although aircraft-mounted video systems have been under development for several years, the relatively high cost and short dwell time has limited its widespread application. Thus, most video measurements for research applications are obtained through methods whereby arrays of video cameras are fixed on land and oriented obliquely to the surf zone region of interest. The typically high-oblique imagery is limited in spatial ground coverage by rapidly degrading resolution in the far field, as well as lay-over problems associated with a fluctuating sea surface and high incidence look-angle. In order to alleviate these problems, researchers have attempted mounting video (or photographic) sensors on tethered balloons where long time series can be obtained over large regions of the surf zone without limiting resolution in the far field. In our research we have developed a technique for mounting a video system onboard a tethered helikite, a combination kite and helium-filled blimp (Allsopp Helikites, Ltd.). The video system consists of a downward-looking video camera in a custom weather-proof housing mounted on the keel of the helikite. Also included are a differential GPS receiver, tilt and heading sensor for accurate geometrical transformation, micro-processor, onboard power supply, and wireless data link. In this presentation, we will discuss the system in more detail, the image resolution and accuracies, and the expected applications to nearshore processes research. This work is sponsored by the Office of Naval Research.
OS11A-0479 0800h
Circulation on the Inner-Shelf of Long Bay, South Carolina: Vertical Current Variability and Evidence for Cross-Shelf Variation in Near-Bed Currents
Cross-shelf variations of near-bed currents and variations in vertical flow were investigated on the inner shelf of Long Bay, South Carolina during the spring and fall of 2001. Current meters sampled near-bed currents at six locations as well as vertical current profiles at three of the sites. The observations showed that the tides accounted for approximately 45-66% of the flow variability. The dominant tidal component, the semi-diurnal constituent M2, exhibited tidal ellipse orientations that are increasingly aligned with the coast closer to the shore. The largest M2 current magnitudes were identified closest to shore and over the top of a sand shoal located 5.5 km offshore of Myrtle Beach. The remaining flow variability was associated with sub-tidal flows which respond to the passage of low-pressure systems across the region. These weather systems were characterized by periods of southwesterly winds in advance of low-pressure centers followed by northeasterly winds as the systems passed over the study area. When strong southwesterly winds persisted, surface flow was oriented approximately in the direction of the wind. At the same time near-bottom flows were also directed to the northeast in the direction of the wind except during periods of stratification when vertical current profiles suggest near-bed onshore flow. The stratified flows were observed mainly during the spring deployment. For periods of strong northeasterly winds, currents were directed alongshore to the southwest and exhibited little variation throughout the water column. These observations are consistent with recent field and modeling studies for the inner-shelf. Comparison of the near-bed flow measurements during the fall deployment revealed a cross-shore gradient in alongshore flow during periods of strong northeasterly winds. During these episodes flows at the offshore measurement stations were oriented in the direction of the wind, while flows closest to shore occurred in the opposite direction. These observations reveal 1) conditions which contribute to cross-shore transport and 2) the presence of an alongshore flow gradient which may affect sediment transport patterns during certain meteorological conditions.
OS11A-0480 0800h
Progress on a Real-Time Storm Surge Forecasting System
As part of an ongoing NOPP project entitled Real-Time Forecasting System of Winds, Waves and Surge from Tropical Cyclones, we are developing a storm surge prediction scheme. 2004 has proven to be an excellent year for testing the model with one of the most active hurricane seasons on record. In developing our storm surge predictions, we seek to couple the wind, pressure and wave stress with the circulation model. We use the Advanced Circulation model, ADCIRC, to calculate the change in surface elevation. We began testing our system on hurricane Georges (1998). In these tests we found that including the wave stresses in a storm surge prediction scheme was important. The waves were found to contribute about 30% to the total water level increase. In some areas the waves can contribute significantly more on the order of 50%. Including the wave forcing reduced the normalized RMS error by 20 to 50%. For the case of hurricane Georges we are satisfied with the predictive results which yielded a MSE of less that \( 0.10 m^{2} \). We test our system next by running a hindcast of hurricane Isabel (2003). In this model scenario we hold all variables constant with those used in the run of hurricane Georges and only vary the wind, pressure and wave field. We are working for a model system that minimizes error without having to tune the model to specific storms. Our results for hurricane Isabel are compared to the measured sea level response. From a series of plots we see that with no tuning of the model formulations we achieve agreement with the recorded data. We will also present preliminary results from the 2004 hurricane season. This includes real-time model forecasts of Charlie, Frances and Ivan. These results are compared with the field data recorded at tidal stations.
OS11A-0481 0800h
Wind-Wave Variability in the Northeast Pacific, 1981-2003
The dominant characteristics of wave energy variability in the eastern North Pacific are described from NOAA NDBC buoy data collected from 1981-2003. Ten buoys at distributed locations were selected for comparison based on record duration and data continuity. Long period, LP [$T > 12$]\,s, intermediate period, IP [$6\leq T\leq 12$]\,s, and short period, SP [$T < 6$]\,s wave spectral energy components are considered separately. Empirical orthogonal function (EOF) analyses of monthly wave energy anomalies reveals that all three wave energy components exhibit similar patterns of spatial variability. The dominant mode represents coherent heightened (or diminished) wave energy along the West Coast from Alaska to Southern California, as indicated by composites of the 700\,hPa height field. The second EOF mode reveals a distinct ENSO-associated spatial distribution of wave energy, which occurs when the North Pacific storm track is extended unusually far south, or has receded to the north. Monthly means and principal components (PCs) of wave energy levels indicate that the 1997-98 El Ni\~{n}o winter had the highest basin-wide wave energy within this record, substantially higher than the 1982-83 El Ni\~{n}o. An increasing trend in the dominant PC of LP wave energy suggests that storminess has increased in the NE Pacific since 1980. This trend is particularly emphasized at central eastern Pacific locations. Patterns of storminess variability are consistent with increasing activity in the central North Pacific, as well as the tendency for more extreme waves in the south during El Ni\~{n}o episodes, and in the north during La Ni\~{n}a.
OS11A-0482 0800h
Continuing Studies of Turbulent Bore Wave Evolution
Turbulent bore waves formed after wave breaking on beaches have been studied in the field with natural incident waves and in laboratory wave tanks for monochromatic and realistic wave spectra. The present research extends our previous research on the subject of turbulent bore evolution. In our current research, turbulent bores are generated in the Max Hammond Wave Tank at SRI, on an $\alpha$ = 0.04 sloped linear beach using TMA incident spectra. Instrumentation employed for measurement of bore propagation includes 28 capacitive wave height gauges and Ku-band Doppler radar. In this paper, we continue our study of the evolution of individual turbulent bores in greater detail using our capacitive wave height gauge data, to determine the effects of breaker type, bottom depth, and other factors on both the height and propagation velocities of the bores. New scalings, and scalings derived previously, for non-dimensionalizing the height and propagation velocity are tested with our new data, using analysis tools with improved logic for determining wave breaker type and wave breaking status. Results are also presented for more detailed wave "extractions" which test the universality of the suggested scalings and probe the physics of bore creation and propagation. Finally, radar reflectivity measurements are presented for turbulent bores with the results compared against the capacitive wave height gauge measurements, in order to determine how the radar can be used to observe the changing physics of the littoral region. The ongoing goals of this research are to improve prediction of turbulent bore waves in realistic conditions and develop techniques for retrieving bathymetry and other surf-zone properties via remote sensing.
OS11A-0483 0800h
Shelf-Slope Exchange in the Mid-Atlantic Bight
A high resolution model (ROMS, the Regional Ocean Modeling System) of U.S. East Coast circulation from Newfoundland to Cuba is used to explore features of alongshelf freshwater transport, residence time, and shelf-sea/deep-ocean exchange. The focus of the analysis is the Mid-Atlantic Bight (MAB) shelf-slope system which, like continental shelves throughout the world, contributes to the oceanic budgets of heat, salt, and fresh water. In addition, the "continental shelf pump" transfers carbon from the atmosphere to the deep ocean through fluxes of dissolved organic carbon and particulate organic carbon off the shelves. Solar radiation, evaporation, rainfall, riverine input, gas exchange with the atmosphere, and biological production all modify the character of shelf waters. In the MAB, the shelf-slope front separates shallow coastal waters from slope waters and the Gulf Stream, extending residence times on the shelf and maintaining coastal salinities at significantly lower levels than those offshore. The southwestward coastal mean flow exchanges weakly with slope waters along most of the MAB, with the strongest offshore flow occurring at Cape Hatteras where much of the flow is entrained into the Gulf Stream front. The shelf circulation is influenced by input from the Hudson and Delaware Rivers and Chesapeake Bay. Along the shelf break, exchange is modulated by warm-core rings from the Gulf Stream and variability of the shelf-break front. Key features of the seasonal circulation such as the MAB "Cold Pool" are captured by the simulation. Measurements suggest that DOC in shelf and shallow slope waters of the MAB include both old marine carbon and a young terrestrial-riverine-estuarine component, and these carbon cycling processes are being studied with a companion primary production, nitogen and carbon cycle model directly coupled to ROMS. Results showing salinity, idealized dye and Lagrangian float tracking results from a ROMS simulation of the MAB shelf circulation are presented. Shelf and slope water freshwater budgets are quantified, including regional variability in onshore/offshore exchange rates, and trhese are compared to observational estimates.
OS11A-0484 0800h
Wave Transformation and Undertow Over a Barred Beach
The mean cross-shore flow (undertow) over a barred beach is examined by calibrating an existing wave and circulation model which will provide predictions that will be compared to in-situ measurements. The wave model applied to the experiment is a linear shoaling model with wave breaking dissipation according to Dally, Dean, and Dalrymple (2002); following the calibration of the wave model the undertow model by Garcez Faria et al. (2000) is calibrated. The experiment was conducted in the long wave flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University (OSU). Regular waves with a wave height of 0.60 meters and a frequency of 0.25 Hz were generated and vertical velocity profiles were collected at seven different cross-shore locations each with eight to nine points per profile. Three Acoustic Doppler Velocimeters(ADV) were used to collect the cross-shore flow velocities and six fixed wave gauges were used to measure the water surface elevation. The data collected from the wave gauges were processed using a zero-upcrossing analysis followed by ensemble averaging the waves to determine the average wave height of each cross-shore gage. This approach was taken since regular waves are assumed to be similar to one another and would yield higher numbers of realizations, making it a more statistically reliable estimate. In total, 49 different time series were analyzed for each wave gage, yielding a reliable wave height estimate. The calibration of the wave model was accomplished by altering the three free parameters: higher breaking threshold $\gamma$, lower breaking threshold $\gamma$$_{s}$, and an empirical constant $\kappa$, until the wave model best fit the data. Subsequently, the undertow model was calibrated by varying the eddy viscosity ($\mu$) for each profile. The presented work was performed as part of the Research Experience for Undergraduates (REU) program during the summer of 2004 at OSU.
OS11A-0485 0800h
Using Radium Isotopes to Evaluate Cross-shelf Dispersion in the Coastal Ocean: Application to San Pedro Bay, CA
The short-lived radium isotopes Ra-223 (11 day half life) and Ra-224 (3.6 day half-life) are potentially useful for evaluating cross-shelf dispersion rates in the coastal ocean. A requirement for this application is that their source function and its variability in time and space must be defined. The primary mechanisms for introducing radium into coastal surface waters include: (1) wave and tide-driven circulation of water through permeable beach sands, (2) input from the seafloor due to molecular diffusion and circulation of bottom water through surficial sands, (3) flow of water rich in Ra from marshes and estuaries, and (4) net advection of groundwater. The importance of these inputs to San Pedro Bay was determined from concentrations in waters collected from each of these potential sources. In most of the region, mechanism (2) supplies 90% of the input, although mechanisms (1) and (2) may become dominant locally as the coastal morphology varies in the longshore direction. Longshore variations in the composition of beach sand and the presence of persistent coastal eddies create longshore gradients in Ra concentration that are significant in this region. Temporal variations in shoreline concentrations on time scales 6-8 hours reflect variations in mechanism (1) as tides rise and fall, with drainage of water from the beach face creating higher concentrations during the falling tide. Despite these complications in characterizing the source function, the distribution of short-lived Ra isotopes is useful in constraining the rate of horizontal mixing. A two-dimensional advection-diffusion model was best fit with an eddy diffusivity of 1.3+/-0.2 m2/s over length scales of several km offshore, with a value about 50% smaller in the littoral zone. The scale dependence of eddy diffusivity is also apparent in the distribution of Ra-228, which requires lower eddy diffusivities in the nearshore than in the offshore region. A budget for Ra-226 indicates that little groundwater directly enters the ocean in this region, although some groundwater may enter marshes and estuaries that are adjacent to the coast.
OS11A-0486 0800h
Inclusion of Wave Direction in Bathymetry Inversion Algorithms
With the increasing focus on understanding, and managing the response of natural coastlines to rising sea level, there is an ongoing need for accurate measurements of nearshore bathymetry. Traditional in-situ surveys, while accurate, are logistically hard, and time-consuming. An attractive alternate method that has seen increasing application is the use of video images to calculate bathymetry based on wave celerity. However, most accepted algorithms only exploit the magnitude of the wave number, k, and ignore refractive turning of waves as an exploitable signal. We propose a method that uses snap shots of shoaling waves to determine the directional components of the wave number (kx, ky), and the resulting bathymetry. Wave phase is determined using Hilbert transforms of the 2D image. From the spatial gradients of the wave phase, we calculate kx, ky, wave angle, and the resulting bathymetry. The method is initially tested using a simple model to generate a wave field on a known bathymetry, and then using the above techniques to regenerate the input. The model is also tested with field data.
OS11A-0487 0800h
Wave Friction Factors from Energy Flux Comparisons Outside of the Surf Zone
Pressure and current measurements were recorded at 5.5 m, 8 m, and 13 m water depths in the outer surf zone and inner continental shelf region off the coast of Duck, NC. The instrumentation was deployed in October, 1997 and remained operational through December, 1998. This data set was analyzed to investigate wave dissipation and evolution by comparing estimates from linear wave theory with field measurements. The analysis was restricted to six significant storm events that occurred during four months to ensure that significant energy was present for meaningful measurements. The range of significant wave heights included is 1.75 m $<$ H$_{s}$ $<$ 3.5 m. Energy flux calculations combining shoaling and refraction theory that did not account for energy dissipation, showed smaller measured than predicted energy flux values at the two inshore locations. Some of these values were smaller by more than one third of the total energy flux, emphasizing the importance of bottom friction in wave energy loss. A representative friction factor for each 34-minute record was determined by accounting for frictional energy loss in the energy flux calculations, using velocity time series measured 20 cm above the sea floor. Estimated friction factors varied throughout storm events, but most fell within a range of 0.002 to 0.03. This range of values was compared to other wave friction factor estimates both inside the surf zone and outside. A representative value of 0.021 was identified through the use of energy flux and energy loss values averaged over all storm events. This study emphasizes the importance of including bottom friction when considering wave energy dissipation.
OS11A-0488 0800h
Resolving Kuroshio/Near Boundary Interaction With Nested Numerical Models: Application to the Kyucho
The Kyucho (lit.~swift current) is marked by sudden strong currents and an associated internal temperature rise which propagates counterclockwise at 1-2 m/s around Sagami Bay, Japan. Sagami Bay is a semicircular bay just southeast of Tokyo Bay. It has been hypothesized that the Kyucho is the manifestation of a semigeostrophic shock wave formed at the leading edge of a density current associated with the intrusion of warm Kuroshio water into Sagami Bay (T.~Yamagata, Tellus, 1980, 32, pp.~73-76; A.~Kubokawa and K.~Hanawa, J.~Ocean.~Soc.~Japan, 40, 1984, pp.~247-270). Because the Kyucho causes an average of about 2 million dollars per year of damage to fishing nets in Sagami Bay, and because it has been associated with the arrival of fishable yellowtail in Sagami Bay, Japanese researchers have been interested in the Kyucho for at least the past 75 years. It has been definitely linked to variations of the Kuroshio and to the passage of the typhoons, but little is known about the structure of the current itself or how Kyucho induced by the Kuroshio differ from those induced by typhoons. We first mention our attempts to study the Kyucho by analyzing available data: coastal station current and temperature records, recently installed HF radar surface current data, satellite SST data, and the output of the JCOPE numerical model which resolves the Kuroshio. We present some instances in which Kyucho can be linked to the advection of warm water by the Kuroshio (using the reanalysis SST product of H. Kawamura at Tohoku University). However, none of the available data has sufficient time and space resolution to clearly capture the formation and propagation of the Kyucho. In some of the near-coast HF radar data, reliability is also an issue. We therefore discuss recent work on numerical models which can resolve the details of Kyucho formation: principally, a high-resolution nested hydrostatic model based on POM which uses data from the coarser JCOPE model (Japan Coastal Ocean Predictability Experiment). It is hoped that this model will eventually be used for operational forecasting. We will also present analysis of an September-October 2004 JCOPE experiment, which will have a horizontal resolution of 3km near Sagami Bay, possibly enough to see features of the Kyucho.
OS11A-0489 0800h
Shoreline Setup Observations and Predictions
The importance of wave rollers to the breaking-wave-driven increase of the mean water level (setup) in shallow water is investigated by comparing observations with predictions of one-dimensional, depth- and time-averaged models that include (neglect) wave rollers. Wave setup was measured near Duck, NC (August to December 1997) with 9 Paroscientific pressure gages (+/- 0.5 cm accuracy for mean water levels) located on a 200-m-long cross-shore transect extending from the shoreline to approximately 4-m water depth. The setup models are driven with the cross-shore distribution of radiation stresses. To obtain radiation stress estimates every meter, observed centroidal frequencies and local bathymetry are linearly interpolated, observed wave directions are interpolated using Snell's Law, and wave heights are calculated by fitting parametric wave models to observations (least squared error less than 8%). The beach profile was surveyed approximately every other day. Offshore wave heights ranged from 20 to 200 cm. Consistent with previous observations, the models accurately predict setup in water depths greater than 1 m. However, the setup is increasingly underpredicted with decreasing water depth, partly owing to the assumption that energy dissipated by wave breaking is transferred immediately into the water column. The addition of a roller, defined as a passive region of circulating water carried shoreward by the breaking waves, in the radiation stress formulation delays the transfer of energy and momentum causing an increase or decrease in the predicted setup depending on whether the transfer is into shallower or deeper water. Preliminary analysis indicates that inclusion of a wave roller usually improves setup predictions in water depths less than 40 cm when offshore wave heights are greater than 100 cm (on average, errors are reduced by about 30%). This project was funded by ONR and NSF.
OS11A-0490 0800h
Remote Sensing of Radiation Stress Gradients from NCEX Optical Imagery
As waves break in the surf zone, radiation stress gradients are created, corresponding to a local net addition of momentum to the water column and resulting in a driving force for nearshore currents and subsequent sediment transport. Consequently, estimation of radiation stress gradients throughout a given domain is of great interest for both the initilization and analysis of nearhore circulation model results. Currently, such estimates are based on first-order differences in cross-shore momentum flux calculated from {\it in situ} data recorded at discrete locations. We are investigating the potential for optical signals of wave breaking to provide an alternate remote sensing technique for the direct estimation of radiation stress gradient fields. For a beach with straight, parallel contours, the individual components of radiation stress gradients can be derived from linear wave theory and depend on local celerity, wave angle, and rate of dissipation, all of which have optical signatures. Our current work focuses on extending these equations to include spectral wave information over more complex bathymetry. In addition, the inclusion of a more sophisticated breaker intensity model allows for more accurate and robust estimation of dissipation. Initial results for the cross-shore gradient in longshore directed momentum compare well with numerical model results during the NCEX field experiment.
OS11A-0491 0800h
Characterising the Surf Zone Wave-Breaking Environment Using Passive Acoustics
Preliminary results are presented of an experiment to measure the ambient sound within the surf zone through varying tidal states and wave environments. Surf zone ambient noise from 20 Hz-20 kHz is predominantly generated by breaking waves through injection of oscillating bubbles into the water column. Breaking wave noise is related to the wave energy dissipation and it has been demonstrated that passive acoustic methods are a viable means of monitoring local wave breaking. Ambient noise was recorded using an omnidirectional broad-band hydrophone deployed just above the seabed in the inter-tidal zone for a four-hour period over high water when the hydrophone was submerged. Measurements of velocities, wave heights and suspended sediment concentrations were acquired simultaneously about 2 m away from the hydrophone at the same cross-shore distance. In the frequency range of interest, 20 Hz to 20 kHz, the predominant source of noise was from waves breaking directly above the hydrophone. Simultaneous land-based video recordings of the sea surface above the hydrophone confirm that individual breaking waves are associated with discrete noise events above the ambient background noise. These noise events from individual breakers evolve with the shape of wave crests and correlate with the pressure time series at gravity and infra-gravity frequencies. Throughout the sampling period the ambient noise reflects the type of breaker, water depth and sound propagation conditions. Mean sound intensities at all frequencies vary with the water depth changes with the tide. Literature reports of ambient noise measured within the surf zone are sparse and this work contributes valuable insights into temporally varying breaking wave characteristics in the surf zone and the associated acoustic noise and propagation conditions.
OS11A-0492 0800h
Energy Flux Balance of Surfzone Infragravity Waves
Infragravity waves, with periods (between roughly 20-200 s) longer than sea-swell (between about 2-20 s), are ubiquitous on ocean shorelines. Generation by groups of sea and swell, dissipation, shoreline reflection, and refractive trapping have been suggested to contribute significantly to the infragravity energy balance in shallow, nearshore waters. Methods used previously to analyze infragravity wave data either have assumed unidirectionality or have neglected dissipation (i.e. associated all trapped energy with edge waves). Here, spectra of cross-shore infragravity energy fluxes in alongshore wavenumber - frequency space are estimated from pressure and cross-shore velocity measured on alongshore-oriented transects using a new method based on the WKB approximation. The energy fluxes of infragravity waves are studied using field observations from five alongshore arrays deployed for 4 months across the shoaling and surf zones (between 1- and 5-m water depth) of a gently sloping sandy beach. Estimates of cross- and alongshore infragravity energy flux allow calculation of the distribution of the flux reflection coefficient in the alongshore wavenumber - frequency space, as well as bulk (frequency-integrated) reflection coefficients for trapped and leaky waves. Results suggest dissipation occurs over a broad range of infragravity frequencies, and at oblique angles with alongshore wavenumber within the range of theoretically trapped waves. Supported by ONR and NSF.
OS11A-0493 0800h
Theory of Edge Capillary-Gravity Waves
We consider a body of fluid in equilibrium in a gravitational field and having a free surface and a plane-sloping beach with a straight coastline. If, under the action of some external disturbances, the surface is moved its equilibrium position, motion will occur in the fluid. This motion will be propagated along the coast in the form of waves, which are driven under the action of gravity and surface tension forces. We call these waves edge capillary-gravity waves, if their amplitude decays exponentially with distance from the coast. The fluid is considered inviscid, irrotational and incompressible. Under these conditions the velocity potencial satisfies the Laplace's equation everywhere in the fluid. The boundary conditions are such that the normal velocity at the bottom is zero and on the free surface in the presence of surface tension the linearized kinematic and dynamic boundary conditions are satisfied. The main difficulty for solution of this problem is that the variables are not separated. We present explicit solutions for all modes of the edge capillary-gravity waves and the dispersion equation. Capillary forces affect markedly the edge gravity waves profiles over the high frequency range. The peaks and lows have become larger as compared to pure edge gravity waves, dependence on the radial coordinate becomes more complicated, and a number of zeros of a mode might not coincide with the number of the mode. When ignoring capillary forces, our results are in complete agreement with the classic results of Ursell (1952) for the edge gravity waves on a sloping beach.
OS11A-0494 0800h
Observations of Extreme Wave Events in a Shallow Coastal Embayment
Lunenburg Bay is a coastal inlet located on the southern shore of Nova Scotia, Canada, and is approximately 8 km long and 4 km wide. It has irregular and shallow bathymetry characterized by a typical depth of 10 m and is exposed to wave energy from the North Atlantic Ocean. Within the bay, an array of sensors has been deployed to collect physical, biological and atmospheric data as part of a real-time coastal observing system. The wave array consists of bottom-mounted instruments including two acoustic doppler current profilers capable of resolving wave spectra, two single-point velocimeters with co-located pressure sensors and standard acoustic doppler profilers, and a surface-moored directional wave-rider buoy. The set of instruments is capable of observing directional spectra at 0.5-hr intervals for wave events at 5 locations in Lunenburg Bay. This poster summarizes the dataset collected in fall 2002, summer-fall 2003 and summer-fall 2004. Extreme wave events are highlighted and compared with respect to differences in atmospheric forcing (wind, pressure, duration, storm type and scale) and attenuation within the bay from observed wave spectral properties (peak direction and directional spread, significant wave height and peak period). Extreme events include the close passage of several hurricanes (Gustav, Fabian, Juan), stronger but more distant hurricanes (Isabelle) and larger-scale mid-latitude low pressure systems. The most notable wave event in the bay was driven by Hurricane Juan with a deep-water significant wave height of 9 m, more than double that of the second largest event in the observation period. Strong attenuation between instrument locations inside the bay is attributed to refraction and breaking. Observed significant wave heights and near-bed orbital velocities are compared to numerical model simulations from the SWAN model for peak conditions for various extreme events and are in good agreement.
OS11A-0495 0800h
Boussinesq Modeling of Nonlinear Waves and Surf-Zone Currents over a Permeable Beach
The study introduces a complete set of Boussinesq-type equations suitable for water waves and wave-induced nearshore circulation over an inhomogeneous, permeable bottom. The derivation starts with the conventional expansion of the fluid particle velocity as a polynomial of the vertical coordinate $z$ followed by the depth integration of the vertical components of the Euler equations for the fluid layer and the volume-averaged equations for the porous layer to obtain the pressure field. Inserting the kinematics and pressure field into the Euler and volume-averaged equations on the horizontal plane results in a set of Boussinesq-type momentum equations with vertical vorticity and $z$-dependent terms. A new approach to eliminating the $z$-dependency in the Boussinesq-type equations is introduced. It allows for the existence and advection of the vertical vorticity in the flow field with the accuracy consistent with the level of approximation in the Boussinesq-type equations for the pure wave motion. Examination of the scaling of the resistance force reveals the significance of the vertical velocity to the pressure field in the porous layer and leads to the retention of higher-order terms associated with the resistance force. The equations are truncated at $O(\mu^4)$ where $\mu$ is the measure of frequency dispersion. An analysis of the vortical property of the resultant equations indicates that the energy dissipation in the porous layer can serve as a source of vertical vorticity up to the leading order. In comparison with the existing Boussinesq-type equations for both permeable and impermeable bottoms, the complete set of equations improves the accuracy of potential vorticity as well as the damping rate. The new equations retain the conservation of potential vorticity up to $O(\mu^2)$. Such a property is desirable for modeling wave-induced nearshore circulation but is absent in existing Boussinesq-type equations. The study has been sponsored by the Office of Naval Research.
OS11A-0496 0800h
Pressure Gradients on a Barred Beach
Pressure gradients generated by free surface oscillation and wave breaking are examined. With the purpose of defining when and how pressure gradients most significantly create instability, Madsen (1974) proposed that the critical value of normalized pressure gradient needed for sand bed instability was 0.5. Cox et al. (1991) conducted a laboratory experiment and suggested that the occurrence probability of the momentary landward and seaward failure might occur at the steep rear face of near-breaking and breaking random waves. The laboratory experiment was conducted at O.H. Hinsdale Wave Research Laboratory of Oregon State University in a 104 m long by 3.7 m wide by 4.6 m high concrete-walled large wave flume equipped with a flap-type wavemaker. In this experiment, a bar was installed on a 1/36 slope beach area with rough impermeable bottom slope. 4 pressure transducers, 2 three-component Acoustic Doppler Velocimeters and 3 wave gages were installed on the movable cart and 6 wave gages were directly fixed on the flume. Nine positions (2 for offshore, 4 on the bar, and 3 for onshore) were chosen to understand how pressure gradients behave as waves propagate across barred beaches. Both regular and irregular wave conditions were involved in the measurements. For the analysis, pressure gradients are calculated from four sensors to obtain better estimates and are compared with water surface elevation and velocity. In regular wave case, phase-averaged quantities (i.e., water surface elevation, pressure gradients, and acceleration) are used to discuss the relationship to maximum pressure gradient. Exceedance probabilities of pressure gradients are considered at each cross-shore location using irregular wave data to evaluate the critical value for bed instability on barred beach. Also, large but rare pressure gradients are compared with acceleration to figure out their relationship. Funding for Research Experience for Undergraduates (REU) was provided by the National Science Foundation. The O.H. Hinsdale Wave Research Laboratory was partially supported by the NSF as part of the George E. Brown NEES program.
OS11A-0497 0800h
Scattering And Reflection Of Semidiurnal Internal Tide In Uchiura Bay
To clarify scattering process of semidiurnal internal tide in Uchiura Bay, located at the head of Suruga Bay in Japan, mooring observations using ADCP were performed at two stations near the southern coast of Uchiura Bay in summer, 2002. These mooring systems were arrayed in cross-shore direction, and water depths of the stations are 74m and 84m, respectively. Semidiurnal internal tide was predominant throughout the observation period. The alongshore currents for semidiurnal internal tide at both stations showed maxima in upper and lower layers and the upper layer currents were approximately out of phase to the lower layer current, implying the structure of the first vertical mode. On the other hand, the cross-shore currents with semidiurnal period were larger in the middle layer at both stations and their phases delayed in the upper layer. The depth of maximum amplitude of the cross-shore current at offshore station was about 6m deeper than that at near-shore station. These features indicate the downward energy propagation and upward phase propagation, i.e., vertical propagation of internal wave. Next, we carried out numerical experiment using a three-dimensional level model with real topography to investigate the generation process of the downward propagating internal wave. The experiment result well reproduced the observed result. The downward propagating wave was found to be regenerated at shallow region of the southern coast of Uchiura Bay by a scatting process of the first-mode internal wave that has a characteristics of internal standing wave in the bay. The scattered waves were also regenerated at the bay mouth and along shore currents associated with the scattered waves were strengthened just above the sea bottom at the center of the bay, because the bottom slope is near critical for the semidiurnal internal tide. About 20 percent of the energy for the semidiurnal internal tide in the bay was estimated to be transported to small-scale internal wave by the scattering and reflection processes.
OS11A-0498 0800h
Generation and Evolution of Solibores on Slopes
The present study focuses on understanding the generation and propagation of solibores formed as a result of the interaction of a nonlinear first-mode internal wave field with a sloped coastal shelf break, using high-resolution two-dimensional numerical simulations. Our study shows the development of a nonlinear bolus that moves upslope as a result of the interaction and breaking of the baroclinic wave with the slope, consistent with earlier numerical results by others (Slinn & Riley 1998a, 1998b, Legg & Adcroft 2003). These solibores that propagate onto the shelf have large onshore velocities contained within their cores indicating that there is sufficiently high shear in these solibores that might create shear instability. The minimum Richardson number at the interface of these solibores is less than the critical value of 0.25 for much of the interface at the leading edges of these solibores, indicating the high probability for shedding of heavy fluid via shear instability. Enhanced dissipation rates are noted at these leading edges. An analysis of the mass exchange between the solibores and the surrounding fluid indicates that the bores can shed a significant fraction of their initial mass as they propagate shoreward, highlighting their importance in diapycnal mixing and mass transport. The results may support ongoing efforts in the oceanographic community to understand the small-scale dynamics associated with these highly nonlinear internal waves.
OS11A-0499 0800h
Canyon Effects on Nearshore Infragravity Waves During NCEX
Infragravity waves become increasingly important as the water depth gets shallower and wind generated waves become saturated due to wave breaking. Infragravity wave energy is composed of wave-group forced long waves and reflected leaky waves and trapped edge waves. Typically conditions on a approximately alongshore uniform beach are consisdered (e.g. Herbers et al., 1994, van Dongeren et al., 2003). Here we examine the alongshore variability in the infragravity conditions induced by nearby canyons utilizing a 2D-surfbeat model (Reniers et al., 2004). The model simulates the propagation of both leaky and trapped infragravity waves that are generated by directionally spread wave groups. Model computations are used to examine the potential reflection (Inman et al., 1976, Huntley et al., 1981) of shore-trapped edge waves from the canyon walls by considering various model-scenarios with and without the canyons. Computational results will be compared with observations of infragravity conditons obtained from an alongshore array of pressure and velocity meters situated just north of the canyon (MacMahan et al., 2004, this conference). References Herbers, T.H.C., Steve Elgar and R.T. Guza, 1994: Infragravity-frequency (0.005 0.05 Hz) motions on the shelf. Part 1: Forced waves. J. Phys. Oc., 25, 1063-1079. Huntley, D. A., R. T. Guza and E. B. Thornton, 1981, "Field Observations of Surf Beat: Part I, Progressive Edge Waves", J. Geophys. Res., 86, 6451-6466. Inman, D.L., C.E. Nordstrom and R.E. Flick, 1976: Currents in sub-marine canyons: An air-sea-land interaction, Ann. Rev. Fluid Mech., 8, 275-310. MacMahan, J., E.B. Thornton, A. Reniers and T.P. Stanton, 2004, The Torrey Pines Rip-currents, this conference. Reniers, A.J.H.M., E.B. Thornton and J.A. Roelvink, 2004: Morphodynamic modeling of an embayed beach under wave-group forcing, J. Geophys. Res., 109, C01030, doi:10.1029/2002JC001586. Van Dongeren, A.R., A.J.H.M. Reniers, J.A. Battjes and I.A. Svendsen, 2003, "Numerical modeling of infragravity wave response during Delilah." J. Geoph. Res, 108 (C9), 4-1-19
OS11A-0500 0800h
Validation of an Optical Swash Velocity Measurement Algorithm
Along coastlines, damage to infrastructure and homes is caused by foreshore erosion which is, in turn, dependent on the swash flow velocity. However, velocity data is difficult to obtain in natural swash zones with an in situ instrument due to the temporal variation in bed and water levels, the potential flow disruption by such a sensor, and the overall hostile conditions. Last year an optical swash velocity algorithm was introduced based on the trajectories of foam tracers in the the swash. Argus time-space imagery (time stacks) are high-pass filtered to retain only the foam signatures, then transformed in the space domain using a Hilbert transform to yield phase and wave number as a function of time and space. Cross-shore velocity is found from the time rate of change of these signals. This year, development has continued and the algorithm has been tested with Acoustic Doppler velocity data from the swash zone in the wave flume of the Hinsdale Wave Research Laboratory, Oregon State University. Comparisions of swash velocities from the optical and in situ sensors will be made and accuracy and sensitivities of the technique determined. Image registration errors due to finite amplitude water surface displacement error will be corrected using wave gauges. Detergent was used to increase the duration of surface foam.
OS11A-0501 0800h
A Boussinesq-Type Wave Model as a Wave Driver for a Morphodynamic Model
The current generation of operational morphodynamic models is based on the short-wave averaged depth-integrated Reynolds equations. Because the short waves are averaged out, a separate module, the wave driver, is needed to provide the wave-depentdent forcing in the momentum equations. Most operational models use a steady wave driver based on linear theory, which means that certain aspects relevant to the sediment transport formulations need to be parametrized. We will present the results of using the phase-resolving Boussinesq-type wave model TRITON as a wave driver, which computes the wave forces. These are transferred to the short-wave averaged free-surface model DELFT3D for the computation of flow, sediment transport, and morphological changes. In this way, the time-integrated effects of intra-wave properties such as individual wave height transformation (including breaking), wave skewness and wave asymmetry, and drift velocities are communicated online with DELFT3D. The changes in bathymetry predicted by DELFT3D are transferred back to TRITON to include this effect in the simulation of the wave dynamics. In particular, we will discuss the online separation of waves into long and short waves (this must be done in time domain to get the time-integrated effects of the intra-wave properties), and reconstruction of the velocity over the vertical coordinate. Validation of the wave part of the methodology is done by comparison with high-resolution wave flume experiments by Boers (1996). The combined strength of both models will improve the prediction capabilities for nearshore morphodynamics in response to wind wave forcing on the time scale of a storm event. This work is funded by the U.S. Office of Naval Research under contract N00014-02-C0075.