OS13D-1220
A Remote-Control Airship for Coastal and Environmental Research
The University of Delaware recently acquired an 18 m (60 ft) remote-control airship capable of carrying a 36 kg (120 lb) scientific payload for coastal and environmental research. By combining the benefits of tethered balloons (stable dwell time) and powered aircraft (ability to navigate), the platform allows for high-resolution data collection in both time and space. The platform was developed by Galaxy Blimps, LLC of Dallas, TX for collecting high-definition video of sporting events. The airship can fly to altitudes of at least 600 m (2000 ft) reaching speeds between zero and 18 m/s (35 knots) in winds up to 13 m/s (25 knots). Using a hand-held console and radio transmitter, a ground-based operator can manipulate the orientation and throttle of two gasoline engines, and the orientation of four fins. Airship location is delivered to the operator through a data downlink from an onboard altimeter and global positioning system (GPS) receiver. Scientific payloads are easily attached to a rail system on the underside of the blimp. Data collection can be automated (fixed time intervals) or triggered by a second operator using a second hand-held console. Data can be stored onboard or transmitted in real-time to a ground-based computer. The first science mission (Fall 2008) is designed to collect images of tidal inundation of a salt marsh to support numerical modeling of water quality in the Murderkill River Estuary in Kent County, Delaware (a tributary of Delaware Bay in the USA Mid-Atlantic region). Time sequenced imagery will be collected by a ten-megapixel camera and a thermal- infrared imager mounted in separate remote-control, gyro-stabilized camera mounts on the blimp. Live video- feeds will be transmitted to the instrument operator on the ground. Resulting time series data will ultimately be used to compare/update independent estimates of inundation based on LiDAR elevations and a suite of tide and temperature gauges.
OS13D-1221
Predicting Nearshore Waves
Predicting wave transformation over a variable sea bottom is a difficult task. Analytical solutions limit themselves to simple geometry; numerical models, unless in the context of long waves, must base their calculations on the assumption of 'slowly varying' sea depth. Artificial Neural Network (ANN) methodology, because of its capability of dealing with highly nonlinear phenomena, is an attractive approach to predict wave transformation in a complex bathymetry. In this paper, we demonstrate and discuss the use of ANN in predicting nearshore waves from offshore observations. The wave field of the study area, Redondo Beach, California, characterized by the presence of a deep submarine canyon, presents an excellent challenge for this purpose. Incoming waves converge and diverge when traveling over the canyon, which extends perpendicular to the coast for 10 km offshore with maximum depth of 400 meters. This paper presents 2, 4, 6, 12 and 24 hours ANN predictions for comparisons with numerical model results and prototype measurements.
OS13D-1222 [WITHDRAWN]
Extraction of Coastal Wavefield Properties from X-Band Radar
The dynamic wave field in a high-energy coastal environment is investigated using nautical X-band radar imagery. Radar observations and wave analysis were carried out by the Wave and Surface Current Monitoring System WaMoS II which was deployed from February-October 2005 at the US Army Corps of Engineers Field Research Facility (USACE-FRF) in Duck, North Carolina. Obtained directional wave spectra represent spatial mean sea state conditions over an area of 3.7 square km outside the surf zone in a water depth of 8m to 10m. The radar measurements were compared with those obtained from an 8-m depth pressure gauge array located in the radar analysis area by means of a wave spectral partitioning analysis. Preliminary results show that the WaMos II system adequately captures the temporal evolution of the individual windsea and swell wave components entering the surf zone. A statistical error analysis of the isolated windsea and swell wave systems yields a quantitative assessment of WaMoS II performance in a coastal setting.
OS13D-1223
Current Measurements From an Airborne Remote Optical Spotlight System Imaging Multispectral Polarimeter (AROSS-MSP)
Tidal flat regions are dynamic in their physical and biological processes. The transport of sediment and organisms in these regions are dependent on the dynamics of waves, tides, turbulence, and the currents associated with rivers that empty into and flow through them. We present preliminary measurements of currents from remote sensing of a tidal flat region using our Airborne Remote Optical Spotlight System- Multispectral Polarimeter (AROSS-MSP). AROSS-MSP is a 12-channel sensor system that measures 4 color bands (RGB-NIR) and 3 polarization states for the full linear polarization response of the imaged scene. Color and polarimetry, from remotely-sensed time-series imagery, provide unique information for retrieving dynamic environmental parameters relating to sediment transport processes over a larger area than is possible with typical in situ measurements. Current retrievals from our MSP imagery are presented with an emphasis on sediment-laden features in tidal flats.
OS13D-1224
Observation of wave celerity evolution in the nearshore using digital video imagery
Celerity of incident waves in the nearshore is observed from oblique video imagery collected at Myrtle Beach, S.C.. The video camera covers the field view of length scales O(100) m. Celerity of waves propagating in shallow water including the surf zone is estimated by applying advanced image processing and analysis methods to the individual video images sampled at 3 Hz. Original image sequences are processed through video image frame differencing, directional low-pass image filtering to reduce the noise arising from foam in the surf zone. The breaking wave celerity is computed along a cross-shore transect from the wave crest tracks extracted by a Radon transform-based line detection method. The observed celerity from the nearshore video imagery is larger than the linear wave celerity computed from the measured water depths over the entire surf zone. Compared to the nonlinear shallow water wave equation (NSWE)-based celerity computed using the measured depths and wave heights, in general, the video-based celerity shows good agreements over the surf zone except the regions across the incipient wave breaking locations. In the regions across the breaker points, the observed wave celerity is even larger than the NSWE-based celerity due to the transition of wave crest shapes. The observed celerity using the video imagery can be used to monitor the nearshore geometry through depth inversion based on the nonlinear wave celerity theories. For this purpose, the exceeding celerity across the breaker points needs to be corrected accordingly compared to a nonlinear wave celerity theory applied.
OS13D-1225
Long Period Edge Waves Generated by Hurricane Landfall
Direct observations of the storm surge induced by Hurricane Wilma 's landfall on the West Coast of Florida revealed a formation of an edge wave pulse propagating downstream (in the direction of Kelvin wave). The wave height exceeded 1.5 m in the detided sea level. The duration of this wave pulse was ~6 hrs and the propagation speed was of O(10) m/s. A set of numerical experiments has been conducted to delineate a generation of edge waves with large spatial and temporal scales by a fast-moving storm system. The coastal ocean model was set in a 2-D configuration with the continental shelf and slope topography reminiscent of the West Florida shelf. A moving cyclonic system in the gradient wind balance was prescribed analytically. In order to identify a long-wave response in the model, a linear boundary problem was solved yielding dispersion characteristics and a structure of edge wave modes for the arbitrary depth profile. A fast-moving storm system crossing the shelf at a right angle produces a nearly symmetrical response of two edge wave trains propagating both downstream and upstream. Typically, zero-mode edge waves dominate the response. As the translation speed of the storm becomes lower, its Eulerian time scale becomes longer and the waves are more affected by the Earth's rotation. In that case the wave energy propagates predominantly downstream. When the storm trajectory deviates from the normal approach, the edge wave response is not symmetric: most of the energy propagates in the direction of the alongshore component of the storm translation velocity.
OS13D-1226
Infragravity wave generation and dynamics over a mild slope beach : Experiments and numerical computations
Charasteristic frequencies of gravity waves generated by wind and propagating towards the coast are usually
comprised between 0.05Hz and 1Hz. Nevertheless, lower frequecy waves, in the range of 0.001Hz and
0.05Hz, have been observed in the nearshore zone. Those long waves, termed as infragravity waves, are
generated by complex nonlinear mechanisms affecting the propagation of irregular waves up to the coast.
The groupiness of an incident random wave field may be responsible for producing a slow modulation of the
mean water surface thus generating bound long waves travelling at the group speed. Similarly, a quasi-
periodic oscillation of the break-point location, will be accompained by a slow modulation of set-up/set-down
in the surf zone and generation and release of long waves. If the primary structure of the carrying incident
gravity waves is destroyed (e.g. by breaking), forced long waves can be freely released and even reflected at
the coast. Infragravity waves can affect port operation through resonating conditions, or strongly affect
sediment transport and beach morphodynamics.
In the present study we investigate infragravity wave generation mechanisms both, from experiments and
numerical computations. Measurements were conducted at the 70-meter long wave tank, located at the
Instituto Nacional de Hidraulica (Chile), prepared with a beach of very mild slope of 1/80 in order to produce
large surf zone extensions. A random JONSWAP type wave field (h0=0.52m, fp=0.25Hz, Hmo=0.17m) was
generated by a piston wave-maker and measurements of the free surface displacements were performed all
over its length at high spatial resolution (0.2m to 1m). Velocity profiles were also measured at four verticals
inside the surf zone using an ADV. Correlation maps of wave group envelopes and infragravity waves are
computed in order to identify long wave generation and dynamics in the experimental set-up. It appears that
both mechanisms (groupiness and break-point oscillation) are clearly present in this experiment while
spectral analysis evidences the reorganization of energy density from the original narrow spectrum into the
infragravity band.
This experiment provides an opportunity to test numerical models that would in principle be able to reproduce
infragravity wave generation and dynamics. We compare numerical results (free surface and velocities)
produced by a fully nonlinear Boussinesq model including breaking and runup to the experimental data and
show that the complex infragravity wave dynamics is adequately reproduced by the model.
OS13D-1227
Wave Forced Normal Modes on Fringing Reefs
In an effort to assess wave-driven coastal inundation at the shoreline of fringing reefs, pressure and current observations were collected at reefs on Guam (Ipan) and Oahu, Hawaii (Mokuleia) as part of the PILOT (Pacific Island Land-Ocean Typhoon) experiment. Similar to dissipative sandy beaches, nearshore surface elevation at both reefs is dominated by energy in the infragravity frequency band. Coherent infragravity oscillations across the reef tend to occur at discrete frequencies and with standing wave cross-shore structures that are consistent with open basin resonant modes. The modes are forced by swell wave groups, similar to a time-dependent setup. The resonant modes are most apparent during energetic wave events, in part because wave setup over the reef increases the low mode resonant frequencies to a range that is conducive to wave group forcing. Evidence of the excitation of resonant modes during tropical storm Man-Yi at Ipan, Guam is presented.
OS13D-1228
Low Frequency Wave Generation in a Spectral Model
The nearshore and surf zone areas of the coast often have considerable low frequency energy, which serves to modulate breaking events and can be linked to the formation of larger-scale bottom features. One likely mechanism for the generation of this energy is subharmonic interactions, in which high frequency waves interact to augment energy in frequencies below the spectral peak. The performance of most phase-resolved frequency domain models has not been verified for subharmonic interactions, despite the fact that the necessary terms for these interactions are present in the models. In this study we investigate the generation of low frequency energy in two laboratory experiments and discuss the suitability of frequency domain phase resolved models for predicting this energy transfer. The effect of bottom slope and breaking (and their representation in the predictive models) will be evaluated. Of particular interest is the rate of energy transfer from spectral peak to subharmonic frequencies, and further down to very low frequencies. It has been surmised that energy in these very low frequency bands have potential for coastal flooding during storm events over reef systems; the application of this work to predictive capabilities in this regard will be discussed.
OS13D-1229
Diurnal Cross-Shore Exchange on the Inner-Shelf in Southern Monterey Bay, CA
The effects of a strong diurnal sea-breeze on the cross-shore exchange on the inner shelf is investigated by comparing wind stress estimates and ocean currents over the vertical at three locations in southern Monterey Bay, CA . Cross-shore exchange on the inner shelf significantly impacts the ecosystem by transporting heat, nutrients, pollutants and phytoplankton between the inner-shelf and surf zone. Spectral analysis of surface winds at three coastal locations within the bay indicates a significant diurnal wind component. The observed subaqueous velocity profiles and pressure time series are measured by bottom mounted 1200-kHz Broad- band Acoustic Doppler Current Profilers (ADCPs) deployed at three separate alongshore locations in ~ 13 m water depth. The velocity and pressure signals were collected continuously at 1 Hz for all three locations for over 2 years. The cross-shore wind stress is significantly correlated to the cross-shore subaqueous velocity with onshore flow near the surface and offshore flow near the bottom. Cross-rotary spectral analysis is used to describe the rotational coherence and phase over the vertical with respect to the wind stress. It is further hypothesized that normally-incident sea-swell waves (0.04-0.2 Hz) will modify net cross-shore transport. Cross-shore transport is evaluated for conditions that are dominated by either waves or cross-shore wind stress. Results indicate that when waves are small, the cross-shore wind stress associated with the diurnal sea-breeze is the primary forcing mechanism for cross-shore exchange on the inner-shelf.
OS13D-1230
Impact of Synoptic Meteorological Variations on Nearshore Hydrodynamics
Passage of low pressure atmospheric frontal systems over the Southeast US introduces synoptic variation in meteorological parameters. Nearshore wave climate in this region is influenced by locally generated winds, showing a strong response to moving frontal systems. Accurate prediction of surf-zone response to wave forcing and wind forcing in general and atmospheric fronts in particular is important in quantifying sediment mobility and changes in beach morphodynamics. Energetic wave conditions can be attributed to three atmospheric front systems: 1) Cold Fronts, 2) Warm Fronts, and 3) Tropical Storms. Low pressure systems associated with cold fronts moving from west to east-northeast, change the wind direction from northeast to southwest. Conversely, warm fronts are accompanied by an opposite change in wind direction. Tropical storms moving nearshore rotate the wind direction slowly from southwest to southeast. Long term (2004- 2007) nearshore wind, wave and current information is analyzed from a station located at mean water depth of 5 meters on the coast of SC (Springmaid Pier) to examine the relationship between meteorological forcing and nearshore hydrodynamic conditions. Atmospheric pressure, temperature and wind velocity from the same station were used to identify, 24 cold fronts, 18 warm fronts and 14 tropical storms on average for each year from 2004-2007. In this contribution, a 2-D wave propagation model, Simulating WAves Nearshore (SWAN), coupled with the coastal-circulation model Regional Ocean Modeling System (ROMS v 3.0) is being used to predict longshore current and sediment transport in the surf zone in response to different types of fronts. The performance of the wave propagation model is evaluated with in-situ measurements collected as a part of South Carolina Coastal Erosion Study (SCCES) along the coast. Synthetic atmospheric fronts and tropical storms, developed from the climatic analysis, are used as input to the coupled models. The results are used to examine the overall impact of the different type of atmospheric fronts and tropical storms over a year in terms of net alongshelf sediment transport in surf zone.
OS13D-1231
Impacts of Post-Breach Tidal Circulation within Pleasant Bay, Massachusetts via Numerical Simulation
The barrier island located off the coast of Chatham, Massachusetts, known as Nauset Beach, extends approximately 14.5 kilometers north to south terminating at Chatham Inlet. This inlet was formed during a major storm in 1987, and since then has been the only inlet into the Pleasant Bay estuary. This estuary exhibits a recurring process of breaching and closing, and in 2007 a large storm combined with low lying topography caused a breach in the barrier island approximately four kilometers north of the existing inlet. Since then this breach has deepened and widened significantly forming a new inlet into Pleasant Bay. Using a numerical model, as well as recent and historic data, this study explores what impacts this breach may have on the Pleasant Bay system in terms of water quality, tidal hydrodynamics and the overall stability of the system. Results of the breach include an increase in peak high tide on the order of 15 %, a decrease in the system residence time from 0.9 to 0.7 days, and decreases in the peak ebb and flood currents through the pre-existing inlet of approximately 10 %.
OS13D-1232
Simulation and Validation of a Storm Surge in Bays and Estuaries With Complex Coastline Geometry
Simulation of nearshore ocean dynamics in a region with a complex coastline, commonly found along bays and estuaries, has been a challenging task for modelers. Over the last decade, unstructured grid models based on finite-element and finite-volume methods have been gaining popularity within the ocean modeling community. Application of this type of model facilitates accurate representation of the nearshore processes in model domains with bays, estuaries, and river channels. This study demonstrates the utility of applying an unstructured grid model for simulation and analysis of a storm surge in such a region. A high-resolution storm surge model of Apalachee Bay in the northeastern Gulf of Mexico is developed using an unstructured grid Finite-Volume Coastal Ocean Model (FVCOM) with wetting and drying capabilities. The model is applied to the case of Hurricane Dennis (July 2005). This storm caused underpredicted severe flooding of the Apalachee Bay coastal area and communities located inland up rivers that has yet to be adequately explained. Accurate resolution of the complicated geometry of the coastal region and waterways in the model reveals substantial spatial variability in the amplitude and timing of the maximum water level and processes responsible for the unanticipated high storm tide in the area. In this study, a methodology of validating a storm surge simulations using high-water marks is illustrated. Model experiments suggest that excessive flooding in the coastal zone during Dennis was caused by additive effects of coincident high tides, wave setup, and a propagating shelf wave that added to the locally wind generated surge.
OS13D-1233
Wave-Averaged and Wave-Resolving Simulations of the RCEX Experiment: Mean Flows and Drifter Dispersion
The RCEX experiment, conducted in Monterey in 2007, provided a detailed look at rip current circulation over a bathymetry consisting of persistent shoals and rip channels. The experiment employed a large fleet of GPS drifters to map mean flows and to develop one and two-point dispersion statistics, and revealed flow patterns ranging from closed recirculation cells to meandering alongshore currents. The experiment revealed instances where a well developed rip current circulation pattern did not lead to frequent transport of drifters beyond the outer edge of the surf zone, indicating that cross-shore transport and mixing between surfzone and offshore waters is occasionally weaker than would be expected. In this talk, we describe the application of a wave- resolving nearshore model (Funwave) and a wave- averaged nearshore model (Delft-3D) to the study of the RCEX flow field. The wave-averaged model does a qualitatively good job of reproducing observed flow patterns over the range of conditions which were present in the experiment. Here, mean current patterns and associated vorticity and swirl from the two modeling techniques are compared to each other and to data. In particular, we will examine whether the differences in vorticity generating processes in the two modeling techniques (curl of a smooth wave forcing field in Delft3D vs. bore-related generation in Funwave) leads to significant, detectable differences in structure of the vorticity field at the scale of the surfzone width. The talk will also describe the statistical analysis of drifter dispersion based on simulated drifter trajectories.
OS13D-1234
On the Time-dependence of the Lagrangian Surfzone Cross-shore Diffusivity
GPS-tracked surfzone drifters have recently been used to calculate surfzone single-particle Lagrangian statistics. Drifter-derived surfzone cross-shore diffusivities have an unanticipated time-dependence as they are not monotonic. Although cross-shore diffusivities initially grow like t (ballistic diffusion) and eventually reach a constant (Brownian diffusion), they reach a maximum before gently decreasing toward the constant. Explanations for this maximum are investigated using Fickian diffusion models, i.e. φt = ∂x [κ(x,t) ∂x φ]. A constant Fickian diffusivity in a semi-infinite domain, representing the shoreline, results in a time-dependent measured diffusivity, and suggests that observed Lagrangian surfzone diffusivities must be interpreted with caution and should not be directly used in a Fickian diffusion equation. A time- dependent drifter-derived diffusivity is also found where the Fickian diffusivity is space-dependent. In particular, with two constant diffusivities, κ(x<x0)=κ1 and κ(x≥x0)=κ2. with the smaller diffusivity seaward of the surfzone. Time-dependent diffusivities are required in order to produce ballistic diffusion. Adding time-dependent growing diffusivities (similar to observed alongshore diffusivities) into these models, so that the diffusivity is both a function of space and time, yields measured time- dependent cross-shore diffusivities with many of the features of the observed Lagrangian cross-shore diffusivity.
OS13D-1235
Surf zone Exchange on a Rip Channeled Beach
The dispersion and surf zone exchange of GPS-equipped surface drifters observed during the Rip Current EXperiment (RCEX) is examined with help of Lagrangian Coherent Structures (LCSs). LCSs allow for the detection of transport barriers in unsteady flows and are typically applied to shelf-scale circulation systems. Here LCSs are specifically computed to detect the effects of surfzone-originated Very Low Frequency motions (VLFs) with O(10) minute time scale on the cross-shore exchange of floating material using numerical model calculations of the Lagrangian surface velocity at the wave group timescale. After verification with RCEX field observations, the model is run for a range of environmental conditions experienced during the field experiment to assess the effects of VLFs on the cross-shore surf zone exchange. Results are relevant for (but not restricted to) sediment and nutrient exchange, human health, water clarity, and swimmer safety.
OS13D-1236
A Surfzone Circulation Model With Influence of Surf-swash Interaction
Existing wave-averaged nearshore circulation models often neglect swash zone dynamics. A shoreline boundary in a wave-averaged circulation model is usually specified at the location where the mean total water depth is zero, which is not consistently defined in a sense of wave-averaged property in both theory and practical applications. In this study, we redefined a shoreline boundary in a wave-averaged circulation model at the wave run-down position estimated by a residual bore height (Brocchini and Bellotti, 2002). The surf- swash interaction is presented by the mass and momentum exchange between the surf zone and swash zone at the newly defined shoreline boundary. The swash zone dynamics are modeled using integral swash zone properties that can be evaluated based on the parameterizations given by Antuono et al. (2007). A case study with alongshore non-homogenous wave conditions shows that swash zone motions strongly influence the circulation patterns in the inner surfzone.
OS13D-1237
Surf zone prediction and assimilation using a Bayesian network
Prediction of surf zone processes, including waves, currents, and sediment transport, can be implemented with a variety of detailed process models to obtain predictions with significant skill. This detailed modeling capability supports a wide range of scientific and practical applications, including coupled interdisciplinary modeling, detailed evaluation of complex process interactions, and environmental safety and management. However, even if the models themselves utilized accurate parameterizations, prediction accuracy is often limited by the availability and accuracy of the data used to drive the models. Inaccuracies in data are unavoidable and useful predictions, whether for hypothesis testing or practical applications, must be accompanied by estimates of the prediction uncertainty. Here, we demonstrate how a Bayesian network model can be used to provide accurate predictions of wave height evolution in the surf zone given very sparse and sometimes inaccurate boundary condition data. The approach is based on a formal treatment of the data assimilation problem. The application takes advantage of significant reduction of the dimensionality of the model system. We demonstrate that (1) the results of a detailed forward model of the wave evolution are reproduced accurately; (2) uncertainty in the model inputs are accurately transferred to uncertainty in output variables; and (3) the Bayesian network can assimilate remote sensing data to improve predictions and estimate boundary condition data.
OS13D-1238
Alongshore Sinuous Currents on a Rip-Channeled Beach
The presence of alongshore sinuous currents, coupled to the underlying bathymetry, is investigated for a rip- channeled beach. Observations were obtained during a Rip Current EXperiment (RCEX), from late April to late May, 2008, at a beach in Sand City, Monterey Bay, CA, which is comprised of shore-connected shoals with incised, quasi-periodic, O(125m), rip channels. In-situ co-located velocity and pressure measurements were obtained with a cross-shore array of instruments deployed in a rip channel, extending 150m offshore, and an alongshore array spanning two neighboring shoals and rip channels. In addition, surfzone drifters were deployed on seven days during various wave and tidal conditions to spatially map the flow field. Three flow patterns were observed throughout the experiment: 1) a rip current circulation pattern; 2) an alongshore sinuous current pattern; and 3) a wandering current pattern that is composed of both a rip current circulation and alongshore sinuous flow. The three flow patterns were easily identified using the more spatially synoptic drifter results, and served as a basis to further classify the flow field for the entire experiment utilizing the velocities measured with the in-situ instrument array. Surprisingly, the surfzone current was found to be primarily alongshore sinuous for the majority of the experiment, rather than the expected rip current circulation pattern. It was found that rip current circulation systems typically develop during large waves and/or low tides. There were practically no occurrences of alongshore sinuous currents during tidal elevation extrema (i.e. low or high tides). The effect of tidal elevation and offshore wave conditions for alongshore current events are investigated further using a numerical model, XBeach. The specific hydrodynamic mechanisms that result in sinuous alongshore current patterns will be discussed.
OS13D-1239
Vertical Behavior of Rip Current Pulsations Outside the Surf Zone
The vertical and temporal behavior of rip currents outside of the surf zone is poorly understood due to a paucity of comprehensive observations. Previous studies have found that rip currents are relatively depth- uniform within the surf zone and tend to be surface-dominated outside of the surf zone. Observations of an upward-facing Acoustic Doppler Current Profiler (ADCP) deployed in 5 m water depth just offshore of a rip current (nominally 1.2 surf zone widths from the shoreline) were obtained during April-May 2008 as part of the Rip Current EXperiment (RCEX) at Sand City, Monterey Bay, CA. The ADCP was sampled continuously at 1 Hz. The morphology is comprised of shore-connected shoals with incised rip channels. Energetic seaward- directed episodic pulses associated with the rip current obtained velocities up to 0.5 m/s with a frequency of occurrence that varied from 1-15 times a day depending upon the coincident wave conditions and tidal elevations. These observations are consistent with the spatially (in the horizontal) synoptic observations by Smith and Largier (1995). Vertical variations of the episodic rip current pulsations ranged depth-uniform to surface-dominant. Complex correlation was performed in the vertical to describe rotational behavior and temporal lags. The mean vertical profiles, based on averaging velocities within defined velocity range, tend toward near-surface dominance for more energetic pulses.
OS13D-1240
Further consideration of the generation of rip currents by instability.
Rip currents have significant influence on the development of alongshore variability in shorelines, and on transport and mixing in nearshore waters. Theoretical understanding of their generation mechanisms, however, has not been satisfactory, in particular on beaches lacking obvious alongshore variations. We reconsider the generation of rip currents due to a hydrodynamic instability owing to the dynamical interaction of waves and currents, in a system initially alongshore uniform. The preliminary results show that for typical conditions of waves and beaches, the instability can lead to rip current circulations with alongshore wavelength of a few hundreds of meters and growth time of a few tens of minutes. The alongshore wavelength is intrinsically determined by the instability process and does not depend on the empirical parameters in the model. Because of its instability origin, the mechanism has the potential to explain the transient and sporadic development of rip currents, at unpredictable, seemingly random locations on beaches.
OS13D-1241
Rip Current Velocity Structure in Drifter Trajectories and Numerical Simulations
Estimates of rip current velocity and cross-shore structure were made using surfzone drifters, bathymetric surveys, and rectified video images. Over 60 rip current trajectories were observed during a three year period at a Southern California beach in July 2000, 2001, and 2002. Incident wave heights (Hs) immediately offshore (~7 m depth) were obtained by initializing a refraction model with data from nearby directional wave buoys, and varied from 0.3 to 1.0 m. Tide levels varied over approximately 1 m and winds were light. Numerical simulations using the non-linear shallow water equations and modeled over measured bathymetry also produced similar flows and statistics. Time series of drifter position, sampled at 1 Hz, were first-differenced to produce velocity time series. Maximum observed velocities varied between 25 and 80 cm s-1, whereas model maximum velocities were lower by a factor 2 to 3. When velocity maxima were non-dimensionalized by respective trajectory mean velocity, both observed and modeled values varied between 1.5 and 3.5. Cross-shore location of rip current velocity maxima for both shore-normal and shore-oblique rip currents were strongly coincident with the surfzone edge (Xb), as determined by rectified video (observations) or breakpoint (model). Once outside of the surfzone, observed and modeled rip current velocities decreased to 10% of their peak values within 2 surfzone widths of the shoreline, a useful definition of rip current cross-shore extent.
OS13D-1242
Wave Dissipation Formulations in Frequency Domain Models
Incorporation of wave dissipation due to breaking in both time-domain and frequency domain models have long been a subject of study. Until recently, the formulation of wave breaking in frequency domain models have been based on lumped-parameter dissipation models based on a Rayleigh distribution function for the wave heights in the surf zone (Battjes and Janssen, 1978; Thornton and Guza, 1983). Modifications to improve the dissipation model include allowing for nonlinear energy transfer (Mase and Kirby, 1992), which leads to improvements in predictions of the skewness and asymmetry. Bredmose (2004) show how time- domain wave breaking models can be included in a frequency domain version of the Boussinesq model using a time-domain inversion of the roller model. However, frequency domain versions of the Boussinesq model tend to perform poorly until the waves are close to breaking. In this study, we will show how time-domain wave breaking models can be included in frequency domain models that are based on the mild-slope formulation. We will then compare the results of using such a breaking model to the empirical bulk-dissipation formulation. Comparisons will include wave height distributions, skewness, and asymmetry. We will also discuss implications of using different breaking models on sediment transport.
OS13D-1243
Observations of Ocean Surface Wave Damping on a Muddy Continental Shelf
It is well known that the presence of mud deposits on the continental shelf can cause dramatic damping of ocean surface waves, but quantitative field observations are scarce. Preliminary results are presented of a recent field experiment on the wide and muddy shelf of Western Louisiana. An extensive array of instruments was deployed during February-March, 2008, that included two directional waverider buoys, six bottom tripods equipped with a pressure-velocity sensor and a current profiler, and six bottom tripods equipped with a pressure sensor. The two-dimensional array consisted of two cross-shore transects and an alongshore transect spanning a 40 by 25 km area in depths ranging from 13 to 4 m. The dataset includes numerous local wind sea events with wave directions predominantly from the south (i.e. onshore propagation). Box cores were collected at all instrument sites to characterize the surficial sediment properties (Garcia-Garcia et al., 2008, in review). Preliminary analysis generally shows a gradual decay of waves from the deeper to the shallower instruments, consistent with earlier observations (Sheremet and Stone, J. Geophys. Res., 2003; Elgar and Raubenheimer, Geophys. Res. Let., 2008), with the strongest decay at the muddiest site. The wave spectra evolution shows strong decay (as much as an order of magnitude) of high-frequency wind sea spectral levels and weaker decay at the lower swell frequencies. This research is supported by the Office of Naval Research-Coastal Geosciences Program.
OS13D-1244
A comparison of directional wave measurements from an ADCP, AWAC, and pressure sensor array
Acoustic Doppler Current Profilers (ADCP) are commonly used by coastal researchers for directional wave measurements, as well as current profiling. An inter-comparison of wave measuring capability was made between an RD Instruments (RDI) ADCP (Monitor Workhorse 1200 kHz), a Nortek AWAC, and an array of pressure sensors in 8m depth at the Army Corps of Engineers Field Research Facility (FRF), Duck NC. The ADCP's were located within the FRF's 8m-array of 15 near-bottom pressure sensors. This array consists of nine alongshore and 6 cross-shore pressure sensors and uses an IMLE algorithm to make high resolution (nominal 5 degree resolution) directional wave measurements. ADCP wave processing was done with vendor software (RDI's Wavesmon and Nortek's QuickWave) and Matlab wave processing toolboxes. The work presented here compares directional wave measurements made from these three systems. Data were collected between December 2007 and April 2008 with a variety of wave conditions: narrow band swell, building seas in a storm, high and low energy. Bulk statistics (e.g., significant wave height, peak and mean period, mean direction) and spectra parameters such as directional resolution and spreading were compared. Results indicate good correlation of the bulk statistics. A more detailed examination was done with a spectra partitioning method using the XWaves Ocean Wavefield Analysis Toolbox. IMLE processing of ADP data was capable of resolving multiple wave directions, at the same or overlapping frequencies, superior to PUV analysis (direct Fourier transform method) but not as well as the 8m-array IMLM analysis. The affect of currents changing the local wavenumber and directional spectra estimates was also examined. Advantages and disadvantages of each system are presented.
OS13D-1245
Jets Driven by Wave Breaking Over a Shoal
Field observations and model simulations are presented of a jet forced by waves breaking over a shoal at the entrance to a shallow b ay. The shoal is composed of a series of steep and narrow bedrock ridges with depths of 2-8 m at the ridge crests. During a storm in October 2006 observations from an instrument array on the shoal indicated peak significant wave heights were 2.5-4.0 m across the array, this spatial variability due to wave breaking over the ridges. A well defined jet was observed behind the shoal with mean axial speeds up to 0.4 m/s, exceeding the 0.1 m/s maximum tidal current speed. The hydrodynamic model Delft3D, coupled to the wave model SWAN, was used to simulate the waves and depth-averaged wave-driven flow over the shoal and throughout the entire bay. Wave model results were generally in reasonable agreement with observations and confirm that the jet is forced by the radiation stress gradients over the shoal. However, discrepancies between model and data suggest that the breaking distribution function is inadequately represented. The model over-predicts wave breaking in deeper water as a result and the current speeds in the jet are slightly over-estimated by the model. The direction and transient nature of the flows were in agreement with observations.
OS13D-1246
Three-Dimensional Modeling of Nearshore Hydrodynamics for a Synthetic Cape Attached Shoal
One of the most dominant features of the Carolina's shoreline is the large-scale cuspate forelands and the associated capes. Attached to each of these capes are significant systems of shoals. These capes and shoals undoubtedly play an important role in the evolution and transformation of shoreline of the Carolinas. In order to help understand the influence of the shoals on the surf zone flows, the newly developed version of ROMS with the surf zone wave forcing is applied to a synthetic test case of a cape attached shoal with similar scales to Cape Hatteras and Diamond shoals. Various cases scenarios are simulated; different orientations of the shoreline, with and without the shoal, a continuous shoal and discontinuous shoal. Numerous wave conditions are used to simulate the variety of possible wave fields such as waves from the north associated with nor"easter storms or waves from the south associated with the passage of frontal weather systems. The intention is to provide insight into the range of possible wave driven circulation patterns that may be formed around the shoals, thereby providing guidance for a focused field study of the flows on and around the shoal.
OS13D-1247
Two-phase Approach for Simulating Surfzone Bubbles
Wave breaking in the surfzone entrains large volumes of air bubbles into the water column, which strongly influences the optical and acoustical properties of the water, and subsequently makes optically-based observations difficult. In this presentation, we discuss a full two-phase model to study the evolution and transport of a bubble size distribution in a three-dimensional, turbulent flow. The model employs a VOF method to track the free surface. k-epsilon or LES models are used to describe turbulent closures. For validation, the model is used to simulate a pseudo-two-dimensional gas-liquid flow in a bubble column. The results show that, when the drag, lift and virtual mass forces are all included, the oscillation behavior and flow structures that were observed in the experiment can be captured.