OS14B-01 16:00h
Observations of Wave Reflection from a Submarine Canyon
Observations in 20-m water depth near the sides of the steep-walled (350-m wide, 120-m deep) La Jolla submarine canyon suggest abrupt shallow-water bathymetry can reflect surface-gravity waves. Reflection coefficients estimated by applying a newly developed inverse algorithm to observations of pressure and velocity at each side of the canyon are consistent with theoretical predictions of reflection by a rectangular canyon in shallow water [Kirby and Dalrymple, {\it J. Fluid Mech.}, {\bf 133}, 1983]. The inverse algorithm estimates reflection as a function of the frequency and directional spectrum of the incident waves. Here, analysis was restricted to infragravity frequencies (periods of order one minute, wavelengths of several hundred meters), because predicted reflection is maximized for wavelengths similar to the canyon width. For the rarely observed cases of normally incident (relative to the canyon axis) waves with narrow directional spread, the observations agree well with theory, including the absence of reflection for wavelengths that are integer multiples of the canyon width. In the more commonly observed cases of obliquely incident waves with large directional bandwidth, the observed reflection is less than predicted, and the complete reflection hypothesized to occur for highly oblique waves is not observed. When averaged over all observed wave conditions, La Jolla Canyon reflected more than half of the incident infragravity energy, and thus may be important to infragravity motions at the nearby shoreline. Supported by the Office of Naval Research and the National Science Foundation.
OS14B-02 16:15h
Wave Propagation over a Submarine Canyon: Field Observations
Although abrupt bottom features such as submarine canyons, reefs, banks and shoals are present on many continental shelves, field measurements of their effects on ocean surface waves are scarce. During September-December 2003 an extensive array of wave-measuring instruments was deployed near La Jolla, California, as part of the Nearshore Canyon Experiment (NCEX). The field site is characterized by two submarine canyons, La Jolla Canyon and Scripps Canyon, that strongly affect the propagation of long period Pacific swell. An array of 7 Datawell Directional Waverider Buoys, 17 bottom pressure recorders and 12 Nortek Vector pressure-velocity sensors was deployed in depths ranging from about 10-100 m. The three-month-long observations include numerous long-period swell arrivals from both southern and northern hemisphere storms. Preliminary analysis indicates extreme spatial variations in wave energy in the vicinity of Scripps Canyon. In particular, swell arriving from the west is effectively blocked by the canyon. The observed amplification of swell energy on the north side of the canyon and extremely low energy levels on the south side are consistent with refraction predictions. This research is supported by the Office of Naval Research and the National Science Foundation.
OS14B-03 16:30h
Wave Propagation Over a Submarine Canyon: Model-Data Comparisons
Observations of the transformation of ocean swell over Scripps Canyon, collected during the NCEX experiment, are compared with predictions of a three-dimensional coupled-mode model for wave propagation over steep topography (Athanasoullis and Belibassakis, J. Fluid Mech., 389, 275-301,1999; Belibassakis et al., Appl. Ocean Res., 23, 319-336, 2001). To examine the importance of steep bottom slope effects, which are fully accounted for in this model, results are compared with two earlier models which assume a gently sloping bottom: a parabolic refraction/diffraction model (Kirby, J. Geophys. Res., 91, 933-952, 1986) and a spectral refraction model based on backward ray tracing (O'Reilly and Guza, Coast. Eng., 19, 263-282, 1993). Preliminary results are presented for long period swell arriving from the west at the canyon at a large oblique angle. Model results are compared with data from directional wave buoys and bottom pressure gauges deployed around the rim of the canyon and over the canyon. The coupled-mode model yields accurate results over and behind the canyon where the parabolic refraction/diffraction model and the ray model overestimate and underestimate the wave height, respectively. These differences suggest that diffraction effects are important in the region behind the canyon that is sheltered by the topography from direct swell arrivals. At some locations seaward of the canyon both the refraction/diffraction and coupled-mode models overpredict the wave height. This reseach is supported by the Office of Naval Research and the National Science Foundation.
OS14B-04 16:45h
Vertical Phase Variation in Mean and Oscillatory Surf Zone Currents
Considerable effort has been expended in measuring and modeling the vertical variation of mean flow patterns in and near the surf zone. In general, mean cross-shore currents are characterized by an undertow profile, whereas alongshore currents have an approximately logrithmic boundary layer spanning the water column. Additionally, temporal variations in flow patterns occur on time scales associated with changes in wave forcing, instabilities of the mean flow, and through non-local circulation induced by bathymetric irregularities. In the presense of strong, sometimes pulsating, horizontal circulation, such as that associated with rip currents, the typical undertow profile can modulate between an undertow and logrithmic form. In this work, we examine the temporal variation and phase structure in vertical profiles of the mean flow patterns on time scales ranging from a few minutes to several hours. Observations used in this work were obtained during the Duck94 field experiment held in Duck, NC, from a vertical stack of 2-axis electromagnetic flow sensors deployed on a movable sled, and surface flow estimates obtained from PIV analysis of video data. This research was supported by the Office of Naval Research.
OS14B-05 17:00h
Vertical Distribution of Radiation Stress for Non-linear Shoaling Waves
The flux of momentum directed shoreward by an incident wave field, commonly referred to as the radiation stress, plays a significant role in nearshore circulation and, therefore, has a profound impact on the transport of pollutants, biota, and sediment in nearshore systems. Having received much attention since the seminal work of Longuet-Higgins and Stewart in the early 1960's, use of the radiation stress concept continues to be refined and evidence of its utility is widespread in literature pertaining to coastal and ocean science. A number of investigations, both numerical and analytical in nature, have used the concept of the radiation stress to derive appropriate forcing mechanisms that initiate cross-shore and longshore circulation, but typically in a depth-averaged sense due to a lack of information concerning the vertical distribution of the wave stresses. While depth-averaged nearshore circulation models are still widely used today, advancements in technology have permitted the adaptation of three-dimensional (3D) modeling techniques to study flow properties of complex nearshore circulation systems. It has been shown that the resulting circulation in these 3D models is very sensitive to the vertical distribution of the nearshore forcing, which have often been implemented as either depth-uniform or depth-linear distributions. Recently, analytical expressions describing the vertical structure of radiation stress components have appeared in the literature (see Mellor, 2003; Xia et al., 2004) but do not fully describe the magnitude and structure in the region bound by the trough and crest of non-linear, propagating waves. Utilizing a three-dimensional, non-linear, numerical model that resolves the time-dependent free surface, we present mean flow properties resulting from a simulation of Visser's (1984, 1991) laboratory experiment on uniform longshore currents. More specifically, we provide information regarding the vertical distribution of radiation stress components ($S_{xx}$ and $S_{xy}$) resulting from obliquely incident, non-linear shoaling waves. Vertical profiles of the radiation stress components predicted by the numerical model are compared with published analytical solutions, expressions given by linear theory, and observations from an investigation employing second-order cnoidal wave theory.
OS14B-06 17:15h
Laboratory observations of waves and turbulence over a barred beach
Turbulence in the surf zone plays an important role in many nearshore processes. Turbulence generated by wave breaking dominates wave dissipation and affects the drag felt by waves as they move towards shore. Turbulence may also play an important role in unsteady effects important to cross-shore sediment transport over barred beaches. These effects, which are not incorporated into quasi-steady models for cross-shore sediment transport, include the interactions of breaking wave turbulence with the bed and forward phase shifts of bed shear stress relative to the fluid velocity outside the boundary layer. It is difficult to obtain spatially dense observations of turbulent stresses in the field. Furthermore, separation of turbulent motions from wave-induced motions is technically difficult, and existing field-applicable techniques (Trowbridge, 1998; Trowbridge & Elgar, 2001) for this separation have not been directly compared with laboratory techniques such as phase-averaging. A rigid, impermeable barred beach bathymetry was setup in the large wave flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University for these purposes. The flume is 104 m long, 3.7 m wide, and 4.6 m deep. The bar is modeled as a feature of elevation atop an underlying 1:36 slope, with a 0.45 m crest height and an 0.69 m water depth over the bar crest. Regular and random waves corresponding to an approximately 1:3 scale of field observations at Duck, NC were run over the barred beach. Waves and vertical profiles of velocity were measured at many cross-shore locations. Results to be presented include comparisons of the different techniques for estimation of Reynolds stress components and evaluations of the errors associated with each technique. The cross-shore distribution of near-bed estimates of Reynolds shear stress will be compared with acceleration-based parameterizations for boundary shear stress in the nearshore. The balance of turbulent kinetic energy fluxes with turbulent production and dissipation will be explored along with comparisons of observed stress and dissipation estimates with those arising from turbulent closure or drag coefficient approximations.
OS14B-07 17:30h
Wind Effects on Shoaling Wave Shape
In the nearshore, cross-shore winds strongly affect the location of the breakpoint and the breaking-wave height. From casual observation from the beach, wind direction (onshore or offshore) and speed also appears to affect wave shape (\ie skewness and asymmetry), although as of yet this has not been quantified in the nearshore. The effect of wind on shoaling wave shape is investigated with laboratory experiments using monochromatic waves and onshore directed wind. Wind increases the shoaling wave energy at discrete multiples of the primary frequency, and has a significant effect on the wave shape at both a deeper and shallower shoaling locations. At the shallower location, the ratio of wave energy at twice the primary frequency to the primary frequency is also a function of wind speed, indicating interaction between the wind and the nonlinear wave shoaling process. Nearshore wave models do not account for these wind effects. Incorrect predictions of $3^\mathrm{rd}$ order velocity moments (wave shape), believed to control wave driven sediment transport, would result in incorrect beach morphological evolution predictions.
OS14B-08 17:45h
Breaking Waves in the Gulf of Tehuantepec
We present observations of ocean surface breaking wave variation with wind speed and fetch. Breaking waves play an important role in air-sea interaction: enhancing momentum flux from the atmosphere to the ocean; dissipating wave energy that is then available for turbulent mixing; injecting aerosols and sea spray into the air, and entraining air in the water. A better understanding of wave breaking kinematics and dynamics is important for wave modeling, remote sensing in the microwave and visible wavelengths, and furthering our understanding of surface waves. Surface wave and atmospheric boundary layer data were collected during the Gulf of Tehuantepec experiment (GOTEX) off the Pacific coast of Southern Mexico in February 2004. A nadir-looking mega-pixel video camera, along with a scanning lidar, laser altimeter, and inertial measurement unit, was mounted onboard the NCAR C-130Q Hercules aircraft. The imaging and scanning system recorded digital videos of the breaking sea surface and corresponding surface wave heights during strong (10-25m/s), steady off-shore winds over fetches from 0 to 500km. Boundary layer wind profiles were measured using GPS dropsondes and a pressure transducer array on the aircraft radome. Whitecaps are identified in the images using brightness thresholding, the image brightness gradient, and the brightness cumulative probability function. The observed dependence of whitecapping on fetch and surface wind speed is presented.