OS13C-01 INVITED 13:40h
The Torrey Pines Rip Currents
Field observations during the Nearshore Canyon Experiment documented the hydrodynamics of a morphologically-controlled migrating rip current utilizing a coherent cross- and alongshore array of 13 co-located pressure and velocity sensors on a near planar beach at Torrey Pines Beach, CA. The rip current morphology had $\sim 150m$ channel width and $\sim 300m$ rip spacing with a depth variation of 0.003m/m alongshore. The rip channel morphology is very broad with the bathymetric variation smaller than the camber of a football field. The rip currents obtain mean velocities up to 0.5m/s at lower tidal elevations for 0.5 m significant wave heights. Argus Images suggest that the rip channels were present prior to the instrument deployment on yearday 303. The rip channels migrated southerly at 12.5 m/day and migrated past our cross-shore array. The cross-shore velocity profile along the axis of a rip channel in the field has not been well documented. Preliminary analysis found that cross-shore velocity increased to a maximum inside the break point, and then decreased offshore. Velocity vectors suggest a flow reversal close to the shoreline, similar to laboratory measurements by Haller et al., 2002. We will discuss the flow dynamics of the morphologically-controlled, migrating Torrey Pines rip currents. This work was supported by the Office of Naval Research.
OS13C-02 INVITED 14:00h
Detailed Characterization of Nearshore Processes During NCEX
Recent technology advances have allowed the coupling of remote sensing methods with advanced wave and circulation models to yield detailed characterizations of nearshore processes. This methodology was demonstrated as part of the Nearshore Canyon EXperiment (NCEX) in La Jolla, CA during Fall 2003. An array of high-resolution, color digital cameras was installed to monitor an alongshore distance of nearly 2 km out to depths of 25 m. This digital imagery was analyzed over the three-month period through an automated process to produce hourly estimates of wave period, wave direction, breaker height, shoreline position, sandbar location, and bathymetry at numerous locations during daylight hours. Interesting wave propagation patterns in the vicinity of the canyons were observed. In addition, directional wave spectra and swash / surf flow velocities were estimated using more computationally intensive methods. These measurements were used to provide forcing and boundary conditions for the Delft3D wave and circulation model, giving additional estimates of nearshore processes such as dissipation and rip currents. An optimal approach for coupling these remotely sensed observations to the numerical model was selected to yield accurate, but also timely characterizations. This involved assimilation of directional spectral estimates near the offshore boundary to mimic forcing conditions achieved under traditional approaches involving nested domains. Measurements of breaker heights and flow speeds were also used to adaptively tune model parameters to provide enhanced accuracy. Comparisons of model predictions and video observations show significant correlation. As compared to nesting within larger-scale and coarser resolution models, the advantages of providing boundary conditions data using remote sensing is much improved resolution and fidelity. For example, rip current development was both modeled and observed. These results indicate that this approach to data-model coupling is tenable and may be useful in near-real-time characterizations required by many applied scenarios.
OS13C-03 14:20h
Observations of shoaling internal tidal waves during NCEX
Energetic semidiurnal internal waves are evident in moored current and temperature observations obtained from September to December 2003 on the southern California shelf near Scripps Canyon as part of the Nearshore Canyon Experiment. Isotherm displacements of 10 m or more in water depths of 20 m were common. Associated cross-shelf currents exhibited opposite flows in the upper and lower half of the water column (mode-1 vertical structure) with velocities of ~0.1 m/s. Alongshelf currents were uniform with depth (barotropic), also with velocities of ~0.1 m/s. The semidiurnal internal waves propagated onshore at speeds ranging from ~0.3 m/s when the stratification was strong in September to ~0.1 m/s when stratification was weaker in late November and early December. The internal tide shoals at an angle to the local isobaths, arriving at the 10-m isobath near Scripps Canyon 1-2 hours earlier than at a site 2 km to the north.
OS13C-04 14:35h
Offshore Directed Flows in the Surf Zone From Video
Pixel intensity time series from surf zone video imagery are used to explore identification of rip currents and characterization of temporal variability in rip velocity based on localized reduction in shallow water wave celerity owing to interaction between incident gravity waves and offshore-directed currents. Time-varying occurrence and velocity of offshore-directed surf zone flows such as rip currents interact with incoming waves and bores reducing wave celerity from theoretical shallow water speed. Wave celerity is estimated using cross-spectral analysis of 17 minute pixel intensity time series sampled across the surf zone at Blacks Beach, California. Variation in estimated celerity over several hours is compared with expected variation in celerity given measured bathymetry and change in water level. Deviation from expected change in celerity is mapped across the surf zone with the aim of identifying regions of significant rip current flow. Estimation of wave celerity and variation over shorter time scales (1-2min) is explored in an attempt to identify short period fluctuations in rip current velocity.
OS13C-05 14:50h
Observations of a semi-permanent rip current system during NCEX
During the 2003 Nearshore Canyon Experiment (NCEX) a recurring rip, located at the southern end of the study site, was visually observed in video images by the traces of offshore advected residual foam during larger wave events. Additional evidence of this rip was inferred from measured convergences of longshore currents from an array of 24 optical current meters, centered on the rip location. Preliminary examination of the alongshore current time series revealed interesting time and spatial variability during rip activity, including significant oscillations of the alongshore current magnitude with periods of hundreds of seconds. Our main objectives of this study are to provide quantifiable observations on the time and spatial variability of this rip current system and to examine forcing mechanisms that may control observed rip variability. Particularly, we will address how this rip's occurrence and configuration changes with wave conditions through the NCEX study. We also plan to examine the extent of direct forcing of observed rip current variability in the infragravity frequency band by surf zone pumping. We will use the optical signature of time varying surf zone width as a proxy for wave group forcing.
OS13C-06 15:05h
Modeling and Understanding Remotely Forced Rip Current Systems at the Nearshore Canyon Experiment (NCEX)
Evaluation of data collected during the Nearshore Canyon Experiment, has shown that our time-dependent circulation model (OK-Model; \"{O}zkan-Haller and Kirby, JGR, 1999), forced using radiation stress gradients derived from spectral wave models, can predict the spatial location of large offshore directed flows (rip currents). The locations of these transient rip currents, which are not controlled by the local nearshore bathymetry, are visible in remote sensing time exposure and variance images due to the advection of foam on the water surface (Eos Trans. AGU, 84(46), Ocean Sci. Meet. Suppl., Abstract OS21I-04, 2003). Further investigation has indicated that optical measurements of circulation patterns coincide with the numerical predictions. With this validation, the numerical models are being used to evaluate the sensitivity of the dynamic rip current system at the NCEX site to changes in wave conditions and nearshore bathymetry. Additionally, the walls of the offshore submarine canyon contain an undulating pattern that may be important in generating small-scale variations in waveheight, thereby driving rip currents where the water surface is at a local minimum, even in regions where the wave height is generally high. Theoretical tests using bottom boundaries with anomalies in intermediate and deep water are being performed to determine the effect these offshore contours can have in dictating nearshore circulation patterns. These results will provide insight regarding the role of patterns along canyon walls in prescribing the spatial location of the rip currents.
OS13C-07 15:20h
Rip-Currents and Large Beach Cusps
Well-developed, large-scale beach cusps O(200 m) associated with rip currents are studied along 18 km of Southern Monterey Bay, California. Low-tide terraced bars incised by rip channels characterize the nearshore morphology. There is a large increase in wave height going from small wave heights at the southern most part of the bay in the shadow of a headland, to the center of the bay where convergence of waves owing to refraction over Monterey Bay Canyon results in increased wave height. The strongly refracted waves result in near normal incidence along the shoreline and well-developed rip currents. The large alongshore gradient in wave climate results in a concomitant alongshore gradient in morphodynamic scale. The alongshore spatial and temporal variations of cuspate shoreline, measured by a Kinematic GPS equipped ATV, and rip channel spacing, as determined from opportunistic shore parallel jet-ski runs and video image analysis, are cross-correlated. Approximate bi-monthly shoreline surveys were initiated in Dec 03, along with directional wave spectra measured at two alongshore locations in 12 m depth. Rip current morphology and cuspate shoreline are significantly correlated. Decorrelation times and migration rates are calculated from cross-correlations of the shoreline spatial series. Between shoreline surveys on yearday 343, 2003 and 7, 2004, a major storm occurred (7 m significant wave height offshore on yearday 344 during time of spring tides) and the decorrelation time was less than the time between surveys. However, the decorrelation time thereafter (yeardays 7 - 100, 2004) exceeded 90 days, from which it is determined that bi-monthly surveys are sufficient. Based on the cross-correlations, the cusp system at the southern extreme of the bay in the shadow of the headland was stationary and stable. The cusps in the middle were initially stationary for 22 days, migrated rapidly at ~2 m/day to the south for 24 days and then returned to stationary. It is noted that the largest waves (> 8 m) occurred on yearday 91 during a time of neap tides, and the shoreline remained correlated between surveys. It is concluded the bi-monthly surveys are sufficient under "normal waves", but the additional surveys need to be performed after storms. The migration direction is found correlated with the net alongshore sediment transport. A research version of the Delft3D morphodynamic model is used to explain the relationship between alongshore length scale of cusps and wave climate of the incident waves. It is shown that the length-scales of the large cusps are associated with both the directional bandwidth and spectral energy of the incident waves.