Jamie MacMahan

Surf zone surface retention on a rip‐channeled beach

The retention of floating matter within the surf zone on a rip-channeled beach is examined with a combination of detailed field observations obtained during the Rip Current Experiment and a three-dimensional (3-D) wave and flow model. The acoustic Doppler current profiler–observed hourly vertical cross-shore velocity structure variability over a period of 3 days with normally incident swell is well reproduced by the computations, although the strong vertical attenuation of the subsurface rip current velocities at the most offshore location outside the surf zone in 4 m water depth is not well predicted. Corresponding mean alongshore velocities are less well predicted with errors on the order of 10 cm/s for the most offshore sensors. Model calculations of very low frequency motions (VLFs) with O(10) min timescales typically explain over 60% of the observed variability, both inside and outside of the surf zone. The model calculations also match the mean rip-current surface flow field inferred from GPS-equipped drifter trajectories. Seeding the surf zone with a large number of equally spaced virtual drifters, the computed instantaneous surface velocity fields are used to calculate the hourly drifter trajectories. Collecting the hourly drifter exits, good agreement with the observed surf zone retention is obtained provided that both Stokes drift and VLF motions are accounted for in the modeling of the computed drifter trajectories. Without Stokes drift, the estimated number of virtual drifter exits is O(80)%, almost an order of magnitude larger than the O(20)% of observed exits during the drifter deployments. Conversely, when excluding the VLF motions instead, the number of calculated drifter exits is less than 5%, thus significantly underestimating the number of observed exits.

Low-Cost Handheld Global Positioning System for Measuring Surf-Zone Currents

Abstract Low-cost, handheld, L1 (1575.42 MHz) global positioning systems (GPSs) provide scientists with the opportunity to acquire position and velocity estimates at reduced expense (order of [O]

Observing Ocean Surface Waves with GPS-Tracked Buoys

100), size (∼cell phone), weight (O[70 g]), and engineering time. Two different low-cost, handheld GPS units and four different position-correcting configurations are evaluated here to determine their practicality in measuring surf-zone currents. Three of the simpler configurations result in relative position and velocity errors of O(2 m) and O(0.5 m s−1) for stationary tests. Surf-zone position and velocity signal-to-noise spectral ratios for the three configurations suggest that only motions <0.01 Hz can be confidently estimated for these surf-zone systems. For the fourth configuration, a GPS handheld unit that internally records GPS carrier phase is postprocessed using more sophisticated software for position corrections to obtain absolute position and velocity estimates. Simple modifications are required to improve the position accuracy by reducing patch antenna signal multipathing. For this configuration, the absolute position error for dynamic surveys was ∼0.40 m, and the velocity error on land relative to a survey-grade GPS system was 0.01 m s−1. The handheld GPS was attached to a surf-zone drifter and evaluated in the field. The flow field of a rip-current system was obtained with 24 surf-zone drifters. The drifters tracked simultaneous dye releases well, verifying that the observations are valid Lagrangian estimates. Owing to the low cost and small size of the handheld GPS, a large number of drifter systems can be deployed for absolute position tracking and velocity estimates of surf-zone currents.

Modeling of very low frequency motions during RIPEX

AbstractSurface-following buoys are widely used to collect routine ocean wave measurements. While accelerometer and tilt sensors have been used for decades to measure the wave-induced buoy displacements, alternative global positioning system (GPS) sensor packages have been introduced recently that are generally smaller, less expensive, and do not require calibration. In this study, the capabilities of several GPS sensors are evaluated with field observations in wind-sea and swell conditions off the California coast. The GPS buoys used in this study include Datawell Directional Waverider and Mini Directional Waverider buoys equipped with a specialized GPS Doppler shift sensor, and a low-cost experimental drifter equipped with an “off the shelf” GPS receiver for absolute position tracking. Various GPS position receivers were attached to the Waverider buoys to evaluate their potential use in low-cost wave-resolving drifters. Intercomparisons between the Datawell GPS-based buoys, the experimental GPS drifter,...

Surf zone diffusivity on a rip‐channeled beach

Numerical computations are used to explain the presence of very low frequency motions (VLFs), with frequencies less than 0.004 Hz, in the rip current velocity signals observed during the Rip-current field Experiment (RIPEX) field experiment. Observations show that the VLF motions are most intense within the surfzone and then quickly taper off in the offshore direction. By comparing computed VLF intensity (URMS,vlf) distributions in both the cross-shore and alongshore direction with observations in a qualitative sense, the most important contributions to the VLF dynamics are established. VLF motions at neighboring rip-channels are seen to interact in the computations, with stronger surfzone intensity for increasing bathymetric variation. The intermittent forcing by spatially varying wave groups is essential in obtaining the correct URMS,vlf distribution in the cross-shore direction, suggesting this is the predominant mechanism responsible for the generation of the VLF motions observed during RIPEX. Computations also suggest that VLF motions can occasionally propagate offshore but are mostly confined to the surfzone corresponding to surfzone eddies. A quantitative comparison shows good correspondence between model computations and measurements of URMS,vlf with a model skill of O(0.7), with generally increased (decreased) URMS,vlf during mean low (high) water levels.

Rip?current pulses tied to Lagrangian coherent structures

Absolute and relative diffusivity are measured on a rip-channeled beach using 30 position-tracking drifters released in clusters (4–12 drifters) deployed on 7 days with different wave forcing and tidal elevations at Sand City, Monterey Bay, California. Diffusivity and dispersion were found to be larger on days with rip current flow patterns and larger waves. Rip currents cause material to diffuse quickly for t 0.9]. Two independent methods are used to measure the small-scale turbulent diffusion contribution (kxy), which are found significantly correlated (R2 = 0.95) with each other and calculated surf zone wave breaking induced turbulent eddy viscosity. Here kxy is small relative to the total dispersion (Ke/kxy = 3–30), indicating that the shear flow is the primary mechanism responsible for dispersion in a rip current system.

Observations and modeling of steep-beach grain-size variability

The trapping and ejection of surfzone floating material is examined by unveiling Lagrangian Coherent Structures (LCSs) hidden in the pulsating rip?current surface velocity field produced by a three?dimensional numerical model resolving wave?group induced Very Low Frequency motions (VLFs). LCSs explain the typically observed patchiness of flotsam within the surf zone and the streaky distribution outside of the surf zone. The ejection of surfzone material occurs when filament?like LCSs separate form the main rip?current circulation corresponding to a situation where eddies temporarily extend the rip current beyond the surf zone and subsequently detach. The LCSs support the idea that VLFs form the dominant exchange mechanism of surfzone floating material with the inner shelf.

Vortical surf zone velocity fluctuations with 0(10) min period

Novel observations of surface grain-size distributions are used in combination with intra-wave modeling to examine the processes responsible for the sorting of sediment grains on a relatively steep beach (slope?=?1:7.5). The field observations of the mean grain size collected with a digital camera system at consecutive low and high tides for a 2 week period show significant temporal and spatial variation. This variation is reproduced by the modeling approach when the surf zone flow-circulation is relatively weak, showing coarse grain sizes at the location of the shore break and finer sediment onshore and offshore of the shore break. The model results suggest that grain size sorting is dominated by the wave-breaking-related suspended sediment transport which removes finer sediment from the shore break and transports it both on-shore and offshore. The transport capacity of wave-breaking-related suspended sediment is controlled by the sediment response time scale in the advection-diffusion equation, where small (large) values promote onshore (offshore) transport. Comparisons with the observed beach profile evolution suggest a relatively short time scale for the suspended sediment response which could be explained by the vigorous breaking of the waves at the shore break.

Field Observations of Surf Zone-Inner Shelf Exchange on a Rip-Channeled Beach

Observations of velocity fluctuations with periods between about 4 and 30 min, thus longer than infragravity waves and referred to as very low frequency (VLF) surf zone motions, are described and compared with numerical simulations. The VLF motions discussed here exclude instabilities (generated by the wave-driven alongshore current velocity shear) that occur in the same frequency range by selecting cases with weak alongshore currents only. Numerical simulations are based on the linear shallow water equations including friction and forced by nonlinear difference-frequency interactions between incident sea and swell waves. The model is initialized with sea and swell frequency directional spectra observed seaward of the surf zone. Modeled and observed VLF velocity fluctuation magnitudes agree within a factor of 2; both increase approximately linearly with increasing incident wave height and rapidly decay seaward of the surf zone. Observed frequency-wave-number, f-ky, spectra of VLF velocity fluctuations, estimated with instrumented alongshore arrays, are energetic in a broad range of ky in the vortical band. Observed and modeled VLF pressure fluctuations are relatively weak. Still, the model momentum balance suggests that VLF pressure gradients are as important as the nonlinear wave group forcing by sea and swell in accelerating/decelerating the VLF velocities. Model calculations demonstrate that the VLF-f-ky response is a function of the modulations of short-wave forcing associated with the frequency directional distribution of the incident sea and swell spectra. This results in VLF motions which span the surf zone and have O(50–1000 m) alongshore scales with O(200–2000 s) time scales. Given the fact that modulations of short waves resulting from directionally spread incident waves are common during field conditions we expect VLFs to be ubiquitous.

Bathymetric control of surf zone retention on a rip-channelled beach

Cross-shore exchange between the surf zone and the inner shelf is investigated using Lagrangian and Eulerian field measurements of rip current flows on a rip-channeled beach in Sand City, California. Surface drifters released on the inner shelf during weak wind conditions moved seaward due to rip current pulses and then returned shoreward in an arcing pattern, reentering the surf zone over shoals. The cross-shore velocities of the seaward- and shoreward-moving drifters were approximately equal in magnitude and decreased as a function of distance offshore. The drifters carried seaward by the rip current had maximum cross-shore velocities as they exited the surf zone and then decelerated as they moved offshore. The drifters moving shoreward accelerated as they approached the surfzone boundary with maximum cross-shore velocities as they reentered the surf zone over shoals. It was found that Stokes drift was not solely responsible for the onshore transport across the surfzone boundary. The cross-shore diffusivity on the inner shelf was greatest during observations of locally contained cross-shore exchange. These field observations provide evidence that the cross-shore exchange between the surf zone and inner shelf on a rip-channeled beach is due to wave-driven rip current circulations and results in surface material being contained within the nearshore region.