Jenna Brown

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.

Observations and modeling of steep-beach grain-size variability

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

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.

An Introduction to Rip Currents Based on Field Observations

Abstract Rip currents are fascinating, natural, surf zone phenomena that occur daily on many beaches throughout the world. My colleagues, students, advisors, and I have been studying rip currents for more than 10 years and have performed more than 10 comprehensive field experiments on various beaches throughout the world using different observational techniques and model simulations to improve our understanding and prediction of rip currents. We have written a series of scientific articles describing the intricacies and complexities of rip current behavior using statistical and mathematical equations. These manuscripts are typically published in professional journals, which often do not communicate our results to those who would benefit from the information—the beachgoing public and ocean swimmers. Herein, we summarize our findings to help people of all ages gain a better understanding of currents at the coast.

Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities

Larvae of intertidal species develop at sea and must return to adult habitats to replenish populations. Similarly, nutrients, detritus and plankton provide important subsidies spurring growth and reproduction of macroalgae and filter-feeding invertebrates that form the foundation of intertidal communities. Together, these factors determine the density and intensity of interactions among community members. We hypothesized that spatial variation in surfzone hydrodynamics affects the delivery of plankton subsidies. We compared entire zooplankton communities inside and outside the surf zone daily while monitoring physical conditions for one month each at two shores with different surfzone characteristics. Opposite cross-shore distributions of larvae and other zooplankters occurred at the two sites: zooplankton was much more abundant inside the mildly sloping dissipative surf zone (DSZ) with rip currents and was more abundant outside the steep reflective surf zone (RSZ). Biophysical numerical simulations demonstrated that zooplankters were concentrated in rip channels of the DSZ and were mostly unable to enter the RSZ, indicating the hydrodynamic processes behind the observed spatial variation of zooplankters in the surf zone. Differences in the concentration of larvae and other zooplankters between the inner shelf and surf zone may be an underappreciated, key determinant of spatial variation in inshore communities.

Variation in the abundance of Pseudo-nitzschia and domoic acid with surf zone type

Most harmful algal blooms (HAB) originate away from the shore and, for them to endanger human health, they must be first transported to shore after which they must enter the surf zone where they can be feed upon by filter feeders. The last step in this sequence, entrance into the surf zone, depends on surf zone hydrodynamics. During two 30-day periods, we sampled Pseudo-nitzschia and particulate domoic acid (pDA) in and offshore of a more dissipative surf zone at Sand City, California (2010) and sampled Pseudo-nitzschia in and out of reflective surf zones at a beach and rocky shores at Carmel River State Beach, California (2011). At Sand City, we measured domoic acid in sand crabs, Emerita analoga. In the more dissipative surf zone, concentrations of Pseudo-nitzschia and pDA were an order of magnitude higher in samples from a rip current than in samples collected just seaward of the surf zone and were 1000 times more abundant than in samples from the shoals separating rip currents. Domoic acid was present in all the Emerita samples and varied directly with the concentration of pDA and Pseudo-nitzschia in the rip current. In the more reflective surf zones, Pseudo-nitzschia concentrations were 1-2 orders of magnitude lower than in samples from 125 and 20m from shore. Surf zone hydrodynamics affects the ingress of Pseudo-nitzschia into surf zones and the exposure of intertidal organisms to HABs on the inner shelf.

Macrotidal rip current experiment: circulation and dynamics

AUSTIN, M. J., SCOTT, T. M., BROWN, J. W., BROWN, J. A. and MACMAHAN, J. H., 2009. Macrotidal rip current experiment: circulation and dynamics. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), pg – pg. Lisbon, Portugal, ISBN The macrotidal intermediate–dissipative beaches of the southwest UK display strong seasonality whereby extensive low-tide bar/rip systems form during the spring season. These features reach a state of maximum development during the summer months, when strong rip currents are present at various stages of the tide that switch on/off due to tidal translation. This research investigates for the first time the circulation and dynamics of macrotidal rip systems from the perspective of the flow dynamics and surf zone circulation. A field experiment was conducted on a macro-tidal beach (range 6.3 m) with low-tide bar/rip morphology to quantify the circulation and dynamics of the rip current systems. Measurements of flow velocity, water depth and suspended sediment concentration were collected over the channel and shoal of a transverse bar/rip system using an in-situ sensor array, whilst the surf zone circulation patterns were monitored with post-processed GPS drifter floats. The in-situ data show that currents within the rip system are strongly correlated with the stage of the tide and maximum current speeds are recorded during the 3-hour period immediately around low water. During this period, wave breaking is maximised on the adjacent shoal resulting in a longshore gradient in wave dissipation. The GPS drifters indicate that an extensive rotational surf zone circulation pattern exists, whereby the offshore flow within the rip channel returns landwards over the intertidal bar from the outer edge of the surf zone. ADITIONAL INDEX WORDS: Surf zone, Nearshore circulation, Wave breaking, Morphodynamics

Assessing the effectiveness of rip current swimmer escape strategies, Shelly Beach, NSW, Australia

ABSTRACT McCarroll, R.J., Brander, R.W., MacMahan, J.H., Turner, I.L., Reniers, A.J.H.M., Brown, J.A. and Bradstreet, A., 2013. Assessing the effectiveness of rip current swimmer escape strategies, Shelly Beach, NSW, Australia . Rip flow dynamics have important implications for swimmer escape strategies, with recent field studies indicating flow is often contained within the surf zone, re-circulating onto shallow bars. Combining physical measurements in a variety of conditions with various escape strategies allows insights into the safest response by individuals caught in a rip. A three day experiment was conducted at Shelly Beach, NSW in December, 2011. Lagrangian measurements of rip flow using GPS drifters were obtained. Participants with attached GPS were deployed in groups of 3 to 5 in rip currents to simulate various escape actions such as floating with the rip current, and swimming in a parallel direction at varying distances from shore. Day 1 and 2 measurements captured an open beach rip current and an adjacent topographic rip current in moderate energy conditions. Day 3 measurements were obtained on an open beach rip current at a more exposed section of the beach under higher energy conditions. Results were analysed by duration and assessed as being either (i) positive outcome (high chance of safe exit); or (ii) negative outcome (greater potential for hazard). Mean durations were Float (4 min), and Swim (2 min). Of the eight escape scenarios tested, half had potential negative outcomes, including 2 of 3 Float options and 2 of 5 Swim options. Negative outcomes for Floats were related to surf zone exits, and multiple circulations. Negative outcomes for Swims related to swimming against longshore directed circulation. Neither strategy was clearly more likely to result in a successful outcome across all scenarios. Further investigations are currently being conducted in a greater range of hydrodynamic and morphological conditions.

Divergence-Free Spatial Velocity Flow Field Interpolator for Improving Measurements from ADCP-Equipped Small Unmanned Underwater Vehicles

AbstractApplying a two-dimensional (2D) divergence-free (DF) interpolation to a one-person deployable unmanned underwater vehicle’s (UUV) noisy moving-vessel acoustic Doppler current profiler (MV-ADCP) measurements improves the results and increases the utility of the UUV in tidal environments. For a 3.5-h MV-ACDP simulation that spatially and temporally varies with the M2 tide, the 2D DF-estimated velocity magnitude and orientation improves by approximately 85%. Next the 2D DF method was applied to velocity data obtained from two UUVs that repeatedly performed seven 1-h survey tracks in Bear Cut Inlet, Miami, Florida. The DF method provides a more realistic and consistent representation of the ADCP measured flow field, improving magnitude and orientation estimates by approximately 25%. The improvement increases for lower flow velocities, when the ADCP measurements have low environmental signal-to-noise ratio. However, near slack tide when flow reversal occurs, the DF estimates are invalid because the flo...