Surfzone mixing and transport on a sandy, steep (∼1/8 slope), reflective beach at Carmel River State Beach, California, are described for a range of wave and alongshore flow conditions. Depth-limited wave breaking occurred close to the shore due to the steepness of the beach, creating a narrow surf/swash zone (∼10 m wide). Fluorescent Rhodamine dye was released as a slug in the surfzone, and the temporal and spatial evolution was measured using in-situ dye sensors. Dye concentration measured as a function of time reveals sharp fronts that quickly decay resulting in narrow peaks near the dye release, which subsequently broaden and decrease in peak concentration with alongshore distance. The measurements indicate two stages of mixing and transport occur inside the surfzone on the steep beach. 1) In the near-field (<50 m downstream of the dye release location), the dye fully mixed throughout the water column after a few incident waves then continued to disperse in two dimensions, with both advection and diffusion processes being important. 2) In the far-field (>50 m downstream from the dye release location), the mass transport was dominated by advection. The distance to the far-field is much shorter in the alongshore on a steep beach compared with a dissipative beach. Estimates of cross-shore and alongshore diffusion coefficients (κ _x , κ _y ) were found to be similar in magnitude within the surfzone. Outside the surfzone in the far-field, the results suggest that the mixing processes are independent of those inside the surfzone. The mixing and transport of material observed on this steep beach are found to be analogous to that previously measured on dissipative beaches, however the diffusion coefficients within and outside the surfzone were found to be smaller on this steep beach.

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The Deepwater Horizon oil spill damaged some beaches along the Northern Gulf of Mexico (NGoMex) coast more than others, possibly related to the presence of natural protection mechanisms. In order to optimize future mitigation efforts to protect the coast, these mechanisms should be understood. The NGoMex coast is characterized by relatively long stretches of sandy beach interrupted by tidal inlets creating ebb-tidal river plumes featuring frontal zones that may act as transport barriers. This research investigates to what extent these plumes are capable of protecting the adjacent coast. This is done by means of a combination of a 3D Eulerian flow model and a Lagrangian particle model to track oil pathways and visualize Lagrangian Coherent Structures located at the plume front. The models are verified with measurements from a field experiment adjacent to Destin Inlet, Florida. The effects of wind, tidal range and river discharge on the oil fate are discussed. It was found that wind is the dominant parameter. Offshore wind prevents oil from beaching. During onshore winds, oil is pushed to shore, but near the inlet the plume is effective in reducing the amount of oil washing ashore during the ebbing tide. In general, the plume redistributes the oil but is not capable of preventing oil from beaching. For strong winds, the influence of the plume is reduced.

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We investigated whether cross-shore distributions of coastal phytoplankton to the surf zone are controlled by hydrodynamics and their biological characteristics. Data from a rip-channeled beach indicate that concentrations of phytoplankton are higher in the surf zone than offshore. To examine how phytoplankton is transported toward the shore, we used a coupled biophysical model, comprised of a 3D physical model of coastal dynamics and an individual-based model (IBM) for tracking phytoplankton on the rip-channeled beach. Waves and wind in the biophysical model were parameterized by the conditions during the sampling period. Previous studies indicated that growth rates of phytoplankton can be enhanced by high turbulence, which might contribute to high phytoplankton concentration in the surf zone. Some numerical and laboratory works showed that turbulence can also increase the downward velocity of phytoplankton, which could be carried by onshore bottom currents and remain in the surf zone. Furthermore, we adapted the IBM with the theoretical model of diurnal vertical migration (DVM) for phytoplankton. The theoretical DVM works as follows: in the morning, phytoplankton cells adhere to air bubbles and stay at the surface and close to the shore in the daytime because onshore wind and surface current direction is usually onshore; in the late afternoon, the cells switch their attachment from air bubbles to sand grains and sink to the bottom where the water flow is normally onshore at night. Finally, depth-varying growth of phytoplankton was also incorporated into the DVM module. Simulations using neutral passive particles do not give the expected results of observed patterns. All tested mechanisms, i.e., wind- and wave-driven currents, rip-current circulation, turbulence-driven growth and sinking, DVM, and depth-varying growth, enhanced onshore phytoplankton migration and cell concentrations in the surf zone, indicating that both biological traits and physical factors can be essential to phytoplankton cross-shore transport and spatial variability. Our model is open to be modified and re-parameterized, followed by further analysis and validation, so that it can be more adequate for ecological assessment of coastal areas.

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Surf zones, regions of breaking waves, are at the interface between the shore and coastal ocean. Surf zone hydrodynamics may affect delivery of phytoplankton subsidies to the intertidal zone. Over a month of daily sampling at an intermediate surf zone with bathymetric rip currents and a reflective surf zone, we measured surf zone hydrodynamics and compared concentrations of coastal phytoplankton taxa in the surf zones to concentrations offshore. At the intermediate surf zone, ~80% of the variability in the concentration of coastal phytoplankton taxa within the surf zone was explained by their variation offshore; however, concentrations were much higher and lower than those offshore in samples from a bathymetric rip current and over the adjacent shoal, respectively. Hydrodynamics at this intermediate surf zone did not hinder the delivery of coastal phytoplankton to the surf zone, but the bathymetric rip current system appeared to redistribute phytoplankton concentrating them within eddies. At the reflective shore, we sampled surf zones at a beach and two adjacent rocky intertidal sites. Concentrations of typical coastal phytoplankton taxa were usually an order of magnitude or more lower than those offshore, even when offshore samples were collected just 20 m beyond the breakers. The phytoplankton assemblages inside and outside the surf zone often appeared to be disconnected. Surf zone hydrodynamics at the steep, reflective shore coupled with low phytoplankton concentrations in near-surface water appeared to limit delivery of phytoplankton subsidies to the surf zone. Surf zone hydrodynamics may be a key factor in the alongshore variation in phytoplankton subsidies to coastal communities.

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Plankton are transported onshore, providing subsidies of food and new recruits to surf-zone and intertidal communities. The transport of plankton to the surf zone is influenced by wind, wave, and tidal forcing, and whether they enter the surf zone depends on alongshore variation in surf-zone hydrodynamics caused by the interaction of breaking waves with coastal morphology. Areas with gently sloping shores and wide surf zones typically have orders-of-magnitude-higher concentrations of plankton in the surf zone and dense larval settlement in intertidal communities because of the presence of bathymetric rip currents, which are absent in areas with steep shores and narrow surf zones. These striking differences in subsidies have profound consequences; areas with greater subsidies support more productive surf-zone communities and possibly more productive rocky intertidal communities. Recognition of the importance of spatial subsidies for rocky community dynamics has recently advanced ecological theory, and incorporating surf-zone hydrodynamics would be an especially fruitful line of investigation.

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Motivated by the Deepwater Horizon oil spill, the Surfzone and Coastal Oil Pathways Experiment obtained Acoustic Doppler Current Profiler (ADCP) Eulerian and GPS-drifter based Lagrangian “surface” (<1 m) flow observations in the northern Gulf of Mexico to describe the influence of small-scale river plumes on surface material transport pathways in the nearshore. Lagrangian paths are qualitatively similar to surface pathlines derived from non-traditional, near-surface ADCP velocities, but both differ significantly from depth-averaged subsurface pathlines. Near-surface currents are linearly correlated with wind velocities (r =0.76 in the alongshore and r =0.85 in the cross-shore) at the 95% confidence level, and are 4–7 times larger than theoretical estimates of wind and wave-driven surface flow in an un-stratified water column. Differences in near-surface flow are attributed to the presence of a buoyant river plume forced by winds from passing extratropical storms. Plume boundary fronts induce a horizontal velocity gradient where drifters deployed outside of the plume in oceanic water routinely converge, slow, and are re-directed. When the plume flows west parallel to the beach, the seaward plume boundary front acts as a coastal barrier that prevents 100% of oceanic drifters from beaching within 27 km of the inlet. As a result, small-scale, wind-driven river plumes in the northern Gulf of Mexico act as coastal barriers that prevent offshore surface pollution from washing ashore west of river inlets.

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Larvae of intertidal invertebrates need to cross the surf zone to settle in their adult habitat. Onshore transport of invertebrate larvae and detritus at a steep beach was simulated with a biophysical larval tracking model. Hydrodynamic model calculations were performed for 24 h after a 24 h spin-up stage with bathymetry and averaged wave data obtained during the summer of 2011 at Carmel River State Beach, California, and with and without onshore wind. The physical model output was then transferred to a Lagrangian larval tracking model using several types of particles representing larvae. A southward alongshore current controlled particle distribution in the middle and north of the domain. At the southern shore, negatively buoyant particles were trapped by eddies generated between the alongshore current and shore, while positively buoyant particles were carried onshore by wind-driven surface currents. The concentration of modeled detritus in the surf zone was positively correlated with that of negatively buoyant larvae. Additionally, the concentrations of detritus and competent larvae within the surf zone were negatively correlated with wave height, consistent with the observations of the accompanying field study. Some eddies contributed to forming high particle concentration patches by trapping them in the surf zone. More small eddies were generated closer to the shore with smaller waves, leading to high larval and detrital concentration in the surf zone. As waves increased in size, fewer and larger eddies formed, predominantly outside the surf zone, and consequently fewer larvae and detritus particles entered or stayed in the surf zone.

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Surf zone hydrodynamics vary along shorelines potentially affecting the delivery of larvae and zooplankton subsidies to intertidal communities, and, hence, the intensity of postsettlement interactions, growth and reproduction of filter-feeding foundation species and planktivorous fishes. We investigated the ability of zooplankton assemblages to enter the wide surf zone of the rip-channeled, more dissipative beach at Sand City, California, and the narrow surf zone of the steep reflective beach at nearby Carmel River State Beach. Every day for a month, we surveyed zooplankton inside and outside the surf zone and concomitant larval settlement of the dominant invertebrate onshore at each site in this upwelling regime. At the more dissipative surf zone, all zooplankters were far more concentrated inside than outside the surf zone. Many taxa increased in the surf zone and the predominant invertebrate on beaches, Emerita analoga, settled abundantly when prevailing northwesterly winds relaxed and waves were small. At the reflective surf zone, concentrations of zooplankters of most taxa were far greater outside than inside the surf zone, and many taxa increased in the surf zone when waves were small. Twice as many taxa were positively correlated inside and outside the surf zone at the dissipative than the reflective surf zone, indicating that zooplankters were more freely exchanged although behavior also played a role. Thus, spatial and temporal variation in surf zone hydrodynamics may regulate subsidies of zooplankton food and larval recruits to nearshore communities with potential cascading effects on community dynamics and structure.

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Larvae in the coastal ocean are transported toward shore by a variety of mechanisms. Crossing the surf zone is the last step in a shoreward migration and surf zones may act as semipermeable barriers altering delivery of larvae to the shore. We related variation in the structure of intertidal barnacle populations to surf zone width (surf zone hydrodynamics proxy), wave height, alongshore wind stress (upwelling proxy), solar radiation, and latitude at 40 rocky intertidal sites from San Diego, California to the Olympic Peninsula, Washington. We measured daily settlement and weekly recruitment of barnacles at selected sites and related these measures to surf zone width. Chthamalus density varied inversely with that of Balanus, and the density of Balanus and new recruits was negatively related to solar radiation. Across the region, long-term mean wave height and an indicator of upwelling intensity and frequency did not explain variation in Balanus or new recruit densities. Balanus and new recruit densities, daily settlement, and weekly recruitment were up to three orders of magnitude higher at sites with wide (>50 m), more dissipative surf zones with bathymetric rip currents than at sites with narrow (<50 m) more reflective surf zones. Surf zone width explained 30-50% of the variability in Balanus and new recruit densities. We sampled a subset of sites <5 km apart where coastal hydrodynamics such as upwelling should be very similar. At paired sites with similar surf zone widths, Balanus densities were not different. If surf zone widths at paired sites were dissimilar, Balanus densities, daily settlement, and weekly recruitment were significantly higher at sites with the wider, more dissipative surf zone. The primary drivers of surf zone hydrodynamics are the wave climate and the slope of the shore and these persist over time; therefore site-specific stability in surf zone hydrodynamics should result in stable barnacle population characteristics. Variations in surf zone hydrodynamics appear to play a fundamental role in regulating barnacle populations along the open coast, which, in turn, may have consequences for the entire intertidal community.

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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.@en

In recent years, the application of large-scale beach nourishments has been discussed, with the Sand Motor in the Netherlands as the first real-world example. Such protruding beach nourishments have an impact on tidal currents, potentially leading to tidal flow separation and the generation of tidal eddies of
length scales larger than the nourishment itself. The present study examines the characteristics of the tidal flow field around protruding beach nourishments under varying nourishment geometry and tidal conditions, based on extensive field observations and numerical flow simulations. Observations of the flow field around the Sand Motor, obtained with a ship-mounted current profiler and a set of fixed current profilers, show that a tidal eddy develops along the northern edge of the mega-nourishment every flood period. The eddy is generated around peak tidal flow and gradually gains size and strength, growing much
larger than the cross-shore dimension of the coastline perturbation. Based on a 3 week measurement period, it is shown that the intensity of the eddy modulates with the spring-neap tidal cycle. Depth-averaged tidal currents around coastline perturbations are simulated and compared to the field observations. The occurrence and behavior of tidal eddies is derived for a large set of simulations with varying nourishment size and shape. Results show that several different types of behavior exist, characterized by different combinations of the nourishment aspect ratio, the size of the nourishment relative to the tidal excursion length, and the influence of bed friction.@en

Mega-scale, concentrated nourishments are nowadays proposed as mitigation for coastal erosion or to widen beaches for recreation. The intention of such concentrated nourishments is to spread their sediment in both alongshore and cross-shore direction. The mixing process of sediment and other suspended matter by hydrodynamic processes is of key importance for an evaluation and prediction of these coastal projects. Surf zone velocity oscillations and eddies on timescales of up to 30 minutes are contributing factors to this mixing. One of the questions herein is how the artificial altering of the natural profile affects the velocity fluctuations and spatial patterns. This paper will discuss the characteristics of low frequency velocity motions in a new man-made setting; the large Sand Engine nourishment protruding 1 km into the sea. Measurements will be presented from an alongshore array of 6 ADCPs which were deployed during a 6-week field campaign with the intention to investigate amongst others the effect of the wind sea climate and the strong (>0.5 m/s) alongshore tidal currents at the site. The observed magnitudes of both very low frequency (f<0.004Hz) and infragravity motions (0.004