North Sea Infragravity Wave Observations

Coastal safety assessments with wave-resolving storm impact models require a proper offshore description for the incoming infragravity (IG) waves. This boundary condition is generally obtained by assuming a local equilibrium between the directionally-spread incident sea-swell wave forcing and the bound IG waves. The contribution of the free incident IG waves is thus ignored. Here, in-situ observations of IG waves with wave periods between 100 s and 200 s at three measurement stations in the North Sea in water depths of O(30) m are analyzed to explore the potential contribution of the free and bound IG waves to the total IG wave height for the period from 2010 to 2018. The bound IG wave height is computed with the equilibrium theory of Hasselmann using the measured frequency-directional sea-swell spectra as input. The largest IG waves are observed in the open sea with a maximum significant IG wave height of O(0.3) m at 32 m water depth during storm Xaver (December 2013) with a concurrent significant sea-swell wave height in excess of 9 m. Along the northern part of the Dutch coast, this maximum has reduced to O(0.2) m at a water depth of 28 m with a significant sea-swell wave height of 7 m and to O(0.1) m at the most southern location at a water depth of 34 m with a significant sea-swell wave height of 5 m. These appreciable IG wave heights in O(30) m water depth represent a lower bound for the expected maximum IG wave heights given the fact that in the present analysis only a fraction of the full IG frequency range is considered. Comparisons with the predicted bound IG waves show that these can contribute substantially to the observed total IG wave height during storm conditions. The ratio between the predicted bound- and observed total IG variance ranges from 10% to 100% depending on the location of the observations and the timing during the storm. The ratio is typically high at the peak of the storm and is lower at both the onset and waning of the storm. There is significant spatial variability in this ratio between the stations. It is shown that differences in the directional spreading can play a significant role in this. Furthermore, the observed variability along the Dutch coast, with a substantially decreased contribution of the bound IG waves in the south compared to the northern part of the Dutch coast, are shown to be partly related to changes in the mean sea-swell wave period. For the southern part of the Dutch coast this corresponds to an increased difference with the typically assumed equilibrium boundary condition although it is not clear how much of the free IG-energy is onshore directed barring more sophisticated observations and/or modeling

Alongshore variation in barnacle populations is determined by surf zone hydrodynamics

The article of record as published may be found at http://doi.org/10.1002/ecm.1265Larvae 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.National Science FoundationNSF-OCE#09273

Tidal flow separation at protruding beach nourishments

The article of record as published may be found at http://dx.doi.org/10.1002/2016JC011942In 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.STW grantERC-Advanced GrantSTW Grant no. 12686: Nature-driven nourishments of coastal systems (NatureCoast), S1: Coastal SafetyERC-Advanced Grant no. 291206 - Nearshore Monitoring and Modeling (NEMO

Rip Currents

Faculty Showcase Archive ArticleRip currents are fast-moving flows, traveling “outward almost at right angles to the shore” (Shepard 1936), creating a natural hazard for beachgoers, who suddenly find themselves in deep water. Field measurements and instrumentation, laboratory techniques, and numerical modeling have improved with time, enabling a more complete description of rip currents now. Surprisingly, there are many types of rip currents that can occur on beaches, and these currents are created by a wide variety of mechanisms that are presented here, along with numerical, physical, or field validation. We also show the potential for prediction schemes for use by lifeguards and beach managers.Approved for public release; distribution is unlimited

Data from: Alongshore variation in barnacle populations is determined by surfzone hydrodynamics

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 surfzone width (surfzone 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 surfzone 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. Thirty to 50% of the variability in Balanus and new-recruit densities was explained by surfzone width. We sampled a subset of sites < 5 km apart where coastal hydrodynamics such as upwelling should be very similar. At paired sites with similar surfzone widths, Balanus densities were not different. If surfzone 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 surfzone hydrodynamics are the wave climate and the slope of the shore and these persist over time, and therefore site-specific stability in surfzone hydrodynamics should result in stable barnacle population characteristics. Variations in surfzone 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

Numerical simulations of larval transport into a rip-channeled surf zone

The article of record as published may be located at http://dx.doi.org/10.4319/lo.2014.59.4.1434Competent larvae of intertidal invertebrates have to migrate toward shore for settlement; however, their migration through the surf zone is not understood. We investigated larval transport mechanisms at a ripchanneled beach. Because tracking larvae in the surf zone is infeasible, we used a three-dimensional biophysical model to simulate the processes. The coupled model consists of a physical module for currents and waves, and a biological module for adding larval traits and behaviors as well as Stokes drift to Lagrangian particles. Model calculations were performed with and without onshore wind forcing. Without wind, wave-driven onshore streaming occurs in the bottom boundary layer outside the surf zone. With onshore wind, onshore currents occur near the surface. In the surf zone, offshore-directed rip currents and compensating onshore-directed currents over shoals are formed in both no-wind and wind cases. In the biological module, neutral, negative, and positive buoyant particles were released offshore. Additionally, particles either sank in the presence of turbulence or not. Two scenarios achieved successful onshore migration: Negatively buoyant larvae without wind forcing sink in the turbulent bottom boundary layer and are carried onshore by streaming; positively buoyant larvae drift toward shore in wind-driven surface currents to the surf zone, then sink in the turbulent surf zone and remain near the bottom while transported shoreward. In both cases, the larval concentration is highest in the rip channel, consistent with field data. This successful result is only obtained if turbulence-dependent sinking behavior and Stokes drift are included in the transport of larvae.This study is supported by National Science Foundation (NSF) Ocean Sciences (OCE-092735) ‘Collaborative Research: Does coupling between the inner shelf and surf zone regulate larval supply to intertidal populations?’ C. Paris is funded by NSF (OCE-1155698). This is a contribution of the Rosenstiel School of Marine and Atmospheric Science, the Oregon Institute of Marine Biology, the Naval Postgraduate School, and the Bodega Marine Laboratory

Mechanisms of Cross-Shore Transport and Spatial Variability of Phytoplankton on a Rip-Channeled Beach

The article of record as published may be found at http://dx.doi.org/10.3389/fmars.2018.00183We 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.National Science Foundation (OCE-092735)1457769National Science Foundation (1457769) Guam Ecosystems Collaboratoriu

Transport of larvae and detritus across the surf zone of a steep reflective pocket beach

The article of record as published may be found at http://dx.doi.org/10.3354/meps11223Larvae of many intertidal species develop offshore and must cross the surf zone to complete their onshore migration to adult habitats. Depending on hydrodynamics, the surf zone may limit this migration, especially on reflective rocky shores. As a logistically tractable analog of a rocky shore environment, we carried out a comprehensive biological and physical study of the hydrodynamics of a steep reflective sandy beach. Holoplankton and precompetent larval invertebrates were much less abundant within the surf zone than offshore, and their concentrations inside and outside the surf zone were not significantly correlated, suggesting that they were not entering the surf zone. Persistent offshore flow throughout the water column at the outer edge of the surf zone may prevent these organisms from entering the surf zone. In contrast, the concentrations of detritus and a competent larval invertebrate (i.e. cyprids), while also not significantly correlated with concentrations offshore, were frequently more concentrated in the surf zone than offshore. Within the surf zone, the concentration of detritus was significantly correlated with concentrations of competent larval invertebrates (barnacles, gastropods, polychaetes, and bopyrid amphipod) and organisms that may be associated with detritus (amphipods and harpacticoid copepods). These concentrations were significantly negatively correlated with average daily wave height. We hypothesize that detritus and larvae enter the surf zone near the bottom during calm wave conditions by a process of near-bottom streaming. Near-bottom streaming is associated with all surf zones and may be a general mechanism for onshore transport of larvae close to the coast.National Science FoundationGrant NSD-OCE#09273

Low-Energy Rip Currents Associated With Small Bathymetric Variations

The article of record as published may be found at http://dx.doi.org/10.2112/JCOASTRES-D-11-00024We show for the first time that low-energy waves can induce a rip current system over subtle alongshore bathymetric variations. Comprehensive field measurements across a rip current that morphologically migrated (∼ 12 m/day) through a coherent cross- and alongshore array of co-located pressure and velocity sensors were obtained. The rip current is associated with a small bathymetric surfzone non-uniformity (1 in 300 alongshore variation). The circulation was kinematically non-uniform for ∼5% of the time over the course of the 20 day experiment and was present at low tides associated with increases in rip current activity. The presence of the rip current and mild-sloped rip channel induce statistically significant alongshore variations in Hrms, wave direction, directional spreading, infragravity waves, and very low frequency motions. Changes in the directional spreading are correlated with the presence of very low frequency motions influenced by the presence of the rip current.JM was funded by ONR under contract number N00014-05-1-0154 and N00014- 05-1-0352 and the Naval Postgraduate School. EBT and TPS were funded by ONR under contracts N0001405WR20150 and N0001405WR20385. AJHMR was funded by ONR under contract N000140310829 and the Dutch National Science Foundation (NWO) under contract DCB.5856. GS was a National Research Council Fellow during a sabatical funded in part by the ONR under contract N0001405WR20150.JM was funded by ONR under contract number N00014-05-1-0154 and N00014- 05-1-0352 and the Naval Postgraduate School. EBT and TPS were funded by ONR under contracts N0001405WR20150 and N0001405WR20385. AJHMR was funded by ONR under contract N000140310829 and the Dutch National Science Foundation (NWO) under contract DCB.5856. GS was a National Research Council Fellow during a sabatical funded in part by the ONR under contract N0001405WR20150