Winter Swell Research Articles

Re-discovering macerating Posidonia oceanica bottoms: Characterization of meiofaunal community inhabiting a peculiar Mediterranean habitat

The highly efficient carbon storage capacity of beds formed by Posidonia oceanica, an endemic Mediterranean seagrass species, has been widely recognized. Recently, the supra-littoral deposits of leaf litter (i.e., banquettes) have been investigated in terms of their nutrients, biomass and associated community. Nevertheless, an overlooked fraction of the P. oceanica detritus never reaches the shore and sinks far away to deeper seafloor. Additionally, part of the supra-littoral deposit goes back to the sea during winter swells. This deep detrital compartment, mainly composed of sediment mixed with dead leaves and rhizomes coming from P. oceanica beds, has only been described once by Pères in 1953, who focused on the macrofaunal component. Here, we investigated for the first time the meiofaunal community inhabiting sediments characterized by P. oceanica detritus in a deposit located at 65−80 m depth off the Ischia Island (Gulf of Naples, Italy, Tyrrhenian Sea). Our results show that the meiofaunal community appears highly diversified and strongly dominated by nematodes (from 85% to 93%). Differences in meiobenthic assemblage structures were significant only when rare taxa were considered (i.e. taxa found in low abundances and characterized by a sporadic distribution in the study area). The nematode community revealed a very high biodiversity (number of families and genera: 31 and 104, respectively), with a clear prevalence of selective and non-selective deposit feeders that suggest the key role of this habitat in the benthic detrital food web. The richness of meiofauna and the taxonomic and functional diversity of the nematode assemblages account for a “good” to “moderate” ecological quality status. These findings support the high ecological value of the macerating seagrass bottoms, an overlooked component of the blue carbon cycle that deserves to be further investigated.

Bringing Back Reef Fish: Sustainable Impacts of Community-Based Restoration of Elkhorn Coral (Acropora palmata) in Vega Baja, Puerto Rico (2008–2023)

In response to the severe fragmentation of Elkhorn coral, Acropora palmata (Lamarck, 1816), stands caused by a major winter swell (“Holy Swell”) in March 2008, an emergency community-based low-tech restoration was initiated in Vega Baja, Puerto Rico. Over a 15-year period, coral demographic performance and fish assemblages were monitored across four restored and four control (non-restored) 100 m2 plots. The restoration effort proved to be highly successful, leading to successful coral survival and growth, and to sustained recovery of fish assemblages, particularly herbivore guilds. Significantly increased abundance, biomass, and diversity were observed across all trophic functional groups, fishery target species, and geo-ecological functional groups in both restored and control plots. These positive outcomes were attributed to enhanced spatial complexity by long-term coral growth, “nutrient hotspots” within restored plots, the refugia effect from enhanced benthic spatial complexity, and the recovery of fish dispersal paths promoting spillover effects from restored to adjacent non-restored areas. Restoring herbivore guilds and geo-ecological functional groups played a crucial role in restoring vital ecological processes promoting reef ecosystem resilience. Recommendations include integrating fish assemblage recovery into coral restoration strategies, establishing natural coral nursery plots for future coral sourcing, and incorporating the concept of nursery seascapes for a holistic and ecosystem-based approach to restoration.

An early warning system for wave-driven coastal flooding at Imperial Beach, CA

Waves overtop berms and seawalls along the shoreline of Imperial Beach (IB), CA when energetic winter swell and high tide coincide. These intermittent, few-hour long events flood low-lying areas and pose a growing inundation risk as sea levels rise. To support city flood response and management, an IB flood warning system was developed. Total water level (TWL) forecasts combine predictions of tides and sea-level anomalies with wave runup estimates based on incident wave forecasts and the nonlinear wave model SWASH. In contrast to widely used empirical runup formulas that rely on significant wave height and peak period, and use only a foreshore slope for bathymetry, the SWASH model incorporates spectral incident wave forcing and uses the cross-shore depth profile. TWL forecasts using a SWASH emulator demonstrate skill several days in advance. Observations set TWL thresholds for minor and moderate flooding. The specific wave and water level conditions that lead to flooding, and key contributors to TWL uncertainty, are identified. TWL forecast skill is reduced by errors in the incident wave forecast and the one-dimensional runup model, and lack of information of variable beach morphology (e.g., protective sand berms can erode during storms). Model errors are largest for the most extreme events. Without mitigation, projected sea-level rise will substantially increase the duration and severity of street flooding. Application of the warning system approach to other locations requires incident wave hindcasts and forecasts, numerical simulation of the runup associated with local storms and beach morphology, and model calibration with flood observations.

Phase‐Coherent Amplification of Ocean Swells Over Submarine Canyons

AbstractOcean swells can be a significant coastal hazard, potentially causing coastal disasters with costly damage to infrastructure and tragic loss of lives. We have observed this in the coastal region of Toyama Bay in the Sea of Japan, which has been devastated by severe winter swell events (the so‐called YoriMawari‐nami; YM wave). An extreme swell event was recorded in February 2008 in which the wave height at a site in the bay reached 9+ m, which corresponds to 16 times the local climatological average. Substantial efforts by the Japanese coastal engineering community have been expended to reproduce YM wave events with phase‐averaged wave models but, to our knowledge, with no success. During an archetype YM event, we found that the Noto peninsula filtered the incoming wavefield such that in the bay incident waves were quasi‐monochromatic. The swell then refracted over a submarine canyon resulting in a bimodal directional distribution of wave energy. This initiated conditions favorable for coherent interference, violating the assumptions of the phase‐averaged approach. Coherent interference is the key mechanism for understanding the formation of the large amplitudes and for matching observed high space‐time variability. Our results demonstrate the significance of the coherent interference for wave statistics over coastal bathymetry; only by accounting for phase‐resolving phenomena were we able to adequately reproduce the observations.

On the spatial distribution of the wave energy resource in Puerto Rico and the United States Virgin Islands

The present article describes a numerical modeling study of the spatial distribution of the wave energy resource for Puerto Rico and the United States Virgin Islands (USVI). The study consists of an analysis of three years of output from the CARICOOS Nearshore Wave Model, an operational model based on the Simulating Waves Nearshore (SWAN) model. Using the wave model output, the wave power was computed and then averaged for a three year period from between January 1, 2013 to December 31, 2015. Maximum values at nearshore areas range from 7 to 8 kW/m for those locations in Puerto Rico exposed to North Atlantic winter swells, particularly the region between the towns of Aguadilla and Fajardo. This region is likely the most viable location for wave power generation in the US Caribbean given the very narrow shelf and the proximity to some of the most important components of Puerto Rico's electric grid. Wave power extraction certainly presents an interesting possibility for the future, and the present study is a first step towards better understanding the feasibility of such projects for the PR/USVI archipelago.

Different approaches to model the nearshore circulation in the south shore of O'ahu, Hawaii

Abstract. The dynamical interaction between currents, bathymetry, waves, and estuarine outflow has significant impacts on the surf zone. We investigate the impacts of two strategies to include the effect of surface gravity waves on an ocean circulation model of the south shore of O'ahu, Hawaii. This area provides an ideal laboratory for the development of nearshore circulation modeling systems for reef-protected coastlines. We use two numerical models for circulation and waves: Regional Ocean Modeling System (ROMS) and Simulating Waves Nearshore (SWAN) model, respectively. The circulation model is nested within larger-scale models that capture the tidal, regional, and wind-forced circulation of the Hawaiian archipelago. Two strategies are explored for circulation modeling: forcing by the output of the wave model and online, two-way coupling of the circulation and wave models. In addition, the circulation model alone provides the reference for the circulation without the effect of the waves. These strategies are applied to two experiments: (1) typical trade-wind conditions that are frequent during summer months, and (2) the arrival of a large winter swell that wraps around the island. The results show the importance of considering the effect of the waves on the circulation and, particularly, the circulation–wave coupled processes. Both approaches show a similar nearshore circulation pattern, with the presence of an offshore current in the middle beaches of Waikiki. Although the pattern of the offshore circulation remains the same, the coupled waves and circulation produce larger significant wave heights ( ≈ 10 %) and the formation of strong alongshore and cross-shore currents ( ≈ 1 m s−1).

Climate change impact on waves in the Bay of Biscay, France

The knowledge of offshore and coastal wave climate evolution towards the end of the twenty-first century is particularly important for human activities in a region such as the Bay of Biscay and the French Atlantic coast. Using dynamical downscaling, a high spatial resolution dataset of wave conditions in the Bay of Biscay is built for three future greenhouse gases emission scenarios. Projected wave heights, periods and directions are analysed at regional scale and more thoroughly at two buoys positions, offshore and along the coast. A general decrease of wave heights is identified (up to −20 cm during summer within the Bay off Biscay), as well as a clockwise shift of summer waves and winter swell coming from direction. The relation between those changes and wind changes is investigated and highlights a complex association of processes at several spatial scales. For instance, the intensification and the northeastward shift of strong wind core in the North Atlantic Ocean explain the clockwise shift of winter swell directions. During summer, the decrease of the westerly winds in the Bay of Biscay explains the clockwise shift and the wave height decrease of wind sea and intermediate waves. Finally, the analysis reveals that the offshore changes in the wave height and the wave period as well as the clockwise shift in the wave direction continue toward the coast. This wave height decrease result is consistent with other regional projections and would impact the coastal dynamics by reducing the longshore sediment flux.

Judging Surf Culture: The Making of A White Exemplar Masculinity During the 1966 Natal and South African Surfriding Championships Held in Durban

The 1966 Natal and South African Surfriding Championships was a founding moment in the history of the sport of surfing in South Africa and shaped the future trajectory of South African surfing culture. It did so by prioritising the masculine surfing styles of white men who surfed competitively. The championships were held over the first weekend in July in Durban – South Africa’s “Surf City” – during the peak winter swell season. It was the first ever national surf contest organised under the auspices of the newly formed national surfing association, the South African Surfriders’ Association. It was at these national surfing championships that a judging system was created to allow amateur South African surfers to compete for a place in the national Springbok team that travelled to the Third World Surfing Titles held two months later at Ocean Beach in San Diego, California, USA. The introduction of this judging system during the 1966 Surfriding Championships throws light on the development of three trends in the history of (stand-up board) surfing in South Africa : firstly, how local surfing sought out international acceptance as a sport; secondly, why surfing came to be seen as a largely white sporting and leisure activity; and thirdly, why women’s surfing has not receive the same attention as that of men’s. This article explores how the emergence of a competitive surfing culture, typified by the codification of judging competitive surfriding on boards of between nine and eleven feet in length, infused local surf culture with a need for global recognition as a surfing “nation” and, at the same time, accommodated racial segregation and a male-dominated gender order. The contention of this article is that the cultural logic embedded in the 1966 judging criteria has left a legacy for future generations of South African surfers; one that privileged a hegemonic white masculinity located in competitive surfing. Today, though an understanding of the making of surfing’s past and that exemplar masculinity, it becomes possible to promote new cultural configurations in South African surfing that are racially inclusive, gender equitable, and not determined by competitive prowess alone.

Evidence of sediment resuspension by nonlinear internal waves on the western Portuguese mid-shelf

In-situ observations of nonlinear internal waves (NIWs) propagating over the northern shelf of Portugal, near the Nazaré submarine canyon (39°47.4′N/009°11.4′W), and evidence of their action on the bottom, are presented and discussed. Synthetic Aperture Radar (SAR) images from this margin suggest the generation of NIWs over the canyon head (39°35′N/009°25′W) and the subsequent propagation directly to the mid-shelf. An observational program was conducted in this area in the summer of 2004, between August 29th and September 1st, to study these short-period internal waves. Temperature and current high resolution measurements were made with thermistor chains and acoustic Doppler current profilers (ADCPs), moored along the canyon generated NIW track, at the 82 m isobath. A simultaneous ENVISAT SAR image was obtained during the experiment. These observations were complemented by a CTD survey and bottom sediment sampling. Collected data show the propagation of large amplitude internal solitons forcing strong bottom current pulses. Turbidity profiles and ADCP echo variability suggest the remobilization of the mid-shelf bottom sediments induced by these short-period events. Calculations of bottom shear stress, induced by the observed canyon generated NIWs, reveal an energetic internal wave activity that can complement, during summer, the role of winter swell in sediment resuspension over the mid-shelf. This work is a contribution to the European project EUROSTRATAFORM, whose objectives include the study of the specific canyon systems dynamics and related sedimentary impacts on the European continental margin.

Video‐based observations of nearshore sand ripples and ripple migration

Observations of O(1 m) length and O(0.1 m) height sand ripples at Waimea Bay, a steep (slope ∼ 0.05) pocket beach on the north shore of Oahu, Hawaii, are presented. Time series of ripple patterns over a 900 m2 section of the nearshore in depths of 1–2 m are obtained using time‐averaged video images from an Argus station overlooking the bay. Ripples are detected during weak wave conditions in the summer or between winter swell events. The ripple field exhibits narrow‐band wave number structure, with ripple crests oriented parallel to the shoreline. The ripple wavelengths vary with wave orbital displacements, but they are shorter than predicted by orbital vortex ripple scaling relationships. A new suspension‐limited model (Smith and Wiberg, 2006; Traykovski, 2006) in which the suborbital ripple wavelengths vary with wave period appears to describe well the observed ripple wavelength scaling. Lagged correlations between sequential Argus images reveal area‐average ripple migration rates ranging from −3.3 m day−1 (offshore) to 4.5 m day−1. The corresponding estimated sediment transport ranges from −0.3 m3 m−1 day−1 to 0.5 m3 m−1 day−1, similar to previous estimates of cross‐shore transport based on observed volume changes at the subaerial beach. This suggests that the recovery of beach sand following major swell‐driven erosion events may be accomplished in part by the shoreward migration of seabed ripples.

Multiparameter studies of surf infrasound

Infrasound stations on islands or near coastlines routinely detect signals associated with breaking ocean waves. Although the source mechanisms of these infrasonic surf signals are not well understood, they might provide useful insight into coastal processes and permit an assessment of wave energy distribution in the littoral zone. Near-shore infrasonic arrays, ocean bottom sensors, an infrared imager, and a video camera were deployed on rocky and sandy coastlines in Hawaii during the 2004–2005 Winter swell season, with the aim of establishing a relationship between the directional ocean swell height and the infrasonic source distribution, intensity, and spectral content. The cameras targeted acoustically active regions for selected time periods to associate the timing of the infrasonic signal arrivals with the breaking and dissipation of ocean wave sets. A wide range of sea states and weather conditions were captured during two separate deployments. The results of our multiparameter analyses and preliminary comparisons of our observations with mesoscale ocean wave model predictions will be presented. [Work supported by the Defense Advanced Research Projects Agency. Support of the State of Hawaii is acknowledged.]

Effects of shade from multiple kelp canopies on an understory algal assemblage

We examined the effects of shade from multiple kelp canopy layers (surface Macrocystis pyrifera canopy, understory Pterygophora californica canopy), both individually and in combination, on an understory algal assemblage in a central Californian kelp forest. The removal of both kelp canopies resulted in a dense recruitment of the understory brown alga Desmarestia ligulata that formed a third canopy layer, which significantly decreased bottom light and the abundance of under- story red algae. We subsequently created an additional canopy treatment by removing this third canopy layer. In general, the understory red algae fluctuated seasonally with winter swell intensity, changes in kelp canopy cover, and grazing; and although species richness increased significantly with increased bottom light, red algal bottom cover did not respond significantly to the canopy clear- ings until 2 yr after the canopies were initially cleared. Red algal cover within the Pterygophora canopy treatment was similar to that in the control treatment. In the absence of Pterygophora, the Macrocystis canopy treatment and 'no canopy' treatment were found to have greater red algal cover and species richness. Individual understory species were rare, which resulted in small effects sizes and thus low statistical power. However, when grouped post hoc, according to how they responded to the canopy clearings (i.e. response groups), we were able to detect canopy treatment effects as much as 1 yr earlier. This method identified that some understory red algae adapted to areas of canopy removal (light-adapted), and others adapted to a variety of light regimes (shade-tolerant). We were able to classify the light-adapted algae into 2 subgroups: the high-light species and the inter- mediate-light species. Although this method of grouping was done post hoc, our results indicate that it may provide the clearest assessment of how understory algae respond to shading from kelp canopies.

Sea-cliff erosion as a function of beach changes and extreme wave runup during the 1997–1998 El Niño

Over time scales of hundreds to thousands of years, the net longshore sand transport direction along the central California coast has been driven to the south by North Pacific winter swell. In contrast, during the El Niño winter of 1997–1998, comparisons of before and after airborne lidar surveys showed sand was transported from south to north and accumulated on the south sides of resistant headlands bordering pocket beaches. This resulted in significant beach erosion at the south ends of pocket beaches and deposition in the north ends. Coincident with the south-to-north redistribution of sand, shoreline morphology became prominently cuspate with longshore wavelengths of 400–700 m. The width and elevation of beaches were least where maximum shoreline erosion occurred, preferentially exposing cliffs to wave attack. The resulting erosional hotspots typically were located in the embayments of giant cusps in the southern end of the pocket beaches. The observed magnitude of sea cliff retreat, which reached 14 m, varied with the number of hours that extreme wave runup exceeded certain thresholds representing the protective capacity of the beach during the El Niño winter. A threshold representing the width of the beach performed better than a threshold representing the elevation of the beach. The magnitude of cliff erosion can be scaled using a simple model based on the cross-shore distance that extreme wave runup exceeded the pre-winter cliff position. Cliff erosion appears to be a balance between terrestrial mass wasting processes, which tend to decrease the cliff slope, and wave attack, which removes debris and erodes the cliff base increasing the cliff slope.

Steep beach morphology changes due to energetic wave forcing

The three-dimensional variability of the sub-aerial beach is examined for Waimea Bay, Hawaii, a steep-slope pocket beach subject to energetic winter swell events. The beach is mapped using 32 Real-Time Kinematic Global Positioning System (RTK-GPS) surveys collected from March 1996 to July 1997. The survey method provides 4 m horizontal spacing and 2–3 cm vertical accuracy. The primary morphology change is the episodic removal and re-establishment of a foreshore dune during the winter swell season. The average sand volume flux between surveys reaches −8 and 4 m 2/day during peak erosion and accretion periods. The sand volume of the sub-aerial beach is found to be highly correlated (0.96) with the position of the shoreline. Longshore variations in morphology include beach cusps with typical peak to trough amplitudes of 0.5–1.5 m and longshore wavelengths of 30–70 m. The relationships between the energetic swell forcing and the volume and flux of sand on the sub-aerial beach are examined. The highest correlation (−0.88) is found between the total sand volume and the wave energy flux after the latter is filtered following Wright et al. [Wright, L.D., Short, A.D., Green, M.O., 1985. Short-term changes in the morphodynamic states of beaches and surf zones: an empirical predictive model. Mar. Geol. 62, 339–364]. The filter is effective at capturing slow accretion phases following energetic erosion events.

Coexistence in a Kelp Forest: Size, Population Dynamics, and Resource Partitioning in a Guild of Spider Crabs (Brachyura, Majidae)

The population dynamics and resource partitioning of five species of spider crabs (Loxorhynchus crispatus, Pugettia producta, Mimulus foliatus, Pugettia richii, and Scyra acutifrons) were measured in a kelp forest on the central California coasts. The distribution of the five species showed zones of peak abundances along a transect from the intertidal zone through the kelp forest to a deep reef. P. richii was the most abundant species with peak densities of 6 crabs/m2 at the middle and inner edge of the kelp forest. M. foliatus was second most abundant with a peak of 3.5 crabs/m2 at the middle and outer edge of the kelp forest. S. acutifrons had peak densities of about 3 crabs/m2 at the outer edge of the kelp forest. P. producta had a peak of 2 crabs/m2 in the intertidal zone. L. crispatus was present in low densities of about 0.5 crabs/m2 from the middle of the kelp forest to the deep reef. The greatest combined density of spider crabs (11 crabs/m2) occurred at the middle of the kelp forest. Densities of all five species in the main study area in the middle of the kelp forest showed a synchronous cycle with a maximum in late summer and fall and a decline in winter to a low in spring. The cycle appeared to be caused by a combination of density—independent mortality from winter swells and storm activity and by slower development and growth rates during the upwelling period of colder water temperatures. Brooding frequencies of all the species remained high year—round. Size structures of the populations did not change during bimonthly sampling, indicating continuous recruitment. Size—frequency analysis of crab populations along the transect indicated that juveniles of Pugettia producta recruit into the intertidal and shallow Phyllospadix zones and migrate out into the kelp forest as they grow, but the other species did not have zones of recruitment separate from adult distribution. The diversity of predators which take spider crabs as a major portion of their diets indicated that the general level of predation pressure is high and may limit the overall population levels of the crabs. Predation by sea otters probably limits the density of P. producta, and the other four species are probably limited by fish predation. The crabs exhibit different color morphs, color change, and decorating behavior, which are adaptive for concealment from predators. Size at maturity of the five species spanned an order of magnitude in carapace width from small Scyra acutifrons (1 cm) to large Loxorhynchus crispatus (10 cm). Mean body size of mature females and mean body size of the entire population of all pairs of species except Mimulus foliatus/Pugettia richii had ratios larger than Hutchinson's (1959) predicted value of 1.28. Body size may reflect limitations of the crabs' utilization of crevice refuges in the microhabitat. Partitioning of the microhabitat resource was measured by substrate utilization. Mimulus foliatus had the largest microhabitat niche breadth and was found on most types of substrates, with an important refuge in kelp holdfasts. Pugettia richii was found primarily on the alga Cystoseira osmundacea and in coralline algal mats. Scyra acutifrons occurred in the interstices of algal/invertebrate turf, and Loxorhynchus crispatus was found on top of the turf. Pugettia producta occurred upon the kelp plants and had the narrowest microhabitat niche breadth. Analysis of stomach contents showed partitioning of food resources. Pugettia producta was a strict specialist grazing on giant kelp, Macrocystis pyrifera. Mimulus foliatus and Pugettia richii also had narrow diets of mostly drift M. pyrifera. Scyra acutifrons had major food categories of detritus, sponge, and pieces of M. pyrifera trapped in the algal—invertebrate turf. Loxorhynchus crispatus was a dietary generalist foraging on a broad range of invertebrates and giant kelp. Niche separation in the guild is multidimensional, and similar utilization of one resource is generally complemented by dissimilar utilization of another resource. Graphical analysis of niche separation for the three dimensions of microhabitat, food, and body size showed that Pugettia producta is an overall specialist and Loxorhynchus crispatus is a generalist. Scyra acutifrons also has a distinct niche within the algal invertebrate turf, but Mimulus foliatus and Pugettia richii exhibit extensive overlap in all of the parameters measured. However, the degree to which a species is a specialist vs. a generalist for a resource did not relate to the amount of niche overlap with the rest of the guild. Microhabitat resources appear to be the most important for niche separation and may also be explained as adaptations for minimizing predation.