High-energy Swell Research Articles

Observations of coastal infragravity wave characteristics under swell-dominated conditions

This study analyzed the characteristics of infragravity (hereafter IG) waves under a microtidal and swell-dominated wave climate based on the field observations collected in the nearshore waters (water depth: ∼ 13 m) of southern Sri Lanka. The main focus is on the dependence of nearshore IG waves on the incident short wave parameters, with particular attention to the influence of the short wave spectral shape on the IG wave characteristics. The observation results indicate that IG waves are primarily locally excited by incident swell wave energy, and accurate prediction of the IG wave height can be achieved using the parameter HS2Tm−1,0, which represents the short wave energy flux. Bound IG wave energy is much smaller than the free IG wave under conventional sea states but becomes significant under high-energy swell sea states.Higher levels of IG wave energy are found to be forced under narrower spectral shapes and smaller directional spreading short wave conditions, and these conditions typically correspond to stronger coupling strengths in the bound IG wave theory. Compared to the single-peaked spectra wave condition, the forced IG wave energy decreases under the double-peaked spectra wave condition. This is likely mainly because the latter tends to have a broader spectrum and a larger directional spreading. On the other hand, under the narrow-banded spectral short wave conditions, IG wave energy is inclined to be distributed over a lower and smaller frequency range. This is noteworthy because lower-frequency IG waves tend to be more difficult to break. In addition, the bound IG wave energy was found to increase at low tide, but for the intermediate water depths in which the observation site is located in this study, the influence of the small tidal range in the study area on the IG wave energy was not significant.

Variations in short-term statistics of waves observed in the coastal waters of the eastern Arabian Sea

Understanding the short-term statistics of waves is crucial for various marine applications. Present study investigates variations in the short-term statistics of waves in the eastern Arabian Sea, at a water depth of 15 m utilizing 172,704 30-min-long sea state records from 2010 to 2020. The maximum recorded individual wave height during the study period is 8.3 m. Individual wave period, height, length and steepness are higher during the monsoon (June–August). Consequently, the number of waves is lower than it is in the non-monsoon. Moreover, there are high energy swells during high wave events, and during the non-monsoon, the wave spectrum is multi-peaked, with low-frequency swells and high-frequency wind-seas. The crest height of the largest wave in different years during the study period is 0.57–0.60 times the wave height. The characteristic wave height, H1/10, is 1.24 times the significant wave height, and the maximum wave height is 1.6 times the significant wave height. Significant inter-annual variability exists in the exceedance probabilities of waves with higher heights. For the extreme sea state, the probability of the exceedance of normalized wave heights is in good agreement with the Rayleigh, Forristall and Van Vledder distributions. However, the crest heights only follow the Rayleigh distribution.

Short-term patterns and processes of coastal cliff erosion in Santa Barbara, California

Sequential terrestrial LiDAR scans and 3D volumetric change were used to document coastal cliff erosion at several sites along the coast of Santa Barbara, California. Retreat rates along the base, middle, and cliff top were calculated for six different scans throughout the El Niño of 2015–16 to quantitatively link wave action to cliff base erosion, and two seasonal scans were taken during 2017 to investigate patterns in erosion and retreat rates. Retreat rates along the cliff base, middle, and top converged over the course of the study period and observations and patterns in erosion help characterize the fundamental mechanisms that govern the retreat of soft shale cliffs. The base and middle of the cliff preferentially eroded during high-energy swell events and are strongly coupled, whereas cliff top erosion is temporally variable and retreat rates increased with higher seasonal rainfall totals. We observe that the cliff base experiences erosion from both abrasion and mechanical wave action, but the dominant processes is segregated by elevation above the cliff toe. Abrasion was found to be the dominant processes at elevations up to ~0.8 m above the cliff toe, whereas block removal from mechanical wave action was dominant at elevations above 0.8 m. Threshold behavior was observed for cliff base retreat rates, and Spearman correlation coefficients suggest strong relationships with the sum of the wave energy flux imparted to the cliff base (R2 = 0.84), the sum of the wave energy flux >35,000 W/m (R2 = 0.82), and the average total water level above the cliff toe (R2 = 0.85). Threshold behavior and maximum retreat rates occurred when average total water levels were >0.8 m above the cliff toe. Seasonal Schmidt-hammer measurements suggest that energy thresholds are modulated by weathering rates which remain poorly understood. These thresholds, when exceeded, produced the highest cliff base retreat rates during the study period and reinforce the expectation that coastal cliff retreat rates will increase with rising sea levels.

Morphological changes in a cuspate sandy beach under persistent high-energy swells: Reñaca Beach (Chile)

The present work proposes a morphological evolution model for sandy cuspate beaches, which depends on two parameters, swash excursion (S) and offshore wave steepness (H0/L0). The model was derived from observations of morphology and wave climate at Reñaca Beach in central Chile, comprising in-situ instrumentation, topographic surveys, wave modeling, and optical imagery. It was found that wave steepness alone was unable to explain the morphological changes observed on the beach. Rather, results indicate that swash excursion is better to distinguish between erosive and accretionary events, with swash excursion exceeding 29.5 m for more than 12 h being indicative of erosive events. While morphological changes are led by S, wave steepness correlates with the erosive potential of waves, especially when H0/L0 is greater than 0.017. Although derived for a specific beach under a limited range of conditions, this work adds to the understanding of the dynamic behavior of beaches exposed to persistent high-energy swells.

EXTRATROPICAL STORM GENERATED SWELL INDUCED VULNERABILITY EFFECT ON A TROPICAL ISLAND

The high-energy swells in the Northern Pacific are generally due to extratropical cyclones during the winter season. Conserving most of their energy while traveling long distances, strong swells increase the susceptibility of coastal zones of remote islands by increasing their vulnerability. Understanding the vulnerability caused by distant source generated swells allows adaptive attempts to be taken to protect natural, social, cultural and economic assets. The extratropical cyclone trends and its relation to swells requires a need to quantify its effect on island coastlines. The vulnerability is a complex phenomenon that demands multidisciplinary approaches, methods, and data sources to estimates of the impacts objectively. The main problem when it comes to defining vulnerability of small island communities is that even the unforeseen elements affect their susceptibility because they are more prone to the impacts of climatic forcing. Therefore, in this study, we extend an existing approach to quantify how extratropical storm generated swells affect the vulnerability of remotely located tropical islands.

Observations of nearshore infragravity wave dynamics under high energy swell and wind-wave conditions

Infragravity waves (0.005–0.04Hz) can dominate the water motion close to shore on low sloping beaches and play a significant role in beach and dune erosion. A new field data set of water surface elevation at 15 cross-shore locations on a dissipative, fetch-unlimited beach is analysed to investigate the forcing and surf zone behaviour of infragravity waves during a wide range of offshore wave conditions (Ho=0.38–3.88m; Tp=6–20s). Infragravity waves approach the shore as bound waves lagging slightly (~4s) behind the short wave (0.04–0.33Hz) envelope and are released in the surf zone as free waves. Infragravity wave heights of up to 1m are measured close to shore and are best predicted using an offshore forcing parameter that represents the short wave energy flux (Ho2Tp). Considerable infragravity dissipation is observed in the surf zone and dissipation increases with offshore wave energy. Dissipation is highly frequency-dependant and a frequency-domain Complex Empirical Orthogonal Function analysis reveals (quasi-)standing waves at frequencies <0.017Hz, but an increasingly progressive wave pattern at higher frequencies with reflection coefficients <0.1, indicative of more than 90% dissipation. Much of the observed dissipation occurs very close to shore and the dependence of the reflection coefficient on a normalised bed slope parameter implies that energy at high infragravity frequencies is dissipated by wave breaking, since these frequencies fit into a mild sloping regime. This is supported by the results of bispectral analysis which show predominantly infragravity-infragravity interactions in shallow water and the development of infragravity harmonics indicative of steepening and eventual breaking of the infragravity waves.

The influence of fluvial dynamics and North Atlantic swells on the beach habitat of leatherback turtles at Grande Riviere Trinidad

Grande Riviere beach, located on the north coast of Trinidad, West Indies, is internationally recognised as a critical habitat/nesting ground for the endangered leatherback turtles (Dermochelys coriacea). Episodic extreme flooding of the Grande Riviere River led to the shifting of the river mouth and resulted in backshore beach erosion, with the most recent recorded event occurring in 2012. Following this event, the construction of a sand dam to arrest further erosion which threatened coastal infrastructure, precipitated a host of new problems ranging from beach instability to public health threats. In January 2013, high energy swell waves naturally in-filled the erosion channel, and the beach recovery continued over the successive months, thereby rendering the intervention in the previous year questionable. This paper presents a geomorphological analysis of beach dynamics for Grande Riviere, within the context of this erosion event. Data on beach profiles, sediment and coastal processes were collected using standard geomorphological techniques. Beach topographic analysis and water quality tests on impounded water in the erosion channel were conducted. Results indicate that the event created an erosion channel of 4843.42 m3 over a contiguous area of 2794.25 m2. While swell waves were able to naturally infill the channel, they also eroded 17,762 m3 of sand overall across the beach. Water quality tests revealed that the impounded water was classified as a pollutant, and created challenges for remediation. Hydrologic and coastal geomorphologic interplay is responsible for the existence and sustainability of this coastal system. It is also evident that the beach system is able to recover naturally following extreme events. Our results demonstrate that effective and integrated management of such critical habitats remains dependent upon continuous monitoring data which should be used to inform policy and decision making.

Longshore transport estimation and inter-annual variability at a high-energy dissipative beach: St. Trojan beach, SW Oléron Island, France

This study investigates the annual and inter-annual longshore transport at a high-energy dissipative beach (St. Trojan, SW Oléron Island, France), over the period 1997–2006. This study is divided in two parts: (1) a short-term study, based on field measurements carried out during, low, moderate and high-energy swell conditions, and permitting to calibrate a wave propagation model and both empirical longshore sediment transport formulas and a hydro-sedimentary numerical model; and (2) a long-term study, combining global optimization methods using WAVEWATCH III (WW3) model outputs and wave numerical modelling to define the nearshore wave climate and then to compute the annual and inter-annual longshore transport. A good agreement between the different methods was obtained and values for the annual longshore transport ranged from about 50,000±20,000 m 3/yr to 140,000±30,000 m 3/yr. The net annual longshore transport displayed an inter-annual variability, whereas values were always 3–10 times less than those proposed in previous studies. Such differences point out the necessity of calibrating empirical transport formulas and computing accurately the nearshore wave climate. The relatively low values for the net annual longshore transport were explained by a weak contribution of the most energetic swells, which systematically approach the shore with a frontal obliquity and also generate episodes of reverse transport. The methodology itself was also discussed, pointing out the limitations of this study but also the advantages of using WW3 data and simplified wave climates.

Wave-Sediment Interactions on an High Energy Beach System

Please click here to download the map associated with this article. The North coast of Northern Ireland is exposed to high-energy swells and frequent storm events and therefore beach systems found along this coast tend largely to be dissipative in character. Bedrock-framed coastlines formed by basalt cliffs, shore platforms and smaller sandy embayments dominate this study area. West Strand beach at Portrush (located 55°12′N/6°40′W) is a high-energy system shaped by a headland-embayment with a sandy beach to the north and boulder beach to the south. It is characterised by a microtidal regime and predominant local winds are west and south westerly. The research work aims to examine this high-energy beach under multiple storm scenarios and to investigate hydrodynamic and morphodynamic behaviour at the site. For the analysis, a wave propagation model was used to compare wave energy dissipation from storm wave events as well as examining currents and sediment transport through the analysis of the spatial distribution of radiation stress. Real data from deep-water wave parameters (significant wave height, period and direction), wind direction and speed, and recent high-resolution bathymetry data were used as an input to the propagation model. The methodology was applied to West Strand beach (9 km2) at a scale of 1:250.

Identifying storm impacts on an embayed, high-energy coastline: examples from western Ireland

Large sections of the western Irish coast are characterised by a highly compartmentalised series of headland-embayment cells in which sand and gravel beaches are backed by large vegetated dune systems. Exposure to modally high-energy swell renders most of these beaches dissipative in character. A mesotidal range (c. 3.5–4.5 m) exists along much of the coast. Analysis of instrumental wind records from three locations permitted the identification of a variety of storm types and the construction of storm catalogues. Few individual storms were recorded at all three stations indicating a lack of regional consistency in storm record. Of the total storms recorded, only a small percentage are potentially damaging (onshore directed) and even fewer span a high tide and thus potentially induce a measurable morphological response at the coast. Through a combination of historical records, meteorological records, field observations and wave modelling we attempt to assess the impact of storms. Quantifiable records of coastal morphology (maps, air photos and beach profiles) are few in number and do not generally record responses that may be definitely attributed to specific storms. Numerical wave simulations and observations at a variety of sites on the west Irish coast, however, provide insights into instantaneous and medium term (decadal) storm responses in such systems. We argue that beaches and dunes that are attuned to modally high-energy regimes require extreme storms to cause significant morphological impact. The varying orientation of beaches, a spatially nonuniform storm catalogue and the need for a storm to occur at high water to produce measurable change, impart site-specific storm susceptibility to these embayments. Furthermore, we argue that long-period wave energy attenuation across dissipative shorefaces and beaches reduces coastal response to distant storms whereas short-period, locally generated wind waves are more likely to cause major dune and beach erosion as they arrive at the shoreline unrefracted. This apparently variable response of beach and dune systems to storm forcing at a decadal scale over a coastline length of 200 km urges caution in generalising regarding regional-scale coastal responses to climatic change.

Long–wave forcing by the breaking of random gravity waves on a beach

This paper presents new laboratory data on long-wave (surf-beat) forcing by the random breaking of shorter gravity water waves on a plane beach. The data include incident and outgoing wave amplitudes, together with shoreline oscillation amplitudes at long-wave frequencies, from which the correlation between forced long waves and short-wave groups is examined. A detailed analysis of the cross-shore structure of the long-wave motion is presented, and the observations are critically compared with existing theories for two-dimensional surf-beat generation. The surf beat shows a strong dependency on normalized surf-zone width, consistent with long-wave forcing by a time-varying breakpoint, with little evidence of the release and reflection of incident bound long waves for the random-wave simulations considered. The seaward-propagating long waves show a positive correlation with incident short-wave groups and are linearly dependent on short-wave amplitude. The phase relationship between the incident bound long waves and radiated free long waves is also consistent with breakpoint forcing. In combination with previous work, the present data suggest that the breakpoint variability may be the dominant forcing mechanism during conditions with steep incident short waves.

Infragravity-Frequency (0.005–0.05 Hz) Motions on the Shelf. Part I: Forced Waves

Abstract In Part I, the energy levels of ocean surface waves at infragravity frequencies (nominally 0.005–0.05 Hz) locally forced by swell in 13-m water depth were shown to be predicted accurately by second-order nonlinear wave theory. However, forced infragravity waves were consistently much less energetic than free infragravity waves. Here, in Part II, observations in depths between 8 and 204 m, on Atlantic and Pacific shelves, are used to investigate the sources and variability of free infragravity wave energy. Both free and forced infragravity energy levels generally increase with increasing swell energy and decreasing water depth, but their dependencies are markedly different. Although free waves usually dominate the infragravity frequency band, forced waves contribute a significant fraction of the total infragravity energy with high energy swell and/or in very shallow water. The observed h−1 variation of free infragravity energy with increasing water depth h is stronger than the h−1/2 dependence pre...

A system for measuring bottom profile, waves and currents in the high-energy nearshore environment

A new data-acquisition system capable of measuring waves, currents and the nearshore profile in breaking waves as high as 5 m has been developed and successfully field-tested. Components of the mechanical system are a sled carrying a vertical mast, a double-drum winch placed landward of the beach, and a line that runs from one drum of the winch around three blocks, which are the corners of a right triangle, to the other drum of the winch. The sled is attached to the shore-normal side of the triangular line arrangement and is pulled offshore by one drum of the winch and onshore by the other. The profile is measured as the sled is towed along the shore-normal transect using an infrared rangefinder mounted landward of the winch and optical prisms mounted on top of the sled's mast. A pressure sensor and two-axis electromagnetic current meter are mounted on the frame of the sled. These data are encoded on the sled and telemetered to a receiving/recording station onshore. Preliminary results suggest that near-bottom offshore-flowing currents during periods of high-energy swell are important in forcing changes to the configuration of the nearshore profile.

Factors controlling shelf sediment dispersal along the southeast African continental margin

Four major factors control sediment dispersal along the southeast African continental margin: (1) morphology of the upper continental margin; (2) wave regime and wind-driven circulation; (3) influence of the Agulhas Current; and (4) sediment supply. These factors interact on various scales to produce a unique regional sediment dispersal pattern. The continental shelf is very narrow and the adjacent slope extremely steep. The shelf platform is interrupted by a number of structural offsets, and locally the shelf break is dissected by numerous submarine canyons. The nearshore zone down to about 60 m waterdepth is dominated by a high-energy swell regime, and in most places is underlain by a thin wedge of sandy sediment. This wedge appears to have achieved dynamic equilibrium with the prevailing energy regime, and additional sediment is rapidly dispersed and fed into a sandstream, situated on the central shelf, where the Agulhas Current dominates sediment transport. Along some parts of the coast the sediment wedge is dammed behind a drowned coastal dune ridge. Within the sandstream, sediment is transported alongshelf until a structural offset is reached, where sediment spills over the shelf break. Four major localities for off-shelf transport are recognized between Maputo and Port Elizabeth. Most of the suspended load appears to be expelled from the system without being deposited on the shelf. In the lee of each offset a clockwise gyre is developed, which produces return-flow currents capable of moving sediment northward, thereby creating bedload partings. In the vicinity of these partings, depositional features are observed that are similar to those normally associated with tidal regimes, e.g., bedforms migrating in opposite directions. Most of the bedload sediment entering the system is eventually deposited in well-defined sinks situated in the lee of structural offsets. This model applies only to inter-glacial high sea-level situations accompanied by strong western boundary flow of the Agulhas Current.

Morphodynamics of reflective and dissipative beach and inshore systems: Southeastern Australia

Field experiments involving direct measurement of surf and inshore current spectra, inshore circulation patterns, and depositional morphology have been replicated under different energy conditions and in several environmentally contrasting beach localities on the high-energy coast of New South Wales, Australia. The region exhibits compartmentalized beach systems and is dominated by a highly variable wind-wave climate superimposed on persistent high-energy swell ( T = 10–14 sec). Two general types of beach system occur: (1) predominantly reflective systems in which much of the incidentwave energy is reflected from the beach face; and (2) dissipative systems with wide surf zones and high turbulent energy dissipation. Reflective systems are characterized by steep, linear beach faces, well-developed berms and beach cusps, and surging breakers with high runup and minimum setup; rip cells and associated three-dimensional inshore topography are absent. Wave height and current spectra from reflective beaches consistently have their dominant peaks at incident wave and subharmonic frequencies, and cross-spectra indicate the existence of low-mode edge waves at those frequencies. Infra-gravity peaks are negligible. Under low-energy conditions subharmonic peaks are low relative to incident-wave peaks; however, increasing breaker height tends to be accompanied by increasing subharmonic dominance. Analyses of shore-normal currents near the bed show that under all conditions the strongest shoreward motions are induced by the incident waves; however, seaward motion near the beach face is subharmonic-dominated. Dissipative systems characterize the exposed open coast and are fronted by concaveupward nearshore (seaward of break) profiles and wide flat surf zones. Waves break 75–300 m seaward of the beach and dissipate much of their energy before reaching the beach, creating significant radiation-stress gradients and setup. Topography is much more complex and varied than in the case of reflective beaches; one or more bars, three-dimensional inshore topography and different scales of rip cells are normally present. The commonly occurring time-and-environment-dependent morphologic states can be classified into six general types. The greatest total dissipation is associated with Type 1 which prevails in the regions of most abundant inshore sediments or during and immediately after severe storms. Setup is highest and runup is lowest (relative to incident-wave height) with this type and the dominant energy near the beach is in the surf-beat part (80–120 sec) of the spectrum. As the bar migrates shoreward (Types 2 and 3) and beach face steepens and a deep trough develops within which the partially dissipated waves reform. Although the outer surf zone remains dissipative, synchronous and subharmonic resonance occurs near the beach face and, as with reflective beaches, low-mode edge waves form beach cusps. A conspicuous feature of Types 2 and 3 is the occurrence of pronounced resonant spectral peaks at 4 T ( ⋍ 40–50 sec ) within the trough and on the bar. Edge waves at this frequency may be responsible for the development of the crescentic bar forms (Type 3). Lower frequency surf beat peaks also remain present but are secondary. The peak at 4 T attenuates as Type 4 develops and is not present with Type 5 topography but the first subharmonic (2 T) becomes more pronounced, particularly with high tide. Type 6 morphology represents the fully accreted beach state and occurs only after prolonged periods of low swell. This type is a reflective beach with a steep linear beach face and a very high berm which remains continuous for long distances alongshore; rips are always absent. Wave and current spectra are also similar to those described for reflective beaches.