Coastal Barrier System Research Articles

Late Holocene evolution of a coastal barrier system at Bilene, Mozambique

AbstractThis research attempts to provide a contribution to the study on coastal barrier‐lagoons in the SE Africa generated by sea‐level fluctuations, and climate change during the Late Holocene. To achieve this goal, the grain‐size analysis and morphoscopic description helped to detect cyclic facie sequences with sediments sized over 200 μm mixed with silt, bioclasts and organic matter. Moreover, the magnetic susceptibility and X‐ray diffraction (XRD) analyses displayed over 90% of quartz and isolated mica, feldspar and heavy minerals. Additionally, the X‐ray fluorescence (XRF) results display the interplay between terrigenous (>Ti and Al) and marine influence (S, Cl and ~Ca) over the last ~3,500 cal. yrs. BP. The geochemical tracer fluctuations detected suggest five distinct deposits including beach, washover, lacustrine system, aeolian deflation that rest on top of the Pleistocene palaeodunes, which are tentatively associated with the sea‐level variations, extreme marine events or with local floods. Although the multiproxy data record refers to a study in Bilene‐Mozambique, it is interesting to understand the mechanisms and variables related to the evolutionary dynamics of coastal barrier lagoons generated by sea‐level fluctuations and climate change, especially in the SE Africa.

A PATH TOWARDS HOLISTIC COASTAL ZONE MANAGEMENT IN TEXAS LEVERAGING SIGNIFICANT INVESTMENTS IN DATA COLLECTION, PLANNING, AND IMPLEMENTATION

The Gulf of Mexico shorelines of Texas are in a largely regressive state due to an amalgamation of factors including but not limited to: sea level rise, subsidence, littoral drift, and riverine supply disruptions. In some areas the erosion rate is several meters annually, leading to significant loss of coastal barrier island area. The bulk of the coastal barrier systems in Texas is undeveloped. However, barrier island loss is no longer able to go unnoticed as there is ever-growing recognition of their intrinsic value to coastal habitat and economies and their role in coastal storm resilience. Projects with significant merit and sediment needs are being identified for funding, which could result in a firstcome- first-serve extraction of the easily available and less costly sediment resources. Without proper management, the complexity and cost of future projects will naturally escalate, leading to a reduction in investments due to lower returns. The following questions need to be addressed: how can lower returns be avoided to incentivize investment and maximize coastal restoration efforts in Texas? And what can be done to galvanize support for beneficial use (BU) placement of beach-quality dredge material to minimize its removal from the littoral system? What can be done now to inform and leverage the existing and future knowledge so that management decisions lead to better outcomes for the environment and Texas constituents? A well thought out and executed plan maximizing returns and coastal resilience will enable other regions to more easily replicate the success.

Coastal erosion in temperate barriers: an anthropized sandy beach in Buenos Aires, Argentina

Coastal erosion is a common issue in urbanized areas located on sandy barriers in Latin America, making them highly vulnerable to both natural and human induced disasters. This pose a significant threat to the long-term sustainability of these coastal systems. The main objective of this study was to evaluate the medium-term shoreline evolution of a highly-anthropized beach sector located within a coastal barrier system of Buenos Aires, Argentina. For this purpose, a geospatial analysis based on aerial imagery and high-resolution satellite data was conducted for the 1965–2021 period. Additionally, the study aimed to identify critical erosive sectors and to provide estimations for the near-future shoreline evolution of these beaches. The results showed contrasting patterns, with sectors presenting shoreline retreat rates up to 0.7 m/yr while others demonstrated accretion rates exceeding 0.6 m/yr. Despite these along-shore variations in erosion/accretion balances, more than 70% of the shoreline presented an erosional trend for the studied period. This led to the identification of an erosional hot spot covering 30% of the urbanized waterfront. Furthermore, the 2030 forecast suggests that a highly urbanized beach sector could suffer a shoreline retreat of about 20 m. These results could be directly linked to the historical land use/land cover changes, mainly related to dune stabilization processes and to the implantation of urban surfaces. At this point, erosive patterns could be mainly caused by local cross-shore imbalances due to these anthropogenic disturbances. It is expected that the results of the present study will serve as a basis for coastal management policies, and the proposed method will be a useful tool for assessing coastal erosion hazard.

Eco-morphological evolution of the Bolivar Peninsula (Texas, U.S.A.) during the last 2,000 years: A multi-proxy record of coastal environmental changes

In the face of rapidly changing climate and increasing natural and anthropogenic disturbances, the long-term dynamics of coastal ecosystems in relation to hurricanes and human activities has become a focus of research for ecologists and geomorphologists across the Gulf of Mexico. This study integrates diatom, remote sensing, geochemical, and hydrodynamic analyses to document the ecological and morphological development of the barrier-island and estuary-wetland complex bordering Galveston Bay, Texas, from the late Holocene to the Anthropocene. The multi-proxy dataset revealed two major ecological shifts between 1320 and 840 cal yr BP and between 230 cal yr BP and 2008 AD in Galveston Bay. The first shift was driven by the landfall of a backdoor hurricane at ∼1000 cal yr BP in combination with the aperiodic and intense long-shore current that caused the barrier complex to retreat over fluvial deposits while the Bolivar Peninsula was growing toward the west. These morphological changes resulted in a narrower inlet, hence, restricting the marine influence from entering the bay and converting Galveston Bay to a wave-dominated estuary that was shallower and fresher. The second shift was caused by increasing modification to the waterway and infrastructures since 1800 AD. As a result, rapid sediment accretion was induced near the North Jetty, forming the current Bolivar Flats and setting the stage for mangrove colonization. The warming climate and hurricanes in recent decades acted as catalysts that accelerated mangrove expansion on the Bolivar Peninsula. Our results imply that the proposed coastal barrier system will likely increase the sediment deficit, alter the biogeochemical environment, and impact the primary productivity in the bay. However, on a regional scale, more and more mangrove stands are expected to appear around estuaries and lagoons, and along man-made jetties and barriers along the Gulf.

Vegetation Reconfigures Barrier Coasts and Affects Tidal Basin Infilling Under Sea Level Rise

AbstractWorldwide, many tidal basins associated with barrier coasts have infilled over the past millennia due to the combination of sediment supply, wave‐tidal sediment transport, and eco‐engineering effects of vegetation. However, the biogeomorphological interactions between saltmarsh and the morphodynamics of an entire coastal barrier system are poorly understood, especially under sea level rise (SLR). Here, we study the evolution of a barrier coast for combinations of mud availability, presence of vegetation, and SLR. We developed a novel biogeomorphological model of an idealized barrier coast enclosing a tidal basin with sandy‐clayey sediments that was subjected to tides and waves for a century. The morphodynamic Delft3D model was coupled to a vegetation code which accounts for the dynamics of marsh‐type vegetation. Initially, vegetation contributed to reducing the tidal prism while sediment was imported. However, with SLR this trend was reversed and the tidal basins started to export sediment for vegetated runs after about 50–60 years while the unvegetated scenarios continued to infill in pace with the SLR. The sediment export was caused by cascading biomorphodynamic feedback effects triggered by vegetation which modified channel and shoal dynamics. Even under higher mud supply, the SLR resulted in vegetation collapse. The hypsometries, similar to natural systems, showed that vegetated systems converge to an alternative stable state condition. We conclude that the long‐term resilience of the tidal basin associated with sediment infilling under SLR can be reduced by cascading large‐scale effects of vegetation on the morphodynamics of barrier coasts.

Volume Estimation From Planform Characteristics of Washover Morphology

AbstractOverwash is the cross‐shore transport of water and sediment from a waterbody over the crest of a sand or gravel barrier beach, and washover is the resulting sedimentary deposit. Washover volume, and alongshore patterns of washover distribution, are fundamental components of sediment budgets for low‐lying coastal barrier systems. Accurate sediment budgets are essential to forecasting barrier system sustainability under future climate‐driven forcing. However, comprehensive surveys of three‐dimensional washover morphology are challenging to deliver. Here, we use the results of a physical experiment, analysis of lidar data, and examples of washover characteristics reported in the literature to develop scaling relationships for washover morphometry that demonstrate volume can be reasonably inferred from planform measurements, for washover in natural (non‐built) and built barrier settings. Gaining three‐dimensional insight into washover deposits from two‐dimensional information unlocks the ability to analyze past aerial imagery and estimate contributions from washover flux to sediment budgets for past storms.

Natural and nature based features for environmental enhancement and coastal storm risk management: a case study on Marco Island, Florida, United States

Natural and Nature-Based systems provide an opportunity for adaptive response to Coastal Storm Risk Management and Sea Level Rise. The Tigertail Lagoon/Sand Dollar Island Restoration on Marco Island, Florida, presents a case study designed to maintain and enhance an existing coastal barrier system consisting of a 3-km-long sand spit and tidal lagoon ecosystem that is otherwise evolving toward closure. The case study is part of a nature-based adaptive management plan to restore and stabilize the sandspit and tidal lagoon through cyclic use of sediment within the system. This approach seeks to preserve existing protective habitats and landforms that also serve as natural coastal barriers to protect upland development. Design of the restoration plan considers the functions of a wildlife nature preserve and evolution of complex tidal inlet morphologic features bordered by a heavily developed barrier Island. The design aims to restore and enhance a sandspit degraded by a sequence of storms since Major Hurricane Irma impacted Southwest Florida in 2017 and improve the existing deteriorated habitat by enhancing tidal exchange through restoration of the lagoon flow channel. Total wetland area will be increased by relocating the sand spit seaward of its present location to where it was located in approximately 2017. The reconstructed beach berm will provide enhanced resiliency to high frequency weather events. Sediment will be sourced from the existing sand spit and an innovative sand trap that will maintain the lagoon entrance open while providing beneficial re-use for excess sediment that continues to accumulate at the end of the spit. Components of the project were analyzed using existing engineering models and methods such as the Coastal Modeling System (CMS) and XBeach. Enhancing and preserving this barrier island feature and productive ecosystem provides an example of the enhanced coastal resiliency provided by natural and nature-based systems that are adaptable and responsive to sea level rise and ongoing coastal processes.

Evolution of an estuarine valley to barrier-lagoon system promoted by tidal prism change during a transgressive event (Lower Permian, Paraná Basin)

In modern marine marginal environments, the coastal morphology directly influences the tidal and wave processes. In this paper we present an example of an ancient coastal zone where the change in coastal morphology controlled the tidal range. This study focuses on the Permian Rio Bonito Formation in the southern Paraná Basin, where the major coal deposits of South America are located. Sedimentological and stratigraphical approaches were applied to cores drilled in the Candiota paleovalley area. We describe the stratigraphic architecture of a transgressive event of the Lower Permian (Rio Bonito Formation). Two third order depositional sequences were identified, which show differences in the paleoenvironmental record. In the lower sequence, associated with the infilling of an incised valley, facies related to fluvial and tidal currents are dominant. In a micro-tidal inner sea context, tidal range is amplified in a funnel-shaped valley. In the upper sequence, already evolving in linear coastal conditions, evidence of tidal depositional processes almost disappear, and the sedimentation becomes dominated by wave processes, thus a coastal barrier system associated with lagoons and swamps is established. Our study, besides detailing the paleoenvironmental evolution of the sedimentary infilling of an incised valley, also provides for a better understanding of the origin of southern Brazil coal.

Self‐Organization of Coastal Barrier Systems During the Holocene

AbstractUnderstanding the response of coastal barrier systems to sea level rise is a crucial societal need. Despite the problem having been studied extensively, major knowledge gaps remain. For example, neither the sedimentary record nor existing numerical models have been conclusive in explaining the formation of barrier islands. Here I present a comprehensive 2D model that seamlessly couples cross‐shore and along‐shore transport, tidal transport, storm surges, and wind waves, and use it to simulate an idealized passive margin during the last 7,000 years. In the early Holocene, when sea level was rising ∼20 mm/yr, shoals and ephemeral barrier islands formed, periodically drowned, and then formed again at a landward location. Shoal emergence was triggered by the disequilibrium of the recently submerged shelf, especially for large waves and mild shelf slopes. About 5,000 years ago, as sea level rise slowed down to ∼1 mm/yr, barriers stabilized and even prograded seaward. The combination of excess sediment in the nearshore and storm surges allowed barriers to accrete above mean high water. When barriers eventually equilibrated to the new sea level rise rate and started to retreat, their retreat rate was highly variable in space and time due to autogenic processes such as inlet formation and backbarrier channel interception. This variability also included multi‐decadal periods of localized progradation. Both lag dynamics and autogenic processes confound the relationship between barrier retreat and sea level rise rate.

Late Holocene evolution of São Tomé cape (Rio de Janeiro, Brazil): Insights from geomorphological, geophysical and geochronological data

At wave-dominated coasts it is common to find coastal barriers that might be in a progradational pattern and can compose sedimentary capes. Those prograding coastal barriers can be characterized by beach ridges or barrier spits, or even by a more complex pattern depending on the processes controlling their formation. In this study geomorphological mapping, topographic surveys, ground-penetrating radar profiles and optically stimulated luminescence (OSL) dating were performed on the coastal barrier systems of São Tomé cape (north of Rio de Janeiro state, Brazil). The aim was to reconstruct its spatial and temporal evolution and, by this, better understand the interplay between sedimentary cape formation and its driving mechanisms as well as their function as potential archives for long-term environmental dynamics. The study site is characterized by lagoonal and fluvio-lagoonal depressions, and a lagoon surrounded by beach ridges that are interleaved with barrier spits and erosional truncations. Both types of coastal barriers reach up to 6 m above sea-level. Shallow geophysical analysis identified seven radar facies that characterize the stratigraphy of the targeted morphologies. Beach ridges are mainly composed by planar, seaward dipping, subparallel, continuous reflectors which represents their progradation, while barrier spits have planar to convex up, parallel, landward migrating reflectors. Geochronological results demonstrate that the coastal barriers were formed during the Late Holocene. Beach ridge formation until ~2700 years ago was interrupted by the formation of a barrier spit system between ~2300 and ~2000 years ago. This barrier spit system was responsible for the formation of an enclosed lagoon, the Salgada lagoon, and was truncated by a set of beach ridges formed since ~2000 years ago and continuing to the present shoreline. The different types of coastal barriers indicate a change in hydrodynamic processes during the Late Holocene, with an alternation between the predominance of wave-induced cross-shore transport, responsible for the formation of beach ridges, and the predominance of longshore transport, responsible for the formation of barrier spits. Besides that, erosional truncations and unconformities indicate the interruption of shoreline progradation and the impact of erosion due to the storms. This illustrates the complex and highly dynamic development of São Tomé Cape with wave climate and storms as its most important drivers. More generally, this study shows that sedimentary capes may be regarded as geoarchives with potential for the reconstruction of coastal processes over Holocene timescales.

ESTRATIGRAFIA E EVOLUÇÃO DE UM ESPORÃO ARENOSO EM AMBIENTE MACROMARÉ: PONTA DA AREIA - SÃO LUÍS - MARANHÃO - BRASIL

Na Ilha do Maranhão os sistemas de barreiras costeiras podem ser divididos em dois morfotipos principais: (i) Barreiras de Praias Anexadas, que se caracterizam por uma praia anexada transversalmente a topografia antecedente exibindo um gradiente íngreme onde há perda contínua de areia na costa erodida; e (ii) Barreiras de Esporões Arenosos ligadas a Cabeços de Promontórios, que se caracterizam por um sistema laguna-barreira-esporão ancorado longitudinalmente à topografia antecedente. Este último morfotipo origina-se de um amplo transporte de sedimentos via deriva litorânea, cujo prolongamento é geralmente interrompido pelo efeito espigão hidráulico, junto a elevados prismas de maré nas desembocaduras fluviais. O presente estudo utiliza-se de sondagens do tipo percussão e vibrocore para recuperação do registro sedimentar, o qual é descrito quanto à sua compactação, textura, estruturas e fácies sedimentares. Os resultados obtidos demonstram a natureza estratigráfica da barreira costeira na Praia da Ponta da Areia como uma sequência retrogradacional caracterizada pela migração em direção ao continente de um esporão arenoso. A datação radiométrica de sedimentos lagunares aflorantes na praia atestou a idade de 7.240±30 anos AP indicando que neste tempo, este sistema laguna-barreira-esporão estendia-se mar adentro. Recentemente a construção de um espigão costeiro alterou esta natureza estratigráfica, passando a comportar um sistema progradacional nas imediações do espigão, onde ambientes distais (antepraia) são sotopostos por ambientes proximais (foreshore).

Transgressive Architecture of Coastal Barrier Systems in the Ofanto Incised Valley and Its Surrounding Shelf in Response to Stepped Sea-Level Rise

Coastal deposits/barriers react to sea-level rise through rollover or overstepping. Preserved coastal deposits/barriers allow us to examine coastal responses to sea-level rise, an important aspect within the context of climate change. This study identifies the Ofanto incised valley and examines the possible factors that caused the considerable difference in shape between this valley and adjacent valleys: the Carapelle and Cervaro incised valley and Manfredonia incised valley. In addition, this study assesses the response of transgressive units to stepped sea-level rise with a focus on the evolution of palaeo-barriers/shorelines on the continental shelf and within the infill of Ofanto incised valley. We identified the traces of two slowstands in sea-level rise: the first, short-lived at a centennial scale, interrupted Meltwater Pulse 1A; the second is referable to part of Bølling-Allerød and Younger Dryas. During these two slowstands, two barrier-shoreface/estuarine-backbarrier systems formed. Meltwater Pulse 1A and Meltwater Pulse 1B led to overstepping and partial preservation of these systems in the form of aligned topographic highs. The second slowstand gave rise to continuous landward backstepping of the coastal barrier system; during the following Meltwater Pulse 1B (MWP-1B), landward rollover of the coeval barrier/backbarrier system occurred.

O SIGNIFICADO PALEOGEOGRÁFICO DAS TURFEIRAS COSTEIRAS DO RIO GRANDE DO SUL

Na Planície Costeira do Rio Grande do Sul (PCRGS) depósitos de turfas afloram ao longo do litoral indicando os setores onde as barreiras costeiras exibem uma natureza transgressiva. Ao longo das últimas décadas várias causas foram apontadas para explicar o processo erosivo nestes setores, todas associadas a fenômenos naturais. Estas turfas aflorantes são tidas como elemento chave no entendimento da evolução costeira de longo termo deste litoral. O processamento químico das amostras foi realizado a frio com HCl (5%) e KOH (5%) e separação entre partículas orgânicas e inorgânicas com solução aquosa de ZnCl2. Este estudo se baseou em amostras de turfas obtidas de sondagens geológicas, afloramentos e perfilagens com Georradar nas praias de Itapeva e Hermenegildo. Os resultados indicam que o estrato basal que recobre os sedimentos pleistocênicos é formado por turfas, sinalizando cronologicamente o início da sedimentação holocênica na PCRGS em torno de 10.000 anos AP, quando ainda prevaleciam condições essencialmente lacustres, nas posições que viriam a se desenvolver os sistemas de barreiras costeiras holocênicas. Uma vez instaladas, estas barreiras criaram condições para o desenvolvimento de uma segunda geração de ambientes paludiais, desta vez, vinculados à redução das profundidades dos sistemas lagunares em processo de colmatação, por volta de 710 anos AP. Este estudo fornece evidências de que a linha de costa atual da PCRGS encontra-se num processo contínuo de reorientação, conduzido por um déficit sedimentar nos setores costeiros onde as barreiras retrogradantes migram para o continente e no sentido N-NO, expondo depósitos turfáceos sistematicamente ao longo do litoral.

“Continuous” Backstepping of Holocene Coastal Barrier Systems into Incised Valleys: Insights from the Ofanto and Carapelle-Cervaro Valleys

Two recently recognised incised valleys in the Manfredonia Gulf are described. The first (CCV) is correlated with the current Carapelle and Cervaro streams. The second (OSFV) is correlated mostly with the current Ofanto River. Six seismic facies and seven unconformity-bounded seismic units have been identified, which infilled CCV and OSFV. In CCV, during the sea-level ranges from −29 to −18 and from −18 to −4.7 m b.s.l., two barrier/spit-backbarrier systems formed in the most landward sector of the valley. The lower system was attributed to a time interval between 9.2 ka BP and ca. 8.3 ka BP, chronologically constrained by the ZS2 borehole. In OSFV, during the sea level ranges from −39 to −29, and from −29 and to −18 m b.s.l., two beach/spit-backbarrier systems, arranged in a “continuous” landward backstepping pattern, formed. The phase that contributed most to the beach/spit-backbarrier systems formation is that which is coeval with the formation of the sapropel S1 in the Mediterranean. The conservation of barrier/spit-backbarrier systems arranged in a “continuous” landward backstepping pattern, is due to a strong and continued sediment supply that occurred during the sapropel S1 formation, coupled with low-gradient settings and a regime of slow sea-level rise.

Monitoring the Morphodynamic Cannibalization of the Rossbeigh Coastal Barrier and Dune System over a 19-Year Period (2001–2019)

This research presents a study on the morphodynamic evolution of the Rossbeigh coastal barrier and its dune system, located in Dingle Bay, County Kerry, Ireland. The study examines the evolution of the system over a 19-year period (2001–2019) through remote sensing, geographic information system (GIS) analysis, and field-based surveys. This research provides an ideal opportunity to examine a natural erosion event, referred to as cannibalization on a coastal barrier and its dune system. Since the beginning of this century, significant erosion has been visible on the coastal barrier, with the erosion eventually leading to a breaching event in the winter of 2008/2009. Over the study period, analysis has shown that the vegetated dunes decreased by more than 60 percent, the width of the breached area reached a maximum width of over 1 km and a change in orientation and appearance on the coastal barrier has been quantified. The analysis identifies a growing drift-aligned zone, contrasted with a reduction in the stable swash-aligned zone. Significantly, the point between these zones (i.e., the hinge point) has been shown to have moved by more than 1 km also. The migration of this hinge point and cannibalization of the dunes are illustrated. Finally, the potential mechanism for beach healing is identified, utilizing the rich datasets collected during the study, thus providing an insight into the long-term behavior of a dynamic coastal barrier system undergoing naturally driven cannibalization.

Application of multivariate statistical techniques in alongshore differentiation of coastal barriers

This paper investigates the existence of statistical correlations and relationships between geological and oceanographic parameters along a coastal barrier system in southern Brazil. The existence of few statistical studies on coastal barriers motivated the use of multivariate statistical techniques that take environmental parameters such as coastal morphological-sedimentary features and their morphometric measurements into account. The Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA) were used to assess the spatial variations of fourteen parameters driving the current and past behavior of this coastal barrier system at fourteen locations. Through the application of HCA, these locations were divided into four groups (Groups 1, 2, 3, and 4). The HCA results were analyzed using PCA. The first (PC1) and the second (PC2) principal components together explained at least 76.4% of the variability of all four groups. The most important geological and oceanographic parameters that were correlated to PC1 and PC2 were different in each group. In general, the shoreface slope and shoreface dimensions were common to all groups as PC1-correlated parameters, being the most important ones regarding data variability.In addition, for the progradational (Groups 1 and 4), retrogradational (Group 2) and aggradational barriers (Group 3) PCA indicated that shoreline orientation (wave exposure) and beach type are secondary, but relevant factors controlling the past and current behavior of coastal barriers in southern Brazil.

3D stratigraphic forward modeling of an ancient transgressive barrier system: A case study of accuracy and sensitivity

Accuracy of geological modeling and understanding of the controlling parameters in sedimentary systems evolution are challenges in both academy and industry. Uncertainty in the prediction of spatial and temporal facies distributions and difficulties in quantitative analysis of controlling parameters are current challenges. To increase geological model certainty and sedimentary process understanding, computational modeling has evolved exponentially during the last 50 years. In this study, 3D stratigraphic forward modeling technique is used to simulate an ancient transgressive barrier system of southern Brazil, deposited during the Lower Permian in the Paraná Basin, upper part of the Rio Bonito Formation. In the study area, the depositional systems are part of a large transgressive cycle that includes fluvial-estuarine facies at the base and barrier-lagoon deposits (associated with the development of salt marshes) at the top. Throughout the region, offshore strata cover the barrier-lagoon facies. The simulation successfully represented the retrograde evolution of the coastal barrier system, with longshore currents as the main transport mechanism of sediments. The evolution of the barrier system started with upper shoreface deposits overlying the lagoon system facies in the south. Later, lower shoreface sediments covered upper shoreface deposits. Then, shoreline migration continued landward. Finally, offshore transition deposits covered all previous deposits. Simulations and tests of controlling parameters allowed an adjustment to actual thickness and lithologies up to 83%. These simulations allowed for the analysis of model sensitivity, in particular to the influence of parameters that control sedimentary evolution. The main controlling parameter was sediment supply (volume and composition), followed by time step. Initial bathymetry and sediment velocity had less of an impact.

SURFACE-GROUNDWATER FLOW NUMERICAL MODEL FOR COASTAL BARRIER BEACH

Fine-grain beaches are relatively common where the beach forms part of a larger coastal barrier system. In such a coastal system, different water levels could be present in the seaward side and back-barrier lagoon. The water level changes in the steady-state lagoon may potentially induce groundwater dynamics near the beach which would subsequently affect seepage flow into (or out of) the beach. The exchange of water could generate varying hydrodynamical and morphodynamical behaviour at the seaward boundary. Hence, it is important to gain insight into groundwater flow dynamics which results in exchange of water between sea, barrier and lagoon; especially for coastal engineers responsible for planning and managing such a coastal environment. In present work, a numerical model is developed to simulate flows through a coastal barrier and it is validated against prototype-scale experimental results, which can then be extended to model small time-scale groundwater flows that may be too expensive or impractical to set up in a lab.

Rapid shoreline progradation followed by vertical foredune building at Pedro Beach, southeastern Australia

AbstractAt Pedro Beach on the southeastern coast of Australia a series of foredune ridges provides an opportunity to explore the morphodynamic paradigm as it applies to coastal barrier systems using optically stimulated luminescence (OSL) dating, ground penetrating radar (GPR) and airborne LiDAR topography. A series of sandy dune‐capped ridges, increasing in height seawards, formed fromc. 7000 years ago toc. 3900 years ago. During this time the shoreline straightened as the embayment filled and accommodation space for Holocene sediments diminished. Calculation of Holocene sediment accumulation above mean sea level utilising airborne LiDAR topography shows a decline in average sediment supply over this time period coupled with a decrease in shoreline progradation rate from 1.2 m/yr to 0.38 m/yr. The average ridge ‘exposure lifetime’ during this period increases resulting in higher ridges as dune‐forming processes have longer to operate. Increasing exposure to wave and wind energy also appears to have resulted in higher ridges as the sheltering effect of marginal headlands was diminished. An inherited disequilibrium shoreface profile will drive onshore accumulation of sandy sediments forming a prograded barrier; however, if there is no longer ‘accommodation space’ for sediment, this will be an overriding factor causing the cessation of progradation, as occurredc. 3900 years ago at Pedro Beach. Excess sediment in the nearshore zone after 3900 years ago may have been moved northward to nourish downdrift beaches in the compartment. A high outer foredune has formed through vertical accretion after 500 years ago, evidenced by GPR subsurface structures and OSL ages, with a distinct period of vertical and lee slope accretion and dated to the period 1890–1930 AD. The increased dune sediment transport resulting in foredune building is attributed to recent human disturbance. © 2018 John Wiley & Sons, Ltd.

Spatio-temporal geomorphological and ecological evolution of a transgressive dunefield system, Northern California, USA

Abstract A series of historic aerial photographs in addition to the U.S. Coast Survey 1870 map were used to document evolution of the coastal barrier and transgressive dunefield system at the Lanphere-Ma-le'l Dunes on Humboldt Bay, California, USA, between 1870 and 2016. Geomorphic units (shorelines, foredunes, nebkha, blowouts, parabolic dunes, deflation basins, and dunefields) as well as vegetation alliance were mapped in ArcGIS at approximately decadal intervals and compared among three subareas delineated based on large scale geomorphic differences that coincided with contrasting biological invasion and management histories. Sand inundation events in the 1930s and 1940s led to deterioration of the foredune and burial of deflation basins in the south, with the transgressive dunefield directly abutting the backshore in places. The transgressive dunefield in the north differentiated into parabolic dunes that decoupled from the foredune complex earlier than in the southern subareas, which have retained undifferentiated transgressive dunefields. Shoreline progradation increased dramatically following the historic 1964 flood, but exhibited a north-south gradient consistent with the Mad River to the north acting as the primary source of sediment, and effective north-south littoral drift. Biological invasions of Lupinus arboreus in the north and Ammophila arenaria in the south contributed to stabilization post-1965, but concomitant increases of native vegetation in other areas suggest these biotic processes were subordinate to other forcing factors. A notable switch from an erosional to a depositional system occurred coincident with the Pacific Decadal Oscillation (PDO) shift from a cool to a warm phase in the late 1970s, suggesting climate forcing as a major driver of dune evolution. Changes in the rate of vegetation stabilization per time interval post 1948 are correlated with change in mean monthly PDO index for the same intervals. Alongshore gradients of sediment availability and tectonic subsidence are likely superimposed on climatic controls. Stability of the dune system reached its maximum extent in 2000, despite erosional effects of the 1998–2000 La Nina event and invasive plant removal projects in the 1990s. Increased storminess (2015–2016 El Nino) resulted in decreased shoreline stability in the most recent interval, coincident with sharply declining rates of progradation and a reduction in mean monthly PDO index. This study provides insights into how transgressive dune phases can be formed, sustained and decoupled, as well as how they change their geomorphology over time. The absence of relict foredunes in the study area and elsewhere on the North Spit barrier suggests that the foredune-blowout-parabolic dune complex may build to quite large proportions for some time, and then be destroyed or destabilized to such a degree that the sediments comprising the complex are released to form a new dunefield phase. Ruptures along the Cascadia subduction zone are a likely mechanism for transgressive dunefield initiation, both in the past, and likely in the future.