Barrier Island Migration Research Articles

Predicting Barrier Island Shrub Presence Using Remote Sensing Products and Machine Learning Techniques

AbstractBarrier islands are highly dynamic coastal landforms that are economically, ecologically, and societally important. Woody vegetation located within barrier island interiors can alter patterns of overwash, leading to periods of periodic‐barrier island retreat. Due to the interplay between island interior vegetation and patterns of barrier island migration, it is critical to better understand the factors controlling the presence of woody vegetation on barrier islands. To provide new insight into this topic, we use remote sensing data collected by LiDAR, LANDSAT, and aerial photography to measure shrub presence, coastal dune metrics, and island characteristics (e.g., beach width, island width) for an undeveloped mixed‐energy barrier island system in Virginia along the US mid‐Atlantic coast. We apply decision tree and random forest machine learning methods to identify new empirical relationships between island geomorphology and shrub presence. We find that shrubs are highly likely (90% likelihood) to be present in areas where dune elevations are above ∼1.9 m and island interior widths are greater than ∼160 m and that shrubs are unlikely (10% likelihood) to be present in areas where island interior widths are less than ∼160 m regardless of dune elevation. Our machine learning predictions are 90% accurate for the Virginia Barrier Islands, with almost half of our incorrect predictions (5% of total transects) being attributable to system hysteresis; shrubs require time to adapt to changing conditions and therefore their growth and removal lags changes in island geomorphology, which can occur more rapidly.

Shoreface erosion counters blue carbon accumulation in transgressive barrier-island systems

Landward migration of coastal ecosystems in response to sea-level rise is altering coastal carbon dynamics. Although such landscapes rapidly accumulate soil carbon, barrier-island migration jeopardizes long-term storage through burial and exposure of organic-rich backbarrier deposits along the lower beach and shoreface. Here, we quantify the carbon flux associated with the seaside erosion of backbarrier lagoon and peat deposits along the Virginia Atlantic Coast. Barrier transgression leads to the release of approximately 26.1 Gg of organic carbon annually. Recent (1994–2017 C.E.) erosion rates exceed annual soil carbon accumulation rates (1984–2020) in adjacent backbarrier ecosystems by approximately 30%. Additionally, shoreface erosion of thick lagoon sediments accounts for >80% of total carbon losses, despite containing lower carbon densities than overlying salt marsh peat. Together, these results emphasize the impermanence of carbon stored in coastal environments and suggest that existing landscape-scale carbon budgets may overstate the magnitude of the coastal carbon sink.

An Integrated, Probabilistic Modeling Approach to Assess the Evolution of Barrier-Island Systems Over the Twenty-First Century

Barrier-island systems, spanning ∼7% of the world’s coastlines, are of great importance to society because not only they provide attractive, liveable space for coastal communities but also act as the first line of defense from coastal storms. As many of these unique coastal systems are affected by both oceanic and terrestrial processes, it is necessary to consider the holistic behavior of applicable terrestrial and coastal processes when investigating their evolution under plausible future scenarios for climate change, population growth and human activities. Such holistic assessments, also accounting for uncertainties, can readily be achieved via reduced-complexity modeling techniques, owing to their much faster simulation times compared to sophisticated process-based models. Here, we develop and demonstrate a novel probabilistic modeling framework to obtain stochastic projections of barrier-island evolution over the twenty-first century while accounting for relevant oceanic and terrestrial processes under climate change impacts and anthropogenic activities. The model is here demonstrated at the Chandeleur islands (Louisiana, United States) under the four Intergovernmental Panel on Climate Change (IPCC) greenhouse gas emission scenarios (i.e., Representative Concentration Pathways 2.6, 4.5, 6.0, and 8.5) with results indicating that there are significant uncertainties in projected end-century barrier-island migration distance and available barrier freeboard under the high emission scenario RCP 8.5. The range of uncertainties in these projections underscores the value of stochastic projections in supporting the development of effective adaptation strategies for these fragile coastal systems.

Impacts of Seagrass Dynamics on the Coupled Long‐Term Evolution of Barrier‐Marsh‐Bay Systems

AbstractSeagrass provides a wide range of economically and ecologically valuable ecosystem services, with shoreline erosion control often listed as a key service, but can also alter the sediment dynamics and waves within back‐barrier bays. Here we incorporate seagrass dynamics into an existing barrier‐marsh exploratory model, GEOMBEST++, to examine the coupled interactions of the back‐barrier bay with both adjacent (marsh) and nonadjacent (barrier island) subsystems. While seagrass reduces marsh edge erosion rates and increases progradation rates in many of our 288 model simulations, seagrass surprisingly increases marsh edge erosion rates when sediment export from the back‐barrier basin is negligible because the ability of seagrass to reduce the volume of marsh sediment eroded matters little for back‐barrier basins in which all sediment is conserved. Our model simulations also suggest that adding seagrass to the bay subsystem leads to increased deposition in the bay, reduced sediment available to the marsh, and enhanced marsh edge erosion until the bay reaches a new, shallower equilibrium depth. In contrast, removing seagrass liberates previously sequestered sediment that is then delivered to the marsh, leading to enhanced marsh progradation. Lastly, we find that seagrass reduces barrier island migration rates in the absence of back‐barrier marsh by filling accommodation space in the bay. These model observations suggest that seagrass meadows operate as dynamic sources and sinks of sediment that can influence the evolution of coupled marsh and barrier island landforms in unanticipated ways.

Connectivity in coastal systems: Barrier island vegetation influences upland migration in a changing climate.

Due to their position at the land-sea interface, barrier islands are vulnerable to both oceanic and atmospheric climate change-related drivers. In response to relative sea-level rise, barrier islands tend to migrate landward via overwash processes which deposit sediment onto the backbarrier marsh, thus maintaining elevation above sea level. In this paper, we assess the importance of interior upland vegetation and sediment transport (from upland to marsh) on the movement of the marsh-upland boundary in a transgressive barrier system along the mid-Atlantic Coast. We hypothesize that recent woody expansion is altering the rate of marsh to upland conversion. Using Landsat imagery over a 32year time period (1984-2016), we quantify transitions between land cover (bare, grassland, woody vegetation, and marsh) and the marsh-upland boundary. We find that the Virginia Barrier Islands have both gains and losses in backbarrier marsh and upland, with 19% net loss from the system during the timeframe of the study and increased variance in marsh to upland conversion. This is consistent with recent work indicating a shift toward increasing rates of landward barrier island migration. Despite a net loss of upland area, macroclimatic winter warming resulted in 41% increase in woody vegetation in protected, low-elevation areas, introducing new ecological scenarios that increase resistance to sediment movement from upland to marsh. Our analysis demonstrates how the interplay between elevation and interior island vegetative cover influences landward migration of the boundary between upland and marsh (a previously underappreciated indicator that an island is migrating), and thus, the importance of including ecological processes in the island interior into coastal modeling of barrier island migration and sediment movement across the barrier landscape.

Barrier island migration dominates ecogeomorphic feedbacks and drives salt marsh loss along the Virginia Atlantic Coast, USA

Barrier island migration dominates ecogeomorphic feedbacks and drives salt marsh loss along the Virginia Atlantic Coast, USA

Barrier island migration dominates ecogeomorphic feedbacks and drives salt marsh loss along the Virginia Atlantic Coast, USA

Coupling between barrier islands and their associated backbarrier environments (salt marsh, tidal flats) leads to complex ecogeomorphic feedbacks that are proposed to control the response of barrier island systems to relative sea-level rise. This study tests the applicability of these still-theoretical concepts through investigation of the Virginia barrier islands (eastern United States), which are located in an area of accelerated sea-level rise. Using historical maps and photographs from A.D. 1851 to 2010, we determine that rapid landward island migration (1–6 m yr –1 ) is leading to backbarrier area reduction and large-scale salt marsh loss (63 km 2 or 19%) at a rate of 0.45 km 2 yr –1 . Landward barrier island migration far outpaces upland marsh migration and is responsible for 51% of marsh loss; the remainder is due to backbarrier processes (e.g., edge erosion). In direct contrast to proposed ecogeomorphic feedbacks linking barrier island and backbarrier environments, shoreline retreat rates were not related to changes in backbarrier marsh, open-water areas, or tidal prism. Rather, these results indicate that, for barrier island systems already undergoing migration, the primary barrier-backbarrier coupling is the loss of marsh and tidal-flat area because of barrier island migration.

Stratigraphic evolution of an estuarine fill succession and the reservoir characterization of inclined heterolithic strata, Cretaceous of southern Utah, USA

Abstract This study documented the architecture of a paralic succession using outcrops of the Upper Cretaceous John Henry Member of the Straight Cliffs Formation, southern Utah (USA). The 65 m thick interval of interest includes a basal succession of elongate tidal bars, which sits unconformably on the Calico Bed of the Smoky Hollow Member. These barforms are overlain by carbonaceous estuarine bay fill with tidal deposits, a bayhead delta and, ultimately, a coastal plain succession. The estuarine succession demonstrates a decreasing tidal influence through time and this is interpreted to reflect evolution from a mixed-energy to a wave-dominated estuary as a result of the changing morphology of the estuary and/or the migration of barrier islands across its mouth. The regional correlations are also discussed, with implications for sequence stratigraphic interpretations. A detailed interpretation of the ancient bayhead delta highlights the internal architecture and provides data for statistical comparison between this feature, which is dominated by inclined heterolithic strata, and a previously published study of fluvial point bar inclined heterolithic strata. The results emphasize distinct grain size trends from these two examples, as well as differences in the continuity and spatial distribution of mudstone drapes, which act as both baffles and barriers to fluid flow.

A cross-shore model of barrier island migration over a compressible substrate

Barrier islands that overlie a compressible substrate, such as islands in deltaic environments or those that overlay mud or peat deposits, load and consolidate the underlying subsurface. Through time, the elevation and aerial extent of these islands are reduced, making them more susceptible to future inundation and overwash. Sand washed over the island and onto back-barrier marsh or into the bay or estuary begins the consolidation process on a previously non-loaded substrate, with time-dependent consolidation as a function of the magnitude of the load, duration of load, and characteristics of the substrate. The result is an increase in the overwash, migration, breaching, and segmentation of these islands. This research developed a two-dimensional (cross-shore) numerical model for evolution of a sandy barrier island that spans durations of years to decades as a function of erosion, runup, overwash, migration, and time-dependent consolidation of the underlying substrate as a function of loading by the island. The model was tested with field data and then applied to evaluate the effects of a compressible substrate on long-term barrier island evolution. Results illustrate that barrier islands overlying a compressible substrate are more likely to have reduced dune elevation due to consolidation, incur overall volumetric adjustment of the profile to fill in compressed regions outside the immediate footprint of the island, and experience increased overwash and migration when the dune reaches a critical elevation with respect to the prevalent storm conditions.

A numerical model of coastal overwash

Overwash, the flow of water and sediment over the crest of a beach, contributes to flooding and the deposition of sand landward of the beach crest. Washover, the sand deposited by overwash, contributes to the sediment budget and migration of barrier islands. The ability to predict the occurrence, location, and thickness of overwash deposits is important for coastal residents, coastal town planners, environmental planners, and engineers alike. In this study, a numerical model that simulates the sediment transport and one-dimensional barrier profile change caused by overwash was developed. The magnitude of overwash and the morphology of washovers are dependent on the overwash regime. New formulae are developed to estimate the sediment transport rate over the beach crest for both run-up overwash, using ballistics theory, and inundation overwash, treating flow over the crest as weir flow. Two-dimensional flow is described on the back barrier by considering the continuity of a block of water at steady state, taking into account lateral spreading, friction, and infiltration. The model is tested against 26 different beach profile sets from several different locations, and several different storms, exhibiting a variety of initial morphologies. The model is capable of reproducing varying overwash morphology responses including dune crest erosion, dune destruction, barrier rollback, the thinning of a washover deposit on the backbarrier, and overwash over a multiple dune system.

Shoreline Characterization of the Geomorphological Impact of Hurricane Georges on the Chandeleur Islands: Southeast Louisiana

ABSTRACT The Chandeleur Islands located in southeast Louisiana are the oldest and longest transgressive barrier island arc in the Mississippi River Delta plain. This north-south trending barrier island system was created by the abandonment and transgression of the St. Bernard Delta Complex of the Mississippi River (Penland et al., 1985; Suter et al., 1988). The St. Bernard Delta Complex was abandoned 1.8 ka due to the process of delta switching which resulted in the formation of the Chandeleur Island barrier arc system (Frazier, 1967). The Chandeleur Islands form an almost perfect arc facing the dominant wave approach from the southeast with sediment transport running south to north along the barrier island chain (Ritchie et al., 1992). The Chandeleur Island arc is a Stage II transgressive barrier island arc (Penland et al., 1988), and consists of the main Chandeleur Island followed by Curlew Island, Grand Gosier Island, and Breton Island to the south. The objective of this study is to develop a storm impact geomorphic classification for Hurricane Georges in order to map the geomorphic storm response patterns. This study utilized aerial videotape surveys (AVS) and will provide insight into the processes of hurricane impact and recovery in the Chandeleur Islands. Hurricanes and cold fronts are important to barrier island migration and geomorphology. Storm overwash can occur when hurricanes impact these barrier islands. The fans created by overwash form a new barrier back platform for the island to migrate onto as it continues its landward transgression. The overwash character of an island is controlled partly by its geomorphology (Sallenger, 2000; Morton, in press; Sallenger et al., in press). The geomorphic pattern seen on the Chandeleur Islands is a reflection of previous hurricane impact processes. The AVS were conducted on July 18, 1996, October 10, 1998, and August 28, 1999. Hurricane Georges passed over the Chandeleur Islands on September 28-29, 1998. The three AVS were analyzed and compared to the 1996 basemap. The basemap consists of the linework representing the mean high water line produced from mosaicked 1996 aerial photography of the Chandeleur Islands. Data obtained from AVS makes it possible to identify and map detailed geomorphic changes at a higher resolution than vertical aerial photography (Debusschere et al., 1991). Data from the field studies were used as ground truth to supplement and ground truth the AVS and linework. The information obtained from viewing the AVS was then mapped on the base map in order to create a set of three descriptor bars on the final products. The shoreline characterization includes only the Gulf of Mexico shoreline. The information obtained and mapped in the descriptor bars (one for each AVS) included the placement and relative size of washover channels, tidal inlets, fringing beaches, presence and type of dune system, washover flats and terraces, intertidal flats, outcropping marsh platforms, flood-tidal deltas, ebb-tidal deltas, and barrier destruction. The descriptor bars are made up of symbols assigned to each of the descriptors and a legend was created. Asample of the characterizations made is included in Figure 1. The scale for all plates is 1:12,000. The linear shoreline measurements made from the characterization are included in Table 1. These numbers show the change in abundance of features following the impact of Hurricane Georges (1996-1998) and upon the initial recovery (1998-1999). Storm impacts control island morphology and, in turn, morphology controls the response of an island to storms. Hurricanes may cause extensive amounts of erosion, but hurricanes create the opportunity for sediment to be ransported and deposited in the back barrier lagoons. This sedimentation allows for the creation and extension of the back barrier platform. Therefore, hurricanes help islands to maintain width as they migrate landward. End_Page 451------------------------

Foraminifera as an indicator of overwash deposits, Barrier Island sediment supply, and Barrier Island evolution: Folly Island, South Carolina

Micropaleontological analysis of four vibracores from back-barrier marshes of Folly Island, South Carolina, detected intervals of storm-deposited sediment. Individual washover intervals were not detectable from lithology alone, but only in conjunction with associated foraminifera. Washovers contained more modern offshore and Oligo-Miocene genera and total calcareous foraminifera than `background' marsh samples. The presence of Tertiary foraminifera in the back-barrier cores from Folly Island supports the hypothesis that much of the sediment for barrier island migration is derived from offshore. Foraminiferal analyses of back-barrier cores were also used to calculate relative bioturbation rates by the method of Officer and Lynch [Officer, C.B., Lynch, D.R., 1983. Mixing, sedimentation rates and age dating for sediment cores. Mar. Geol. 46, 261–278, eqs. 10–14]. In all cores, the current depositional environment correlated well with the expected mixing parameter ( G): the highest mixing parameter (greatest signal attenuation) was recorded in a core from a low-marsh/creek-side environment whereas the lowest mixing parameter (least smearing of signal) was recorded in a high-marsh core.

Stratigraphic Setting of Transgressive Barrier-Island Reservoirs with an Example from the Triassic Halfway Formation, Wembley Field, Alberta, Canada

Established models for landward barrier-island migration focus primarily on the preservation pattern of transgressive facies in the shoreface, which are typically thin and buried beneath finer grained marine lower shoreface-to-offshore sediments. In contrast, transgressive barrier-island sandstones in the Triassic Halfway Formation of Wembley field in Alberta are preserved interbedded with, and overlain by, backbarrier and nonmarine sediments. These transgressive barrier sandstones formed from coalescing washover fans during shoreface retreat and were subsequently as the shoreline stabilized and resumed progradation. The abandoned transgressive barrier sandstones were subsequently blanketed by backbarrier and nonmarine sediments as the coastline continued to p ograde. Abandoned transgressive barrier island sandstones in the Halfway Formation are 2-6 m thick, up to 2 km wide, and form paleocoastline-parallel trends tens of kilometers in length. The trends define the paleolandward limit of transgressive events. The updip pinch-out of these sandstones in backbarrier mudstones forms a stratigraphic trap for hydrocarbons in Wembley field. Top seal is provided by nonmarine mudstones and evaporites which buried the abandoned transgressive barrier island. The sandstone has porosities and permeabilities averaging 11% and 63 md, respectively. By using well logs and cores to correlate individual parasequences in the Halfway Formation to their updip termination, it is possible to define the extent of associated marine flooding events and therefore identify exploration targets for abandoned transgressive barrier-island sandstones.

The last interglacial in the Mediterranean as a model for the present interglacial

Deposits of the Last Interglacial on the south and southeastern coasts of Spain are shallow marine and coastal sediments, with a warm fauna of Strombus bubonius. These units exhibit a diversity of morpho-sedimentary models controlled by the tectonic activity of the Mediterranean area, which is closely related to the approximation of Africa and Iberia during the Quaternary. There are three well-dated peaks of maximum sea level (T-I: isotopic substage 7a, T-II: isotopic substage 5e, T-III: isotopic substage 5c). A younger episode, T-IV, probably corresponds to the isotopic substage 5a. Episodes T-II, T-III and T-IV were laid down during Last Interglacial age. In addition, three Holocene peaks of maximum sea level: H 1 ca. 5100 yr B.P., H 2 ca. 3500 yr B.P. and H 3 ca. 2400 yr B.P. were found. The three main peaks of the Last Interglacial correspond to the morpho-sedimentary Tyrrhenian units T-II, T-III and T-IV, deposited during a time span of some 45,000 years. Several smaller oscillations can be distinguished within each of these units as subunits separated by erosional surfaces. At least three of such mapable subunits were distinguished within the peak T-II (5e); each lasted ca. 10,500 yr. As the positive oscillations of sea level (H 1, H 2, and H 3) recorded during the present Interglacial (Holocene) are much shorter, we infer that they are smaller-scale fluctuations (2500-1100 yr cycles) within the first oscillation (duration: ca. 10,500 yr) of the first Holocene peak of sea level which has not yet been completed. In addition to changes of sea level, the vertical and lateral arrangement of morpho-sedimentary units, which can be designated as the stratigraphic architecture, depends on tectonics and oceanography, including geoidal and steric changes and coastal dynamics. The coastal dynamics factor largely depends on the exchange of waters between the Atlantic and the Mediterranean. Maximum incursions of water coincides with warm periods (highstands) when the coastal accretion increases. The tectonic factor greatly influences and modifies the effects of sea-level changes in the coastal area of tectonically-active regions such as the Mediterranean. Areas with tectonic uplift will be characterized by a staircase of prograding gravelly beaches, whereas sinking areas will favour the deposition of vertically stacked sequences with coastal onlap of barrier island and lagoon deposits. Rates of sea-level rise for the coming years of 1 cm/yr, have been suggested by some authors. These gradients greatly exceed those produced by any tectonic factor in the Spanish coast during the last 100 kyr. Shoreface erosion and transgression with landward migration of barrier islands and lagoons will occur in subsiding areas (Murcia-Alicante and Valencia), even with relatively low rates of sea-level rise (less than 0.5 cm/yr). Higher rates of sea-level rise (0.5–1 cm/yr) will increase the transgressive trend. Areas with subsidence rates higher than 7.5 cm/kyr (Mar Menor and Oval of Valencia) are prone to transgression and erosion of barrier islands and lagoons both in the cases of stable and rising sea levels. Risks are smaller in areas with lower rates of subsidence (La Mata,Santa Pola and Torrevieja lagoons) when stable sea level is considered; however, any rise of sea level will trigger coastal erosion.

Estuarine, barrier-island to strand-plain sequence and related ravinement surface developed during the last interglacial in the Paleo-Tokyo Bay, Japan

Recognition of sequence boundaries and transgressive surfaces (i.e. ravinement surfaces, RS) is now known to be of great importance in stratigraphy. The sedimentary features of deposits immediately above a transgressive surface are well exposed in the Upper Pleistocene Kioroshi Formation of the Kanto Plain in central Japan. The formation comprises mainly coastal and shallow-marine deposits (estuarine, barrier-island and the strand-plain systems) which accumulated along a wavedominated coast in the Late Pleistocene, i.e., the last interglacial to last glacial period. The Kioroshi Formation is bounded above and below by sequence boundaries that formed in the lowstand periods correlative to the glacial periods of oxygen isotope stages 4 and 6, respectively. A significant transgressive surface that was formed by landward migration of barrier islands during the transgressive interval, the ravinement surface (RS), is found within the deposits of the upper shelf environment. This ravinement surface is characterized by the exotic nature of the overlying sediment veneer (pebbles, shells and scattered mud clasts) which is poorly sorted. The RS shows a very flattened erosional surface in the shore-parallel sense, and the gradient of the surface in shore-normal sense is calculated as 0.0021, where the syndepositional tectonic movement is revised. The RS commonly cuts through the lower sequence boundary. However, in the places where the river or tidal channel valleys incised, the valley-filling sediment shows a deepening-upward sequence recognized as a transgressive systems tract and the RS can be clearly distinguished from the lower sequence boundary.

Role of inlets in geomorphic evolution of the south shore barriers of Long Island, New York, USA

A study of historical storm-induced changes along the south shore barriers of Long Island (Fire Island Inlet to Southampton), New York, USA, was undertaken in an effort to determine the relative roles of different transport processes in barrier migration. Inlets were found to have a profound effect on the barrier system, largely controlling its landward migration, based upon maps, charts, and aerial photographs from the seventeenth century to the present. Salt marshes became established principally on the broad intertidal bay shoals left by a closed or slowly migrating inlet. Inlets along this microtidal coast are now stabilized by jetties, which may have a negative effect on long-term barrier-island migration.

Barrier island migration and onshore sediment transport, southwestern Delaware Bay, Delaware, U.S.A.

A detailed sediment budget for part of a rapidly migrating (3 m yr −1) barrier system of southwestern Delaware Bay demonstrates that longshore drift and other principal sources of sediment are insufficient to balance the sediment budget during 1926–1981. Changes in bathymetry from 1964 to 1984 indicate that erosion is occurring in shallow areas outside the surf zone. If approximately half of the sediment eroded offshore is transported onto the beach, the sediment budget balances. These results suggest that sand and gravel is supplied to the barriers of the study area from offshore. Several processes probably combine to carry sediment landwards. Tidal currents are weak, and probably rarely entrain sediment without the assistance of waves. Once entrained, sediment may be carried onshore by the combined influence of dominant ebb-tidal currents and frequent northwest winds. This process is probably also aided by a residual onshore shear stress associated with 2nd-order Stokes waves.

Barrier island migration and onshore sediment transport, southwestern Delaware Bay, Delaware, U.S.A.: Pizzuto, J.E., 1986. Mar. Geol., 71(3–4):299–325. Dept. of Geol., Univ. of Delaware, Newark, DE 19716, USA

Barrier island migration and onshore sediment transport, southwestern Delaware Bay, Delaware, U.S.A.: Pizzuto, J.E., 1986. Mar. Geol., 71(3–4):299–325. Dept. of Geol., Univ. of Delaware, Newark, DE 19716, USA

Holocene-Pleistocene Stratigraphy of the Inner Shelf Off Fire Island, New York: Implications for Barrier-Island Migration

ABSTRACT Prevailing hypotheses pertaining to the migrational behavior of the south shore of Long Island barriers are examined, and a more supportable hypothesis is advanced based on existing and new geophysical data. Intermittent landward migration by shoreface retreat is responsible for barrier-island migration from at least the mid-Holocene period. Concave features interpreted to be relict Holocene tidal-inlet-filled channels were found throughout the inner-continental shelf off Fire Island in depths from -12 to -30 m below mean sea level (MSL) approximately 1 to 7 km offshore. Additional supporting evidence for intermittent landward migration by shoreface retreat includes preservation of back-barrier deposits on the inner shelf, presence of linear shoals across the shoreface and shelf, and continuous low-angle Holocene-Pleistocene interface.

Interaction Between Overwash and Eolian Processes on Migrating Barrier Islands: ABSTRACT

Landward barrier-island migration is accomplished by three processes: (1) inlet dynamics, (2) overwash, and (3) eolian transport. Although these processes are well understood conceptually, few studies have been designed to define their relative roles and thus determine the actual mechanics of barrier-island migration in recent times. From field surveys of sites on Nauset Spit, Cape Cod, Massachusetts, and Assateague Island, Maryland, an interaction between the two subaerial sediment transport processes can be recognized. Overwash surges during storm conditions deliver fairly large quantities of sand each year (often exceeding 10 cu m of overwash deposition per meter of dune breach). A large part of this material is then redistributed by the wind, eolian transport being largely governed by the winter northwest (offshore) winds. The net result at Assateague Island is the transport of most of the sand back to the beach face. A small part (less than 10%) of the overwash sand is deposited on the backside of the primary barrier dunes. Although this amount of accretion may seem quantitatively insignificant, this sand may serve as the major source of material to the dunes for their landward translocation concurrent with the migration of the island. This same general model can be applied to the Cape Cod barrier beaches except that drift-line deposits can initiate dune development on the washover fans. These studies can be applied to barrier-island management as well as improving End_Page 486------------------------------ our basic understanding of barrier-island dynamics. End_of_Article - Last_Page 487------------