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In addition to their being vital components of mid- to high-latitude coastal ecosystems, salt marshes contain 0.1% of global sequestered terrestrial carbon. Their sustainability is now threatened by accelerating sea-level rise (SLR) that has reached a rate that is many times greater than the rate at which they formed and evolved. Modeling studies have been instrumental in predicting how marsh systems will respond to greater frequencies and durations of tidal inundation and in quantifying thresholds when marshes will succumb and begin to disintegrate due to accelerating SLR. Over the short term, some researchers believe that biogeomorphic feedbacks will improve marsh survival through greater biomass productivity enhanced by warmer temperatures and higher carbon dioxide concentrations. Increased sedimentation rates are less likely due to lower-than-expected suspended sediment concentrations. The majority of marsh loss today is through wave-induced edge erosion that beneficially adds sediment to the system. Edge erosion is partly offset by upland marsh migration during SLR. ▪ Despite positive biogeomorphic feedbacks, many salt marshes will succumb to accelerating sea-level rise due to insufficient mineral sediment. ▪ The latest multivariate marsh modeling is producing predictions of marsh evolution under various sea-level rise scenarios. ▪ The least well-known variables in projecting changes to salt marshes are suspended sediment concentrations and net sediment influx to the marsh. ▪ We are in the infancy of understanding the importance and processes of marsh edge erosion and the overall dynamicism of marshes. ▪ This review defines the latest breakthroughs in understanding the response of salt marshes to accelerating sea-level rise and decreasing sediment supply. ▪ Climate change is accelerating sea-level rise, warming temperatures, and increasing carbon dioxide, all of which are impacting marsh vegetation and vertical accretion. [ABSTRACT FROM AUTHOR]

Thick bay-fill sequences that often culminate in strandplain development serve as important sedimentary archives of land-ocean interaction, although distinguishing between internal and external forcings is an ongoing challenge. This study employs sediment cores, ground-penetrating radar surveys, radiocarbon dates, palaeogeographic reconstructions and hydrodynamic modelling to explore the role of autogenic processes - notably a reduction in wave energy in response to coastal embayment infilling - in coastal evolution and shoreline morphodynamics. Following a regional 2 to 4 m highstand at ca 5·8 ka, the 75 km2 Tijucas Strandplain in southern Brazil built from fluvial sediments deposited into a semi-enclosed bay. Holocene regressive deposits are underlain by fluvial sands and a Pleistocene transgressive-regressive sequence, and backed by a highstand barrier-island. The strandplain is immediately underlain by 5 to 16 m of seaward-thickening, fluvially derived, Holocene-age, basin-fill mud. Several trends are observed from the landward (oldest) to the seaward (youngest) sections of the strandplain: (i) the upper shoreface and foreshore become finer and thinner and shift from sand-dominated to mud-dominated; (ii) beachface slopes decrease from >11° to ca 7°; and (iii) progradation rates increase from 0·4 to 1·8 m yr−1. Hydrodynamic modelling demonstrates a correlation between progressive shoaling of Tijucas Bay driven by sea-level fall and sediment infilling and a decrease in onshore wave-energy transport from 18 to 4 kW m−1. The combination of allogenic (sediment supply, falling relative sea-level and geology) and autogenic (decrease in wave energy due to bay shoaling) processes drove the development of a regressive system with characteristics that are rare, if not unique, in the Holocene and rock records. These findings demonstrate the complexities in architecture styles of highstand and regressive systems tracts. Furthermore, this article highlights the diverse internal and external processes and feedbacks responsible for the development of these intricate marginal marine sedimentary systems. [ABSTRACT FROM AUTHOR]

FitzGerald, D.M., Georgiou, I., and Kulp, M., 2016. Restortation of the Chandeleur Barrier arc. Louisiana. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1282 - 1286. Coconut Creek (Florida), ISSN 0749-0208. The north-south trending barrier arc of the Chandeleur Islands protects valuable wetlands and reduces hurricane impacts in southeastern Louisiana. Avulsion of the lower Mississippi River ~ 1500 ka led to reworking of the St. Bernard deltaic headland concentrating sand and producing the proto-Chandeleur Islands. During the same period, landward interior delta plain erosion and subsidence formed Chandeleur Sound. Since the 1980's, isolation from new sand sources, high rates of relative sea-level rise (5 mm/yr), and hurricane impacts have produced a low transgressive barrier, periodically breached by numerous hurricane passes. Much of the sand eroded during storms is transported laterally to the ends of the barrier arc and lost from the system. A sustainable restoration plan for the barrier chain involves massive sand nourishment (from Hewes Point and St Bernard shoals ~ 285 × 106 m3) while accommodating its natural landward migration. Sand would be placed along existing barrier segments to build beaches and foredune ridges. Additional sand would widen and elevate recurved spits at hurricane passes. Sand would also be pumped behind the barriers to create a broad intertidal and subtidal sand sheet would provide a platform for barrier migration during transgression. Foredune ridges, recurved spits, and intertidal rear platforms would be vegetated to help stabilize these sand deposits and provide integrity. Finally, 15 large subtidal troughs would be excavated perpendicular and landward of the barrier arc, each filled with a 106 m3 of sand. This plan emphasizes landward sand transport during storms and gradual reintroduction of sand during transgression. [ABSTRACT FROM AUTHOR]

Tidal point bars are commonly developed in coastal plain meandering channels. They form by the same basic processes as fluvial point bars but are further modified by tidal action. This study analyses the sedimentary facies of six active tidal point bars and their relationships. The bars are situated along the Georgia coastal plain, USA, in the lower and upper reaches of the fluvial marine transition and in tidal channels without upstream river input. Based on millimetric logging of 31 cores, nine sedimentary facies were identified, which are proposed to be used for the characterization of tidal point bars in other depositional settings. These bars share common sedimentary facies; however, the internal organization of these facies and their associations differ appreciably in response to complex local tidal processes and fluvial–tidal interactions. The increase in tidal influence results in prominent heterogeneous architectures, with greater concentration of heterolithic stratification (mostly inclined), bioturbation and evidence of flow reversals. In contrast, when freshwater river discharge overcomes tidal stages in the fluvial marine transition bars, structureless coarse‐grained facies with attendant erosional surfaces and decreased bioturbation are predominantly developed. In the lower reach of the fluvial marine transition, along the estuarine turbidity maximum, alternations of fluvial and tidal sedimentation are evidenced by a bimodal facies association, where alternations of gravel lag and muddy facies (massive to heterolithic) dominate. Moreover, despite differences in local hydrodynamics, coarsening‐upward sequences, mostly composed of structureless coarser facies, are consistently distributed along the upper parts of the bars, which may indicate overprinting of regional events such as storm surge‐related floodings in the sedimentological record. This study provides a basis for better identifying the hydrodynamic processes that shape coastal systems, such as estuarine and backbarrier environments. Additionally, those deposits are analogues for certain hydrocarbon reservoirs, thereby increasing knowledge of their facies distribution and aiding production optimization. [ABSTRACT FROM AUTHOR]

Coastal morphologic features associated with past shoreline transgressions and sealevel highstands can provide insight into the rates and processes associated with coastal response to the modern global rise in sea level. Along the eastern and southern Brazilian coasts of South America, 6000 years of sea-level fall have preserved late-stage transgressive and sea-level highstand features 1-4 m above present mean sea level and several kilome ters landward of modern shorelines. GPS with real-time kinematics data, ground-penetrating radar, stratigraphy, and radiocarbon dating within a 2-3-km-wide river-associated strandplain in central Santa Catarina (southern Brazil) uncovered a diverse set of late-stage transgressive and highstand deposits. Here, the highstand took the forms of (1) an exposed bedrock coast in areas of high wave energy and low sediment supply; (2) a 3.8-m-high transgressive barrier ridge where landward barrier migration was prohibited by the presence of shallow bedrock; and (3) a complete barrier-island complex containing a 5.2-m-high barrier ridge, washover deposits, a paleo-inlet, and a backbarrier lowland, formed in a protected cove with ample sediment supply from small local streams and the erosion of upland sediments. Similar signatures of the mid-Holocene highstand can be traced across all coastal Brazilian states. This study presents the fi rst complete compilation of the diversity of these sedimentary sequences. They are broadly classifi ed here as exposed bedrock coasts (type A), back barrier deposits (type B), transgressive barrier ridges (type C), and barrier-island complexes (type D), according to localized conditions of upland migration potential, wave exposure, and sediment supply. These Brazilian systems present a paradigm for understanding future coastal response to climate change and accelerated sea-level rise: the recognition of a minimum threshold sea-level-rise rate of ~2 mm yr-1 above which transgression proceeded too rapidly for the formation of these stable accretionary shoreline features demonstrates the nonlinearity of coastal response to sea-level change, and the site specifi city of conditions associated with the formation of each highstand deposit type, even within a single small embayment, demonstrates the non-uniformity of that response. [ABSTRACT FROM AUTHOR]

Tidal inlets are defined as major tidal channels separating individual barrier islands or barrier spits and adjacent headlands. Two types of barrier islands are recognized: (1) those that consistently migrate landward (transgressive), and (2) those that build seaward (prograding or regressive). The most common types include those that formed from (1) elongation of sand spits from major headlands; (2) drowning of coastal sand ridges; and (3) landward migrating transgressive barriers that stabilized and then prograded seaward during the mid-Holocene on the interfluves between the major lowstand valleys. Large tidal inlets developed in these former river valleys. The influence of tidal range vs. average wave height plays an important role in determining the morphology of the barrier islands and the character of the tidal inlets. As an example, the outer margins of the Georgia Bight, where the tidal range is microtidal and wave heights are greatest, most barrier islands are long (28-30 km), wave-dominated, and transgressive. Toward the mixed-energy head of the Bight, where the tidal range is mesotidal and waves are relatively small, barrier islands tend to be short (<15 km), regressive, and drumstick-shaped. Tidal inlets having large flood-tidal deltas are mostly restricted to the microtidal margins of the Bight, whereas inlets with large ebb-tidal deltas dominate the mesotidal head of the Bight. Most tidal inlets form by the following mechanisms: (1) storm-generated scour channels; (2) spit growth across the entrances to flooded valleys during the mid-Holocene; (3) intersection of major tidal channels by landward-migrating transgressive barrier islands; and (4) tidal prism-controlled, evenly spaced inlets that evolve in regions lacking major former river valleys. [ABSTRACT FROM AUTHOR]

Clastic, depositional strandplain systems have the potential to record changes in the primary drivers of coastal evolution: climate, sea-level, and the frequency of major meteorological and oceanographic events. This study seeks to use one such record from a southern Brazilian strandplain to highlight the potentially-complex nature of coastal sedimentological response to small changes in these drivers. Following a 2 to 4 m highstand at ca 5·8 ka in southern Brazil, falling sea-level reworked shelf sediment onshore, forcing coastal progradation, smoothing the irregular coastline and forming the 5 km wide Pinheira Strandplain, composed of ca 500 successive beach and dune ridges. Sediment cores, grab samples and >11 km of ground-penetrating radar profiles reveal that the strandplain sequence is composed of well-sorted, fine to very-fine quartz sand. Since the mid-Holocene highstand, the shoreline prograded at a rate of ca 1 to 2 m yr−1 through the deposition of a 4 to 6 m thick shoreface unit; a 1 to 3 m thick foreshore unit containing ubiquitous ridge and runnel facies; and an uppermost beach and foredune unit. However, the discovery of a linear, 100 m wide barrier ridge with associated washover units, a 3 to 4 m deep lagoon and 250 m wide tidal inlet within the strandplain sequence reveals a period of shoreline transgression at 3·3 to 2·8 ka during the otherwise regressive developmental history of the plain. The protected nature of Pinheira largely buffered it from changes in precipitation patterns, wave energy and fluvial sediment supply during the time of its formation. However, multiple lines of evidence indicate that a change in the rate of relative sea-level fall, probably due to either steric or ice-volume effects, may have affected this coastline. Thus, whereas these other potential drivers cannot be fully discounted, this study provides insights into the complexity of decadal-scale to millennial-scale coastal response to likely variability in sea-level change rates. [ABSTRACT FROM AUTHOR]

Abstract: Details of the internal architecture and local geochronology of Plum Island, the longest barrier in the Gulf of Maine, have refined our understanding of barrier island formation in paraglacial settings. Ground-penetrating radar and shallow-seismic profiles coupled with sediment cores and radiocarbon dates provide an 8000-year evolutionary history of this barrier system in response to changes in sediment sources and supply rates as well as variability in the rate of sea-level change. The barrier sequence overlies tills of Wisconsinan and Illinoian glaciations as well as late Pleistocene glaciomarine clay deposited during the post-glacial sea-level highstand at approximately 17ka. Holocene sediment began accumulating at the site of Plum Island at 7–8ka, in the form of coarse fluvial channel-lag deposits related to the 50-m wide erosional channel of the Parker River that carved into underlying glaciomarine deposits during a lower stand of sea level. Plum Island had first developed in its modern location by ca. 3.6ka through onshore migration and vertical accretion of reworked regressive and lowstand deposits. The prevalence of southerly, seaward-dipping layers indicates that greater than 60% of the barrier lithosome developed in its modern location through southerly spit progradation, consistent with a dominantly longshore transport system driven by northeast storms. Thinner sequences of northerly, landward-dipping clinoforms represent the northern recurve of the prograding spit. A 5–6-m-thick inlet-fill sequence was identified overlying the lower stand fluvial deposit; its stratigraphy captures events of channel migration, ebb-delta breaching, onshore bar migration, channel shoaling and inlet infilling associated with the migration and eventual closure of the inlet. This inlet had a maximum cross-sectional area of 2800m2 and was active around 3.5–3.6ka. Discovery of this inlet suggests that the tidal prism was once larger than at present. Bay infilling, driven by the import of sediment into the backbarrier environment through tidal inlets, as well as minor sediment contribution from local rivers, led to a vast reduction in the bay tidal prism. This study demonstrates that, prior to about 3ka, Plum Island and its associated marshes, tidal flats, and inlets were in a paraglacial environment; that is, their main source of sediment was derived from the erosion and reworking of glaciogenic deposits. Since that time, Plum Island has been in a state of dynamic equilibrium with its non-glacial sediment sources and therefore can be largely considered to be in a stable, “post-paraglacial” state. This study is furthermore the first in the Gulf of Maine to show that spit accretion and inlet processes were the dominant mechanisms in barrier-island formation and thus serves as a foundation for future investigations of barrier development in response to backbarrier infilling. [Copyright &y& Elsevier]

The evolution of coastal systems during the Holocene resulted from complex interactions and temporal shifts in the relative contribution of sea-level changes, climate change, and sedimentary processes. Along the Red Sea Coast, a 0.5-2 m highstand of sea level at 5 ka can be directly attributed to far-field effects resulting from the reduction in land ice following the last glacial maximum. At the ancient Egyptian harbor of Mersa/Wadi Gawasis, the site of the world's oldest archaeological evidence of long-distance seafaring, stratigraphic and geomor-phologic evidence has been identified for this highstand. Here, wadi sediment input, enhanced by a period of wetter climate of the African Humid Period (early to mid- Holocene), forced the closure of coastal embayments, despite ongoing, relatively rapid sea-level rise. A stable, shallow bay persisted at Mersa/Wadi Gawasis as a result of coincidental aridization and a high-stand of sea level during the mid-Holocene. This bay served as the primary harbor for ancient Egyptian trade along the Red Sea coast. During the late Holocene, shoreline progradation was dominated by sea-level fall, driven by isostatic processes. These results demonstrate the interplay of various global (sea level), regional (climate, sea level), and local (sedimentation, bathymetry) controls on the coastal evolution of the Red Sea and how these controls dictated the response of a complex civilization. Furthermore, they highlight the crucial role played by sedimentation in governing coastal response to changing sea levels. [ABSTRACT FROM AUTHOR]

During the 2005 hurricane season, the storm surge and wave field associated with Hurricanes Katrina and Rita eroded 527 km² of wetlands within the Louisiana coastal plain. Low salinity wetlands were preferentially eroded, while higher salinity wetlands remained robust and largely unchanged. Here we highlight geotechnical differences between the soil profiles of high and low salinity regimes, which are controlled by vegetation and result in differential erosion. In low salinity wetlands, a weak zone (shear strength 500-1450 Pa) was observed ∼30 cm below the marsh surface, coinciding with the base of rooting. High salinity wetlands had no such zone (shear strengths > 4500 Pa) and contained deeper rooting. Storm waves during Hurricane Katrina produced shear stresses between 425-3600 Pa, sufficient to cause widespread erosion of the low salinity wetlands. Vegetation in low salinity marshes is subject to shallower rooting and is susceptible to erosion during large magnitude storms; these conditions may be exacerbated by low inorganic sediment content and high nutrient inputs. The dramatic difference in resiliency of fresh versus more saline marshes suggests that the introduction of freshwater to marshes as part of restoration efforts may therefore weaken existing wetlands rendering them vulnerable to hurricanes. [ABSTRACT FROM AUTHOR]

The survival of salt marshes depends on their ability to maintain vertical elevation and areal extent. In the lateral direction, marsh edges can expand laterally or undergo edge erosion through mass failure or continuous particle erosion through waves and tidal processes. In this study, we evaluate possible relationships between marsh shoreline type within the Great Marsh in Massachusetts and major geotechnical parameters along the marsh edge. We also explore if wave energy, using fetch as a proxy, affects the presence, type, and distribution of shoreline type. We mapped and classified the marsh into four categories: slumping, vertical and abrading, stable/accretionary, and bedrock or gravel, and sampled the marsh edge at 98 sites. Using over 450 measurements, we present typical ranges of values at these marsh edges for bulk density (0.10–1.43 g/cm3), organic content (0.99–55.07%), belowground biomass (0.11–36.76%), and shear strength (4.04–136.49 kPa at 20 cm into the marsh bank, and 4.04–131.03 kPa at 40 cm into the bank). We show that there are no significant differences in fetch or geotechnical properties for the different marsh edge classes. Thus, none of these parameters explain or correlate with edge erosion, even though the majority of previous edge erosion studies focus on these parameters as determinants of edge erosion. We further emphasize the heterogeneity of the marsh, as edge erosion can occur in highly exposed or sheltered areas alike with no trends in geotechnical properties, and that complex interactions between parameters not generally studied may be responsible for edge erosion. • This large-scale field study examined the relationships among geotechnical properties, fetch, and edge erosion at over 90 sites. • Neither fetch nor the geotechnical properties of marsh banks explained the type and distribution of marsh edge erosion. • Previously identified parameters do not explain the observed system, suggesting more complex interactions are causing edge erosion. [ABSTRACT FROM AUTHOR]

Abstract: The morphodynamics of a sand wave field in a flood-dominant channel inside Moriches Inlet was monitored for eight weeks during the summer of 2005. Bathymetric data show sand waves on average are 15 m long and 39 cm high with shallow slip faces. The sand waves remained stationary over the eight-week study. The maximum peak current speeds recorded during this study only reached 60 cm/s. This velocity was not sufficient to cause measurable sand wave migration. Analysis based on work by van Rijn suggests that the sand waves of this size would be created when the current velocity exceeds 80 cm/s, conditions not observed during this study. Water level data from Sandy Hook, NJ, and Shinnecock Inlet, NY, show that large tidal ranges occur when a drop in barometric pressure increases the elevation of high tide and strong north winds decrease the low-tide elevation. During these events the current velocity may reach 80 cm/s, which is predicted to be strong enough to produce these sand waves. This study demonstrates that high-energy, episodic events may control sand wave morphology at Moriches Inlet. [Copyright &y& Elsevier]

The Intergovernmental Panel on Climate Change (2007) recently estimated that global sea level will rise from 0.18 to 0.59 m by the end of this century. Rising sea level not only inundates low-lying coastal regions but also contributes to the redistribution of sediment along sandy coasts. Over the long term, sea-level rise (SLR) causes barrier islands to migrate landward while conserving mass through offshore and onshore sediment transport. Under these conditions, coastal systems adjust to SLR dynamically while maintaining a characteristic geometry that is unique to a particular coast. Coastal marshes are susceptible to accelerated SLR because their vertical accretion rates are limited and they may drown. As marshes convert to open water, tidal exchange through inlets increases, which leads to sand sequestration in tidal deltas and erosion of adjacent barrier shorelines. [ABSTRACT FROM AUTHOR]

Understanding of long-term dynamics of intense coastal storms is important for determining the frequency and impact of these events on sandy coasts. We use optically stimulated luminescence (OSL) dates on relict scarps within a prograded barrier sequence to reconstruct the chronology of large-magnitude erosional events in the western Gulf of Maine. OSL dates obtained on quartz-rich sediments immediately overlying relict scarps indicate severe beach erosion and retreat due to erosional events ca. 1550, 390, 290, and 150 cal yr B.P. Our data provide new evidence of increased storm activity (most likely frequency and/or intensity of extratropical storms) during the past 500 yr, which was preceded by a relatively calm period lasting ∼1000 yr. The width of the coastal sequence preserved between successive paleoscarps shows strong correlation with the time interval elapsed between storms. Our findings indicate that diagnostic geophysical and sedimentological signatures of severe erosional events offer new opportunities for assessing the impact and timing of major storms along sandy coasts. [ABSTRACT FROM AUTHOR]

Abstract: New England salt marshes are dominated by the supratidal high marsh grass, Spartina patens. This grass forms a nearly planar surface, which makes it highly vulnerable to the predicted regime of accelerated sea-level rise (SLR). If the high marsh cannot keep pace with rising sea level it will be transformed to intertidal environments, leading to unusually rapid coastal evolution. Winter processes such as ice-loading of surface peat may degrade the marsh surface. Large volumes of snow and ice compress peat, resulting in shallow compaction and a net loss of elevation in some areas. On the Webhannet marsh in Maine, a simulated ice compaction experiment indicates that thick ice can compress marsh peat (46cm simulated ‘ice’ produced 6.9±0.3mm of compaction; 24cm ‘ice’ produced 3.0±0.8mm). Experimental data suggest that ice thicknesses greater than 10cm depress the marsh surface by 2mm for each cm of total ice thickness. However, surface elevations rebounded to near-control levels within 2weeks of the removal of simulated ‘ice’ from the surface of the marsh. Normal winter ice accumulations on New England marshes, therefore, do not appear to be sufficient to permanently compact marsh surface peat and lead to loss in marsh surface elevation. [Copyright &y& Elsevier]

Three digital precision bathymetric data sets spanning 16.1 yr enabled a long-term analysis of the geometric, migration, and volumetric changes to large bedforms in a 1 km² region of eastern Long Island Sound. Whereas a 1 yr study of sand wave mechanics during 1987 provided indirect evidence of long-term southwestward transport, this study adds a June 2003 data set and conclusively demonstrates a basinward (southwestward) progression of sand waves between 7 and 17 m in height. In addition, the modus operandi of sand wave migration during various temporal scales involves crestal flexing, crestal rotation, and differential migration along the crest. A volumetric analysis suggests that the sand waves are in a waning phase of migration and early stage of preservation as sea level rises into the basin and flood-dominant tidal currents decrease in strength. [ABSTRACT FROM AUTHOR]

The present-day coast of Louisiana is undergoing unprecedented change when compared with other coastal regions of the United States, Whereas most of its shoreline is retreating and its coastal hays expanding at the expense of wetlands, the Wax Lake and Atchafalaya deltas are prograding and forming new delta plains. Coastal processes responsible for reworking the lower delta plain and modifying the coast are modest, including very low-wave energy and a microtidal regime. However, occasional hurricanes and more frequent frontal systems produce elevated water levels and large waves (heights >1 to 2 metersl, which produce erosion, overwash, and barrier breaching. High subsidence rates coupled with eustatic sea-level rise and wave erosion are converting wetlands to open-water bays. Along barrier shorelines, this process is increasing tidal exchange, which is enlarging tidal inlets and the volume of sand captured in ebb-tidal deltas. Sequestration of sand in offshore ebb shoals depletes sand resources to the barrier chain. The segmentation, landward migration, and overall decrease in size of the barriers are a product of relative sea-level rise and the lack of contribution of new sediment to the system. Restoration of the barriers should be planned with an understanding that the Louisiana coast is evolving in a transgression. [ABSTRACT FROM AUTHOR]

The Barataria barrier coast formed between two major distributaries of the Mississippi River delta: the Plaquemines deltaic headland to the east and the Lafourche deltaic headland to the west. Rapid relative sea-level rise (1.03 cm year-1) and other erosional processes within Barataria Bay have led to substantial increases in the area of open water (> 775 km² since 1956) and the attendant bay tidal prism. Historically, the increase in tidal discharge at inlets has produced larger channel cross-sections and prograding ebb-tidal deltas. For example, the ebb delta at Barataria Pass has built seaward > 2.2 km since the 1880s. Shoreline erosion and an increasing bay tidal prism also facilitated the formation of new inlets. Four major lithofacies characterize the Barataria coast ebb-tidal deltas and associated sedimentary environments. These include a proximal delta facies composed of massive to laminated, fine grey-brown to pale yellow sand and a distal delta facies consisting of thinly laminated, grey to pale yellow sand and silty sand with mud layers. The higher energy proximal delta deposits contain a greater percentage of sand (75-100%) compared with the distal delta sediments (60-80%). Associated sedimentary units include a nearshore facies consisting of horizontally laminated, fine to very fine grey sand with mud layers and an offshore facies that is composed of grey to dark grey, laminated sandy silt to silty clay. All facies coarsen upwards except the offshore facies, which fines upwards. An evolutionary model is presented for the stratigraphic development of the ebb-tidal deltas in a regime of increasing tidal energy resulting from coastal land loss and tidal prism growth. Ebb-tidal delta facies prograde over nearshore sediments, which interfinger with offshore facies. The seaward decrease in tidal current velocity of the ebb discharge produces a gradational contact between proximal and distal tidal delta facies. As the tidal discharge increases and the inlet grows in dimensions, the proximal and distal tidal delta facies prograde seawards. Owing to the relatively low gradient of the inner continental shelf, the ebb-tidal delta lithosome is presently no more than 5 m thick and is generally only 2-3 m in thickness. The ebb delta sediment is sourced from deepening of the inlet and the associated channels and from the longshore sediment transport system. The final stage in the model envisages erosion and segmentation of the barrier chain, leading to a decrease in tidal discharge through the former major inlets. This process ultimately results in fine-grained sedimentation seaward of the inlets and the encasement of the ebb-tidal delta lithosome in mud. The ebb-tidal deltas along the Barataria coast are distinguished from most other ebb deltas along sand-rich coasts by their muddy content and lack of large-scale stratification produced by channel cut-and-fills and bar migration. [ABSTRACT FROM AUTHOR]

Coastal-morphological, geophysical (ground-penetrating radar [GPR]), and sedimentological data document extreme storm events along the sandy barriers of Maine''s south–central (Hunnewell and Flat Point barriers) and southwestern (Saco Bay barriers) coastal compartments. The Hunnewell barrier contains four equally spaced buried storm scarps behind the exposed scarp of the Blizzard of 1978, a 100-year storm that eroded more than 100 m of shoreline, causing extensive property loss. These scarps dip 3–5° steeper than the normal beachface slope and consist of sands with more than 50% heavy minerals. The heavy minerals produce distinct subsurface reflections that facilitate the location of buried supratidal parts of storm scarps and the mapping of ancient poststorm shoreline positions. The imaged scarps likely formed within the past 1.5–2.0 ka BP. The Flat Point barrier consists of a prograded sequence overlain by a laterally extensive, seaward-thinning layer of freshwater peat and capped by aeolian sands. This stratigraphy suggests that the bog varied in size through time, contracting during overwash events and aeolian deposition and expanding across washover sheets during extended periods of barrier stability. The main overwash event accompanied by barrier planation and wetland expansion may be linked to the first historical storm in New England, the “Great Colonial Hurricane” of 1635.Evidence of near-modern and mid-Holocene storm events along Saco Bay includes washover units and marsh ridges. Washovers interfinger with saltmarsh peat that ranges in age from 4.5 ka BP to modern. The presence of isolated sandy ridges behind existing and former tidal inlets reflects overtopping of marshes and high intertidal mudflats during major storms. Radiocarbon ages indicate that this process took place at different locations along the Saco Bay barrier complex from 3 to 1 ka BP. [Copyright &y& Elsevier]

Washovers, dune scarps and flattened beach profiles with concentrations of coarse-grained sediment or heavy minerals are the diagnostic geological signatures of large storms on barriers today. It is clear that storms are a major force driving transgressive barriers onshore, but what is not as well understood is the role these powerful erosive events play in the evolution of prograding barriers. Application of ground-penetrating radar (GPR) and a combination of coring techniques have significantly improved our ability to image barrier architecture. Results of these studies reveal a more complex evolution than previously recognized. It is now possible to precisely locate and map storm deposits within prograded barrier lithosomes.A comprehensive study of northern Castle Neck, Massachusetts was performed using 15 km of GPR surveys, a 120-m-long seismic line, 11 cores, and several radiocarbon dates. Storm-related layers are the most prominent horizons contained in the barrier stratigraphy. The geometry and sedimentology of these layers closely resembles those of a present-day post-storm beach. Twenty closely spaced, curvilinear heavy mineral layers imaged in the landward portion of the barrier suggest that the Castle Neck barrier was heavily influenced by storms during its initial phase of progradation beginning ∼4000 years BP. Approximately 1800 years BP, two intense storms impacted the coast depositing two extensive coarse-grained units. These layers mimic the flat-lying sand and gravel deposits that occur in front of a nearby eroding till outcrop following major storms. The great number of storm deposits in the early history of Castle Neck is related to either a period of greater storm activity and/or a slow rate of barrier progradation. The occurrence and preservation of these earlier storm layers are likely a product of the exposure of nearby drumlins resulting in greater availability of iron oxide and ferromagnesian sands. The supply of heavy-mineral sands has gradually diminished as the barrier prograded and the proximal drumlin source was buried by beach and dune sands. [Copyright &y& Elsevier]

In recent years, numerous technological advances have made field studies and laboratory analyses of tidal inlets more time efficient while also substantially improved data quality. Mapping channel bathymetry was once a labor-intensive task that was accomplished by measuring water depths along a detailed network of channel profiles. Now aircraft-operated Light Detection and Ranging (LIDAR) can accurately map the bathymetry of most tidal inlets in a few hours' time. If greater detail of the channel bottom is required, then a multibeam acoustic system is utilized, although this technique is deployed from a boat and is less time efficient. The topographic version of LIDAR enables the construction of inlet shoreline maps and determination of volumes associated with shoreline gains and losses. In another advancement, side scan sonar mosaics now provide a picture of the bottom indicating different sediment types, bedrock outcrops, and bedform distributions. These surveys have also become essential for identifying scour holes next to rock jetties and assessing the susceptibility of these structures to collapse. The accretionary history and/or migrational trend of a tidal inlet can be determined from ground-penetrating radar transects on land and from shallow seismic surveys in water. These records provide a representation of the layers of sediment left behind as the inlet shifted positions. The hydraulic regime of a tidal inlet can be much more accurately determined using the Acoustic Doppler Current Profiler (ADCP). This instrument produces velocity profiles for an entire inlet cross section and provides almost instantaneous discharges for small and moderately sized inlets. In a slightly different mode, ADCP technology has been used for directional wave measurements. Finally, significant upgrades in the manipulation of information, including Geographic Information System (GIS) and image analysis methods, have greatly facilitated the analyses and interpretation of the field data. These advancements have provided scientists with more effective means of gathering data, testing numerical models, and finding solutions to coastal engineering problems. [ABSTRACT FROM AUTHOR]

Relocation of Mason Inlet, North Carolina was necessitated due to severe shoaling of navigable waterways and southerly migration of the inlet that threatened a resort complex and infrastructure on northern Shell Island. The historical decrease in tidal prism and inlet cross sectional area was caused primarily by long-term sedimentation in the backbarrier tidal channels, which eliminated the hydraulic connection between Mason Inlet and the Atlantic Intracoastal Waterway. Revitalization of the Mason Inlet involved relocation of the inlet channel 920 m to the north and dredging Mason Creek. These efforts increased the tidal prism by 3.5 × 10[sup 6] m³ resulting in a larger equilibrium sized tidal inlet compared to the old inlet. The increase in tidal discharge at Mason Inlet may diminish the tidal prisms of adjacent inlets. Closure of the old inlet resulted in the landward transport of sand comprising its ebb delta. Tidal flow measurements taken at the new inlet indicate shorter flood than ebb tidal durations and stronger flood than ebb current velocities. Recent observations of sand accumulating in the backbarrier tidal channels are evidence of net landward sand transport into Mason Inlet. This loss of sand to the backbarrier coupled with the growth of the new ebb-tidal delta will reduce sand supply to the downdrift shoreline. [ABSTRACT FROM AUTHOR]

Presents several pictures of the coast of Ireland. Cliffs of Moher; Antidune migration at the Gowlaun Pocket Beach.

Nauset Spit was breached during a moderate northeast storm on 2 January 1987, forming New Inlet. Factors leading to the breach included a long-term narrowing of the barrier; a 0.5-m storm surge superimposed on near perigean, perihelion, and spring tide conditions; restricted flow through existing inlets, and large differences in tidal range and tidal phase between the ocean and Pleasant Bay. Ultimately, destruction of the foredune ridge and a large hydraulic slope across the barrier (> 1.2 m) facilitated the development of an overwash channel, which led to tidal exchange and inlet formation. As the channel captured an increasingly larger portion of the bay tidal prism, the inlet grew in size from 0.5 km after two months to almost 2.0 km wide by early 1988. Opening of New Inlet increased the tidal range from 1.2 to 1.5 m, which drastically changed the hydraulic character and sediment transport patterns in Pleasant Bay. The opening of New Inlet washed much of the sand from the eroding barrier into the bay. As the dimensions of the inlet increased, the ebb-tidal delta grew in volume reducing the amount of sediment bypassing the inlet. Sand continues to enter the backbarrier building sand shoals and bedforms. The decrease in sand supply to the downdrift barrier caused 100 m to 300 m of shoreline recession immediately south of the inlet from 1990 to 2000. During the past six years the rate of erosion has decreased slightly due to more active transport of sediment past the inlet. The major mechanism of inlet sediment bypassing is accomplished through the breaching of hydraulically more efficient channels through the outer portion of the ebb delta. This process results in the formation of swash bars along the downdrift portion of the delta. These bars are 100 m to 300 m long and attach to the landward beach every 1 to 3 years. Permanent breaching of the spit platform has not occurred at New Inlet due to rapid accretion of Nauset Spit and backbarrier shoals directing ebb flow away from the spit platform. [ABSTRACT FROM AUTHOR]

Geological controls on the origin and preservation of organic-rich facies in Holocene barrier sequences of northern New England are documented using sedimentological and geophysical databases. In addition to backbarrier marsh interfingering with washover/aeolian deposits, several distinct modes of organic accumulation are recognized in association with clastic barrier facies. These include: 1) basal lake gyttja or wetland peats (thickness: 0.1-2.5 m); 2) intra-barrier saltwater/freshwater horizons (0.01-1.2 m); and 3) foreshore peat exposures (up to 1.2 m). Freshwater peat and gyttja underlying barrier lithosomes below contemporaneous Mid-Holocene sea level suggest extensive backbarrier lake and wetlands, possibly due to a wetter climate. Present freshwater organics accumulate up to several meters below lake level, which is controlled by the elevation of groundwater table and, over the long term, sea level trends. In some areas, the saltwater-freshwater peat transition is attributed to cessation of long-term saltwater input into the backbarrier, commonly as a result of tidal inlet closure, rather than relative sea-level fall. The association of well-preserved tree stumps and saltmarsh peat exposed on the foreshore suggests drowning of the upland fringe by rising sea level around 2.5-3.0 ka BP and subsequent barrier rollover. Whereas basal ages of high-marsh peats are conventionally used for sea-level reconstruction, radiocarbon dates of the top portions of in-situ organic units may provide near-maximum ages of burial by barrier sediments. This information is independent of compaction and type of dated material and may be used to estimate the timing of barrier emplacement at or near its present position. Age estimates are less constrained when dating allochtonous organic material, but may still provide upper age limits for overlying clastic sequences. Similar to other mid- to high-latitude coastal settings, parts of maritime bogs and ponds in mid-coastal Maine have been buried episodically by storm overwash or dune migration over the past 5,000 years. The chronology of these events varies over a short distance along the coast as a function of barrier exposure and morphology, antecedent geology, and changes in sediment supply and vegetation cover. [ABSTRACT FROM AUTHOR]

Tests the hypothesis that the geologic and hydraulic complexity of the Kennebec River estuary produces an intraestuary unidirectional, net ebb-oriented sediment-transport system. Location of the Kennebec-Androscoggin drainage basin within the state of Maine; Characteristics of the estuary.

Examines the historical position of the shorelines in East Cambridge, Massachusetts. Stratigraphy; Mean-high water prior to urban development; Historical charts.

Duxbury Beach is a retrograding barrier system located along the South Shore of Massachusetts Bay. The barrier fronts a large embayment consisting of a broad supratidal marsh in the northern sector grading to intertidal flats and open water to the south. Radiocarbon dates indicate that the barrier began forming sometime prior to 3,700 14C years BP. The paucity of sediment combined with rising sea level have produced a relatively low barrier ranging in width from 95 to 285 m. The foredune is discontinuous and storm washovers are common. Historical maps and coastal charts indicate the presence of several former tidal inlets along the length of the barrier, although none exist today. Ground-Penetrating Radar (GPR) surveys suggest a minimum of 18 paleo-inlets along the barrier as indicated by large-scale channel cutand- fill structures. Former inlet channels range from 60 to 285 m in width and are typically 3 to 6 m deep. The ephemeral character of the inlets and their diminutive size are a product of their inability to capture a significant portion of the bay tidal prism. Seven channel fill patterns were documented in the GPR records; the three major types include prograded, conformable, and complex. The patterns of subsurface reflectors reveal that former inlet channels infilled by a variety of processes including simple spit extension from one or both sides and channel aggradation through landward sediment transport. Statistical analyses indicate that generally the position and geometry of former tidal inlets are unrelated to present barrier dimensions or offshore bathymetry, however, there are correlations with bay morphology and hydraulics as well as barrier stratigraphy. In one instance, a relatively wide paleo-inlet coincides with a moderately large bay tidal channel. Deepening of another inlet appears to be related to migration of its channel toward a resistant till headland. This study demonstrates that tidal inlet fill deposits can comprise a significant portion of the coastal lithosome along retrograding barriers (>25% at Duxbury Beach). [ABSTRACT FROM AUTHOR]

Evidence of barrier progradation is found throughout the world, although the modes of seaward growth and their stratigraphic signatures can be quite variable. Based on high-resolution ground-penetrating radar (GPR) images of several New England barrier systems, a mesoscale model of coastal progradation is proposed which consists of four styles: 1) uniform; 2) non-uniform; 3) punctuated, and 4) complex. The first two modes are typically represented by sigmoidal-oblique geometry of reflectors similar in angle to beachface slope, and are differentiated based on observable uniformity of reflector spacing. Punctuated progradation is manifested by: 1) sharp tangential-oblique reflector geometry representing the paleo-beach surface and 2) steeply dipping straight reflector configuration interpreted as a buried upper berm/dune scarp. Significant accumulations of heavy minerals (>30% by volume) resulting from severe storm erosion or riverine floods are responsible for strong return of the electromagnetic radar signal. Complex progradation-aggradation mode reflects an abundant sediment supply and can be recognized by complex sigmoidal-oblique radar facies with occasional toplap. The geometry of individual bounding surfaces may be used to document shifts in sediment supply. The present study indicates that seaward accretion of a jetty fillet may proceed with a time-averaged rate of over 3 m/yr, ultimately producing a progradational sequence comparable in scale and reflector characteristics to that of many natural coastal systems. Along paraglacial coastlines, changes in sediment supply (fluvial, offshore, or local glacial deposits) may introduce various amounts of texturally and compositionally heterogeneous sediments, which can alter the rates of seaward accretion and preserve a record of individual progradation episodes. Local sea-level history and storm and flood frequency exert a major control on the magnitude of accretionary phases and the extent of barrier erosion and retreat. A survey of prograded sequences reported in the literature indicates widespread application of the proposed classification. [ABSTRACT FROM AUTHOR]

The Great Marsh in northern Massachusetts and southern New Hampshire is the largest salt marsh in New England and includes the marsh surrounding Plum Island Sound, one of the most studied coastal wetlands in the United States. A primary goal in marsh research today is predicting their resilience in a regime of sea-level rise, which has become increasingly based on modeling studies with sparse or site-specific field data input. Here, we present the results of a vertical accretion study using 210Pb and 137Cs geochronology developed from 50-cm long auger cores taken at 15 locations throughout the PIS marsh. Two significant outliers were omitted from the final analyses due to their unusual depositional settings. The 137Cs spike produced the shortest-term record and lowest average accretion rate (2.5 mm/yr). Of the various 210Pb accretion models (Constant Flux-Constant Sedimentation: CF-CS, Constant Rate of Supply: CRS, Constant Initial Concentration: CIC), we chose the CF-CS model accretion rate for comparison to other environmental parameters and future marsh projections due to the robust R2 linear regression values and its wide use. The 13 stations exhibited an average accretion rate of 3.13 mm/yr, which is 0.6 mm/yr greater than the average rate reported elsewhere in New England. Individual accretion rates show no correlation with geography, elevation, bulk density, loss on ignition, distance to tidal channels, or peat thickness, indicating the complexity of the interacting factors that control marsh growth. Using moderate sea-level rise predictions, we show that large portions of the high marsh are likely to be converted to low marsh by 2050, and that the entire marsh high platform will become low marsh prior to 2070. This is the first time that wide spread field data are used to project a major transformation of the Plum Island Sound platform marsh that undoubtedly will lead to dramatic changes to marsh drainage, backbarrier hypsometry, tidal current dominance, and redistribution of sediment reservoirs, that will include the fronting barrier islands. [ABSTRACT FROM AUTHOR]

The New England coast harbours a wide variety of barrier forms, which we organize into six barrier-coastline types. The barriers develop in response to the relative importance of several spatially and temporally variable parameters, particularly antecedent topography and geology, sediment abundance and size, exposure to wave and tidal energy and sea-level history. The six coastline types can also be identified in other paraglacial regions. Existing barrier-coastline classification schemes do not allow consistent subdivision of paraglacial barrier coasts. This paper presents a new scheme that is applicable to paraglacial and non-paraglacial barrier coasts alike. Aside from hydrodynamic regime, which forms the basis of the barrier classification most commonly used to date, it includes a compartmentalization factor. Sediment-starved ‘isolated’ (‘type 1’) barrier coastlines are characterized by short, widely spaced barriers. Small, localized updrift and offshore sources have provided sediment for short barriers along ‘clustered headland-separated’ (‘type 2’) barrier coastlines. Various amounts of sediment from larger updrift and offshore glaciofluvial deposits or directly from rivers have formed the longer barriers along ‘wave-dominated mainland-segmented’ (‘type 3a’), ‘mixed-energy mainland-segmented’ (‘type 3b’), ‘wave-dominated inlet-segmented’ (‘type 4a’) and ‘mixed-energy inlet-segmented’ (‘type 4b’) barrier coastlines. Geomorphic form, grain size and stratigraphy can be used to characterize individual barriers along barrier coastlines. Most paraglacial barriers form as spits, but many are transformed subsequently. Welded barriers are common along ‘type 1’ and ‘type 2’ coasts. Baymouth barriers are characteristic of ‘type 3’ coasts, and barrier islands... [ABSTRACT FROM AUTHOR]

Presents a volumetric analysis of a New England barrier system along Saco Bay in Maine using ground-penetrating-radar (GPR) and coring techniques. Geologic and physical setting; Facies distribution; Differentiation of the barrier lithosome; Depth calibration of GPR data.

Analysis of a large data base of ground-penetrating-radar (GPR) profiles from both natural and developed paraglacial barriers along the coast of New England has allowed identification of eight reflection configurations that characterize this type of mid- to high-latitude coastal environment. Bedrock anchor points yield primarily hyperbolic configurations, whereas glacial anchor points and sediment-source areas are characterized by chaotic, parallel, and tangential-oblique configurations. Beaches and dunes produce predominantly sigmoidal oblique, hummocky, reflection-free, and bounding-surface configurations. Back-barrier sediments may yield basin-fill configurations, but generally include abundant signal-attenuating units. The GPR data, calibrated with information from cores, were collected across swash-aligned and drift-aligned barriers in a variety of wave- and tidal-energy settings. Application of a 120-MHz antenna, as used in this study, enables portrayal of a range of sedimentary units, from individual bedforms (on single records) to entire barrier elements (using large numbers of intersecting GPR sections), at maximum vertical resolutions that vary between 0·2 m and 0·7 m. The most important drawback of GPR in the coastal environment is attenuation of the electromagnetic (EM) signal by layers of salt-marsh peat or by brackish or salty groundwater, primarily along barrier edges. This disadvantage is offset by many benefits. Data can be collected at rates of several km per day, making GPR an excellent reconnaissance tool. A core that is used in the calibration of GPR data can be matched with great accuracy to its position on the complementary GPR record, allowing detailed correlation between lithostratigraphy and reflection configuration. [ABSTRACT FROM AUTHOR]

Five small islands offshore of the State of Kuwait in the northern Arabian Gulf (Miskan, Awhah, Kubbar, Qaruh, and Umm Al-Maradem) occur in differing structural settings and have evolved in a dynamic environment of shifting fluvial depocenters, fluctuating Holocene sea level, and variable tidal conditions. As a first step in reconstructing the origin and evolution of the islands, bathymetric charts, historical photographs, topographic profiles, and sediment samples were used to investigate their geomorphology and sedimentary environments. The subaerial portion of the islands varies in area from 0.04 to 0.57 km2. The plan-shape ellipticity ranges from nearly circular (0.10) to oval-shaped (0.54). The northern two islands (Miskan and Awhah) experience mesotidal conditions and are situated on a shallow platform (<5 m below mean sea level), which is part of a larger Failaka Island structure. In contrast, the three southern sites have a microtidal regime and sit atop small isolated platforms of aggrading perched reef origin, which descend steeply to more than 20–30 m depth within 3 km of the islands. Topographic profiles indicate that island interiors are composed of beach-ridge sets, whereas modern shoreline consists of segments of beachrock platforms alternating with depositional salients and flying spits in areas of longshore transport convergence. The proportion of total shoreline length represented by exposed beachrock varies from 4 to 46%. Beach sediments consist of very coarse to medium sand and variable sorting, primarily due to the degree of attrition of bioclastic material. A simple conceptual model is proposed to describe the general evolutionary trajectory of end-member islands. Both their accretion during the Late Holocene sea-level stillstand and decadal-scale persistence of sediment transport convergence sites (salients) suggest a morphosedimentary stability of the islands in the near future. • First comprehensive study of the offshore islands of Kuwait. • Unique archives of storm-forced erosional/depositional trends. • Field-based morphologic and textural database. • Persistent historic coastal landform positions. • Limited satellite image database and challenge of re-visiting the field sites since 2001 (post-9/11) make this a unique database. [ABSTRACT FROM AUTHOR]

A review of the article "Longshore Sediment Transport Directions and Rates from Northern Maine to Tampa Bay, Florida: Literature Compilation and Interpretation," by J. F. van Gaalen, S. F. Tebbens, and C. C. Barton, which appeared in the "Journal of Coastal Research" in 2016, is presented.

Hein, C.J.; Fitzsimons, G.G., FitzGerald, D.M., and Fallon, A.R., 2016. Records of Migration and Ebb-Delta Breaching at Historic and Ancient Tidal Inlets along a River-Fed Paraglacial Barrier Island. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 228-232. Coconut Creek (Florida), ISSN 0749-0208. Tidal inlets link backbarrier environments to the coastal ocean and play a dominant role in both longshore and cross-shore sediment transport. Additionally, inlet-fill sequences comprise up to 50% of barrier lithosomes in wave-dominated settings. This study uses historic records to investigate centennial-scale cycles of inlet dynamics and barrier shoreline adjustment at Merrimack River Inlet (Plum Island, Gulf of Maine). Geophysical and sedimentological data reveal geometric and stratigraphic signatures of these recent changes, and allow for comparison to similar records preserved within a nearby 3600-year-old inlet-fill sequence. Driven by processes of longshore transport, spit elongation and ebb-delta breaching, the Merrimack River Inlet once actively migrated across a 2.5-km long section of Plum Island. An ebb-delta breaching event in the mid-1800s caused abandonment of the former inlet channel and the onshore welding of a large sand bar, which developed into the northern 1.5 km of the island. The inlet stabilized by jetties in the late 1800s. Ground-penetrating radar profiles and sediment cores across this former inlet channel capture the details of changes prior to breaching, including the seaward deflection of the otherwise southerly and landward-migrating channel, in response to onshore bar migration and welding. Similar details of inlet migration, bar welding, and ebb-delta breaching are observed stratigraphic records of an ancient inlet in central PI, located 7 km to the south. Comparison between the ancient and historical sequences provides clues to decipher the complex inlet dynamics preserved in the stratigraphic and sedimentologic record. [ABSTRACT FROM AUTHOR]

Abstract: Many saltmarsh platforms in New England and other northern climates (e.g. Canada, northern Europe) exhibit poor drainage, creating waterlogged regions where short-form Spartina alterniflora dominates and stagnant pools that experience tidal exchange only during spring tides and storm-induced flooding events. The processes related to pool formation and tidal creek incision (via headward erosion) that may eventually drain these features are poorly understood, however it has been suggested that an increase in pool occurrence in recent decades is due to waterlogging stress from sea-level rise. We present evidence here that saltmarshes in Plum Island Estuary of Massachusetts are keeping pace with sea-level rise, and that the recent increase in saltmarsh pool area coincides with changes in drainage density from a legacy of anthropogenic ditching (reversion to natural drainage conditions). Gradients, in addition to elevation and hydroperiod, are critical for saltmarsh pool formation. Additionally, elevation and vegetative changes associated with pool formation, creek incision, subsequent drainage of pools, and recolonization by S. alterniflora are quantified. Pool and creek dynamics were found to be cyclic in nature, and represent platform elevation in dynamic equilibrium with sea level whereby saltmarsh elevation may be lowered (due to degradation of organic matter and formation of a pool), however may be regained on short timescales (101–2 yr) with creek incision into pools and restoration of tidal exchange. Rapid vertical accretion is associated with sedimentation and S. alterniflora plant recolonization. [Copyright &y& Elsevier]

Abstract: Ice rafting is an important secondary sedimentation process that redistributes sediment form tidal flats, channel beds, and ponds to the vegetated marsh surface in northern temperate climates. Source location of ice-rafted sediment is identifiable based on distinct sediment properties. In New England salt marsh systems, ice raft thickness and entrained sediment load vary both during the season and interannually as a function of severity and duration of winter conditions; however, 97% of ice rafts carry measurable sediment loads. Thick rafts move sand or peat up to 100m from source areas, whereas thinner rafts tend to transport mud still further onto the marsh platform, sometimes reaching the upland border. Based on these observations, we present relationships defining the theoretical sediment-carrying potential of ice rafts as well as empirical parameterizations for ice-rafted sediment with respect to ice volume. Our results suggest that ice-rafting deposits a volume of sediment contributing up to 5% of annual vertical accretion, an important input in a region where rates of vertical accretion barely compensate for sea-level rise. We provide conceptual models of ice-raft formation and sediment entrainment linking these processes to the general geomorphic evolution of northern temperate marshes, which must be understood in light of the modern acceleration in rates of sea-level rise. [Copyright &y& Elsevier]

A large deficit in the coastal sediment budget, high rates of relative sea-level rise (~0.9 cm/year), and storm-induced current and wave erosion are forcing barrier shoreface retreat along the periphery of the Mississippi River delta plain. Additionally, conversion of interior wetlands to open water has increased the bay tidal prism, resulting in degradation of barrier islands due to inlet widening, formation of new inlets, and sediment sequestration at ebb-tidal deltas. Single-beam bathymetric surveys along a 165-km stretch of south-central Louisiana barrier coast, from Raccoon Point in Terrebonne Parish to Sandy Point in Plaquemines Parish, were conducted in 2006. These data, combined with historical bathymetry from three time periods (dating to the 1880s), provide a series of digital elevation models that were used to calculate sediment volumetric changes and determine long-term erosional-depositional trends. Dominant patterns during the 125-year period include (1) erosion of ~1.6 × 109 m3 from the shoreface, forcing up to 3 km of shoreface retreat, (2) sediment deposition in coastal bights and at ebb-tidal deltas, and (3) a combined increase in tidal inlet cross-sectional area from ~41,400 m2 to ~139,500 m2. Bathymetric and shoreline change datasets separated by shorter time periods (sub-annual) demonstrate that these long-term trends are driven by processes associated with major hurricane impacts, and that rates of shoreface erosion are an order of magnitude greater during active hurricane seasons compared to long-term trends. [ABSTRACT FROM AUTHOR]

Bathymetric surveys conducted before and after the 2005 hurricane season at Little Pass Timbalier in the Mississippi River delta plain, United States, demonstrate that 9.1 (±2.4) x 106 m³ of sediment was eroded from a 47.9 km² area. Between the two surveys, Hurricanes Cindy, Katrina, and Rita passed within 300 km of the tidal inlet. Comparison of before and after bathymetric data sets shows that the distal portion of the ebb-tidal delta (ETD) was the site of 63% of the total erosion, locally resulting in 400 m of shoreface retreat. Shoaling (0.75-1.0 m) in the seawardmost portion of the ebb channel and erosion in the landward portion (∼1.5 m) resulted in a 160 m landward shift of the inlet throat. Collectively, these processes forced the landward migration of the entire tidal inlet and ETD system. There has been considerable discussion about large volumes of mineral sediment deposited on the Mississippi River delta plain interior marsh surface as a result of hurricanes; however, the origin of this sediment is unknown. We identify the distal portion of an ETD as one possible sediment source and hypothesize that ETD and shoreface sediment is mobilized by hurricane waves and transported landward by surge-induced currents. Our results emphasize the role of frequent and intense hurricanes in long-term coastal evolution and as a mechanism for regional sediment retention within the transgressive system beyond typical barrier overwash processes. [ABSTRACT FROM AUTHOR]

Principal component analysis was used to identify dominant patterns of change in morphology of five recent LIDAR surveys of Shinnecock Inlet. The first principal component accounted for 55% of the ebb shoal variability over a period of 6 years (21 June 1994 through 3 July 2000) and identified three accretionary features: (1) an updrift bar complex, (2) a downdrift bypass bar, and (3) shore perpendicular bars along the downdrift barrier. The evolution of these features, accounted for in the first principal component, appears to be the result of both a natural deflection of the main ebb channel and realignment of the channel by dredging. Evidence of channel migration is seen in comparison of 14 sequential historical photographs of the inlet from 24 September 1938 to 22 April 1997. This study also documents that sand waves (length = 100 to 200 m) within the outer channel region were displaced because of variations in incident wave climate. [ABSTRACT FROM AUTHOR]

To survive rising sea level, salt marshes must accrete vertically, migrate laterally, or undergo a combination of the two. If sufficient sediment is available for marsh accretion, the slope of the surrounding area is relatively flat, and edge erosion is minimal, then a marsh can theoretically maintain its areal extent through a combination of vertical accretion and upland expansion. However, in cases where sediment supply is limited and the marsh is backed by steeper slopes, it is unclear whether accretion and inland migration will be sufficient to counteract the combined effects of rising sea level and edge erosion. Given these barriers to marsh expansion, inland migration may not be a viable solution to marsh vulnerability to sea-level rise. We quantify the potential changes in areal extent under future sea-level rise scenarios for the Great Marsh in northern Massachusetts, where the marsh has a limited suspended sediment supply and relatively steep upland topography. Salt marsh is identified and classified into low or high marsh using LiDAR elevation and validated using aerial photography and vegetation surveys. We generate a simple 1D-H model using locally-measured accretion rates and their relationship to marsh elevation, to determine change in elevation and dominant plant species over time. A maximum inorganic sediment available to the marsh is prescribed for certain model scenarios to test the impact of sediment limitation. This limit is calculated based on the volumetric contribution of mineral sediment to the present marsh accretion rates. Predicted changes in marsh area over a 100-year model period are determined using the surrounding elevation gradients, calculated sediment availability, projected edge erosion, and local rates of sea-level rise (SLR). The two Representative Concentration Pathway (RCP) SLR scenarios used are based on the most recent IPCC report and also include the latest information concerning responses to ice sheet melting in Greenland and Antarctica. We find that as the rate of sea-level rise increases, the areal extent of the marsh decreases due to a lack of the suspended sediment needed to maintain marsh surface elevation and the inability of the marsh to encroach upon steep upland slopes. By comparing a model assuming constant accretion rates to one with mineral sediment-limited accretion, we find that when sediment if limited, marsh habitat conversion and loss occur earlier and more rapidly. • Migration on to uplands is not a viable solution for marshes surrounded by steep uplands. • Systems dominated by high marsh undergoing sea level rise will rapidly convert to low marsh, delaying marsh loss but changing ecosystem services. • Coastal systems with a limited suspended sediment supply will experience earlier and more rapid habitat conversion and marsh loss. [ABSTRACT FROM AUTHOR]

Comprehensive onshore-offshore surficial and sub-surface mapping of a composite barrier (combination of prograded, aggraded, and/or transgressive segments) have provided a better understanding of the (i) mechanisms responsible for the formation and development of coastal barrier systems, (ii) relationships and interactions among individual parts of those systems, and (ii) the overall stratigraphic framework of subaerial and subaqueous segments of the barriers. Here, we investigate these facets of barrier evolution through integration of stratigraphic data from subaqueous high-resolution seismic and subaerial ground-penetrating radar, sedimentology (terrestrial cores and seafloor surface samples), and merged topographic and bathymetric mapping of the Miquelon-Langlade Barrier (northwest Atlantic Ocean, south of Newfoundland). This barrier system has two open coasts and evolved in a paraglacial setting, influenced by the reworking of glaciogenic sediment (glacial moraines) in a regime of complex sea-level changes. The barrier stratigraphic sequence is placed within the context of a shifting period from shoreline transgression to one of regression; the resulting sedimentary units reflect the isolated position of the Saint-Pierre-and-Miquelon Archipelago distal from continental influence. Seismic profiles reveal the position of the lowstand shoreline, located 20–25 m below modern sea level, further refining the existing lowstand model of southern Newfoundland. Continuous onshore-offshore subsurface geophysical mapping of the barrier allows for the identification of the relative positioning of distinct sedimentary units interpreted as subaerial barrier (beaches, dunes, spit), shoals, and shoreface deposits, and allows for estimation of the total barrier sediment volume (235 × 10 6  m 3 ) and its relative subaqueous (90%) and subaerial (10%) components. Moreover, it reveals the three distinct morphological units comprising the Holocene barrier: (i) central, regressive, swash-aligned beach-ridge plains developed atop both thin (westward-prograding) and thick (eastward-prograding) shoreface deposits, (ii) drift-aligned, elongating spits located in the northwest and northeast of the island, and (iii) a transgressive barrier located adjacent to the northwest spit, pinned on its landward side to parabolic sand dunes, and currently experiencing erosion and limited overwash. Finally, this study places evolution of this system in the framework of paraglacial barrier evolutionary typology. [ABSTRACT FROM AUTHOR]

Constructional sedimentary features can provide insight into past changes in relative sea-level (RSL) in regions where traditional bio-stratigraphic markers are absent. The paraglacial beach-ridge plain at Miquelon-Langlade, located 50 km south of Newfoundland, is an example of a well-preserved regressive barrier. Initiation of this plain correlates with a decrease in the rate of RSL rise (from + 4.4 mm/yr to ~ 1.3 mm/yr) at around 3000 years ago. It developed under conditions of normal regression during a period of slow RSL rise (< 1.3 mm/yr). The barrier is composed of two oppositely prograding mixed sand-and-gravel beach-ridge systems, which evolved contemporaneously along two open coasts. The growth of these features reflects high rates of sediment influx that was sourced from the erosion of proximal glacigenic sediment (moraines) and reworked alongshore and across-shore by wave action. The combination of stratigraphic (ground-penetrating radar and sediment cores), topographic (RTK-GPS) and chronologic (optically stimulated luminescence, OSL) data provide a detailed understanding of the constructional history of the plain. The well-defined contact between coarse-grained, wave-built facies and overlying aeolian deposits is used to demonstrate the dominant influences of RSL change in the development of the barrier system and produce a RSL curve over the period of its formation. A net increase of 2.4 m in the surface elevation of wave-built facies is observed across the plain, corresponding to the increase in mean sea-level during its formation. Coupled with OSL dates, trends in elevation of the wave-built facies across the plain are used to reconstruct the relative sea-level history during this period. Acknowledging the uncertainties inherent in the method applied in this study, three distinct periods of sea-level rise can be distinguished: (1) an increase from 2.4 to 1 m below modern MSL between 2400 and 1500 years (average rate of + 1.3 mm/yr); (2) relatively stable or slowly rising RSL (<+ 0.2 mm/yr) from 1400 to 700 years; and (3) a rise of ca. 0.7 m during the past 700 years (+ 1.1 mm/yr). This study not only produces the first RSL reconstruction for the Saint-Pierre-et-Miquelon archipelago but also provides: (i) additional details of RSL changes in a region exhibiting great spatial variations in RSL histories (Newfoundland); (ii) field confirmation that wave-built/aeolian stratigraphic contacts in beach ridges can provide a powerful tool for sea-level reconstructions in mixed clastic systems; and (iii) evidence that sediment influxes can outpace the rate of accommodation creation producing a broad, progradational coastal system. [ABSTRACT FROM AUTHOR]