Christopher J. Hein

Morphodynamics and Facies Architecture of Tidal Inlets and Tidal Deltas

Tidal inlets are highly dynamic systems marking positions along barrier coasts where dominant wave and longshore sand transport processes are juxtaposed with a tide-dominated regime in which onshore-offshore sand movement is manifested in the formation of flood- and ebb- tidal deltas. The morphodynamics of tidal inlets and distribution of their associated sand shoals are governed by the tidal prism, wave versus tidal energy, and the regional geological framework. Sand that is delivered to the inlet channel via longshore transport can be sequestered in the backbarrier, moved onto the ebb-tidal delta, or can bypass the inlet. Such bypassing is accomplished through wave and tidal processes and ultimately results in the landward migration and welding of large sand bar complexes to the downdrift shoreline. Tidal inlet-fill deposits typically exhibit a sharp basal contact with underlying units and consist of a fining-upward sequence in contrast to the generally coarsening-upward barrier lithosome. The preservation potential of inlet and associated tidal-delta deposits is high in regressive sequences, but relatively poor in transgressive systems due to the shallow nature of inlet-fill deposits compared to the base of the erosional wave- or tidal- ravinement surfaces. Exceptions occur in paleotidal inlet regions having large bay tidal prisms and deep inlet channels. Although tidal-inlet deposits have been reported in the rock record and may serve as important petroleum reservoirs, to date they are not readily recognized. High-resolution geophysical and sedimentological research of both active and relict inlets is providing a wealth of information necessary to improve the inlet facies models for ancient sedimentary sequences.

10.8 Morphodynamics of Barrier Systems: A Synthesis

The morphodynamics of open-ocean barrier systems (barrier islands, barrier spits, and mainland or headland beaches), synthesizing classic studies, current scientific knowledge, and future research directions regarding a number of barrier systems globally are reviewed. Within a coastal tectonic framework, the authors address: (1) Amero-trailing-edge coasts (USAs New England coast, mid-Atlantic Bight coast, North Carolina Outer Banks, Georgia Bight coast, and Florida Atlantic coast; Brazils Santa Catarina coast; German Bight coast; and southern and western Australian coasts); (2) marginal-sea coasts (USAs Florida Gulf Coast; Gulf Coast of Alabama, Mississippi, and Louisiana; Texas Gulf Coast; and eastern Australian coast); and (3) collision coasts (USAs Alaskan Pacific coast and New Zealand). Moreover, the chapter includes a glossary and robust current set of references.

Anthropogenic controls on overwash deposition : evidence and consequences

Accelerated sea level rise and the potential for an increase in frequency of the most intense hurricanes due to climate change threaten the vitality and habitability of barrier islands by lowering their relative elevation and altering frequency of overwash. High-density development may further increase island vulnerability by restricting delivery of overwash to the subaerial island. We analyzed pre-Hurricane Sandy and post-Hurricane Sandy (2012) lidar surveys of the New Jersey coast to assess human influence on barrier overwash, comparing natural environments to two developed environments (commercial and residential) using shore-perpendicular topographic profiles. The volumes of overwash delivered to residential and commercial environments are reduced by 40% and 90%, respectively, of that delivered to natural environments. We use this analysis and an exploratory barrier island evolution model to assess long-term impacts of anthropogenic structures. Simulations suggest that natural barrier islands may persist under a range of likely future sea level rise scenarios (7–13 mm/yr), whereas developed barrier islands will have a long-term tendency toward drowning.

Coastal response to late-stage transgression and sea-level highstand

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.

Evolution of paraglacial coasts in response to changes in fluvial sediment supply

Abstract Paraglacial coastal systems are formed on or proximal to formerly ice-covered terrain from sediments with direct or indirect glacial origin. This review addresses the roles of tectonic controls, glacial advances and retreats, sea-level changes, and coastal processes in sediment production, delivery and redistribution along the paraglacial Gulf of Maine coast (USA and Canada). Coastal accumulation forms are compositionally heterogeneous and found primarily at the seaward edge of the Gulfs largest estuaries; their existence is directly attributable to the availability of glacial sediments derived from erosion of weathered plutons within coastal river basins. Multiple post-glacial sea-level fluctuations drove the redistribution of these sediments across the modern lowland and inner shelf. Central to the formation of barrier systems was the paraglacial sand maximum, a time-transgressive phase of relative sea-level fall and enhanced fluvial sand export c. 2000–4000 years following deglaciation. Vast quantities of sand and gravel were reworked landward during the subsequent transgression and combined with additional riverine sediments to form the modern barrier systems. Today, reduced fluvial sediment loads, anthropogenic modifications of barrier and river systems, and sea-level rise have combined to exacerbate long-term coastal erosion and may eventually force these barriers toward a state of rapid landward migration.

Runaway Barrier Island Transgression Concept: Global Case Studies

The regime of accelerating sea-level rise forecasted by the IPCC (2013) suggests that many platform marshes and tidal flats may soon cross a threshold and deteriorate/drown as back-barrier basins transform to intertidal and subtidal areas. This chapter explores how marshes may succumb to rising sea level and how the loss of wetlands will increase the extent and the overall depth of open water in the back-barrier, causing greater tidal exchange. Here, we present a conceptual model that depicts how increasing tidal prism enlarges the size of tidal inlets and sequesters an increasingly larger volume of sand in ebb-tidal delta shoals. The conceptual model is based on empirical relationships between tidal prism and inlet parameters, as well as field and theoretical hydraulic studies of tidal inlets showing that long-term basinal deepening intensifies the flood dominance of existing inlet channels and transforms some ebb-dominated channels to flood-dominated channels. This condition leads to sand movement into the back-barrier, which builds and enlarges flood-tidal deltas, filling the newly created accommodation space. The model hypothesizes that sand contributed to the growth of the ebb and flood tidal delta shoals will be at the expense of barrier reservoirs. This will result in diminished sand supplies along the coast, eventually leading to fragmentation of barrier island chains and the transition from stable to transgressive coastal systems. Several historical studies of barrier island systems throughout the world demonstrate barrier response to changing tidal prism and illustrate different stages of this conceptual model.

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.

Barrier shoreline evolution constrained by shoreface sediment reservoir and substrate control: The Miquelon-Langlade Barrier, NW Atlantic

ABSTRACT Billy, J., Robin, N., Certain, R., Hein, C. and Berné, S., 2013. Barrier shoreline evolution constrained by shoreface sediment reservoir and substrate control: the Miquelon-Langlade Barrier, NW Atlantic. The Saint-Pierre-et-Miquelon Archipelago (France) is located in the NW Atlantic Ocean, proximal to the Cabot Straight outlet of the Gulf of Saint-Lawrence, and 50 km south of Newfoundland (Canada). The Miquelon-Langlade Barrier is a 12-km-long, 100–2500-m-wide, north-south–oriented isthmus connecting two bedrock islands (Miquelon to the north; Langlade to the south). This study aims to improve our understanding of shoreface-shoreline sediment exchange processes by comparing medium-term (1949–2011) shoreline changes, determined from aerial photographs and differential GPS data, with total shoreface sediment reservoir volumes estimated using seismic along the west coast of the Miquelon-Langlade Barrier. Spatial variability between the northern and southern sectors of the study site are seen both in the volumes of shoreface sedimentary reservoirs and in multi-decadal shifts of the shoreline position. The northern region has the lowest shoreface sediment volume and the highest rate of shoreline retrogradation. By contrast, the center and southern regions contain the largest volume of sediment in the shoreface and have demonstrated either long-term stability or progradation. This study demonstrates the primary roles of geological control and the distribution of shoreface sediments in local shoreline change at multi-decadal time scales. The sedimentary reservoir, in conjunction with shoreline-monitoring studies and knowledge of transport patterns, may provide a good alternative proxy.

Evolution of a Pharaonic harbor on the Red Sea: Implications for coastal response to changes in sea level and climate

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 world9s oldest archaeological evidence of long-distance seafaring, stratigraphic and geomorphologic 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 highstand 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.

CYCLICAL SHORELINE EROSION: THE IMPACT OF A JETTIED RIVER MOUTH ON THE DOWNDRIFT BARRIER ISLAND

Beaches and inlets throughout the U.S. have been stabilized for purposes of navigation, erosion mitigation, and economic resilience, commonly leading to changes in shoreline dynamics and downdrift erosion/accretion patterns. The developed beach of Plum Island, Massachusetts is highly dynamic, experiencing regular complex erosion / accretion patterns along much of the shoreline. We analyzed > 100 years of high-water line positions derived from satellite imagery, t-sheets, historical maps, and aerial photography. Unlike most beaches, the river-proximal sections of Plum Island are not uniformly retreating. Rather, the beach undergoes short-term erosion, followed by periods of accretion and return to a long-term mean stable shoreline position. These cycles occur over different timeframes and in different segments of the beach, creating an ephemeral erosion ‘hotspot’ lasting as briefly as one year. The highly dynamic and spatially diverse nature of erosion along Plum Island provides insight into the complex nature of coupled inlet-beach dynamics over multiple timescales.