Search

Beach‐ridge systems are geomorphological features common in every type of coast and have been widely interpreted as geo‐archives of past sea‐level, climatic, or tectonic changes. These relict morphological elements are well‐known by their facies, architecture, stratigraphy, and evolution. However, the coastal context where beach ridges are formed has never been evaluated. Coastal beach‐ridge systems were classified into four main types corresponding to deltaic strandplains, non‐deltaic strandplains, spits, and barrier islands. Our study consisted of three scales of analysis: (1) the entire beach‐ridge system; (2) beach‐ridge sets; and (3) individual beach ridges. Several beach‐ridge systems having formed in different types of coastal settings are compared by their present characteristics. Geomorphic data generated from processing and interpreting satellite images combined with previous studies are used to quantify multi‐scale attributes of beach‐ridge systems. These attributes include the area, length, width, number of sets, number of ridges per set, set rotation, ridge spacing, and ridge elevation. Our findings demonstrate that significant differences define beach‐ridge systems including deltaic strandplains, large non‐deltaic strandplains, small non‐deltaic strandplains, large spits (cuspate forelands), small spits (flying spits, bay‐mouth spits and tombolos) and barrier islands. A more comprehensive characterization of the factors controlling beach‐ridge variability will improve our ability to recognize the sedimentary record of ancient of these systems. The proposed basic platform can be used to isolated different beach‐ridge types and systems to facilitate future process and morphodynamic studies. [ABSTRACT FROM AUTHOR]

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]

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]

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]

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]

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]

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]

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]

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]