Breaking of storm waves on sand and reef zone in the Lesser Antilles Arc
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The most part of the exposed coastal zone of the Lesser Antilles Arc are composed by sand and coral reef. The high frequencies of passage of cyclones near these islands and anticyclone’s swell subject them to waves of large amplitude. These waves are 4 to 5 times lager to the normal conditions. The weak slopes observed on these zones are particularly sensitive to this type of waves and cause the process of surfing. The mode of dissipation of these waves influenced the run-up and the floods on the coast. The surf zones are situated in 5 in 20 meters of the line of coast. A displacement of sea water towards the coast line is provoked by the breaking of the waves. These quantities of water are held by the particularly bathymetry of these islands and provoke a raised of the sea level. The propagation of the waves are allowed by the sea elevation in the surf zoneKeywords:
Swell
Surf zone
Anticyclone
Fringing reef
Storm Surge
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ABSTRACT Study of the response of an active oolite sand shoal (Lily Bank), located along the northeastern margin of Little Bahama Bank, to its physical environment and the Holocene sea-level rise has provided a developmental sequence from initial formation to present maintenance for this marine sand belt. Lily Bank is oriented parallel to the bank margin and is independent of underlying bedrock topography. It has developed immediately bankward of two large, intrareef bank-margin reentrants which allow a relatively unrestricted flow of tidal and storm-generated currents on and off the bank top. Subsequent to the initial sea-level rise over the bank approximately 5,700 yr BP, these reentrants funnelled bankward-directed flow which resulted in two zones of bedforms: (1) an outer zone of linear, p rallel-to-flow sand ridges (200-500-m spacing, 3-4 km long, 2-3 m in height) oriented in a radiating pattern away from the reentrant throats; (2) an inner belt of sand waves oriented transverse to flow (90-300-m spacing, 1-2 m in height). These now-relict bedforms developed approximately 4,000 yr BP. As sea level continued to rise, bankward flow velocities decreased as the reentrant channeling effect decreased. The linear sand ridges, which require a higher flow velocity for development and maintenance, became less active and eventually were stabilized by marine grasses. The adjacent sand-wave field developed rapidly during the deceleration in sea-level rise which occurred around 3,500 yr BP. A portion of this sand-wave field continues to migrate slowly into the shelf lagoon in response o the dominant bankward energy flux and maintains itself in pace with the slowed sea-level rise. The present geometry and sand-transport regime on the oolite shoal appear to result from interplay between storm-generated and tidal currents. Storm-generated currents have developed channels which cross the sand body and terminate in flood- or ebb-oriented spillover-lobes. Between the lobes are wide, flood-dominated zones with flood oriented sand waves protected from ebb-tidal flow by topographically higher shield areas covered with symmetrical sand waves. During storms, the shield areas are flattened and sand is transported bankward onto the shelf lagoon when the dominant energy flux is in that direction. Tidal currents reestablish the shield areas during poststorm recovery. Tidal bar belts oriented perpendicular to bank margins, and marine sand belts, parallel to bank margins, are not necessarily mutually exclusive, and occur together along this same bank edge. The now inactive linear sand ridges seaward of Lily Bank may be analogous to active tidal bar belts found elsewhere in the Bahamas today. Paleoenvironmental ramifications are that, despite differences in sand-body and sedimentary structures, both types of sand body may exist along the same bank margin. The tidal bar field is located near the bank edge and the marine sand belt is adjacent to it but farther bankward. Because reentrants within the bank margin are common, they have played an important role in localizing oolite shoals which then may serve as foundations for future island development, perhaps during a later sea-level fluctuation. Some of the large Pleistocene islands in the Bahamas may be situated upon reentrant-controlled oolitic sand shoals.
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This article is devoted to the study of the basic laws of the formation, structure, morphology and composition of sediments of accumulative landforms at the mouths of small rivers on the northern (fjord) coast of the Kola Peninsula washed by the Barents Sea waters. Along the northern Kola fjord Coast of Barentz Sea, the development peculiarities of lithodynamical processes are geographical location, expansion of old strong rocks with slowly rates of cliffs retreat and with severe hydrodynamical regime. As a result of this alongshore drift flows cannot develop. Normal exchange by sediment with adjoining regions of a sea shores are absent. Abrasive-denudation process is dominant, and it intensifies the sediment deficit within narrow coastal zone of the Sea. In addition, coastal deposition of the shore origin is unlikely along the entire distance by absolute Murmansk shoreline. The main reason is the strong impact of water dynamic and very steep shelf slope. They throw off sedimentary material to deep bottom without sphere of positive waves (surf) impulse for big debris of local rocks. That is why within the Northern coast of the Kola Peninsula the most important lithodynamical action all time were separate hearth’s sites of deposition and short flows of migration. All time the fiord tops have location of sediment debris which undergo wave rubbing, and during sea-ice time discard shore sediment take place. A detailed analysis in the mouths of the Teriberka, Voronya and Kharlovka rivers where modern sandy terraces of the coastal-marine genesis lie have been carried out. It was noticed that the influenced of strong igneous and crystallized rocks and active physical weathering after the fast melting of snow from the catchment area carries away fine-grained material into the rivers (fractions ≤ 3 mm). These materials which are composed mainly of Alluvials leads to the construction of terraces. The productive vector of the wind flow on the coast with an average long-term wind speed ≈ 8 m/s is directed from land to sea. The smallest sediments are blown into the sea from the surface of the terraces. In contrast to the eastern shores of the Barents and White Seas, these sediments form sandy tidal dehydrations. Silt debris composed of fine-grained material of (fractions ≤ 1 mm) dominates here. The main part of alluvium ≥ 1.0 mm is retained in river channels. It should be noted that on the studied coast, an average of 15.36% of these fractions erupt into the composition of coastal terraces. The smallest debris and part of the sand goes down to a depth along steep underwater slope (up to i10 ≤ 0,15). On the frontal surface slope of the terraces, fractions of 1.0-0.25 mm on the beach dominate (of which the content is 0.5-1.0 mm = 38.93%). All sand fractions account for 88.78%, including coastal dunes. The dune relief forms are small and are in the germinal state. They are characteristics of the mouths of the rivers where they exist.
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Denudation
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ABSTRACT High-resolution seismic-reflection profiles of the Quicksands, located along a broad ridge on the platform shelf west of Key West, Florida, indicate a significant deposit of non-oolitic carbonate sand occurs in a belt 47 km long by 28 km wide. The surface of the belt is ornamented by large (5 m), migrating tidal bars, oriented in a north-south direction, on which sand waves, oriented in an east-west direction, are superimposed. Some of the sand waves are awash at low tide. The sand waves are formed by strong reversing tidal currents flowing between the Gulf of Mexico and the Straits of Florida. The waves migrate directly over Pleistocene bedrock to the east, but the deposit thickens to the west and sand waves there overlie non-oolitic Holocene accumulations as thick as 12 m. Westward- ipping accretionary bedding indicates that net migration of the sands is to the west, despite north-south movement of tidal currents. The westward edge of the accumulation has accreted over deeper, muddier deposits. Although tidal currents and resultant bedforms appear identical to those of active ooid deposits in the Bahamas and elsewhere, no oolitically coated grains were found in this study. Thin-section analyses show the principal component (average 48%) of the sands is fragmented plates of species of the green alga Halimeda, followed by particulate coral (average 17%), which increases off the flanks of the main sand body. Short vibracores confirm the presence of cross-bedding.
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Regularly spaced incised shore-normal grooves (gutters) on plane consolidated surfaces in littoral and sublittoral zones are widely observed in the marine geological record. Despite their common occurrence there are few investigations into their origins in contemporary marine environments. While their formation is often attributed to wave-induced currents related to wave swash and backwash on the beach-face, no conceptual model has been advanced to explain the presence of gutters, their morphology or their quasi-regular alongshore spacing. The paper examines gutters cut in soft sandstone at Medmerry near Selsey, UK and argues that their formation is related to wave breaking and swash zone processes during an unprecedented sequence of storms in the winter of 2013/14. During this period exceptionally high near-shore waves (Hs around 6m) were recorded for the south coast beaches and these storm conditions persisted periodically through to mid-February 2014. The consequence was extensive beach erosion and the exposure of underlying substrates. In this study gutter morphology was quantified using terrestrial lidar and a wave-resolving numerical model was used to defined the nearshore wave conditions and swash characteristics. Three of the largest storm events during the winter of 2013/14 were modelled: (a) moderate waves coincident with an exceptionally high tide; (b) exceptionally high waves occurring during neap tides; and (c) high waves occurring during spring tides. The model results showed swash zone shear stress is a dome-shaped function of distance across the beach-face thereby controlling gutter depth. Further, high-speed sheet flows characterised by periodic, shore-normal, high and low speed streaks alongshore are thought to be implicated in the mechanism controlling gutter spacing. However, in any situation, the specific spacing of gutters is moderated by both the local sheet flow characteristics and the larger-scale morphological forcing. Together these factors indicate that gutter spacing is an emergent property which makes spacing unpredictable.
Swash
Longshore drift
Plage
Coastal erosion
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Seabed
Deposition
Crest
Bedform
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Crest
Seabed
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Swash
Swell
Coastal erosion
Beach morphodynamics
Beach nourishment
Longshore drift
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Bedform
Seabed
Ripple marks
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Seabed
Bedding
Flume
Ripple marks
Tidal current
Open sea
Deposition
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Barrier islands on bedrock-framed coasts are poorly known. In this paper we report on the only barrier island system on the high-energy, west coast of Ireland. Bartra Island, County Mayo is approximately 4.5 km long and 0.5 km wide. It is topped by a continuous, well-vegetated dune ridge up to 30m high and is surrounded by intertidal sand beaches. The ocean-facing beach is a gently sloping, generally dissipative sand beach while the back-barrier contains extensive intertidal sand flats up to 800m wide, but more typically 300m. A sandy shoreface fronts the island and it is flanked by tidal inlets with well-developed ebb deltas. These extend directly into the adjacent ocean and appear not to be deflected by waves. Flood deltas as distinctive features are less prominent since the back-barrier contains extensive intertidal sands and has less accommodation space for delta development. There are 10m-deep tidal inlet channels that facilitate regular tidal exchange and the island is backed by an area of open water and intertidal flats. Important differences with better known barrier islands relate to the fact that (i) it is a single island confined to a bedrock-bounded embayment (ii) it is anchored on a relict glacial moraine, (iii) it is topped by high, vegetated dunes that reduce the importance of barrier overwash as an important evolutionary process and (iv) longshore drift is unimportant in its behaviour.
Overwash
Longshore drift
Bedrock
Coastal geography
Beach morphodynamics
Tidal range
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