Sedimentary Coasts
5.4 Barriers, Barrier Islands and Lagoons
5.4 Barriers, Barrier Islands and
Forshore bar rising Transportation of sand particals
Forshore bar emerging
Emerging of a barrier island
© 2011 Hans van der Baan / Ingeborg Scheffers
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Fig. 5.51 Along the barrier of this lagoon individual spits have developed by inner longshore drift (Brazil at 22° 54´S and 42° 14´W, width 40 km). (Image credit: © Google Earth 2010).
Fig.5.52 Patos lagoon in south-eastern Brazil, with a length of about 250 km, is the largest of the world. (Image credit: © Google Earth 2010).
Fig. 5.53 a) A segmented lagoon in north-eastern Siberia (69° 49´N and 175° 46´E) with a width of 35 km, and (b) enlarge-ment of the north-eastern section, where a part of the outer lagoon is segmented. Width here is only 10 km. (Image credit:
© Google Earth 2010).
Fig. 5.54 Lagoons at Nantucket island in NE USA (41° 16´N and 70° 02´W). (Image credit: © Google Earth 2010).
Fig. 5.55 A smooth coastline formed by abrasion of headlands consisting of drift material that is closing the remaining bays by barrier development. Baltic shoreline of Germany and Poland at about 54° 30´N and 16° 41´E. length of the shoreline shown is more than 90 km. (Image credit: © Google Earth 2010).
Fig. 5.56 Lagoons with their long axes more or less perpen-dicular to the coastline and closing barriers are rare closed rias.
Martha´s Vineyard Island, Massachusetts, north-eastern USA at 41° 21´N and 70° 36´W, length about 9 km. (Image credit:
© Google Earth 2010).
Fig. 5.57 A barrier has cut a ria coastline from the open sea in the Gulf of Guinea, west Africa, at 5° 17´N and 4° 31´W. Width of scene is about 70 km. (Image credit: © Google Earth 2010).
Fig. 5.58 Three barriers and three lagoons show a long his-tory of coastal accretion south of Perth in Western Australia at 32° 52´S and 115° 41´E. Width of the image is 10 km. (Image credit: © Google Earth 2010).
Fig. 5.59 A chain of barrier islands in the northern Netherlands about 5 to 10 km in front of the mainland coast, with tidal flats in the shelter of the islands. (Image credit: © Google Earth 2010).
Fig. 5.60 Spiekeroog (nearly 10 km long) as one of the many barrier islands of the German North Sea Coast (Image credit:
© Google Earth 2010).
Fig. 5.61 Barrier island chain around Cape Hatteras, east coast of USA. (Image credit: © Google Earth 2010).
Fig. 5.50 The process of barrier or barrier island development in front of a coastline, where the normal waves touch ground and form offshore sand bars, which later grow to chains of islands.
5.52 5.51
5.53b 5.53a
5.55 5.54
5.57 5.56
5.59 5.58
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have washed over the barriers. In larger lagoons, wind may produce waves with longshore drift that initiate the growth of secondary spits along their inner margins, sometimes separating the lagoons into water bodies of nearly oval form.
Barriers may form by different processes and dependent on the type and availability of sedi-ments, the profile of the coastal slope, tidal range and wave conditions and in addition relative sea level changes (5.49–5.5.58). Many barriers have developed during and after the postglacial sea level transgression by submergence of existing sand ridges and the onshore movement of sea floor sediments. Others have emerged as nearshore bars during relative sea level regressions. Transgressive barriers migrate landward across their back barri-er lagoons othbarri-ers remain stable and grow seaward by progradation of beach or dune ridges. Barriers are found along 13% of the world’s coastline, in general where the tidal range is small. They may become barrier islands where stronger tidal cur-rents maintain open, transverse gaps.
Barrier islands are long, narrow and uncon-solidated sandy islands lying parallel offshore to the main shoreline (Figs. 5.59–5.61). They are separated from the main coast by bays, tidal flats or shallow lagoons. As offshore sandbars build up above the waves, vegetation takes hold and stabilizes the initial islands. Shells, sea grass or algae may accumulate as well, and wind drift may accumulate sand at these obstacles. The process of build-up is accelerated, and finally an island emerges where the first submarine bar developed.
Barrier islands are common along tectonically stable, low-lying coasts where tidal ranges do not exceed 4 m and longshore currents are strong (Otvos, 2005). Yet, a new study identifies the world’s longest chain of barrier islands along a stretch of the equatorial coast of Brazil, where spring tides reach seven meters and identified a total of 2149 barrier islands worldwide using satellite images and Google Earth imagery, topographical maps and navigational charts (Stutz and Pilkey, 2011). All told, the 2149 barrier islands measure 20,783 kilometers in length, are found along all continents except Antarctica and in all oceans, and make up roughly 10% of Earth’s continental shorelines. Seventy-four percent of the islands are found in the northern hemisphere. Prominent barrier islands are found along the coast of the Wadden Sea in Germany, The Netherlands and Denmark. The nation with the
most barrier islands is the United States including the coasts of North Carolina, New Jersey and the Texas coast of the Gulf of Mexico. Most of them have been formed during the Holocene sea level high stand of the past 6000yrs.
The morphology and shape of the islands is con-trolled by a combination of wave and tidal forces.
In wave-dominated, microtidal areas, the barrier islands are typically tens of kilometres long, with widely spaced inlets with large flood-tidal deltas and small ebb-tidal deltas. Along wave and tide dominated coast which occur in mesotidal areas, the islands are shorter and wider with abundant inlets and large ebb-tidal deltas and rather small flood-tidal deltas. Like beaches, barrier islands are in dynamic balance (and equilibrium) with the forces that shape them. Some barrier islands (e.g. along the North Carolina coast of the US) regularly receive the full force of destructive hur-ricanes which can reshape and erode these frag-ile landforms. Moreover, if this natural balance is disturbed – either by natural changes in wave, current or sea level changes or by anthropogenic influences as real estate development (especially in the temperate zone), these landforms are sus-ceptible to erosion and may in cases even disap-pear under the sea surface. Over centennial or even decadal timescales, the shorelines of barrier islands can undergo significant changes by form-ing new inlets, spits or by breachform-ing the existform-ing shoreline. Many homes and other infrastructure are today at risk, but there is little governments or residents can do to prevent these processes from taking their natural course.