Beach Ridge Systems and Cheniers

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.

5.5 Beach Ridge Systems

5.62b 5.62a

Fig. 5.63 A chenier composed of pebbles and shell from the east coast of southern Patagonia. (Photo credit: D. Kelletat).

Fig. 5.65 A beach ridge made from well rounded marble cobbles on the north coast of Ireland. (Photo credit: D. Kelletat).

Fig. 5.64 A fresh beach ridge about 0.8 m high made from coral rubble by the 1999 hurricane Lenny in the Caribbean. It shows singular rubble tongues inland from the overwash process, and an avalanching of the rubble on the steep leeward sides. (Photo credit: A. Scheffers).

Fig. 5.67a,b Portland Bill, a long ridge of pebbles about 3 m above MHW in southern England. (Photo credit: D. Kelletat).

Fig. 5.66 Single steep beach ridge of quartzite cobbles in north-western Ireland. (Photo credit: A. Scheffers).

whereas beach ridges are deposited by wave action usually during storm events. Beach ridges are com-posed primarily of sand, pebbles, cobbles (– gravel) or boulders, or a combination of these sediments and form typically at, or above, the normal spring tide high level and can extend as a continuous linear feature for many kilometres along the shore. (Hesp, 2006). Beach ridge development can take place on very short timescales during one storm event or may proceed rather slow over decades depending on event frequency, sediment supply or hydrodynamic condi-tions. Several studies from cyclone impacts have for example documented that storm wave action can built up ridges of coral rubble more than 3 m high and more than 30 km long during one or two days (Scheffers et al., 2011). A single beach ridge may per-sist for some time and then be eroded by subsequent storms. Extended beach ridge sequences may form through a series of depositional events over time and contribute to the progradation of the coast (Otvos, 2000; Woodroffe, 2003; Sanjaume & Tolgensbakk, 2009; Taylor & Stone 1996, see also Figs. 5.62–5.76).

Once deposited, wind transport may contribute to form superimposed dunes on the beach ridge crests.

Most of the ridge systems show some truncations where beach ridge progradation has been terminated by a phase of shoreline erosion, and then followed by

renewed ridge growth and thus establishing an often complex pattern of different generations of beach ridges (Figs. 5.70, 5.72 and 5.73).

All beach ridge systems near modern sea level are not older than 7000 years, even those with more than 100 single ridges at one location, when the post-gla-cial sea level rise reached modern levels. in neotec-tonic uplifted or glacio-isostatic rising regions, beach ridge systems may reach more than 50 km inland and up to more than 200 m asl., as in the Hudson Bay area of Canada, and the South East region of South Australia.

Cheniers (French chêne for the live oak trees dom-inating the vegetation on chenier ridges in Louisiana) are similar in morphology and composition to beach ridge system, but differ to the later in that they have been deposited on an alluvial foundation along relatively stable sections in deltas or coastal plains (Figs. 5.62 and 5.63). They are generally emplaced during storm events and can consist of sand, shells or a mixture of both. Cheniers vary in height and width but are usually not more than 2–3 m high and up to 50m wide separated by relatively wide intertid-al mud flats. Extended chenier plains occur intertid-along the coasts of Louisiana (US) and French Guiana (South America), and are also typical in Northern Australia along the Van Diemen Gulf.

Fig. 5.68 On the Shetland islands in northernmost Scotland pebble and cobble beach ridges may protect formerly open bays. (Photo credit: D. Kelletat).

Fig. 5.69 Coarse debris beach ridges are also typical in paragla-cial areas, i.e those which store a lot of sediments of all size from glacial and fluvio-glacial times of the ice ages. The image shows the leeward slope of a 5 m high beach ridge in Galway Bay, west coast of Ireland, deposited more than 1000 years ago (Scheffers et al., 2009, 2010). (Photo credit: A. Scheffers).

Fig. 5.70a,b Beach ridge systems composed of coarse coral rubble in the Abrolhos archipelago off Western Australia. (Photo credit:

A. Scheffers).

Fig. 5.71 Excellent preservation of beach ridges from strong wave impact along the west coast of Mexico at about 22°N and 105° 35´W. Width of image is about 20 km. (Image credit: © Google Earth 2010).

Fig. 5.72 A beach ridge system from Brazil at 17° 40´S and 39° 13´W with a width of 20 km. (Image credit: © Google Earth 2010).

5.72 5.71

Fig. 5.73 St. Vincent Island in NW Florida (USA) is a result of beach ridge accretion over several thousand years. Site is at 29° 39´N and 85° 08´W with a width of 13 km. (Image credit: ©Google Earth 2010).

Fig. 5.74 A series of beach ridges closes this lagoon from the open sea in easternmost Russia at 60°N and 170° 09´E. Width of scene is about 25 km. (Image credit: © Google Earth 2010).

Fig. 5.75 A beach ridge system truncated by the modern shoreline in a barrier of the west coast of the Black Sea at 44° 30´N and 28° 46´E.

Width of scene is 25 km. (Image credit: © Google Earth 2010).

Fig. 5.76a,b,c Boulder ridges composed of angular blocks up to 80 tons, 150 m from the cliff edge and at ⁺15 m asl on Inishmaan Island, Aran Islands, central west coast of Ireland. (Photo credit: D. Kelletat).