heat, which increases the melting of the 

 polar ice caps and raises sea level. 



As sea level rises, the barrier 

 island-sound system migrates up the 

 Continental Shelf. This process has been 

 operating for the last several thousand 

 years and continues today. The front side 

 of an island is moved backwards by 

 erosion. The back side of the island 

 grows by two processes (Neal et al . 1984). 

 Storm driven waves can carry sand from the 

 beach across the island to the back side 

 in a process known as overwash (Godfrey 

 and Godfrey 1977; Knutson and Finkel stein 

 1987). In essence, islands migrate by 

 rolling over on themselves like a tank 

 tread. Secondly, storms often form 

 inlets, new passageways between the sound 

 and the ocean. Tidal currents often carry 

 large quantities of sand through these 

 inlets into the sound, forming a tidal 

 delta much thicker than the original 

 island. Longshore currents cause these 

 inlets to migrate, and as they do the 

 island is thickened over the distance of 

 the migration. This process continues as 

 long as the inlet is open. Overwash areas 

 and tidal deltas become stabilized with 

 vegetation and a new sound-side border to 

 the island is formed. 



The distance a barrier island 

 migrates with a given rise in sea level is 

 a function of the slope of the Continental 

 Shelf. In much of the South Atlantic 

 Bight the slope is so gradual that a 0.3 m 

 rise in sea level produces a landward 

 migration of the barrier island-sound 

 system of from 30 to 300 m (Pi 1 key et al . 

 1975, 1980). This means that current 

 rates of sea level rise translate into a 

 landward migration of 0.3 to 30 m per 

 year. This migration is what beach 

 engineers call erosion. 



The size and shape of barrier islands 

 depend on the relative magnitude of tidal 

 and wave energies (Nummedal et al . 1977). 

 The difference in the forms of a tide- 

 dominated and a wave-dominated coastline 

 reflects the ability of the tidal currents 

 to transport sediments through inlets, 

 versus the ability of wave-generated 

 longshore currents to transport sediments 

 along the coast. Along wave-dominated 

 coasts, the longshore currents produce 

 long, continuous barrier islands with 



small ebb-tidal deltas (sand bodies 

 seaward of inlets) because waves rapidly 

 disperse the sediments. Sediments carried 

 landward into inlets by tidal currents 

 accumulate in large back-barrier flood- 

 tidal deltas because these areas are 

 sheltered from wave dispersal. With an 

 increase in tidal range along tide- 

 dominated coasts, the tidal currents 

 through the inlets increase in strength. 

 Consequently, they can support larger ebb- 

 tidal deltas against the destructive 

 influence of the waves. 



In the South Atlantic Bight, barrier 

 islands along the North Carolina coast 

 typify the wave-dominated coastline. The 

 mean tide range is only 0.9-1.2 m (Figure 

 4). The islands are long, generally 

 narrow, and cut by widely separated tidal 

 inlets with large flood-tidal deltas. 

 They are low in elevation and frequently 

 overwashed (Neal et al . 1984). Islands 

 along the southern South Carolina and 

 Georgia coasts typify a tide-dominated 

 coastline. Here the tidal range is 1.5- 

 2.2 m (Figure 4). The islands are 

 relatively short and stubby and are 

 separated by stable tidal inlets. Large 

 ebb-tidal deltas are associated with all 

 inlets. The islands usually have a well- 

 developed row of sand dunes parallel to 

 the beach that is sufficient to block 

 overwash (Neal et al . 1984). 



In the South Atlantic Bight, ocean 

 waves generally hit the coast at an angle 

 which produces longshore currents from 

 north to south. This is the direction in 

 which sand and inlets migrate, especially 

 along wave-dominated coastlines. However, 

 these waves can be refracted by large ebb- 

 tidal deltas, producing south to north 

 currents just south of the inlet. Ebb- 

 tidal deltas may also produce a wave dead 

 zone just south of the inlet. Islands in 

 South Carolina and Georgia that are 

 sufficiently long have the shape of a 

 drumstick as sand is lost (to the ebb- 

 tidal delta of the next inlet) at the 

 south end and sand accumulates at the 

 north. In contrast, islands in North 

 Carolina are of similar width throughout. 



For a geologist, beaches extend from 

 the base of the first row of dunes to a 

 depth of 10-15 m offshore. What we 

 usually walk on is only the upper beach. 



48 



