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The greatest shoreline changes of barrier islands along the South Carolina coast occur in the vicinity of tidal inlets. Depositional processes at these inlets can be categorized as those associated with (1) migrating inlets, (2) stable inlets, and (3) inlets whose main ebb channels breach new positions through their ebb-tidal deltas.
At migrating inlets, curved beach ridges are added to the updrift island while the downdrift island erodes. These processes occur most commonly at shallow inlets whose main ebb channels do not scour into the marine or lagoonal muds underlying the barrier-island sands. Shoreline breachings during storms are also important at inlets with histories of rapid migrations.
Stable inlets have deeper main ebb channels which are entrenched in resistant clays. Morphologic changes associated with these inlets are predominantly the result of wave processes. The coalescing of wave-built swash bars in the outer part of the ebb-tidal delta and the subsequent landward migration of these bar complexes can cause inlets to have either a downdrift or updrift offset, or a straight configuration.
The well-developed ebb-tidal deltas of the South Carolina inlets normally have a single main ebb channel and two or more marginal flood channels. The dominant northeast wave approach causes southerly longshore transport of sand along most of the South Carolina coast and a preferential addition of sediment to the north side of the ebb-tidal delta, which results in a southerly migration of the outer part of the main ebb channel. Because the southerly course is longer and less efficient than a straight course through the ebb-tidal delta, the main channel eventually breaches a new position through a northern marginal flood channel. The accumulation of sand which flanked the old channel is transported landward and accretes to the downdrift beach.
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