Common Shoreline Features of Beaches and Barrier Islands
Figure 12.27 illustrates common shoreline features associated with beaches and barrier islands.
A beach is an accumulation of mostly sand (and some gravel) along a shoreline where wave action winnows away finer sediment. Beaches occur in the intertidal zone (the zone between highest and lowest tides). Above the high tide line the upper supratidal part of the beach is mostly impacted by wind (forming dunes) and storm surges (Figure 12.28).
|Figure 12.27. Coastal environments extend from offshore to inland estuaries and bays.||Figure 12.28. Beach and coastal dunes at Point Reyes National Seashore, California|
A barrier island is a long and typically narrow island, running parallel to the mainland, composed of sandy sediments, built up by the action of waves and currents (Figure 12.29). Barrier islands serve to protect the mainland coast from erosion by surf and tidal surges. Examples include the Outer Banks in North Carolina and Padre Island in Texas. Barrier islands are most common on submergent coastlines associated with low-relief regions such as is present along the Atlantic Coast and Gulf Coast of the eastern United States. They form where the sea floor remains shallow for a long distance offshore.
An estuary is the mouth of a river or stream where the tide-driven flow allows the mixing of freshwater and ocean saltwater (Figure 12.30). A lagoon is a saltwater-filled bay or estuary located between a barrier island and the mainland.
A tidal flat is a nearly flat coastal area (at or near sea level) that is alternately covered and exposed by the tides, and consisting of unconsolidated sediments.
|Figure 12.29. Fire Island, NY, is a barrier island on the south shore of Long Island, NY.||Figure 12.30. Tidal marshes and tidal flats along an estuary, Elkhorn Slough, CA.|