Institute for Water Resources

Home
Home > Missions > Coasts > Tales of the Coast > Coastal Histories > Dynamic Sustainability > Barrier System Basics

Tales of the Coast
Coastal Histories

Dynamic Sustainability: Maryland's Atlantic Coast

Ocean City and Assateague Island

Dynamic Sustainability: Shoreline Management on Maryland's Atlantic Coast

Barrier System Basics

Fenwick and Assateague Islands are long and narrow landforms found off the coast of Maryland. They were formed after the last Ice Age when rising seas eroded sediment from the Delaware headlands, which was then deposited along the coast as these barriers. Fenwick Island is a barrier spit, which is attached to the mainland, and Assateague Island is a true barrier island.

These barriers are gradually eroding as longshore currents and rising seas continually move sediment to the south. At the southern end of Assateague Island there is a growing, hooked portion of the island upon which sediments are deposited.

Waves moving perpendicular to the shore change barriers. High-energy waves remove sediment from the beach and deposit it offshore in sand bars. Then when low-energy waves prevail, they push that sediment back onto the beach. These high- and low-energy waves correspond to seasonal winds and storm activity.

High-energy waves during storms also erode sediment and move it into offshore bars. Storm waves can push sediment from the ocean side over to the bay side of the island, essentially "rolling over" the island.

Strong storms cause the formation of flood-tidal deltas when waves cut through the barrier, causing an inlet to open. Sediment moving along the shore then gets swept into the inlet, and some is deposited in underwater sand bodies known as shoals. The shoal to the landward side of an inlet is called the flood-tidal delta. As sediment continually moves down the shore, it eventually clogs the inlet and the inlet closes. Then the flood-tidal delta slowly attaches to the barrier as overwashed sediments are piled up on the bay side and plants become established in the shoal. This substrate and plant matter grows together into new land on the bay side of the island.

There are ecosystems throughout barriers that are adapted to the dynamic conditions. The nearshore, which is the underwater area just off the beach, contains sand bars and a limited amount of marine life. The beach is continually changing because of seasonal variations, and it remains a sand-dominated environment. Landward of the beach is the dune area, where a large ridge or ridges of sand are piled up. When overwash flattens the dunes, it leaves thin layers of sediment across the barrier, which are known as barrier flats.

The thicket zone is landward of the dunes and barrier flats. These ecosystems can grow into forests with pine trees and shrubs because they haven't been flooded by salt water and they are protected by the dunes. Many types of mammals populate this forest environment on Assateague Island.

On the bay side of the barrier there are two types of salt marshes: low and high. The low marsh is almost constantly flooded, and the high marsh is flooded only at high tide. Plants known as cordgrasses dominate these ecosystems because they can live in salt water. These areas are extremely important for all kinds of wildlife because there is water and land so closely intermingled. These marshes then lead into the bays, which are Assawoman and Isle of Wight Bays to the west of Fenwick Island and Sinepuxent and Chincoteague Bays to the west of Assateague Island.

Assateague Island

Assateague Island
Source: IAN Image Library

Fenwick and Assateague

For thousands of years Fenwick and Assateague were one barrier spit connected to the Delmarva Peninsula. Fenwick Island is still attached to the mainland, thus it is not a true island, but Assateague Island was severed from Fenwick in 1933 by a hurricane.

Continual Movement

Through the processes of overwash and flood-tidal-delta absorption, barriers are continually moving westward in a long-term geological time frame. In the short term they change from season to season with the removal and deposition of sediment.

Constantly Changing

Barriers are constantly changing landforms. They move westward very slowly; the beaches grow and shrink with the season; and the environments change depending on how much overwash takes place.