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Skip Navigation LinksDNREC : Division of Watershed Stewardship : Shoreline and Waterway Management

 
Barrier Island/Sand Cycle

 

Barrier beaches are delicately balanced systems that have evolved since the last ice age, about 14,000 years ago, under the rigorous and never ending assault by ocean waves. Over time the barrier beaches have slowly migrated landward as a result of rising sea level. As sea level rises, the seaward edge of the barrier is eroded and major storms wash sand over the barrier island. Evidence of this process is sometimes apparent after major storms when marsh peat and stumps of old trees are exposed along the beaches. These are remnants of forests and marshes that grew behind an older, more seaward barrier that has since moved inland.

Though the long-term net movement of the beach and dune system goes unnoticed over the short term, annual sand movement and seasonal variations of the beach width are noticeable. Generally the dry beach area is wide during the calmer summer months when gentle ocean swells predominate. These calmer waves transport sand from offshore bars and the surf zone to the beach causing the beach to gradually build up or accrete. (Dune Fun Program through Seagrant on the web.)

Over time sand is blown inland onto the dune where it is trapped by vegetation and stored until removed by storms. In the winter storm season, beaches become narrow as larger waves impinge against the coast, carrying sand offshore and depositing it offshore bars. This process of moving sand offshore helps to dissipate much of the wave energy by causing waves to break further offshore. During storms, large waves washing against the base of the dune erode sand causing the seaward face of the dune to collapse, at times completely destroying the dune. If the supply of sand remains constant this natural process will repeat itself and the dunes will eventually rebuild in width and height

The dunes are less susceptible to seasonal fluctuation in the beach since they are only exposed to the erosive forces of wave action during the more severe storms. The dunes are, however, very susceptible to wind erosion and depend upon beach grass for their growth and survival.  Beach grass traps windblown sand causing it to accumulate around the plants and lowers wind velocities at the dune surface to keep accumulated sand from blowing away. The network of underground roots extends the plants laterally, while the plants continue to grow upwards through accumulating layers of sand.

Visitors to Delaware's beaches may not realize the damage they are causing by walking or driving indiscriminately over the dunes to get to and from the beach rather than using the authorized crossovers. Although beach grass is a hardy plant, amazingly tolerant to high salinity conditions, direct sun, extreme heat, lack of fertile soil and a fluctuating water supply, it can not survive being trampled by vehicle or man.

As part of its resistance to salinity and drying conditions the plant has developed a thick brittle stalk which unfortunately snaps easily when trampled or driven upon. The passage of only one vehicle or a few people over the dune at the same point will kill a strip of grass. Without the anchoring protection provided by the grass, the wind quickly erodes a cut into the dune that enlarges with time. If not repaired, this cut becomes susceptible to breaching by waves during a coastal storm.  Once the dune is breached water flows through flooding low-lying areas directly behind the dune while at the same time widening and deepening the breach.

 

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